United States- Region 7 

Environmental Protection Emergency Response and Removal Branch 

Agency Kansas City, KS 

CHEMICAL ACCIDENT 

INVESTIGATION REPORT 

TERRA INDUSTRIES, INC. 

NITROGEN FERTILIZER FACILITY 

PORT NEAL, IOWA
TERRA CHEMICAL ACCIDENT INVESTIGATION REPORT

TABLE OF CONTENTS

SECTION 1: EXECUTIVE SUMMARY.........................................l

SECTION 2: BACKGROUND ................................................ 5

Figure 2-1, Site Location Map ............................... . 6

Figure 2-2, Terra Facility Process Flow Diagram ................... 7

Figure2-3, Facility SiteMap .................................. 8

Photo 2-1, Post Explosion Aerial Photo ........................ 11

SECTION 3: CHEMICAL ACCIDENT INVESTIGATION OVERVIEW ............... 1 2

Photo 3-1, AN Plant Area After Explosion ...................... 14

Figure 3-1, AN Plant Mounting Bolt Deformation ................. 15

SECTION 4: AMMONIUM NITRATE (AN) INFORMATION ...................... 17

Decomposition and Detonation Hazard .................................... 17

Ammonium Nitrate Sensitization ......................................... 18

Safe Practices for Manufacturing and Storage of Ammonium Nitrate .............. 19

Investigation Objectives for Determining AN Sensitization ...................... 19

SECTION 5: AMMONIUM NITRATE PLANT OPERATIONS ................ 21

Neutralizer . . . . . . . . . . . . . . . . . . . . ........ ....... 21

Figure 5-1, AN Plant Process Flow Diagram................ 22

Figure 5-2, AN Plant Schematic Drawing ................ 23

Photo 5-1, Neutralizer and Rundown Tank . ................ 24

Photo 5-2, Scrubber . . . . . . . . . . . . . . . . . ................ 25

Figure 5-3, Neutralizer . . . . . . . . . . . . . . . ................ 26

Rundown Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................ 27

Figure 5-4, Rundown Tank . . . . . . . . . . . . ................ 28

Ammonium Nitrate Scrubber . . . . . . . . . . . . . . . . . . . . . ................ 29

Figure 5-5, AN Scrubber . . . . . . . . . . . . . . . ................ 30

Distributed Control System (DCS) . . . . . . . . . . . . . . . . . . ................ 31

Figure 5-6, AN Plant P&ID. . . . . . . . . . . . . ................ 32

SECTION 6: FACILITY EVENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

SECTION 7: INVESTIGATION-DERIVED FACTS AND CONDITIONS .............. 35

Acidity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Contaminants .......................................................... 36

i 
Chlorides ................................................... .. 36

Petroleum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 6

Photo 7-1, EPA Item#14 ................................... 38

Low Density Areas ................................................... .39

Temperature ...................................................... .. 3 9

Confinement .................................................... .... 39

Concentration ....................................................... .40

Materials of Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Figure7-1, Titanium Sparger .............................. .41

Table 7-1, Metal Corrosion Rates ............................. 42

Photo 7-2, Neutralizer Fragment ............................. .44

Photo 7-3, Neutralizer Fragment ............................. .44

Photo 7-4, Sparger Fragment ............................... .45

Rundown pH Probe ................................................. ..4 6

Steam Distribution System ............................................. .48

Figure 7-2, pH Measurements (Comparison) ..................... 49

Figure 7-3, Ambient Temperature Graph ........................ 50

Figure 7-4, Steam Distribution System ........................ .51

Air and Steam in Spargers .............................................. 52

Nitric Acid Supply .................................................... 53

Figure 7-5, Nitric Acid Valve Position .......................... 54

Figure 7-6, AN Titration Curve ............................... 55

“No Flow” Condition in the AN Plant ...................................... 56

Figure 7-7, Neutralizer Feeds vs. pH ........................... 57

Distributed Control System ............................................. 58

AN Operator Process Monitoring ......................................... 59

ScrubberOperations ................................................ .. 5 9

Table 7-2 Neutralizer pH and Concentration ..................... 61

SECTION 8: EXPLOSION CAUSE SCENARIOS ................................ 64

Table 8 - 1, Explosion Cause Scenarios ......................... 65

Photo 8-1, AN Plant Craters ................................. 66

Photo 8-2, 83% Product Pump Pads ........................... 66

Photo 8-3, AN Scrubber Pad and Vessel Floor ................... 67

Scenario 1 - AN Plant Pumps ........................................... .68

AN Scrubber Recirculation Pump ................................... 68

Photo 8-44 416J Shaft View ................................ 69

Photo 8-4B, 4165 Suction View .............................. 70

Product Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Photo 8-5, 416J External Damage ............................. 72

Photo 8-6, 416J External Damage ............................. 72

Photo 8-7, 416J Casing ..................................... 73

Photo 8-8, 416J Impeller .................................... 73

i i
Figure 8-1, Rundown Tank Level .............................. 74

Scenario 2- AN Scrubber ............................................ . . 75

Scenario 3 - Neutralizer ................................................ 76

Photo 8-9, AN Scrubber .............................. ... ... 77

Photo 8-10, AN Scrubber Mounting Bolts ...................... 78

Photo 8-11, AN Scrubber Mounting Bolts ..................... 78

Figure 8-2, Scrubber Level .................................. 79

Photo 8-12, Water Tank Damage (Ammonia Tank Background) ...... 80

Photo 8-13, Ammonia Bullet Tank Damage ..................... 80

Photo 8-14, Chemical Stripper Trajectory ....................... 81

Photo 8-15, Neutralizer Base Fragments ........................ 83

Photo 8-16, Ammonia Sparger Plate ........................... 83

Figure 8-3, Fragment Location Map ........................... 84

Spargers ...................................................... 85

Neutralizer Contents ............................................ 87

Figure 8-4, Neutralizer Thermal and Low-density Zones ............ 89

Scenario 4- Rundown Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Photo 8-17, EPA Item #8 (Neutralizer) ......................... 91

Photo 8-18, EPA Item #10 (Rundown Tank) .................... 91

Figure 8-5, Rundown Sank Level, Last 6 Minutes ................. 93

SECTION 9: CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

SECTION 10: RECOMMENDATIONS ........................................ 96

Figure 10-1, Contributing Factors ............................. 99

SECTION 11: BIBLIOGRAPHY, WORKS CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

iii 
Terra Chemical Accident Investigation Team 

Principal Investigators 

Mark J. Thomas, Ph.D. -- Team Leader

On-Scene Coordinator, U.S. EPA

Alan Cummings - Codes and Emergency Response Specialist

Dynamac Corporation

Mariano Gomez, EIT, MBA -- Chemical Engineer

Technical Assistance Team, Ecology and Environment

Technical Consultants 

W. J. Bradford, PE -- Loss Prevention Expert 

Rudolph B. Spanholtz -- NACE Certified Metallurgist

St. Louis Testing Laboratories

Angel Salinas -- Drafting and Graphics

Technical Assistance Team, Ecology and Environment

iv 
Glossary of Terms and Acronyms 

AN Ammonium nitrate. 

Blocked in A phrase indicating that a piece of process equipment has been isolated from 

the rest of the process, usually by closing valves leading into and away from 

the equipment. 

Blowdown Transferring AN solution from the scrubber into the neutralizer. 

CAS# Chemical abstract service number (also CASN). 

DCS Distributed control system. 

Hot work permit A written permit that is required before personnel can conduct maintenance 

or other activities that could provide an ignition source. Examples are welding 

and grinding. 

Offgas A gaseous byproduct of some urea production plants that typically contains 

ammonia, carbon dioxide and steam. 

Prilling A process in which a hot, concentrated liquid is dropped in a countercurrent 

flow of heated air to solidify the liquid drops into solid spheres. 

Turnaround A scheduled maintenance period when an entire plant or facility is shut down 

for maintenance. 

Sparger Device used to uniformly introduced a fluid into a second body of fluid 

V 
PhD.; Mariano Gomez,

emergenq

SECTION 1: EXECUTIVE SUMMARY 

At approximately 0606 hours on December 13, 1994, an explosion occurred in the ammonium 

nitrate plant at the Terra International, Inc., Port Neal Complex. Four persons were killed as a direct 

result of the explosion, and 18 were injured and required hospitalization. The explosion resulted in 

the release of approximately 5,700 tons of anhydrous ammonia to the air and secondary containment, 

approximately 25,000 gallons of nitric acid to the ground and lined chemical ditches and sumps, and 

liquid ammonium nitrate solution into secondary containment. Off site ammonia releases continued 

for approximately six days following the explosion. Chemicals released as a result of the explosion 

have resulted in contamination of the groundwater under the facility. 

The U. S. Environmental Protection Agency (EPA) Region VII was directed by EPA 

Headquarters to conduct an investigation to determine the cause of the explosion and to develop 

recommendations that would help prevent future similar occurrences in ammonium nitrate production 

facilities. This report contains conclusions reached by the EPA chemical accident investigation team 

regarding the cause of the explosion at the Terra International, Inc., Port Neal Complex that occurred 

on December 13, 1994, and recommendations for preventing future similar occurrences in ammonium 

nitrate facilities. This report is the culmination on ten months of work by EPA’s investigation team. 

The investigation was conducted principally by EPA On-Scene Coordinator (OSC) Mark Thomas,

 Alan cummings, Dynamac Corporation; and Ecology and Environment, Inc. 

Both Dynamac Corporation and Ecology and Environment, Inc , are EPA contractors. Dr. Thomas 

and Mr. Cummings participated in EPA’s response activities at Terra that included 

assistance in air monitoring, chemical stabilization oversight. and tank integrity assessment. 

Once the emergency situation had been stabilized, the investigation team began the process 

of gathering information, conducting interviews and depositions, and attempting to determine the 

cause of the December 13 explosion. Some of the documents EPA attempted to obtain were 

destroyed in the explosion, some did not exist, and others did not reflect the construction or operation 

of the ammonium nitrate plant at the time of the explosion. In large part, piecing together the events 

leading up to the explosion was done by talking to Terra employees about what was going on in the 

hours, days, and in some cases, months prior to the explosion. Repetitive interviews were necessary 

to: 

1. Reconstruct reasonably accurate drawings of the plant because of the lack of current drawings 

provided by Terra; and 

2. Identify operating procedures used in the ammonium nitrate plant that were not written 

procedures. 

Investigation team conclusions were reviewed by scientists and engineers before the report 

was released. Multiple reviews were conducted to ensure that conclusions were reasonable based 

upon the information gathered during the investigation. 

The investigation team concluded that the explosion resulted from a lack of written, safe 

Executive Summary Page 1 
operation procedures at the Terra Port Neal ammonium nitrate plant. The lack of safe operating 

procedures resulted in conditions in the plant that were necessary for the explosion to occur. The 

significant conditions that caused the explosion were: 

1. Strongly acidic conditions in the neutralizer and rundown tank; 

2. Prolonged application of 200 psig steam to the neutralizer nitric acid spargers; 

3. The creation of bubbles and low density zones in the neutralizer; 

4. Lack of flow in the neutralizer and rundown tank; 

5. The presence of chlorides in the neutralizer and rundown tank; and 

6. Lack of monitoring of the ammonium nitrate plant after the plant was shut down with the 

process vessels charged. 

No process hazards analysis had been completed on the ammonium nitrate plant, and 

interviews with Terra personnel indicated that they were not aware of many of the hazards of 

ammonium nitrate. The two conditions identified by Terra personnel as concerns were oil 

contamination of ammonium nitrate and excess heating of ammonium nitrate. No one engineer was 

assigned responsibility for overseeing operation of the ammonium nitrate plant and reviewing 

operating procedures in the plant or procedures that might impact the ammonium nitrate plant. 

Information gathered during the investigation indicated that overall communications and 

working relationships were poor between operations and engineering personnel. In the months 

preceding the explosion, the ammonium nitrate plant was converted to a distributed control system, 

(DCS). The engineers involved in hooking up the DCS communicated very little with most 

operators, and some of the operators felt very uncomfortable with the new system once it was up and 

running. They stated that they had received very little training on operation of the AN plant with the 

DCS system. 

In the days and weeks just prior to the explosion, the equipment failures and maintenance 

problems were chronic. The pH probe in the neutralizer rundown line appeared to be malfunctioning 

and there were no spares in stock. Both the ammonium nitrate product pumps which transports 

ammonium nitrate to storage were leaking or otherwise malfunctioning. There were numerous 

problems in the nitric acid plant, and maintenance was having a hard time keeping up with the repair 

requests. 

From interviews with Terra employees, no one believed that oil contamination could have 

gotten into the ammonium nitrate plant and sensitized the ammonium nitrate, even though large 

amounts of oil had been lost upstream in the ammonia plant. Terra did not monitor ammonium nitrate 

feedstreams for contaminant presence. Chloride contamination of the nitric acid had not been 

analyzed since 1980; although the nitric acid plant is a logical source of chloride contamination. Terra 

did not periodically monitor feed streams into the ammonium nitrate unit for contamination. 

The investigation team developed recommendations to reduce the likelihood of future similar 

occurrences in ammonium nitrate plants.The detailed information that led to these recommendations 

is included in the report. The recommendations are that facility management, with the involvement 

Executive Summary Page 2 
of engineers, operators and maintenance personnel: 

1. Conduct a thorough, formal process hazard analysis (PHA) of the ammonium nitrate process 

according to industry guidelines and practices. Current process safety information including 

piping and instrumentation diagrams, plant drawings, process chemistry, chemical hazard 

information and expertise in the technique used are necessary to conduct this evaluation.- The 

findings and recommendations generated by the PHA should be promptly addressed and 

resolved and should address modifications, safeguards or controls to eliminate, reduce or 

manage chemical and process hazards. 

2. Establish safe operating parameters for all activities in the ammonium nitrate process based 

on the PHA. Parameters for this plant should at least include pH, temperature, and acceptable 

contaminant levels. 

3. Develop, implement and keep up-to-date written safe operating procedures for all operations 

and activities, including normal startups, normal and emergency shutdowns or idling and 

routine operation of the ammonium nitrate unit. These written procedures should be based 

on the PHA and require that critical process parameters identified above be monitored and 

specify actions to be taken when parameters deviate from acceptable ranges. 

4. Develop a management of change process for all changes in process equipment, procedures 

and operating parameters or ranges in the ammonium nitrate unit.A prestartup safety review 

should be conducted prior to operation using changed equipment, procedures or parameters. 

5. Develop a program to maintain the on-going mechanical integrity of the ammonium nitrate 

unit. Facility management should consider use of predictive failure analyses and aggressive 

preventive maintenance systems as part of their mechanical integrity program. 

6. Develop and implement training programs on operating and maintenance procedures for 

operators and maintenance personnel involved in the ammonium nitrate unit. 

7 . Ensure that management, engineers, operators and maintenance personnel develop lines of 

communication to ensure that these recommendations are implemented and maintained. The 

investigation team further recommends that corporate management monitor facility 

performance in implementing these programs and conduct periodic audits to ensure program 

effectiveness. 

8. Share information on the hazards of the substances handled, the prevention measures in-place 

or planned to prevent accidental releases and the emergency response measures to be taken 

for the ammonium nitrate unit with the State Emergency Response Commission (SERC), 

Local Emergency Planning Committee (LEPC), first responders, and the public surrounding 

the facility. 

These recommendations reflect accidental release prevention requirements contained in the 

Executive Summary Page 3 
OSHA Process Safety Management (PSM) regulations and in current industry guidelines and 

practices for prevention of chemical accidents and emergencies. EPA also intends to build on the 

OSHA PSM requirements and is currently considering how best to capture these recommended 

practices in the Risk Management Programs for Chemical Accidental Release Prevention rule to 

promulgated in March 1996. 

Executive Summary Page 4 
SECTION 2: BACKGROUND 

The Terra Industries, Inc., Port Neal Complex is a nitrogen products facility located in the 

western portion of Woodbury County, Iowa. The facility is located in a primarily rural area 

containing some industry, in Section 24 of the Liberty Township, Woodbury County, Iowa (Figure 

2-1). The major activities at the Port Neal Complex include the synthesis of anhydrous ammonia, 

operation of a nitric acid plant, and urea and ammonium nitrate production (Figure 2-2). The plant 

is composed of two major areas. Area I consists of the ammonia plant, tank farm, and utilities 

(Figure 2-3). Area II consists of the nitric acid plant, the urea plants, and the ammonium nitrate 

plant. 

The ammonia plant manufactured ammonia from methane, water, and air. Carbon dioxide 

was recovered as a by-product of ammonia production. Ammonia was transferred to storage for sale, 

to the nitric acid plant for nitric acid production, to the urea plant for urea production, and was 

available to the ammonium nitrate plant for ammonium nitrate production. 

The nitric acid plant oxidized ammonia in air in the presence of a precious metal gauze to 

produce 55%-56% nitric acid. The nitric acid was transferred to a storage tank and was subsequently 

used as a raw material in the ammonium nitrate plant. 

The three urea plants at the Terra facility produced urea solution and urea prills from ammonia 

and carbon dioxide. A by-product of two of the urea plants. Offgas, contained primarily ammonia, 

carbon dioxide, and steam. This offgas was transferred to the ammonium nitrate plant as an ammonia 

source during normal ammonium nitrate plant operations 

The ammonium nitrate plant produced an 83% ammonium nitrate (AN) solution by reacting 

ammonia and nitric acid in a vessel called a neutralizer. The nitric acid plant supplied the nitric acid. 

Urea plant offgas supplied the ammonia for normal operations. The AN plant could also use 

ammonia stored in two pressurized storage vessels (bullet tanks) and ammonia from storage. The 

AN solution was sold or mixed with urea to form a urea-ammonium nitrate solution. 

Terra Industries, Inc., has owned and operated the Port Neal Complex since its original 

construction beginning in 1965. The facility began manufacturing activities in late 1967. Terra built 

the facility with the intent of providing agricultural chemical users within a 200 mile radius of the 

facility with ammonia and other nitrogen crop chemicals. The facility shipped approximately 70% 

of the manufactured nitrogen products from the facility by truck, and the balance by rail. The Urea 

II process was added to facility operations in 1974, with major modifications in 1978. Terra replaced 

the original C&I Girdler neutralizer in the ammonium nitrate plant with a Mississippi Chemical 

Corporation neutralizer in 1980. 

A major modification of the ammonia plant that included the installation of a Honeywell TDC-3000 

distributed control system (DCS) and process equipment upgrades was completed in 1992. The 

replacement of process equipment increased the ammonia process capacity from 800 tons per day 

(TPD) to 1,000 TPD. An ammonia scrubber and a Honeywell TDC3000 distributed control system 

Background Page 5 
FIGURE 2-l

SITE LOCATION MAP
PRODUCTION FLOW DIAGRAM AT THE PORT NEAL, IOWA FERTILIZER FACILITY
FIGURE

2-3

SITE MAP
were added to the ammonium nitrate plant during the facility turnaround conducted in September 

1994. Future references to the ammonia scrubber will be as the AN scrubber. 

The Port Neal Complex comprises approximately 70 acres. Finished products at this facility 

include anhydrous ammonia, ammonium nitrate, urea, and urea ammonium nitrate solution. Finished 

products are sold primarily to wholesale distributors. Raw materials included natural gas, water, and 

air. Material transportation to and from the facility included highway, rail, and pipeline conveyances. 

The facility was staffed 24 hours per day with a minimum of two shift supervisors and eight 

operators. The facility employed approximately 100 persons. 

The U. S. Environmental Protection Agency (EPA) Region VII conducted a chemical safety 

audit (CSA) on the ammonia synthesis process in the ammonia plant at the Port Neal Complex in 

February 1994, as a result of a reported 600-pound ammonia release that occurred on February 24, 

1993. The chemical safety audit was conducted by a team of engineers and emergency response 

specialists. The CSA team reviewed documents provided by Terra that were representative of 

ammonia plant activities in the areas of operations, maintenance, and process safety. The audit 

provided a snapshot of conditions that existed in the ammonia plant at the time of the audit, and did 

not reflect planned or anticipated changes in the plant. The resulting audit team recommendations 

were not mandatory. The CSA team did not conduct follow up inspections to determine whether or 

not any changes were made by Terra pursuant to the audit team recommendations. The CSA was 

conducted with the full and voluntary cooperation of Terra Industries, Inc., corporate and facility 

management. 

At approximately 0606 hours on the morning of December 13, 1994, an explosion occurred 

at the Terra Port Neal Complex within the ammonium nitrate plant. Four persons were killed and 18 

persons were hospitalized as a result of the explosion. Initial reports indicated that the explosion had 

occurred in the ammonium nitrate plant. Prior to the explosion, the ammonium nitrate plant had been 

shut down because of a nitric acid shortage and had remained shut down until the explosion. The 

nitric acid plant had been shut down since approximately 0500 hours on December 12, and Terra was 

in the process of starting up the acid plant when the explosion in the AN plant occurred. 

The explosion caused extensive damage to the Terra facility and resulted in the release of 

approximately 5,700 tons of anhydrous ammonia into the atmosphere and on the ground throughout 

the facility before the emergency phase of the resulting response was completed on December 19, 

1994 (photo 2-1). Plumes and clouds of varying ammonia concentrations were monitored as far as 

five miles from the facility. Approximately 25,000 gallons of nitric acid were released from a 

damaged storage tank into lined chemical ditches and sumps on the facility. 

U.S. EPA Region VII responded to an Iowa Department of Natural Resources’ (IDNR) 

request for assistance during the emergency phase of the incident. EPA on-scene coordinators 

assisted IDNR by conducting air monitoring off site, conducting limited air monitoring at the Terra 

facility, entering areas of the Terra facility where chemicals had been released to assess the extent of 

release and assess the integrity of remaining chemical storage tanks, responding to citizen complaints 

of air quality problems, providing photo-documentation of site activities, and assisting with oversight 

Background Page 9 
of response and recovery activities performed by Terra personnel and contractors. EPA personnel 

and contractors worked closely with the local government incident commander to ensure that public 

safety issues were addressed in a comprehensive and timely manner. 

At the conclusion of the emergency response, EPA Region VII was directed to conduct an 

investigation to determine the cause of the explosion at the Terra facility and to develop 

recommendations in an effort to prevent future similar occurrences in ammonium nitrate production 

facilities. The investigation was conducted pursuant to authorities contained in the Comprehensive 

Environmental Response, Compensation and Liability Act (CERCLA), as amended by the Super-fund 

Amendment and Reauthorization Act (SARA) of 1986, 42 U.S.C. §9601 et seq., and the Clean Air 

Act (CAA) as amended by the Clean Air Act Amendments of 1990, 42 U.S.C. §7401 et seq. 

Background Page 10 

SECTION 3: CHEMICAL ACCIDENT INVESTIGATION OVERVIEW 

On December 19, OSC Mark Thomas, U. S. EPA Region VII, was assigned by EPA 

Headquarters to conduct an investigation into the cause of the explosion. The purpose of the 

investigation was to determine, if possible, the cause of the explosion and to develop a list of 

recommendations to help prevent future similar occurrences. The investigation included numerous 

trips to Sioux City/Port Neal, Iowa, as well as to other ammonium nitrate production facilities to 

gather information. The investigation was conducted under authorities contained in CERCLA, 

Section 104, 42 U.S.C. 9604 and the CAA Section 114, 42 U.S.C. 7414, Section 112R. 

Independent investigations to determine the, cause of the explosion were conducted concurrent 

to the EPA investigation by Iowa OSHA, Iowa Fire Marshal, and Terra Employees along with 

Wilfred E. Baker Engineering (retained by counsel for Terra). 

Terra employees advised the investigation team at the beginning of the investigation that all 

information provided to the EPA investigation team and all photographs taken of the Terra Port Neal 

facility, equipment and debris were confidential business information (CBI). All materials provided 

to EPA by Terra pertaining to this investigation were handled as CBI. Terra reclassified much of the 

material provided to the investigation team to a non-confidential status in August 1995. 

The investigation team advised Terra personnel at the onset of the investigation that the 

investigation would be conducted to determine the cause (or possible causes) of the explosion, 

identify circumstances that facilitated the explosion, and to develop recommendations to help avoid 

future similar occurrences. Terra personnel were advised that the investigation would be conducted, 

to the extent possible, in a manner so as to minimize any interruption of Terra’s recovery and 

investigation activities. To avoid unnecessary duplication of effort, Terra agreed to share information 

obtained during the investigation. The investigation team further advised Terra that, if warranted, 

EPA may pursue enforcement actions. 

The initial investigation included a visual overview of the Terra facility and surrounding areas 

to determine the area of origin of the explosion, gathering information specific to the ammonium 

nitrate (AN) plant design specifications, and gathering information about plant operations and 

maintenance for a period of at least seven days preceding the explosion. The initial phase included 

viewing physical evidence in and around the facility and requesting technical, operating and 

maintenance information from Terra. The investigation team determined that recent changes in the 

AN plant included the addition of a AN scrubber and the implementation of a Honeywell distributed 

control system (DCS). Investigation activities included analyzing the impact of these items on normal 

AN plant operations. 

Initial site investigation included sketching and photographing the Terra facility to determine 

the location of the explosion(s) and photographing explosion debris and damage to facilities off site. 

Initial observations included sketching collateral damage patterns to determine the location of the 

explosion(s). The investigation team concluded that more than one explosion occurred in the area 

identified by Terra personnel as the AN plant. Craters in this area were located in the AN plant at 

Investigation Overview Page 12 
locations identified by Terra personnel as previous locations of the AN neutralizer and the AN 83%

rundown tank (photo 3-1). The presence of at least two distinct craters indicated that more than one

explosion occurred in the AN plant. Investigators measured and sketched the area of explosion

origin, approximate crater dimensions, and the resulting directions in which remaining steel,

reinforcing bar and bolts were positioned (Figure 3-1). Other site documentation activities included

visits to the forensic warehouse to view critical process vessel pieces that had been recovered,

identified and assembled by Baker Engineering personnel.

Investigation activities included the request of numerous documents from Terra to reconstruct 

activities and events at the Terra Port Neal facility for a period of one week prior to the explosion 

as well as maintenance activities for a period of one year before the explosion. The purpose of this 

activity was to identify actions and conditions that may have led to the explosion in the AN plant. 

Documents requested from Terra on December 28, 1994 included: 

1. Current drawings of the AN plant that showed equipment locations, piping location and 

elevations, process vessel construction, and the most current piping and instrumentation 

diagram (P&ID). These drawings were requested before and after the installation of the AN 

scrubber. 

2. The equipment history records for the process vessels and active components in the AN plant 

and maintenance records for the AN plant for a period of one year. This request included all 

hot work permits issued for a period of one week before the explosion. 

3. Operator logs and distributive control system records for the AN plant for a period of one 

week before the explosion. 

4. The management of change file for the addition of the AN scrubber to the AN plant. 

5. Copies of all written standard operating procedures for the AN plant for startup, normal 

operation, normal shutdown, emergency shutdown, and turnaround activities. 

6. All completed checklists for activities in the AN plant, 

Other written requests for additional documents were submitted as the investigation progressed. 

The investigation team conducted a series of interviews and depositions of Terra employees that 

included plant management personnel, engineers, operators, supervisors, and maintenance personnel. 

Interviews and depositions were conducted at the Terra Port Neal facility, Terra Corporate offices 

in Sioux City, Iowa, Woodbury County Emergency Services offices in Climbing Hill, Iowa, and at 

EPA Region VII Headquarters in Kansas City, Kansas. These interviews focused on Terra policies 

and procedures for AN plant operations and clarification of events and conditions at the Terra Port 

Neal facility prior to the explosion. The investigation team conducted interviews with BECO 

Engineering personnel pertaining to the design, installation and operation of the AN scrubber at the 

Terra Port Neal facility. The investigation team also interviewed management, maintenance and 

Investigation Overview Page 13 
Photo 3 - 1

AN Plant Area After Explosion

Investigation Overview Page 14

operations personnel at other facilities producing 83% ammonium nitrate solutions to determine any 

additional information pertaining to the known hazards of manufacturing, transferring, and storage 

of AN solutions; and to determine how persons at other AN production facilities avoided or mitigated 

those identified hazards. The investigation team interviewed personnel form some of the contractors 

who had performed work at the Terra facility within one year prior to the explosion. 

The EPA investigation team utilized contractors and agency employees for consultation 

pertaining to the chemical and physical properties of ammonium nitrate, analysis of facts obtained by 

the investigation team, and assistance in the development of conclusions based on those facts. This 

expertise included chemical and metallurgical analysis of process vessel fragments. 

The investigation team and consultants prepared an initial draft of the technical sections of 

this report and submitted the report for two distinct review phases. The first phase was a scientific 

review of the conclusions and the evidence supporting the conclusions. A panel consisting of EPA 

scientists and engineers from Region VII and the Office of Research and Development (ORD) 

reviewed the data gathered by the investigation team and evaluated the conclusions contained in the 

report for accuracy. The second phase consisted of a final review by EPA personnel from Region VII 

and Headquarters. 

The results of the investigation are contained in Section 9 of this report. 

Investigation Overview Page 16 
SECTION 4: AMMONIUM NITRATE (AN) INFORMATION 

83% ammonium nitrate (AN) solution was produced at the Terra Port Neal facility for 

blending with urea to form urea ammonium nitrate solution (UAN) for use as a fertilizer. Terra 

discontinued AN prilling operations necessary for solid AN production in the early 1980’s. Most of 

the information, incident investigations, and AN behavioral studies located by the investigation team 

address AN in its solid form or molten, whether pure or contaminated. Ammonium nitrate (AN) 

[CAS 6484-52-2], NH4NO3, formula wt 80.04, is a white crystalline salt in its solid state. 

83% AN solution was manufactured at the Terra Port Neal facility by mixing 55-56% nitric 

acid with gaseous ammonia in a neutralizer. The neutralization reaction is highly exothermic, 

liberating 46.5-52.4 MJ of heat per mole of AN formed. This reaction was carried out at atmospheric 

pressure and produced an approximately 61% AN solution, with the resulting heat driving off a 

portion of the water to produce 83% AN solution. The operation of the neutralizer at the Terra Port 

Neal facility was continuous, and typical raw material feed stocks were ammonia from the offgas of 

two Weatherly urea plants and nitric acid from the nitric acid storage tank. 

The design neutralizer operating temperature during production at Terra was 2670F, with the 

pH of the 83% AN solution to be held in the range of 5.5-6.5. As designed, ammonia was the 

reactant available in slight excess during normal neutralizer operations. There was no control of 

offgas flow into the neutralizer at the AN plant. Offgas production varied directly with production 

rates and efficiency in the urea plants. pH control in the neutralizer was maintained by controlling 

nitric acid feed. 

Decomposition and Detonation Hazard 

Endothermic dissociation of ammonium nitrate from lowering pH normally occurs above 

3360

F, and is represented by: 

NH 4 NO 3 = HNO3 (1) + NH3 (g) H = 41.8 kcal/mol. [16] 

As the AN is heated to a temperature of approximately 3920F, exothermic decomposition 

occurs. The reaction is rapid and is represented by: 

NH4NO3 (1) - N2O (g) + 2H2O(g) H = -8.8 kcal/mol. [16] 

Above 4460F, exothermic elimination of N2 and NO2, begins, and is represented by: 

4NH4NO3 - 3 N2-+ 2 NO2 +-8 H2O H = -24.4 kca.l/mol. [16]. 

The final violent exothermic reaction occurs with great rapidity when ammonium nitrate 

detonates. This is represented by: 

2NH4NO3 - 2N2+4H2O+O2 H = -28.3 kcal/mol. [16] 

Ammonium Nitrate (AN) Information Page 17 
Ammonium nitrate is normally classified as an oxidizing agent. The pure salt is not classed

as an explosive because it is difficult to detonate. Spark, flame, or friction normally do not cause

detonation in solid AN, and it is relatively insensitive to shock. However, a variety of substances,

such as chloride and oil, are known to sensitize the material [16].

When used in blasting, AN is mixed with fuel oil, and sometimes sensitizers such as powdered

aluminum. Lower density AN is preferred for explosive formulation because it absorbs the oil more

effectively. When detonated, these mixtures have an explosive power of 40% to 50% of TNT.

Ammonium Nitrate Sensitization 

Information collected by the investigation team indicated that the energy necessary to initiate 

thermal decomposition decreased if the AN was sensitized. Conditions that can sensitize AN and 

increase the hazards of thermal decomposition, deflagration and detonation are as follows: 

1. Concentration - Literature reviewed by the investigation team indicated that past AN 

explosions have occurred on solid product or in solutions with concentrations greater than 

95% [18]. However, violent decomposition reactions of AN solutions have been documented 

with concentrations as low as 80% ammonium nitrate by weight in water [42]. 

2. pH - As AN becomes more acidic, AN stability decreases [1,2]. Free acid in AN solution is 

a significant source of or a contributing factor to AN solution decomposition. 

3. Temperature - Increased temperatures can result in increased sensitivity. Reduced 

temperatures can result in AN crystallization (salting out), and solid AN may be less difficult 

to initiate than the 83% AN solution [12,13,14,21]. 

4. Contamination - AN contaminated with organic materials is more easily detonated [ 14].. 

Inorganic contaminants also sensitize AN and include chlorides and some metals, such as 

chromium, copper, cobalt and nickel [3, 5, 6, 14, 21, 42]. These contaminants reduce the 

energy required to initiate a critical ammonium nitrate decomposition reaction. 

Chlorides react synergistically with some metals to further reduce energies needed to initiate 

thermal decomposition. Studies reviewed by the investigation team concluded that synergistic 

catalysis with chloride generally requires a metal capable of forming chloro complexes of 

reasonable stability in two oxidation states differing by one electron unit, and at the same time 

are capable of forming chloro complexes of moderate stability in these states. Chromium was 

considered a special case because oxo complexes were believed to be involved. 

Metals that react synergistically with chlorides to further reduce thermal decomposition 

reaction temperatures include chromium, copper, silver, nickel, iron, palladium, gold, cerium, 

and cobalt. 

Ammonium Nitrate (AN) Information Page 18 
Other sensitizing materials include hypophosphites and thiosulfates. 

5. Confinement - The higher the degree of confinement, the greater the possibility of 

deflagration/detonation. The degree of confinement necessary to support detonation 

decreases, as the presence of other sensitizing factors increase. The mass of the ammonium 

nitrate may create sufficient confinement if the AN is sufficiently sensitized and the threshold 

temperature for thermal decomposition is reached [21]. 

6. Low density areas - Low density pockets in the ammonium nitrate melt, such as gas bubbles, 

reduce the energy required to initiate decomposition reactions [1, 23, 24]. Bubbles also 

enhance thermal decomposition propagation in the ammonium nitrate solution through 

adiabatic compression as the decomposition pressure wave moves through the media, 

generating temperatures as high as 2,000oF. 

Safe Practices for Manufacturing and Storage of Ammonium Nitrate 

Safety documents reviewed by the investigation team that dated from the 1950s and 1960s 

provide the following recommendations for the safe manufacturing and handling of ammonium nitrate 

[14, 17]: 

1 . Contamination of ammonium nitrate with combustibles and catalysts must be avoided. 

2. Confinement must be avoided and free ventilation of gaseous products must be possible. 

3. All processes for the manufacture of ammonium nitrate involve handling concentrated 

solutions of the salt and particular care is required in these stages of the manufacturing 

process. The decomposition of the molten salt is accelerated by acidic conditions as well as 

by contamination. These conditions should be avoided, especially in storage. 

Publications from the early 1950s warn that chlorides are probably an impurity for which. 

great precautions should be taken to avoid since they can act as a catalyst of AN thermal 

decomposition. Chloride sources are identified primarily as nitric acid plants and plant 

cooling water [ 14]. Action levels based on chloride concentrations range from 1 part per 

million to 80 parts per million by volume. 

4. Contamination of feedstreams should be anticipated. Feedstreams should be monitored

accordingly and procedures developed for these occurrences. Documents also warn of the

possibility of operator errors and contamination introduced through maintenance activities

and recommend the development of procedures to address these issues.

Investigation Objectives for Determining AN Sensitization 

One focus of the investigation was to determine the presence, if any, of sensitizing materials 

and the extent of any ammonium nitrate sensitization. This resulted in the review of the following: 

Ammonium Nitrate (AN) Information Page 19 
1. Materials of construction for process vessels, piping, valves and any other component that 

may have contacted the ammonium nitrate. 

2. Events that occurred in the Terra facility that may have introduced sensitizers or sensitizing 

conditions into the ammonium nitrate. 

3. Operating procedures, both written and practiced, in the Terra facility that may have 

introduced sensitizers or sensitizing conditions into the ammonium nitrate. 

4. Analytical data from product and waste samples for the presence of chemical sensitizers. 

5. Metallurgical analytical data form process equipment fragments for the presence of chemical 

sensitizers, forces acting on the metal, temperatures to which fragments were exposed, and 

metal specifications and ahoy constituents. 

Ammonium Nitrate (AN) Information Page 20 
SECTION 5: AMMONIUM NITRATE PLANT OPERATIONS 

The ammonium nitrate (AN) plant at the Terra Port Neal facility produced 83% ammonium 

nitrate solution utilizing ammonia and 55-56% nitric acid feedstreams (Figure 5-l). This reaction 

took place in the neutralizer. The ammonium nitrate solution overflowed from the neutralizer into 

the rundown tank. The rundown tank was a surge vessel. The 83% ammonium nitrate solution (or 

melt) was pumped from the rundown tank to one of two storage tanks to be used as an intermediate 

product in the production of urea ammonium nitrate UAN) solutions and for sale. 

Recent additions to the AN plant included conversion from a Fisher control system to a 

Honeywell TDC-3000 distributed control system (DCS) and the construction and installation of a AN 

scrubber by BECO Engineering. 

Figure 5-2 is a schematic drawing of the Terra ammonium nitrate plant. The investigation 

team requested plant location and piping elevation drawings as well as a current piping and 

instrumentation diagram (P&ID) for the AN plant early in the investigation. Terra provided drawings 

and P&IDs for the original C&I Girdler AN plant constructed in 1966. These drawings did not 

include the relocation of process equipment that occurred during installation of the Mississippi 

Chemical neutralizer in 1980 or the AN scrubber in 1994. The most current site location drawings 

were generated by BECO Engineering for their use in the design and installation of the AN scrubber. 

The schematic drawing was reconstructed by the investigation team from numerous documents and 

interviews and was corrected by Terra employees during interviews occurring late in the investigation 

[ID-1, ID-4, ID-9, ID-14, ID-18, ID-19, ID-21, ID-23]. 

The major vessels in the ammonium nitrate plant were the neutralizer, rundown tank (Photo 

5-l), and the scrubber (Photo 5-2). 

Neutralizer 

The manufacture of 83% AN occurred in the neutralizer (Figure 5-3). Terra constructed the 

current neutralizer in 1980 after licensing the technology from Mississippi Chemical Corporation, 

Yazoo City, Mississippi. Terra purchased the neutralizer license to reduce ammonia emissions 

through the increased efficiency offered in this neutralizer design. Two raw material streams were 

fed into the neutralizer. Nitric acid was provided by the nitric acid plant. Ammonia was supplied for 

normal operations in the offgas from two small urea plants. Ammonia was also available from two 

30,000-gallon liquid ammonia bullet tanks located south of the AN plant or from the two 5,000,000­

gallon atmospheric pressure ammonia storage tanks located in the facility tank farm. The neutralizer 

was constructed of 304L stainless steel, and the nitric acid spargers were fabricated from titanium. 

AN Plant Operations Page 21 


Photo 5 - 1

Neutralizer and Rundown Tank 

AN Plant Operations Page 24 .
_

Photo 5 - 2

AN Scrubber

AN Plant Operations Page 25 

Information provided through interviews indicates that the neutralizer was insulated and steam 

jacketed when built [ID-11, ID-15]. Insulation consisted of 2 inches of calcium silicate. The steam 

coils were not connected to a steam header, thus requiring that a flexible steam line be connected 

between the neutralizer and the steam header for heating. Information gathered during interviews 

indicated that the steam coils in the neutralizer jacket had corroded and could not be used. No one 

interviewed could remember using the steam coils in the past several years [ID-1,ID-11 ]. 

During normal operations, ammonia was introduced into the bottom of the neutralizer at a 

rate determined by the offgas production of the urea plants. The ammonia flowed through an 

insulated 14-inch line labeled “ammonia/offgas” on the schematic with an ammonia sparger plate at 

the line terminus. Nitric acid was introduced into the neutralizer through two titanium spargers 

located in the lower region of the neutralizer. The spargers discharged nitric acid into the neutralizer. 

The neutralization reaction of the ammonia and nitric acid is extremely exothermic, with the heat of 

reaction used to remove excess water to produce the 83% solution. The resulting solution 

overflowed from the neutralizer through a “rundown” line into the rundown tank. 

The Mississippi Chemical neutralizer was designed to operate at a temperature of 

approximately 265°F in a pH range from 2.0 through 6.5. For reasons of efficiency and ammonia 

emission reduction, Terra specified that the neutralizer be designed to operate within a pH range of 

5.5 - 6.5. 

Offgas flow from the urea plants could not be controlled in the AN plant. Nitric acid was 

controlled by a control valve (FCV-401). The plant design provided for the AN solution pH to be 

monitored by a pH probe located in the rundown line between the neutralizer and the rundown tank 

AE-403). The neutralizer contained one temperature sensor (TI-445S). Written communication 

received from Terra in September 1995, stated the temperature sensor was located in the upper 

portion of the neutralizer, approximately 18 inches below the overflow. Offgas and nitric acid flows, 

neutralizer temperature, and rundown pH are indicated and recorded by the AN DCS. 

Rundown Tank 

The rundown tank was constructed of 304L stainless steel, was insulated with two inches of 

calcium silicate, and contained steam coils for heating (Figure 5-4). Information obtained in 

interviews indicated that the steam coils were connected to the 20 psig steam header, but were no 

longer used because of freezing and corrosion problems on the condensate side of the coils. 

The rundown tank served as a surge vessel for the neutralizer for pumping the AN solution 

to the storage tanks located in the tank farm. Original C&I Girdler drawings indicate that the 

rundown tank was equipped with an ammonia sparger to be used to increase the pH of rundown tank 

contents ifneeded. Information gathered from interviews and depositions indicated that no ammonia 

sparger existed in the rundown tank. The rundown tank was also equipped with a local temperature 

indicator, but it had not been used since prilling operations were discontinued. 

AN Plant Operations Page27 

The rundown tank was equipped with a differential pressure sensor and controller (LIC-413) 

to measure and control the liquid level in the tank. Flow from the rundown tank was controlled by 

the associated level control valve (LCV-413), which was located on the discharge side of the AN 

83% product pumps 404JA and 404JB. The product pumps were not equipped with low-flow or 

temperature sensors. 

Ammonium Nitrate Scrubber 

The AN scrubber was designed by BECO Engineering to remove ammonia vapors from the 

neutralizer and rundown tank process gases in the AN plant (Figure 5-5). Since use of a strong acid 

would have resulted in excessive acid aerosol production during scrubber operations, the scrubber 

was designed to operate with dilute nitric acid with a pH above 1.4 and a high recirculation rate, 

approximately 3,000 gallons per minute. The acidic liquor was circulated from the scrubber sump 

through spray nozzles located in the upper portion of the scrubber, where the free ammonia reacted 

with the stream to form a weak AN solution. 

The scrubber was designed to operate efficiently with a 50% AN solution concentration. To 

maintain this concentration, the scrubber contents would be “blown down” (transferred) into the 

neutralizer. The estimated maximum design blowdown rate was 6.5 gallons per minute, with a 

normal constantly operating blowdown range designed to be between 2-3 gallons per minute (ID-Z). 

Scrubber feedstreams included makeup eater consisting of 20-pound condensate and 55-56% 

nitric acid taken from the AN plant nitric acid line Approximately 90% of the nitric acid supply for 

the scrubber was taken from a point upstream of FCV-401 and injected on the suction side of the 

scrubber recirculation pump (416J) through a control valve The remaining 10% of the nitric acid 

supply was taken from a point downstream of FCV-401 and injected on the discharge side of 416J 

through the nitric acid metering pump (417J) The scrubber was designed to operate at a pH between 

1.4 and 2.0, with optimal performance at a pH between 1.79 and 1.89. Operating at a pH less than 

1.4 would cause excessive nitric acid emissions from the scrubber, and operating at a pH greater than 

2.0 would cause an increase of ammonia emissions by reducing scrubber efficiency. BECO 

Engineering recommended setting the low pH alarm in the scrubber at 1.6. 

The scrubber was equipped with a differential pressure sensor for level indication (ET-4 10). 

Condensate was controlled by a flow control valve (FCV-410). Scrubber solution pH was monitored 

by two pH probes located in the discharge line from the recirculation pump (AE-410 and AE-411). 

Nitric acid to the scrubber was controlled by a control valve (ACV-411) and the nitric acid metering 

pump (4173). Blowdown was controlled manually. 

AN Plant Operations Page 29 

Distributed Control System (DCS) 

Since the Terra facility turnaround in September 1994, the AN plant had been operating with 

a Honeywell TDC 3000 Distributed Control System (DCS). As designed, Terra’s DCS was capable 

of monitoring and recording data, alerting operators to process variable deviation, and automatically 

operating the process (Figure 5-6). 

There are three major control modes associated with TDC 3000 operation: manual, 

automatic, and cascade. In manual mode, the operator is in direct control of the output. In automatic 

mode, the DCS automatically adjusts the output to keep the process variable at its desired value (set 

point). In this mode, the operator controls the set point. In cascade mode, the output of one 

controller drives the set point of another controller which automatically adjusts the output to keep 

the variable at its desired value. 

Any changes made to the process within each control mode are recorded in the DCS 

“Operator Process Change Journal.” This journal records all control mode changes as well as any set 

point changes (automatic mode) or output changes (manual mode) of any variable. All other process 

changes, i.e., the disabling and enabling of alarms and the change of alarm priorities, are also recorded 

in this journal. 

The TDC 3000 is capable of alerting the operator when a variable reaches pre-programmed 

critically low or high values as designated by the programmer and/or engineer. These deviations are 

logged in the DCS “Process Alarm Journal.” Each alarm is assigned a priority depending on its 

importance, such as High, Low, and Journal. All process alarms are recorded in the DCS Process 

Alarm Journal, but only the first two categories alert the operator. An alarm assigned a Journal 

priority is recorded, but does not alert the operator. The alarm types, PVLL (process variable 

low/low), PVLO (process variable low) and PVHI (process variable high) are designated and preprogrammed into the DCS as the acceptable limits of operation. Any deviation outside of this range 

triggers an alarm whose priority depends on its pre-assigned importance. 

For security reasons, the TDC 3000 has three distinct access levels (from low to high): 

operator, supervisor and engineer. The difference in access levels refers to the degree of 

authorization to make changes to the control system. The higher the level, the more additions, 

deletions, or changes can be made. The functions that fall within each access level were designated 

by the programmer/engineer when the system was designed. Under normal operations the DCS was 

to be set in operator access level. Within this level, the operator is able to change modes of operation 

and change set points and outputs. Engineering access level allows alteration of the system’s 

algorithm and alarm priority. 

AN Plant Operations Page31 

SECTION 6: FACILITY EVENTS 

The investigation team developed a time line of activities and events that occurred at the Terra 

Port Neal facility prior to the explosion. The time line was developed to provide an overview of these 

events and any possible connection to the explosion. 

On November 27, 1994, operations and maintenance personnel determined that the pH probe 

located in the neutralizer rundown line was defective. There were no spare probes in stock, so the 

AN plant was operated with the apparently defective probe in service until the time of the explosion. 

Shift reports requested by the investigation team and provided by Terra indicated that the 

surface condenser in the nitric acid plant was recorded as leaking from December 5 (1st shift report 

requested by the investigation team) through December 8, 1994. Repairs to the surface condenser 

were performed on December 8. The acid plant was shut down two more times prior to the 

explosion, the last being an emergency shutdown that occurred at approximately 0430 hours on 

December 12. 

At approximately 2200 hours on December 11, the outside operator in the AN plant detected 

an acid condition in the neutralizer. The pH of a grab sample analyzed by the outside operator 

indicated the neutralizer pH was -1.5. Information provided in depositions indicated that nitric acid 

fumes were strong enough in the AN plant to bum the outside operator’s face [ID-3). An off-duty 

operator was called in to collect samples from the neutralizer to monitor pH and concentration of the 

AN solution until the neutralizer was operating within the acceptable pH and concentration range. 

The operator collected a sample about every 10 minutes and used a portable pH probe to determine 

pH and a hydrometer to determine concentration The neutralizer was within operating parameters 

by approximately 0100 hours on December 12. Grab sample results for this period of time were not 

logged. 

At approximately 0200 hours on December 12, the outside AN plant operator observed a leak 

in the condensate line leading to the AN scrubber. He observed that the condensate line was leaking 

where it connected to the scrubber. As he attempted to get a closer look, the operator stated that he 

was sprayed with hot ammonium nitrate solution coming from a leak in a discharge gasket on the 

north AN product pump. The operator shut down the north pump and started the south product 

pump, which also leaked. The operator and shift supervisor changed the discharge gasket on the 

north pump and returned it to service. The operator could give no estimate of the amount of 

ammonium nitrate released from the leaking pump. At approximately 0240 hours, the outside 

operator and shift supervisor shut down the AN scrubber recirculation pump (416J) and blocked in 

the leaking condensate line. The condensate line remained blocked in until the explosion. [ID-l ,ID-3] 

The nitric acid supply began running low on the afternoon of December 12 because the nitric 

acid plant was still out of service. Operators began shutting down the AN plant at approximately 

1410 hours by reducing the nitric acid flow to the neutralizer. The neutralizer was shut down by 

1500 hours. 

Terra Facility Events Page33 
AN product pumps were shut down and blocked in at approximately 1452 hours by the 

outside operator. The operator applied 200 psig steam to the AN product line on the suction side 

of the product pumps. The steam remained applied until the time of the explosion. 

At approximately 1500 hours, operators applied compressed air to the suction side of the 

nitric acid pump located in the nitric acid plant. The “plant air” was applied to the nitric acid line to 

purge the line of nitric acid to prevent freezing. The nitric acid was discharged from the line into the 

AN neutralizer. The air was applied until approximately 2030 hours. Terra employees stated that 

plant air applied to the neutralizer resulted in strong acid gases in the AN plant [ID-3]. This required 

them to wait a short period of time after shutting down the plant air before steam could be applied 

to the spargers to allow the acid fumes to disperse. 

At approximately 2030 hours, the operator applied 200 psig steam to the nitric acid line at 

a point downstream from FCV-401 at a 3/4-inch diameter fitting. The operator stated that a l-inch 

diameter flexible steam line was connected at that location and that the steam valve was fully opened 

[ID-3]. The steam was applied to prevent the backflow of AN into the nitric acid spargers in the 

neutralizer and to prevent the AN from salting up the holes in the sparger. 

At approximately 1650 hours, operators started the AN scrubber recirculation pump and 

started blowdown of scrubber contents into the neutralizer. Blowdown continued until approximately 

1805 hours, then was discontinued. The AN scrubber pump continued to run until the explosion. 

There were no nitric acid or condensate feeds to the scrubber during this time. 

Based on information provided in depositions, the outside operator conducted an inspection 

of the AN plant at approximately 2100 hours on December 12, and a final waIk-through at 0530 

hours on December 13. 

By 0600 hours, December 13, the nitric acid plant air compressor was in “slow roll” in 

preparation for startup. At least two explosions occurred in the AN plant at approximately 0606 

hours on December 13. 

Terra Facility Events Page 34 
SECTION 7: INVESTIGATION-DERIVED FACTS AND CONDITIONS.. 

Acidity 

Information reviewed by the investigation team indicates that the degradation of ammonium 

nitrate is an autocatalytic liquid-phase reaction, the rate of which is proportional to the product of the 

mass of the AN salt and the concentration of acid. The activation energy for this process was 

determined experimentally to be 31.4 kcal/mol at a temperature of approximately 338°F. Results of 

studies indicate that the thermal degradation of ammonium nitrate takes place in the liquid phase and 

is autocatalyzed by acid. Experiments have determined that the mass rate of decomposition of AN 

is a function of the mass of acid present in the melt and is not affected by such products of reaction 

as water vapor and nitrous oxide. The addition of ammonia almost completely inhibits this reaction 

by reacting with the excess acid to form ammonium nitrate. Since the solubility of ammonia in liquid 

ammonium nitrate is much less than that of the acid, the inhibition will not occur until a sufficiently 

high concentration of ammonia has accumulated in the vapor phase. The thermal decomposition of 

ammonium nitrate involves the evolution of ammonia, but the low solubility in ammonium nitrate and 

an atmospherically vented vessel would not allow sufficient ammonia to accumulate in the vapor 

phase to inhibit the reaction [2,18] 

Review of additional materials and interviews with operators and engineers at other facilities 

that produce ammonium nitrate revealed that neutralizers are routinely operated at a pH as low as 

2.4. The resultant AN is “polished” in rundown vessels by sparging anhydrous ammonia into the 

solution prior to storage or further production processes. All information gathered by the 

investigation team indicated that AN solutions should not be stored or processed further until the pH 

had been raised to approximately 6 or higher. Ammonium nitrate scrubbers that operate in low pH 

ranges are so designed because the product will not be stored at that pH for prolonged periods of 

time. 

Prior to shutting down the AN neutralizer, Terra personnel detected an acid condition in the 

neutralizer. Offgas from the urea plant was used to raise the pH in the neutralizer after the nitric acid 

flow had been shut down. Plant air was then used to clear the nitric acid line (3-inch diameter) of 

nitric acid by blowing it into the neutralizer through the acid spargers. The acid plant was located 

approximately 600 feet from the neutralizer. Calculations determined that the line volume of the 

nitric acid line was approximately 220 gallons, and when mixed with the static neutralizer contents 

would have resulted in a pH of approximately 0.8 (calculation). 

Analytical data provided by Terra indicates that the pH of the AN solution in the west storage 

tank at the time of the explosion was approximately 1.4 at standard temperature. The investigation 

team concluded that the pH of the rundown tank contents was less than 1.4. 

Written operating procedures at AN facilities visited. by the investigation team require that 

the pH of all process vessels be adjusted to approximately 6.0 when the plant is shut down. pH is 

adjusted utilizing ammonia spargers in each vessel. If the vessels remain charged during the shutdown 

period, pH is monitored as it would normally be with the process operating. Procedures at these 

Investigation-Derived Facts and Conditions Page 35 
facilities indicate that low pH in AN solutions when the process is down is to be avoided. 

The investigation team determined that the low pH in the neutralizer and rundown tank was

a factor contributing to the explosion.

Contaminants 

Chlorides 

The presence of chlorides reduces the activation energy necessary to initiate thermal 

decomposition of ammonium nitrate. Chlorides also react synergistically with some metals to further 

sensitize ammonium nitrate [3, 4, 5, 6 ,14]. 

Potential chloride sources identified at the Terra facility were the nitric acid plant and facility 

cooling water. Information obtained from employee interviews and depositions, operator logs, and 

maintenance records indicates that there was at least one serious leak in the nitric acid plant surface 

condenser that could have allowed facility cooling water to contaminate the condensate. This leak 

was recorded on operator logs from December 5-8, and led to a nitric acid plant shutdown on 

December 8. The investigation team did not request logs prior to December 5, and was unable to 

determine how long the surface condenser had been leaking prior to December 5. Condensate is used 

as makeup water for the nitric acid absorption column and the AN scrubber. Two nitric acid cooling 

condensers were also indicated as leaking, possibly allowing facility cooling water to enter the nitric 

acid process gas stream. Increased chloride concentrations are also likely during unstable acid plant 

operations, such as during startup and shutdown, and the nitric acid plant had been started twice and 

shut down three times during the week preceding the explosion. Chlorides formed during acid plant 

operations concentrate in the weak acid trays of the absorption column. Following the emergency 

shutdown of the nitric acid plant on December 12, contents of the absorption column were pumped 

to the nitric acid storage tank, introducing chlorides into the AN plant feed stream. 

The investigation team concluded that chlorides were present in the nitric acid used in the AN 

plant. Analytical data provided by Terra indicated chloride concentrations in the west AN storage 

tank were 168 parts per million, and 557 parts per million in the nitric acid absorption column. 

The investigation team concluded that chlorides were present in the neutralizer, rundown tank, 

and west AN storage tank at the time of the explosion. The investigation team determined that 

chlorides were a factor in the explosion. 

Petroleum 

Hydrocarbons, as a reducing agent, increase the sensitivity of AN to detonation initiation. 

Information obtained through Terra employee interviews and documents provided by Terra 

indicated that two significant oil releases had occurred in the ammonia plant around October 1994 

[ID-4, ID-15, ID-21]. 

Investigation-Derived Facts and Conditions Page 36 
The first incident occurred during startup after the September turnaround while ammonia 

plant compressor 103J (syngas compressor) was in slow roll. A back-pressure regulator 

failed and allowed oil to be discharged into areas other than the process stream. Plant 

operators indicated that at the time of the incident, there was no process gas in the synthesis 

loop. Terra personnel indicated that several barrels of oil were recovered and that hot boiler 

feed water had been used to flush the system after oil recovery. 

The second incident occurred sometime in October. Ammonia plant compressor 105J 

released oil into the refrigeration system. Operators stated that there was some 

communication with the process stream, but oil typically would settle in heat exchangers. 

Although a small amount might end up in storage, most would be recovered from the 

exchangers. Approximately 35 barrels of oil was recovered from this incident. 

Hydrocarbon analysis submitted by Terra indicated that trace levels of hydrocarbons were 

present in the acid plant feed stream to the AN plant. Terra submitted laboratory analyses that 

indicated the presence of 2.6% TOC in the offgas line from the urea plant to the AN neutralizer. This 

line had been severely damaged by the explosion. No significant hydrocarbon presence was indicated 

in the AN storage tanks in other analytical data provided by Terra. 

EPA’s metallurgical analysis of a stainless steel pipe fragment with a closed block valve on one 

end and a check valve on the other end (EPA Item #14) revealed that the residue in the pipe 

contained a concentration of 8% total petroleum hydrocarbon (Photo 7-1). Terra employees 

tentatively identified the pipe section as part of a chemical stripper located near the AN plant. The 

investigation team requested a piping diagram and a drawing of the chemical stripper to determine 

whether or not the petroleum hydrocarbons detected in the pipe fragment could have contributed to 

the explosion. The location of this fragment prior to the explosion could not be accurate ascertained. 

Terra had experienced problems with the plant air compressor oil filters during and following 

the September 1994 turnaround. The investigation team could not locate piping associated with the 

plant air system for analysis for petroleum hydrocarbons. Plant air was applied to the neutralizer 

nitric acid spargers for over five hours after the AN plant was shut down on September 12. 

The rundown tank design would possibly have allowed the rundown tank to function as an 

oil separator, minimizing oil transfer to the storage tanks. 

Based on information available to the investigation team when the report was completed, 

hydrocarbon contamination could not be eliminated as a factor in the explosion. 

Investigation-Derived Facts and Conditions Page 37 
-

#

.

Photo 7 1

EPA Item 14, Unknown Fragment 

Investigation-Derived Facts and Conditions Page38 
Low Density Areas 

Bubbles in the AN solution create low density areas. It has been shown that air, steam, and 

offgas bubbles will provide “hot spots” during thermal decomposition or deflagration because the gas 

they contain is adiabatically compressed by the advancing pressure wave front [ 1, 23, 24, 30]. These 

hot spots assist propagation of the detonation.The increase in internal surface area is of considerable 

importance with respect to the sensitization of ammonium nitrate-based explosives. Lowering the 

density of the AN solution also reduces the thermal inertia of the system which, along with the lack 

of flow in the neutralizer, increased the probability of creating local heating problems. 

Compressed air, then 200 psig steam, had been applied to the nitric acid spargers in the 

neutralizer from approximately 1500 hours on December 12 until the time of the explosion. 

Compressed air entering the neutralizer through the nitric acid spargers created bubbles in the AN 

solution. Steam entering the neutralizer through the spargers would also create bubbles in the AN 

solution, and continued steam injection could create a low-density zone in the area immediately 

outside of the nitric acid sparger. 

The investigation team concluded that the creation of local, low-density zones was a factor 

in the explosion. 

Temperature 

The probability of thermal decomposition occurring in AN solutions is proportional to 

temperature. The activation energy required to inmate thermal decomposition decreases with the 

presence of contaminants in the AN solution [21, 34] Potential energy sources present in the AN 

plant after shutdown and before the explosion were limited to the AN scrubber recirculation pump, 

AN product pumps, and the 200 psig steam applied to the nitric acid spargers in the neutralizer. 

The investigation team concluded that the addition of energy to the AN solution via the 200 

psig steam was a significant factor in the explosion. 

Confinement 

Confinement is necessary for AN to deflagrate or detonate [21]. The degree of confinement 

needed for to support deflagration decreases as the presence of contaminants increases. Confinement 

can result from physical containment (containers) or from the mass of the AN. The mass of the liquid 

ammonium nitrate may be sufficient confinement to support the deflagration or detonation of the AN 

in the presence of contaminants and low density areas. 

The investigation team concluded that the mass of the AN solution was sufficient confinement 

to initiate deflagration, that overwhelmed the venting capacity of the vessels due to the presence of 

contaminants, low pH, and a high-temperature local heat source. 

Investigation-Derived Facts and Conditions Page 39 
Concentration 

Operator shift reports and logs indicate that AN solution concentration varied from 82% 

through 85% during normal operations. Lower concentrations occurred when the neutralizer was 

operated at a pH of 2 or less, and higher concentrations occurred when the neutralizer was operated 

at a pH greater than 6. 

The investigation team concluded that the concentration of the AN in the neutralizer and 

rundown tank at the time of the explosion was within this range. 

Materials of Construction 

Process vessels and piping in the AN plant were constructed of 304L stainless steel. 304L 

stainless steel is an alloy containing the following metals in approximate concentration by weight: 

chromium 19%, nickel 9%, carbon no more than 0.08%, with the balance iron. Both chromium and 

nickel ions are listed as metals that have a synergistic effect with chlorides in the catalysis of the 

thermal decomposition of ammonium nitrate [3, 8, 9, 14 ]. One stainless steel piece of the neutralizer 

that showed signs of severe corrosion was submitted for metallurgical analysis to determine if the 

corrosion, and subsequent release of alloy metals, occurred as a result of the explosion or may have 

contributed to the explosion. 

The only process vessel components not constructed from 304L stainless steel were the AN 

neutralizer nitric acid Schedule 40 titanium spargers. Each sparger was 3-inch diameter with a radius 

of curvature of 4 feet and contained 252- l/16-inch diameter holes (Figure 7-1). 

Information gathered by the investigation team indicate that titanium has been used for 

handling and producing nitric acid in applications in which stainless steels have experienced significant 

uniform or intergranular attack. A comparison of the corrosion rates for grade 2 titanium and 304L 

stainless steel in boiling 90% nitric acid is contained in Table 7-1. The presence of some impurities 

in the nitric acid, including the presence of titanium ions, inhibits the corrosion of titanium in nitric 

acid [41]. 

The neutralizer and the nitric acid spargers were inspected during the September, 1994 facility 

turnaround. Interviews and depositions of Terra employees vary in reference to the description of 

the neutralizer and spargers, but all agree that the neutralizer and the nitric acid spargers visibly 

appeared to be in good condition when inspected during turnaround. 

Investigation-Derived Facts and Conditions Page40 
SECTION A-A

NITRIC ACID SPARGER DETAIL
Table 7-1, Metal Corrosion Rates 

Corrosion of Titanium Grade 2 and Type 304L Stainless Steel

Heating exposed to Boiling 90% HNO3

Corrosion Rates of Grade 2 Titanium and Type 3O4L Stainless Steel 

Metal Temperature Grade 2 Titanium Type 304L Stainless Steel 

o C o F mm/yr mills/yr mm/yr mill/yr t 

116 240 0.03 - 0 17 1.2 - 6.7 3.8 - 13.2 150- 520 

135 275 0.04 - 0.15 1.6 - 6 17.2 - 73.7 675 - 2900 

154 310 0.03 - 0.05 1.2 - 2.4 18.3 - 73.7 720 - 2900 

Investigation-Derived Facts and Conditions Page 42 
The investigation team requested written procedures or guidance documents for the visual 

inspection of this vessel to include guidance for conditions that would trigger futher inspection and 

testing. Terra employees stated that there were no written guidelines or procedures for vessel 

inspections or conditions that would warrant closer inspection-or testing of the vessel [ID-21]. The 

investigation team requested a copy of the inspection report that had been completed during the 

September turnaround, and had not received the document by the time this report was issued. 

The investigation team examined fragments from Terra AN plant equipment that had been 

assembled by Baker Engineering personnel in a warehouse located north of the Terra facility. The 

metallurgist retained by EPA examined fragments identified as originating from the AN neutralizer, 

rundown tank, scrubber, and associated piping and appurtenances for further metallurgical 

examination and testing. Items selected included: 

1. EPA Item #8. The fragment was described as stainless steel, approximately 0.375 inches 

thick, approximately 12 inches long by 6 inches wide. The fragment was tentatively identified 

as a reinforcing pad, originally located inside the neutralizer. The fragment displayed a 

reddish-brown discoloration on one surface, with noticeable pitting. 

Metallurgical analytical results indicated that the fragment displayed a high-temperature 

discolored surface and severe erosion in one direction (Photo 7-2). The adjacent area showed 

little discoloration. The opposite surface exhibited severe tearing and fracturing with 

evidence of high-temperature discoloration (Photo 7-3). Unetched cross sections exhibited 

high-temperature oxidation on the inside diameter with severe surface tearing/seams or laps. 

The etched cross-sections displayed evidence of severe surface movement and cold working 

with a one-directional heavy force. Of the fragments analyzed, metalographic analysis 

suggests that the highest temperature, internal pressure and velocity was located in the area 

of Item #8. This sample exhibited the most severe surface distortion and high-temperature 

grain structure change. 

2. EPA Item #4. This fragment was described as originating from, the titanium sparger, and had 

been severely twisted (Photo 7-4). The titanium fragment exhibited a dark-colored surface 

with numerous areas indicating high-temperature oxidation. The interior and exterior surfaces 

displayed numerous mechanical damaged areas and no excessive thinning. 

3. EPA Item #17. This fragment was described as originating from the titanium sparger, and 

displayed the same characteristics as all titanium fragments examined. The fragment was 

severely distorted and exhibited a dark interior and exterior surface. 

Investigation-Derived Facts and Conditions Page 43 
Photo 7 - 2

Neutralizer Fragment 

Photo 7 - 3

Neutralizer Fragment

Investigation-Derived Facts and Conditions Page 44
Photo 7 - 4

Sparger Fragment

Investigation-Derived Facts and Conditions Page 45
The unetched cross-sections of the outside diameter surface displayed surface indications of 

heat cracking and deterioration. The etched cross-sections revealed that the parent material 

had a grain structure typical of titanium which had been heat treated and quenched from 

1,300oF. The external surface exhibited a structure similar to one that had been exposed to 

a 1,700oF temperature and then quenched. The inside surface displayed a slightly lower 

temperature range with slight surface change. 

Titanium sparger fragmentsexamined exhibited an oxide coating on the interior surfaces, and 

some pitting on the exterior surfaces. General metallurgical results indicate that some of the 

fragments were exposed to a temperature equal to or greater than 1,700oF inside the neutralizer. 

Titanium fragments examined indicated that the highest temperatures impinged on the outer surfaces. 

These high temperatures appeared to be a local event inside of the neutralizer, and not an event that 

occurred uniformly throughout the neutralizer. The high-temperature area was proximate to the nitric 

acid spargers. 

The investigation team determined that metallurgical observations and analyses indicated that 

a local area inside the neutralizer had been exposed to a temperature equal to or exceeding 1,700o

F. 

The highest temperatures appear to have been proximal to the nitric acid spargers. Analysis of 

neutralizer and rundown tank fragments indicate that the vessels had been overpressured prior to 

fragmenting, indicating that all or a portion of the vessel contents had deflagrated. 

Analyses confirmed that both the neutralizer and rundown tank exploded. 

Rundown pH Probe 

The pH probe used by Terra to monitor pH in the neutralizer rundown line in the AN plant 

was changed from a monitoring system requiring sample dilution and cooling to a monitoring system 

that inserted the pH probe directly into the process stream. The new probe was designed to operate 

up to a maximum temperature of 140°C (284°F). 

The investigation revealed that the rundown sample line had been replaced when the pH probe 

was changed, that the old pH cell had been removed and the new probe directly inserted in the 

rundown pH line. The rundown pH sample line originated from the rundown line and terminated in 

the rundown tank. The line was stainless steel, 3/4-inch diameter from the rundown line to the pH 

cell, and 1/2-inch diameter from the pH cell to the rundown tank. The AN P&ID showing this change 

indicated that the pH sample line was not insulated or steam traced. This fact was confirmed through 

a written communication by Terra. The investigation team requested drawings and specifications for 

the pH sample line. The investigation team concluded that the design of the pH sample line could 

have contributed to the problems attributed to the pH probe. 

Operations and maintenance personnel stated that the pH probe in the AN neutralizer 

rundown line was not functioning properly on November 27, 1994. Work Order # EM001076 

indicates that there were no more pH probes in stock, and that the replacement pH probe was due 

on December 21, 1994. Interviews with Terra employees indicated that a probe of the new type was 

Investigation-Derived Facts and Conditions Page 46 
expected to have a service life of approximately 30 days, and that two spare probes. were to be 

maintained in inventory [ID-14, ID-23]. No spare probes were available, so the AN unit operated 

from November 27, 1994 until the time of the explosion without a dependable pH feedback control 

loop on the DCS. 

Operators and engineering personnel stated that if pH probe readings were not reliable, grab 

samples of AN from the neutralizer or rundown line would be manually analyzed by the outside 

operator at least every two hours to determine concentration and pH [ID-l, ID-9, ID-19, ID-23] 

Table 7-2, included at the end of this section, summarizes all documented grab sample analyses from 

December 5 -December 13, 1994. Documentation provided by Terra indicates that if grab samples 

were collected every two hours from December 5 through the time of the explosion, that the sample 

results were not logged. 

Information obtained from interviews and depositions indicated that operators detected a 

strong acid condition in the AN neutralizer at 2000 hours on December 11, with pH indications in 

negative numbers by the portable pH probe, as well as by acid fumes burning an operator’s face in the 

AN plant. An off duty operator was called to the plant to assist the board operator in “lining out the 

neutralizer” (adjusting pH to within normal operating ranges). This activity requires the operator who 

was called in to work to collect samples from the neutralizer and relay pH readings to the board 

operator, who would adjust the acid flow into the neutralizer. Approximately 20 samples were 

collected and analyzed by the outside operator from 2200 hours until 0100 hours. No logged grab 

samples were analyzed between 1000 hours and 2000 hours on December 11. 

The investigation team requested all AN plant DCS data from December 5, 1994, through the 

time of the explosion. Terra provided DCS data for the AN plant for December 10 through the time 

of the explosion, with over 24 hours of data missing from 0950 hours on December 11 through 1100 

hours on December 12. Terra employees stated that the missing DCS data contained only steady 

state operational data, therefore the data was not recovered from the DCS digital history [ID-14]. 

EPA investigators determined from operator logs and interviews that the neutralizer had become 

acidic enough during this time (pH -1.5 on log sheet) that an extra outside operator was called to 

work to take grab samples to return the neutralizer to a normal pH operating range. EPA obtained 

the missing data directly from Honeywell with Terra’s permission. 

The investigation team graphed the DCS data and observed that the pH probe appeared to 

be operational. Comparison to the limited operator grab sample analyses conducted confirmed that 

the pH probe was tracking pH trends in the rundown line and accurately reflected reported acidic 

excursions in the AN neutralizer (Figure 7-2). Mass balances and pH calculations conducted utilizing 

DCS data for offgas flow and nitric acid flows accurately followed the indicated, pH curve for the 

neutralizer rundown line: The pH probe readings lagged-behind pH changes; with the lag time 

varying from two to three hours. The investigation team provided graphs generated from DCS data 

provided by Terra to Terra engineers and operators to review. All agreed that the pH probe appeared 

to be working, but could not state whether or not the indications were accurate [ID-4, ID-18, ID21, ID-23]. They did state that the probe appeared to be accurately indicating pH trends in the 

neutralizer. 

Investigation-Derived Facts and Conditions Page 47 
The investigation team contacted persons familiar with pH probe operations, who advised that 

the pH probe did appear to be working and indicated that response delays might be caused by an 

intermittent obstruction in the rundown sample line. The physical configuration of the sample line 

could contribute to this problem, as well as ambient temperatures. Ambient temperatures for the 

Sioux City, Iowa, area are contained in Figure 7-3. 

The investigation team determined that the problems associated with the rundown pH probe 

and sample line contributed to pH problems during normal operations, but that it was not a factor in 

the explosion of the neutralizer. The investigation team determined that the problems with the pH 

probe contributed to the low pH in the rundown tank and the low pH detected in the west AN storage 

tank. 

Steam Distribution System 

The steam system at the Terra Port Neal facility was a multi-level system designed with the 

apparent intent of supplying steam for a diverse range of purposes in an energy efficient manner 

(Figure 7-4). Steam was produced in packaged boilers at a nominal pressure of 600 psig. The 

investigation team assumed that the boilers were equipped with superheaters to ensure a supply of 

dry steam for turbine drivers operating directly off the 600 psig header. 

Steam from the boilers was utilized for a variety of functions within the facility beginning with 

the steam turbine drivers and including a wide range of process, tank and space heating uses. Most 

applications, other than the turbine drivers, utilized 200 psig or 50 psig steam. These two lower 

pressure levels were supplied by varying combinations of turbine exhaust steam and steam 

depressured directly from the 600 psig header Since the steam requirements for the different process 

and heating loads would have varied, both with operations and ambient temperature, and since some 

of the processes were also steam producers, the load on the boilers and the relative amounts of 

depressured and exhaust steam in each header probably varied over a wide range. 

Desuperheating stations were provided to remove the excess superheat from the depressurized 

steam. Presumably, these stations were originally designed for the range of depressuring loads that 

could be foreseen for startup, shutdow and the full range of normal operating conditions. Whether 

or not they were still capable of covering the full range of loads after numerous modifications to the 

plant is not known. It is conceivable, however, that the desuperheater at the 600 psig to 200 psig 

depressuring station could have been temporarily overtaxed by high loads, such as might have 

resulted from startup, upset, or non-standard operations. In that event, there would have been a 

carry-over of excessive superheat downstream of the superheater. 

The investigation team concluded that the steam distribution system had the potential to 

supply superheat during high demand events such as the slow roll of the nitric acid compressor. 

Investigation-Derived Facts and Conditions Page 48 



Air and Steam in Spargers 

At approximately 1500 hours December 12, Terra personnel blocked in the acid line from the 

nitric acid plant and began to blow out the line from the acid plant to the neutralizer with plant air. 

This operation was not included in the standard operating procedures provided by Terra. Information 

obtained during interviews indicates that the purpose of this procedure was to clear the nitric acid 

lines of acid from the acid plant to the neutralizer spargers to prevent the acid from freezing [ID-4]. 

Operators also applied 200 psig steam to the AN line from the 83% process pumps to the storage 

tanks located in the tank farm. 

Information obtained from depositions and interviews indicates that the normally practiced 

procedure was to quickly clear the lines of nitric acid, then remove the air and block in the lines. 

Plant air was injected in the nitric acid line and discharged through the neutralizer nitric acid sparger 

until approximately 2030 hours. Plant air is approximately 110 psig and is approximately ambient air 

temperature. The ambient air temperature for this time period ranged from approximately 24°F 

dropping to 13°F during this time. Air was discharged into the AN solution in the neutralizer creating 

low density zones in the solution. Information obtained both from references and operating 

procedures from other facilities indicate that creation of low density zones in ammonium nitrate 

solutions is a practice that is to be avoided. 

At approximately 2030 hours on December 12, the outside operator disconnected the plant 

air from the nitric acid line leading to the AN plant and blocked in this line. The outside operator 

applied 200 psig steam through a fitting downstream from FCV-401. Fittings to connect the steam 

line were stated to be 3/4-inch in diameter, the steam hose diameter was one inch, and the steam valve 

was completely opened. 

Terra employees stated that the addition of steam was a normal procedure to prevent 

backflow into the neutralizer nitric acid spargers and to keep the spargers from salting out. 

Interviews of personnel ranging from operators to supervisors and managers indicated that there was 

no specific procedure for connecting steam to the spargers [ID-4]. Two steam supplies were 

available in the AN unit, 50 psig and 200 psig As explained to the investigation team, each operator 

performed this task differently, but should only open the steam valve until flow was established. No 

preference was given to either of the two steam supplies. The temperature of 200 psig saturated 

steam is 387o

F, and the temperature of 50 psig saturated steam is 297o F. The investigation team 

determined that since the nitric acid plant was in “slow roll” at the time of the explosion, it was 

possible that there was superheat in the steam applied to the neutralizer prior to the explosion that 

was not present before the nitric acid plant startup. This could have resulted in steam temperatures 

exceeding 400°F being available in the 200 psig supply. 

Drawings provided to Terra for the construction of the neutralizer by Mississippi Chemical 

show block or check valves attached to the nitric acid spargers where they exit the vessel. The 

neutralizer at Terra was not equipped with check or block valves at these locations, but was equipped 

with a barometric loop that allowed backflow into the spargers and into the nitric acid line back to 

the barometric loop. The nitric acid line was not insulated or steam traced. Terra could not produce 

Investigation-Derived Facts and Conditions Page 52 
any design or “as built” engineering drawings for the Mississippi Chemical neutralizer that Terra 

contracted to have constructed. 

The investigation team interviewed engineers and operators at other ammonium nitrate 

facilities in reference to the use of steam in AN vessels and lines. One facility did not use any steam 

in the neutralizer or other AN process vessels other than in steam coils. When the AN plant was shut 

down at one of the facilities, the neutralizer was drained and flushed with water. Other facilities used 

no more than 90-pound steam in any AN vessel and line, and then only under direct supervision. The 

temperature of 90-pound saturated steam is 331°F.This steam is applied through ammonia spargers 

with minimal flow to avoid creating low density zones. No one interviewed recommended the use 

of steam in AN applications where any degree of confinement could be generated. Mississippi 

Chemical neutralizer designs viewed by the investigation team during tours of these facilities were 

all equipped with check or block valves where the nitric acid lines connect to the spargers to prevent 

backflow. 

The investigation team determined that the use of 200 psig steam applied through a 

completely open steam valve to the neutralizer nitric acid spargers was a factor in the explosion. 

Nitric Acid Supply 

The investigation team graphed AN plant DCS data provided by Terra to document 

conditions in the AN plant for three days preceding the explosion. DCS data indicated that from 

0903 hours on December 10 through 2050 hours on December 11, the neutralizer nitric acid control 

valve (FCV-401) was 100% open (Figure7-5). Outside operator grab sample analyses for these 

periods indicate a rundown pH in the 7.0 range, heavily on the free ammonia side of the AN pH curve 

(Figure 7-6). Urea offgas flow during this period ranged from 70-85%. It appeared to the 

investigation team that when the urea plants were operating efficiently, the supply of nitric acid to the 

AN plant was limited and would not maintain the specified pH range of 5.5-6.5. 

Interviews with Terra employees indicated that acid supply to the AN plant had been a 

periodic problem for a few years prior to the installation of the scrubber [ID-14, ID-23]. Terra 

increased the nitric acid pump capacity to overcome the problem, but periodic shortages still occurred 

in the AN plant, apparently due to losses in the piping resulting from a combination of pipe diameter 

and length. The investigation team determined that when the AN scrubber was installed, the nitric 

acid supply for the scrubber was taken from the same nitric acid line that supplied the neutralizer 

without increasing the capability to supply nitric acid to the AN plant. 

Investigation-Derived Facts and Conditions Page 53 

pH-Acidity Relationship of Ammonium Nitrate

FIGURE7-6
When operated in the automatic mode, the AN DCS monitors urea offgas flows and the ratio 

controller adjusts the nitric acid flow rate to follow the offgas fluctuations. This feed-forward control 

loop anticipates pH changes and adjusts nitric acid flow accordingly. In cascade mode, the pH 

control loop, a feed-back loop, monitors the pH of the neutralizer output and fine tunes the nitric acid 

valve to maintain the selected pH (set point). However, the DCS was operating in the manual mode 

for the duration of the graphed data, with one short attempt to put the DCS in cascade mode that 

occurred at 0431 hours on December 10. The DCS data indicates that nitric acid control adjustments 

significantly lagged behind urea offgas fluctuations (reductions) on December 10, December 11, and 

during shutdown on December 12 (Figure 7-7). The DCS was in manual mode, and operator 

adjustments of the acid valve position lagged changes in the offgas flow rate. This caused an acid 

condition in the neutralizer each time. The ratio controller monitored the offgas changes, but was 

not engaged in either the automatic or cascade mode. 

The investigation team determined that the combination of an insufficient acid supply to the 

neutralizer and scrubber and a delayed response in the pH probe contributed to acid conditions in the 

neutralizer during operations, but was not a factor in the explosion of the neutralizer. The 

investigation team determined that this condition was a factor in the explosion of the rundown tank 

and the low pH (1.4) detected in the west AN storage tank. 

“No Flow” Condition in the AA’ Plant 

When the AN plant was shut down, flow stopped in the neutralizer, between the neutralizer 

and the rundown tank, and from the rundown tank to storage. Weak AN solution could still be 

circulated in the AN scrubber. 

A “no flow” condition in the neutralizer and rundown tank reduced the operator’s ability to 

monitor temperature and pH. This allowed local phenomena to occur in the AN solution and 

progress undetected (due to sensor isolation) and escalate because the effect is not circulated 

throughout the entire mass. This is a critical factor in creating local low-density zones and 

concentrating heat in local areas. 

Instrumentation in the AN plant was designed to monitor process parameters when the plant was 

operating continuously. When the neutralizer was shut down, the operator: 

1. Lost the ability to monitor pH in the plant utilizing the DCS because the pH probe was 

located in the rundown line between the neutralizer and the rundown tank. No grab samples 

were collected from the neutralizer or rundown tank after the AN plant was shut down. 

2. Lost the ability to accurately monitor temperature in the neutralizer because the temperature 

probe could only indicate the temperature of the AN solution in the immediate area of the 

probe due to the lack of circulation within the neutralizer. 

Therefore, the operator did not know temperature or pH conditions in the neutralizer or rundown 

tank when the plant was shut down. 

Investigation-Derived Facts and Conditions Page 56 

Information gathered from other AN production facility personnel indicate that “no flow” 

conditions longer than one to two hours are to be avoided. Methods employedin these facilities to 

avoid static conditions include: 

1. Recirculating the AN solution from the rundown tank back to the neutralizer. This procedure 

reduces the opportunity for local events to occur and allows operators to monitor process 

parameters by circulating vessel contents. 

2. Draining the neutralizer and rundown tank and rinsing the system with water. This negates 

the need to monitor the process. 

The investigation team concluded that the lack of circulation in the AN plant while the plant 

was shut down was a factor in allowing acid conditions in the neutralizer to go undetected from 1500 

hours on December 12 until the time of the explosion.The investigation team concluded that the lack 

of circulation in the neutralizer allowed local temperature increases in the neutralizer to go undetected 

from approximately 2030 hours on December 12 until the time of the explosion. 

Distributed Control System 

The DCS operator process change journal indicated that the AN plant DCS was operated 

mostly in the manual mode during December 1994. The journal indicated that operators briefly 

attempted to set some of the control loops in automatic during this tune, but quickly reverted back 

into manual mode. The investigation team could not determine why operators changed parameter 

set point values while in the automatic mode immediately after manually adjusting valve apertures. 

Conversely, operators attempted to manually change valve apertures immediately after changing set 

points while still in automatic mode. It appeared to the investigation team that operators were having 

difficulty operating the DCS, and were having difficulty determining in which mode the DCS was 

operating. 

AN scrubber alarms that indicated low flow in the scrubber recirculation pump and high 

scrubber pH (both primary and trim pH controls) were changed from a low priority setting that would 

alert the operator to a journal setting that would just record the alarm. After the AN plant was shut 

down, the following alarms were disabled: FI-404S (AN product to storage), FI-405S (offgas to 

neutralizer), and RC-401 (AN neutralizer acid/ammonia control). 

The DCS journal indicated that the DCS access level was changed from the operator setting 

to the engineer setting at 0116 hours on December 10, and remained in the engineer access level until 

1509 hours on December 12, 1994. The change back to operator access level coincided with the AN 

plant shutdown. Logs from the Terra guard house indicated that none of the four persons who Terra 

stated could change access levels was present in the plant on December 10 when the access level was 

changed to the engineer level [ID- 14]. 

The investigation team concluded that some operators were experiencing difficulties operating 

the AN plant DCS. The investigation team concluded that altering the scrubber alarm priorities was 

Investigation-Derived Facts and Conditions Page 58 
not a factor in the explosion. The investigation team concluded that the disabled alarms in the AN 

plant were not a factor in the explosion since they were disabled after plant shutdown. The difficulty 

controlling the AN process was not a factor in the explosion of the neutralizer, but did contribute to 

the low pH in the rundown tank and in the west AN storage tank. 

AN Operator Process Monitoring 

The Terra Area II board operator responsible for operating the AN plant was also responsible 

for operating Urea I and II plants. During normal operations, the AN operator should be aware of 

urea offgas fluctuations and adjust the AN plant accordingly when operating the DCS in manual 

mode. 

Statements in interviews and depositions of Terra employees indicate that the AN plant board 

operator on duty from 1900 hours on December 12 through the time of the explosion did not monitor 

the AN plant because it had been shut down prior to his shift and was not operating during his shift 

[ID-6]. At 2100 hours, the outside operator conducted an inspection of the AN plant. Information 

obtained in depositions indicated that the outside operator saw no reason to periodically monitor the 

‘AN plant because the plant was shut down. Both stated that all attention was directed toward 

repairing the nitric acid plant and operating the large urea plant (Stamicarbon plant). Terra employees 

stated that a walk-through inspection of the AN plant was conducted around 0530 hours on 

December 13, but that no samples had been taken and analyzed and no process variables had been 

checked. 

Terra had no written standard operating procedure for operator duties in the AN plant during 

shutdown (including guidance or conditions that would require draining and flushing the neutralizer 

and rundown tank): The only shutdown procedure provided to the investigation team generally stated 

the order in which process feed streams were to be shut down. Neither the SOP nor the additional 

information in the AN, plant manual advised the operator to ensure that the AN solution was on the 

ammonia side, when to leave the neutralizer full, or when to drain the neutralizer. Terra had the 

capability to backfill the neutralizer from the AN storage tanks to speed startup operations. All 

procedures for monitoring the plant when shut down with process vessels charged appeared to be at 

the operator’s discretion. 

Interviews conducted at other AN production facilities indicated that there is always a board 

operator and outside operator on duty in the AN plant when the plant is shut down and the process 

vessels are charged. Process parameters are monitored as they were when the plant was operating. 

The investigation team determined that lack of monitoring in the AN plant and the lack of the 

ability to monitor critical process parameters were factors in the explosion. 

Scrubber Operations 

Because the AN scrubber was a new addition to the AN plant, the investigation team gathered 

information about the operation of the scrubber and conditions in the scrubber at the time of the 

Investigation-Derived Facts and Conditions Page 59 
explosion to determine whether or not the scrubber contributed to the cause of the explosion. The 

investigation revealed that after installing the scrubber and providing some initial training to Terra 

operators, the scrubber manufacturer was not called back to Terra for the scrubber startup or to 

complete operator training under actual operating conditions [ID-2, ID- 12]. 

Comments from the December 5 night shift log indicate that the low flow alarm for the AN 

scrubber recirculation pump continued to indicate a low flow condition, and that the operator could 

not acknowledge the alarm on the DCS. The operator shut the AN scrubber down. Comments from 

the December 7 day shift log indicate that the AN scrubber was returned to service. 

Information obtained from depositions and interviews indicated that the outside operator in 

the AN plant began making rounds in the AN plant at approximately 0200 hours on December 12, 

when he noticed a leak in the condensate line providing makeup water to the AN scrubber. The line 

was observed to be leaking at the point that it entered the AN scrubber. Operators shut down the 

scrubber recirculation pump, 416J, and blocked in the scrubber condensate line and acid feeds. The 

condensate line (scrubber makeup water for level control) and acid feeds remained blocked in until 

the time of the explosion, isolating the AN scrubber from the makeup water supply. 

At 1650 hours on December 12, the AN scrubber recirculation pump was energized. 

Blowdown from the scrubber to the neutralizer commenced at 1652 hours at a rate of 3,400 pounds 

per hour (DCS data) and terminated at 1805 hours. The AN scrubber recirculation pump continued 

to run until the time of the explosion with the AN plant shut down and the scrubber condensate line 

blocked in. 

Interviews with operators indicated that some thought the scrubber pump had to be operated 

to prevent freezing, others that the pump could not be blocked in and drained to prevent freezing, and 

one stated that the pump was always in operation regardless of the AN plant status [ID-6]. Terra did 

not have a written procedure integrating scrubber operations into the overall AN plant operation. 

Interviews with contractors who installed the scrubber and associated piping indicated that the 

scrubber was steam jacketed, lines were steam traced, some were insulated, and steam was connected 

during the installation. Drawings indicated that the pump could be isolated and drained. Inspection 

of components located after the explosion included the block valves indicated on the drawings that 

were required for isolating and draining the scrubber recirculation loop. 

Although records and interviews indicated that there were problems operating the scrubber, 

the investigation team determined that none of these problems contributed to the explosion. 

Investigation-Derived Facts and Conditions Page 60 
Table 7-2 Neutralizer pH and Concentration

AN OUTSIDE OPERATOR GRAB SAMPLE LOGS

NEUTRALIZER pH AND CONCENTRATION

TIME pH CONCENTRATION (%) 

December 5, 1994 

0600 8.6 85.0 

1800 6.6 85.1 

December 6, 1994 

0000 6.5 No Entry 

0400 6.7 85.1 

0800 6.6 85.2 

1400 6.8 85.0 

1600 6.7 84.5 

2000 6.7 84.5 

December 7, 1994 

0200 6.6 85.2 

0800 68 85.0 

1200 6.9 85.0 

1400 6.7 84.5 

December 8, 1994 

0400 6.72 85.0 

0800 6.6 85.0 

1600 6.5 84.5 

2200 7.1 83.4 

December 9, 1994 

0200 7.2 No Entry 

1000 7.3/2.4 83.1 

Investigation-Derived Facts and Conditions Page 6 1 
TIME pH CONCENTRATION (%) 

1200 6.4 No Entry 

1600 7.0 83.5 

1800 7.1 83.0 

2000 6.8 83.0 

December 10, 1994 

0400 7.0 83.5 

0800 6.9 84.2 

1200 6.8 No Entry 

1400 7.0 85.0 

1600 7.1 No Entry 

2000 7.1 84.8 

December 11, 1994 

0200 7.0 85.0 

1000 66 83.6 

2000 -1.5 82.0 

Approximately 20 samples were analyze from 2200 hours on the 11th through 0100 hours on 

the 12th while “lining out” the neutralizer These were not logged. 

December 12, 1994 

1000 5.9 84.8 

Investigation-Derived Facts and Conditions Page 62 
THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK.

Explosion Cause Scenarios Page 63
SECTION 8: EXPLOSION CAUSE SCENARIOS 

The investigation team developed several explosion scenarios based on the facts previously stated in 

this report (Table 8-1). The investigation team focused on the following areas when evaluating each 

scenario: 

1. Events and conditions that could provide initiation energy to these vessels, including the 

vessels and vessel components and peripheral appliances and components. 

2. Sensitization caused by contaminants in the ammonium nitrate (individual effects and possible 

synergistic effects). 

3. Effects of low pH on ammonium nitrate stability. 

4. Potential confinement of ammonium nitrate. 

5. Creation of low-density zones in the ammonium nitrate. 

Physical evidence observed following the explosion indicated that the explosion occurred in 

the ammonium nitrate (AN) plant.The investigation team was able to view the AN plant area within 

four days of the explosion. The AN unit was completely destroyed. The investigation team observed 

two large craters (located underneath the areas formerly occupied by the AN neutralizer and the AN 

rundown tank) approximately 0.5 meters deep and 1.5 meters deep respectively (Photo 8-1). The 

craters provided initial evidence that both of these vessels exploded. All process and recirculation 

pumps were missing from the AN plant area, but pads and foundations remained (Photo 8-2). The 

AN scrubber was missing from its original site, with a portion of the base ring and vessel floor 

remaining (Photo 8-3). The concrete pad on which the scrubber was anchored was intact. 

As a part of the investigative process, the investigation team attempted to reconstruct the 

conditions present in the AN plant prior to the explosion. Although all of the conditions could not 

be determined, the investigation team obtained evidence that the following conditions existed in the 

AN plant prior to and at the time of the explosion: 

1. The AN solution in the neutralizer had been acidified to a pH of approximately 0.8 

(calculations). 

2. The AN plant was shut down with AN solution in the neutralizer and rundown tank. 

3. No nitric acid or urea plant offgas were applied to the neutralizer since shutdown. 

4. The AN scrubber recirculation pump was energized and running. 

5. Chlorides were present in the neutralizer and rundown tank. 

6. The pH of the rundown tank was approximately 1.0. 

7. The rundown tank level was approximately 30%. 

8. 200 psig steam was applied to the nitric acid spargers in the neutralizer, with the steam valve 

fully open from approximately 2030 hours on December 12 until the time of the explosion. 

9. No steam was applied to the steam jackets in the AN scrubber or rundown tank. 

10. 200 psig steam was applied to the AN line from the product pumps to storage. 

11. The AN product pumps had been shut down and blocked in. 

Explosion Cause Scenarios Page 64 
Table 8 - 1, Explosion Cause Scenarios 

Scenario 1 - AN Plant Pumps 

AN Scrubber Recirculation Pump

Product Pumps

Scenario 2 - AN Scrubber 

Scenario 3 - Neutralizer 

Spargers

Neutralizer Contents

Scenario 4 - Rundown Tank 

Explosion Cause Scenarios Page 65 
8 -

AY

Photo 1

 Plant Craters 

Photo 8 - 2

83% Product Pump Pads

Explosion Cause Scenarios Page 66
Photo 8 - 3

AN Scrubber Pad and Vessel Floor

Explosion Cause Scenarios Page 67 
12. The 20-psig condensate line (makeup water) to the AN scrubber had been blocked in. 

13. Some of the AN scrubber contents had been transferred into the neutralizer. 

14. The AN plant was not monitored from 2100 hours on December 12 through the time of the 

explosion, except for one walk-through. 

15. There was no flow in the neutralizer (other than steam) or rundown tank from approximately 

1500 hours through the time of the explosion. 

The lack of as-built drawings of the AN plant, accurate pipe elevation drawings, written 

operating procedures for tasks performed in the AN plant, component design and operating 

specifications, and documentation of parameters made the work of reconstructing the plant and 

conditions in the plant a difficult process. 

Scenario 1 - AN Plant Pumps 

Pumps have initiated AN explosions when allowed to run while blocked in or under low flow 

conditions. Pumps are capable of supplying sufficient energy and confinement to cause an AN 

explosion under these conditions. The investigation team determined that there were at least three 

centrifugal pumps in the AN unit that could have been capable of initiating an explosion under these 

conditions, two product pumps and one AN scrubber recirculation pump. 

AN Scrubber Recirculation Pump 

The AN Scrubber recirculation pump was rated at 3,000 gallons per minute. The pump 

circulated the weak AN solution from the scrubber sump through the spray nozzles in the top of the 

scrubber. One theory investigated was that flow through the pump was reduced or blocked, that 

suficient energy was concentrated in the pump to initiate the decomposition of the AN solution, and 

that this initiated explosions in the neutralizer and rundown tank, 

Facts that support this scenario include: 

1. The pump was running prior to and up to the time of the explosion. 

2 . The scrubber pump had been running without monitoring, without a makeup water supply, 

and without a nitric acid supply. 

3. A flexible coupling on the pump suction side had been expanded and failed from 

overpressurization from the inside. 

Facts that contraindicate this scenario include: 

1. The pump casing was intact and located approximately 800 feet northwest of the AN plant 

as a result of the explosion (Photos 8-4 A and 8-4B). 

Explosion Cause Scenarios Page 68 
Photo 8 - 4A 

4165 Shaft View 

Explosion Cause Scenarios Page 69
Photo 8 - 4B 

4165 Suction View 

Explosion Cause Scenarios Page 70
2. Damage to the pump casing included perforations caused by high-energy missiles from 

outside the pump from the direction of the neutralizer (Photo 8-5 and 8-6). 

3. Damage to the pump impeller and impeller shaft were consistent with an external force from 

the direction of the neutralizer. 

4. Metallurgist’s inspection of the pump failed to reveal any evidence of heating or deformation 

from internal pressures (Photo 8-7 and 8-8). 

The investigation team concluded that the scrubber pump did not initiate the explosion, even 

though the concentration and pH of the AN solution were unknown because of lack of monitoring 

and documentation. Additional research by the investigation team indicated that a small explosion 

or deflagration occurring in a pump would not initiate other vessels via pressure wave through duct 

work similar to that in the Terra AN plant. 

Product Pumps 

The Terra AN unit contained two Wilfley centrifugal product pumps in a parallel 

configuration to pump AN solution from the rundown tank to storage. One pump was active with 

the other in standby. Flow control was achieved with a control valve on the discharge side of the 

pump. The valve position was determined by the level in the rundown tank. A possible scenario was 

that one of the product pumps was running and was blocked in, exploded, and missiles from the pump 

case initiated explosions in the neutralizer and rundown tank. 

Facts supporting this scenario include: 

1. Only one product pump was recovered. The recovered pump displayed clear evidence of 

damage from an external source, but the second pump and its associated piping and valves 

were never recovered. Possibilities for this ranged from: 

A) detonation of pump contents that fragmented the pump, to 

B) the pump was blasted from its mount into building debris where a search could not 

be safely performed. 

2. The indicated level in the rundown tank (DCS data) steadily declined at a rate of 

approximately 50 gallons per hour from the time that the AN plant was shut down until the 

explosion. The rate of decline of the DCS indicated rundown tank level remained constant 

over this time period. (Figure 8-1) 

3. The DCS had the capability of tripping the product pumps under low flow conditions, but this 

feature was not activated when the DCS was installed. 

4. The AN product to storage alarm was disabled. 

5. 200 psig was applied through one of the product pumps to storage. 

Facts that contraindicated this scenario include: 

1. Deposition and interview statements that indicate that both product pumps were shut down 

and blocked in. 

Explosion Cause Scenarios Page71 
-

1165

-

4165

Page

Photo 8 5

 External Damage 

Photo 8 6

 External Damage

Explosion Cause Scenarios 72
-

1165

Photo 8 7

 Casing 

Photo 8 - 8

1165 Impeller

Explosion Cause Scenarios Page 73

2. Notations of shift logs and reports that the pumps had been shut down and blocked in. 

3. Review of damage to other process vessels indicated that the neutralizer exploded first. 

Inspection of recovered neutralizer fragments did not reveal any damage sustained from 

outside impact. 

4. Damage to the recovered pump displayed evidence of a blast originating in the area of the 

neutralizer and did not display any missile damage consistent with pump case fragments 

originating from an adjacent pump. 

Based on the available evidence, the investigation team concluded that the process pump did 

not initiate the explosion. Evidence obtained in other areas of the AN plant indicated, that the 

explosion did not originate in the process pump area. Because the pump could not be located and 

examined, however, the investigation team cannot positively eliminate it as a potential explosion 

initiating source. 

Scenario 2 - AN Scrubber 

The investigation team investigated the possibility that the explosion occurred in the AN 

scrubber for two primary reasons: 

1 . The scrubber was a recent addition to the ammonium nitrate plant, and therefore represented 

a recent change of plant operating procedures, and 

2. The scrubber recirculation pump was running prior to and at the time of the explosion. 

Facts supporting this scenario include: 

1. The scrubber recirculation pump was running at the time of the explosion. 

2. The scrubber level as indicated by the DCS continued to decline from approximately 1630 

hours on December 12 until the time of the explosion. 

3. The scrubber was not monitored. 

Facts that contraindicated this scenario include: 

1. The pH of the scrubber contents was greater than 5.0. 

2. The scrubber contents were circulated at a rate of approximately 3,000 gallons per minute. 

3. The investigation team estimated the concentration of the scrubber contents to be 60% or 

less. 

4 . Damage to the scrubber was consistent with an explosion occurring outside of the scrubber. 

The investigation team observed large portions of the scrubber body and duct work in the field 

west of the AN plant following the explosion. Visual inspection revealed that damage to the 

fragments was consistent with an externally applied force and were perforated by fragments from an 

Explosion Cause Scenarios Page 75 
external source (Photo 8-9). These observations were later confirmed by metallurgist after visual 

inspection. Close observation of the concrete scrubber foundation revealed a large portion of the 

scrubber floor was still present. Mounting bolts on the concrete pad were all bent in approximately 

the same direction, away from the neutralizer location (Photo 8-10 and 8-11); No crater was 

observed in the immediate area around the scrubber base. Investigation team and metallurgical 

observations pertaining to the circulation pump (416J) are listed in the AN Scrubber Recirculation 

Pump section of this report. 

The investigation team reviewed DCS data pertaining to the AN scrubber. The data indicated 

that the scrubber level declined at a steady rate following the blowdown event occurring on December 

12 through the time of the explosion (Figure 8-2). The investigation team determined that two 

plausible scenarios for this decline were: 

1. The blowdown valve was not completely closed when blowdown was completed; or 

2. Level sensors and associated piping were demonstrating cold weather deviation similar to the 

level in the rundown tank. 

The investigation team concluded that neither scenario would have been significant in determining 

the cause of the explosion 

Based on the evidence obtained during the investigation, the investigation team determined 

that the explosion did not initiate in the scrubber. 

Scenario 3 - Neutralizer 

Investigation team observations of the AN plant site and assessments of collateral damage 

indicated that the initial explosion occurred in the neutralizer. Facts and observations supporting this 

conclusion include: 

1. Reverse trajectory sighting of damage to a nearby water tank (Photo 8-12) and the ends of 

two pressure ammonia vessels (Photo 8-13) indicated that the neutralizer was the source of 

the penetrating fragments. 

2. Reverse trajectory sighting of reinforcing bar angles, anchor bolt directions, and prill tower 

footing bolts and pads converged on the original neutralizer site. 

3. Reverse trajectory sighting of a chemical stripper originally located near the AN plant 

indicated the explosion origin to be along a line that intersected with the original neutralizer 

site (Photo 8-14). 

4. Reverse trajectory sighting of scrubber sections and the ammonia vaporizer indicated the 

explosion origin to be along a line that intersected the original neutralizer site. 

Explosion Cause Scenarios Page 76 
Photo 8 - 9

AN Scrubber

Explosion Cause Scenarios Page 77
Photo 8 - 10

AN Scrubber Mounting Bolts 

Photo 8-11 

AN Scrubber Mounting Bolts 

Explosion Cause Scenarios Page 78

Photo 8 - 12 

Water Tank Damage (Ammonia Tank Background) 

Photo 8 - 13

Ammonia Bullet Tank Damage

Explosion Cause Scenarios Page 80
Photo 8 - 14

Chemical Stripper Trajectory

Explosion Cause Scenarios Page 81
5. Visual analysis of recovered neutralizer pieces indicated that a deflagration had originated 

inside the vessel, and none of the pieces examined exhibited impacts from external missiles 

(Photo 8-15). 

6. Analysis of neutralizer fragment dispersion exhibited dispersion over a 360o

 arc, while other 

vessel fragment dispersion were roughly along lines consistent with an explosion at the 

neutralizer site. (Figure 8-3) 

7. Metallurgical examination and testing of selected neutralizer fragments indicated that the 

neutralizer had been subjected to an internal overpressurization before the explosion, 

indicating the possibility of an accelerating thermal decomposition of the AN solution before 

the explosion occurred. Metallurgical analysis also indicated that the internal surfaces of the 

tested fragments had been exposed to temperatures exceeding 1,700oF and had been air 

quenched. A fragment identified as a reinforcing pad displayed a pitting corrosion. These 

corrosion pits were not observed during the September turnaround according to Terra 

employees. 

8. Visual examination of neutralizer fragments by the metallurgist revealed that the fragments 

from the lower third of the neutralizer were large and relatively intact. The ammonia sparger 

plate in the base of the neutralizer was intact and damage was consistent with a force applied 

from the upper section of the neutralizer (Photo 8-16). These pieces indicated damage 

probably due to a deflagration. 

9 . Examination of pieces from the middle section of the neutralizer indicated fragmentation 

damage consistent with a detonation that followed a deflagration. Pieces from the upper 

section of the neutralizer displayed damage consistent with a deflagration followed by a 

detonation. All neutralizer pieces recovered and examined displayed damage consistent with 

a deflagration occurring inside the neutralizer, and none displayed damage consistent with 

external forces or missiles. 

The investigation team determined that the following conditions existed in the neutralizer prior 

to the explosion: 

1. The neutralizer contained approximately 18,000 gallons of ammonium nitrate solution at an 

estimated concentration of 83%-85% and a pH of 0.8. 

2 . Analytical data provided by Terra indicated the presence of chlorides in the AN solution. 

3. The temperature in the neutralizer as indicated by the DCS declined steadily from 238°F at 

1700 hours on December 12 to 221 o

F at 0600 hours on December 13. 

4. 200 psig steam was added to the neutralizer through the nitric acid spargers for a period of 

approximately 9 hours (nitric acid piping not insulated). 

5 . Variation in steam flows in other areas of the facility could generate superheat in the steam 

at the AN plant. 

Explosion Cause Scenarios Page 82 
Photo 8 - 15

Neutralizer Base Fragments 

Photo 8 - 16

Ammonia Sparger Plate

Explosion Cause Scenarios Page 83

6 . No ammonia was added to the neutralizer during this period. 

7. The neutralizer was insulated with two inches of calcium silicate. 

8. The ambient temperatures ranged from 11°F through 5°F during this time period. 

9. The contents of the neutralizer and rundown tank were not circulated, but remained static 

from approximately 1500 hours on December 12 through the time of the explosion. 

The investigation team developed two scenarios for an explosion originating in the neutralizer 

based on the facts as presented. The first scenario evaluated the possibility of AN thermal 

decomposition originating in the nitric acid spargers, and the second scenario evaluated the possibility 

of AN thermal decomposition originating in the neutralizer in an area adjacent to the spargers. 

Spargers 

The investigation team determined that the nitric acid spargers in the neutralizer could present 

an environment in which AN solution could be subjected to both heat and confinement in addition 

to low pH and contamination. The spargers were constructed of Grade 2, Schedule 40 titanium. 

The original piping and instrumentation diagram provided to Terra by Mississippi Chemical 

Corporation indicates that check or block valves were to be installed on the nitric acid spargers where 

they exit the neutralizer. This was not done when Terra fabricated the neutralizer utilizing the 

information provided by Mississippi Chemical Corporation (‘Terra was the general contractor for the 

construction of the neutralizer). This allowed the potential for ammonium nitrate solution to 

backflow into the spargers if the nitric acid flow ceased and the lines were not properly blocked in. 

As built by Terra, some backflow of ammonium nitrate into the spargers was likely. 

The investigation team reviewed the effect of applying 200 psig steam to the nitric acid 

spargers if they contained AN solution. The investigation team determined the following: 

1. The nitric acid lines were not insulated or steam traced and had been purged with plant air for 

a period of over four hours. 

2. Ambient temperatures for this time period ranged from 11°F to 5°F. 

3. 200 psig steam was applied for a period of approximately nine hours at a point immediately 

downstream from FCV-401 and upstream from the barometric loop. This steam may have 

had a component of superheat, especially during the period of time when the acid plant was 

being started. 

4. Temperature of the neutralizer contents (DCS) was 221°F at the time of the explosion. 

A 200 psig steam hose was next attached to the nitric acid piping leading to the acid spargers. 

Calculation results estimated that this would have resulted in a steam flow rate of 2,500 lbs/hr into 

the neutralizer through the nitric acid line. The estimated enthalpy of this steam would have been 

1,170 BTU/lb. This enthalpy is based on saturated conditions at the 200 psig steam header and 

estimated heat losses of 30 BTU/lb between the header and the neutralizer. Any superheat present 

in the 200 psig supply header would have resulted in additional energy being applied to the neutralizer 

by the steam. 

Explosion Cause Scenarios Page 85 
Based on the dimensions provided for the nitric acid spargers and the sparger hole 

configuration, the maximum volume of AN solution that could be trapped in the spargers if the 

spargers were perfectly level was calculated to be approximately 3 liters in each sparger. This volume 

would be reduced as sparger orientation deviated from level. The investigation team concluded that 

any trapped acid and AN solution would have initially been diluted by condensate and consequently 

stripped by turbulent steam flow in the sparger. However, any remaining contents would likely have 

been located in the ends of the spargers, and may have been concentrated due to the presence of 

superheated steam. 

The investigation team requested a metallurgical examination to determine what forces had 

acted on the spargers, determine temperatures to which the spargers had been exposed, determine 

if any unusual corrosion had occurred, and determine whether internal or external surfaces, or both, 

had been exposed to these conditions. The metallurgical report indicated that: 

1. No unusual corrosion had occurred in the spargers. The sparger interior surfaces exhibited 

a titanium oxide coating. Pitting was observed on the outside surface of some sparger pieces. 

The highest corrosion occurred at the gas/liquid interphase on the internal surface. This was 

described as a normal corrosion pattern for this application. 

2. The external surface of the sparger pieces examined had been exposed to temperatures 

exceeding 1,700o

F and air quenched. The internal surface was not exposed to this 

temperature extreme. 

3. The forces that, distorted and destroyed the spargers were applied to the external surface, not 

the interior surface. 

The investigation team concluded that the explosion in the neutralizer occurred on the sparger 

exterior, not internally. 

Since chlorides were present in the neutralizer, the investigation team attempted to determine 

if a synergistic decomposition reaction could have been initiated by the presence of titanium. 

Information obtained by the investigation team indicates that titanium is much more corrosion 

resistant than stainless steel in a nitric acid environment [26]. Titanium corrosion occurs at its highest 

rate, 0.04-0. 15 mm/yr, when the acid flow is constant. When in a static acid environment, titanium 

quickly forms an oxide layer that greatly retards corrosion. Titanium is one of a few substances that 

reacts with its own oxide, or heals in an acid environment. A review of studies that attempted to 

determine synergistic effects on the thernial decomposition of ammonium nitrate indicated that 

titanium did not react synergistically with the chlorides because of its poor solubility in a static system 

[3]. 

Based on the facts available, the investigation team concluded that neither the environment 

in the sparger or the titanium from which the sparger was constructed contributed to the Terra 

explosion. 

Explosion Cause Scenarios Page 86 
Neutralizer Contents 

Thermal decomposition within AN liquid is difficult to initiate. The fact that the neutralizer

did explode (with a resulting crater) illustrates that the liquid within the vessel was sensitized. An

examination of the supporting factors outlined for the neutralizer show that four are of primary

significance:

1. 200 psig steam was present as an energy source; 

2. The pH was low (0.8); 

3. Bubbles were present in the solution; and 

4. Contaminants were present. 

The investigation team evaluated this scenario by first developing a conceptual model of how these

initial conditions would degrade into a deflagration. After the conceptual model was formulated,

standard engineering principles were applied to calculate, when possible, the mechanics of the

scenario.

As described in this section, the initial conditions within the neutralizer were established with 

the addition of nitric acid as a result of clearing a line. Addition of this acid lowered the theoretical 

pH (developed assuming standard temperature and pressure conditions) to a value near 0.8. Since 

the actual temperature of the AN was in the range of 230o to 240o F, the activity of the acid will be 

somewhat higher although the significant factor is that the AN solution is very acidic. This condition 

is generally considered undesirable in handling and storing AN since low pH has the effect of 

sensitizing the product. 

The steam connection was made using a flexible l-inch diameter line using Chicago steam 

connectors. Calculations estimated the steam rate and conditions resulting from this practice. 

Calculation results indicated a steam flow rate of 2,500 lbs/hr into the neutralizer with a minimum 

heat content of 1,170 BTU/lb. The pressure of the steam dropped to approximately 4 psig as it 

passed from the sparger into the neutralizer. The steam flow rate and heat content resulted in 

approximately 2,900,000 BTU/hr of heat input into the neutralizer. 

At the onset of steam application to the neutralizer, the temperature of the AN was 

approximately 230°F as indicated by the DCS. As the steam entered, it would have rapidly collapsed 

-(condensed), transferring heat to the AN at a rate sufficient to raise the AN temperature 

approximately 25oF/hr until the AN solution boiling point was reached. 

The investigation team observed that the neutralizer temperature, as indicated by the DCS, 

continued to decline throughout the shutdown period despitethe addition of steam. This contradicted 

the fact that the temperature should have increased at a rate of approximately 25o

F/hr. The team 

concluded that either the temperature probe was malfunctioning or that the probe had become 

thermally isolated. A malfunctioning probe was discounted based on the consistent response noted 

during the previous AN plant operation and shutdown. Information provided by Terra in September 

1995, indicated that the temperature probe was located approximately 18 inches below the rundown 

Explosion Cause Scenarios Page 87 
line. The investigation team concluded that the probe was thermally isolated during shutdown 

because of no-flow conditions in the neutralizer. 

During the initial phases of steam addition to the neutralizer, the majority of the heat was 

being utilized to raise the temperature of the AN solution. Heating the area of the spargers would 

have initiated convection currents in the AN solution, rising over the spargers along the outside wall 

of the draft tube and returning along the outside wall of the neutralizer (Figure 8-4). This would 

have created a local heating zone immediately above the sparger. Density of the AN and thermal 

inertia in this zone was also reduced. Steam added to the neutralizer during this initial heating phase 

would have added approximately 2,500 lbs/hr of-water to the AN solution, diluting the original AN 

concentration by approximately 2%. 

As the temperature of the AN solution neared the boiling point, energy transfer in the system 

would have altered. Calculations indicate the boiling point was reached approximately two hours 

after steam application. As a boiling state was attained, a quasi-energy balance was established with 

an input of 1,170 BTU/lb of applied steam, and an output of 1,150 BTU/lb (steam). This resulted 

in a small driving force of approximately 20 BTU/lb, which generated a net loss of water from the 

system. This net water loss would have concentrated the solution slightly over the succeeding hours. 

The temperature in the area of the spargers should not have been sufficient to initiate thermal 

decomposition in chemically pure AN. The investigation team determined that low pH, creation of 

low-density zones, the presence of chlorides, lack of circulation in the neutralizer, and possibly the 

presence of metals (especially chromium from the stainless steel) were necessary conditions to initiate 

AN thermal decomposition. 

Calculations indicated that continued steam heating would have resulted in a high temperature 

(near the temperature of the steam) and a low-density zone to develop directly around or above the 

sparger. Sensitizing factors lowered the thermal decomposition activation energy of the AN. The 

combination of energy in the input steam and the addition of superheat (possibly due to high steam 

demands in the nitric acid plant) exceeded the activation energy of the AN, resulting in the onset of 

thermal decomposition. The initial dissociation reaction was endothermic, and would have been 

quickly overcome due to its close proximity to the heating source. As heat continued to be 

transferred into this zone, the reaction became exothermic and self-sustaining [ 13]. At this point, 

large volumes of gases would have been formed.The effect of the rapidly expanding gas would have 

been twofold: 

1. Adiabatic compression of the bubbles in the AN solution resulted in a more rapidly expanding 

wave front (deflagration propagation) [ 1]; and 

2. Overcoming the vent capacity of the neutralizer, rundown tank, and scrubber system that, in 

turn, resulted in additional confinement until the vessels failed [21]. 

Explosion Cause Scenarios Page 88 

The team determined that the following observations supported this scenario: 

1. Metallurgical analysis of the neutralizer fragments indicated that the vessel walls were 

stretched prior to failing; an indication that the pressure increased in a rapid fashion. Such 

an observation would be consistent with a deflagration. 

2. Pitting of some neutralizer fragments that were located near the spargers exhibits 

characteristics of cavitation, which implies that the process causing the deformation involved 

high velocity bubbles (photo 8-17). 

3. The third observation was how the neutralizer failed. Recovered fragments showed that the 

bottom portion remained largely intact while the mid-section (sparger area) was largely 

destroyed. The upper portion was fragmented but less so than the mid-section. The fragment 

sizes imply that once the deflagration started, the bottom section was removed from the vessel 

proper due to pressure induced mechanical failure. Likewise, the upper section failed due to 

pressure. The mid-section was impacted by not only a deflagration but also a detonation 

resulting in small fragments. 

The investigation team determined, based on the facts presented, that the conditions in the 

neutralizer were sufficient to have caused the explosion at the Terra Port Neal Complex. 

Scenario 4 - Rundown Tank 

Initial observations of the AN plant area following the explosion indicated that the largest 

crater was in the immediate area of the rundoun tank. Visual examination of fragments identified as 

from the rundown tank indicated that the vessel had stretched from an internal pressure increase prior 

to failing and that the internal surfaces of the fragments had been exposed to a brief period of high 

heat (Photo 8-18). Metallurgical analysis of a rundown tank fragment confirmed these observations 

and determined that the internal surface had been exposed to a temperature exceeding 1,700o

F for 

a very short period of time and air quenched. The debris dispersion pattern of the rundown tank 

pieces indicated that this vessel exploded after the neutralizer (microseconds). 

The investigation team determined that the following conditions were present in the rundown 

tank prior to the explosion: 

1. The temperature of the contents was unknown. This parameter was not monitored by the 

DCS. Information obtained during depositions and interviews indicated that the rundown 

tank did have local temperature indication, but no one knew if it was working and no one 

monitored the temperature. The temperature was assumed to approximate the neutralizer at 

1500 hours on December 12. 

2. The rundown tank contained approximately 2,000 gallons of AN solution, approximately 45% 

DCS level indication, when the AN plant was shut down. The indicated tank level continued 

to decline at a steady rate to approximately 28% at the time of the explosion. 

Explosion Cause Scenarios Page 90 
Photo 8-17

EPA Item #8 (Neutralizer) 

Photo 8 - 18

EPA Item #10 (Rundown Tank) 

Explosion Cause Scenarios Page 91
3. The rundown tank was equipped with steam coils that were attached to a steam header, but 

steam was not applied because of problems with the coils. 

4. The pH of the rundown tank solution was determined to be approximately 1.0 and analytical 

data indicated the presence of chlorides. 

5. Approximately 2 minutes before the explosion (DCS time), the level indication showed an 

exponential increase to an indicated level of 100% at the approximate time of the explosion 

(Figur e 8-5). 

The investigation team could not determine specifically why the DCS indicated a steadily 

declining level from 1500 hours on December 12 through the time of the explosion. Three plausible 

scenarios for this indication were: 

1. A process pump was operating. The investigation team determined that both process pumps 

had been shut down and blocked in and steam had been applied to the AN product lines from 

the process pumps to storage. 

2. The rundown tank was leaking. The investigation team was unable to totally disprove this 

possibility, but ambient temperatures would contraindicate that ammonium nitrate would 

continue to release from the rundown tank at the low rate indicated without crystallizing. 

3. Impact of cold weather on the rundown tank level sensors and associated piping. The 

investigation team determined that work orders had been issued requesting that these sensors 

be insulated, but they had not been insulated prior to the explosion. The investigation team 

determined that this was the most likely cause of the declining level. 

The investigation team determined that the DCS level increase immediately before the explosion was 

due to a pressure increase in the rundown tank, neutralizer and scrubber due to increasing ammonium 

nitrate thermal decomposition in the neutralizer. 

The investigation team determined that no energy source could be located in the rundown 

tank to initiate the explosion. Analysis of rundown tank fragment dispersion also supports the 

conclusion that the rundown tank did not cause the explosion. The rundown tank could be initiated 

by another strong energy source because of the low pH of the AN solution, the presence of 

contaminants in the AN solution, and the fact that partial crystallization along the tank walls had 

occurred because of heat loss. 

The investigation team determined that the explosion in the rundown tank most likely 

occurred as a result of impacts of high-energy metal fragments originating from the exploding 

neutralizer. 

Explosion Cause Scenarios Page 92 

SECTION 9: CONCLUSION 

The investigation team concluded that the explosion that occurred in the ammonium nitrate 

plant at the Terra Port Neal Complex initiated in the neutralizer, and the explosion in the neutralizer 

initiated an explosion in the rundown tank virtually instantaneously. 

The investigation team developed three scenarios that could have initiated the sensitized 

contents of the neutralizer. Based on the evidence available, the investigation team concluded that 

the explosion initiated in the ammonium nitrate solution in the neutralizer as a thermal decomposition 

reaction that accelerated into a deflagration. 

The investigation team concluded that the explosion occurred as a direct result of a series of’ 

unsafe operating procedures and conditions at the Terra Port Neal Complex. The team concluded 

that the explosion could not have occurred without the presence of the following conditions. The 

conditions were a direct result of unsafe operating procedures. They are as follows: 

1. Strongly Acidic Conditions in the Neutralizer. 

The acidic conditions in the neutralizer resulted primarily from purging nitric acid from the 

acid line beginning in the acid plant and terminating at the neutralizer. 

The investigation team concluded that had the pH of the neutralizer and rundown tank been 

adjusted to a value of 60 or greater, the explosion probably would not have occurred. The 

presence of free ammonia inhibits acid catalysis in thermal decomposition reactions in the 

ammonium nitrate 

2. Application of 200 psig Steam to the Nitric Acid Spargers. 

200 psig steam was applied directly to the nitric acid line in the AN plant, and subsequently 

to the nitric acid spargers. The steam valve was completely open. The only limit to the 

amount of steam applied to the spargers was the ability of the steam system to deliver the 

steam through the one-inch diameter steam line. The temperature of the steam, with the 

addition of any superheat, created temperatures in and around the spargers higher than the 

maximum temperatures of concern provided by Terra personnel. Application of the steam 

in this manner would also allow any additional superheat generated by other demands on the 

steam system to reach the neutralizer. This probably occurred when the nitric acid plant 

compressor was turning in the “slow roll” phase of startup. 

The investigation team determined that the explosion would not have occurred without the 

unrestricted application of 200 psig steam to the spargers. 

3. Introduction of Low Density Zones (Bubbles). 

Low density zones were created by the application of compressed air, then 200 psig steam 

Conclusion Page 94 
through the nitric acid spargers. Low density zones in the AN solution: 

a) Reduce the activation energy necessary to initiate thermal decomposition of the AN; 

b) Lower thermal inertia in these zones allowing local heating; and 

c) Increase propagation of resulting pressure waves through adiabatic compression of 

the bubbles. 

Low density zones were created in the neutralizer for a period of 15 hours before the 

explosion. Terra did not have written operating procedures for introduction of air or steam 

into the neutralizer. 

4. Lack of Plow in Vessels. 

The lack of flow in the neutralizer and from the neutralizer into the rundown tank isolated 

sensors critical for monitoring the AN solution. The lack of flow also contributed to thermal 

and contaminant stratification within the vessels and the significance of local events and 

conditions, such as heating and diffusion. This allowed for local conditions to develop 

without warning to the operator and without being diffused throughout the AN mass. This 

condition existed in the neutralizer for a period of 15 hours before the explosion. 

Plow in the neutralizer and rundown tank was possible using one of the product pumps and 

a recirculation line from the pump to the neutralizer Terra did not have procedures or 

guidelines for limiting the amount of time that the “no flow” condition could exist before 

additional monitoring or procedures were initiated 

5. Contamination. 

Analytical data proved that chlorides were present in the nitric acid and ammonium nitrate. 

The presence of chlorides in ammonium nitrate also decrease the activation energy necessary 

to initiate thermal decomposition, and react synergistically with many metals to further 

decrease this activation energy. Terra did not monitor the nitric acid for chlorides. 

6. Process Monitoring. 

The lack of periodic monitoring of the AN plant when it was shut down with AN in the 

neutralizer and rundown tankallowed otherwise detectable conditions to exist from the time 

the plant was shut down until the explosion. Terra did not have operating procedures that 

required periodic parameter monitoring when the plant was shut down with vessels charged. 

Page 95 
SECTION 10: RECOMMENDATIONS 

The investigation team concluded that the conditions that caused the explosion existed 

primarily because of the lack of safe operating procedures in the Terra Port Neal ammonium nitrate 

unit. Most of the operating procedures that were used in the AN plant were not written. The 

investigation team further concluded that the lack of written safe operating procedures: 

1. Would have made development and evaluation of training programs difficult. 

2. Would have made enforcement of operating procedure use difficult. 

3. Would have made evaluation of the operating procedures for deficiencies and improvements 

difficult. 

Effective safe operating procedures can only be written and evaluated after completion of a 

comprehensive process hazard evaluation. The investigation team could find no evidence that Terra 

had conducted such an evaluation of the ammonium nitrate unit. This resulted in a low pH in the 

neutralizer after shutdown, the unknown presence of contaminants in the neutralizer, the lack of 

monitoring when the plant was shut down with the process vessels charged, the lack of flow in the 

neutralizer, the introduction of high temperatures through the nitric acid spargers, and the 

introduction of low density zones in the neutralizer. 

Terra employees assumed that the DCS in the ammonium nitrate unit was not capable of 

operating in automatic or cascade mode because of a part shortage. A lack of clear communications 

between operations, maintenance and engineering caused the unit to operate without a pH probe from 

November 27 through the time of the explosion. pH probe suppliers indicated that a viable pH probe 

could have been provided in a few days if one had been ordered. Operators were performing 

maintenance tasks, because maintenance was busy in other parts of the facility. 

To avoid the conditions described in Section 9 of this report that resulted in the explosion in 

the ammonium nitrate plant and to minimize the chance of a similar event occurring again, the 

investigation team developed the following recommendations for ammonium nitrate manufacturers. 

1 . The investigation team recommends that engineers and operators conduct a thorough process 

hazard analysis (PHA) of ammonium nitrate plants. This PHA should include a thorough 

literature search to identify materials and conditions that increase the hazards of producing 

AN solutions. This evaluation should be conducted periodically to ensure that the most 

recent information is always available to the engineers and operators conducting process 

hazard evaluations. This activity also requires current piping and instrumentation diagrams, 

unit drawings, materials of construction, equipment histories, and safe operating procedures. 

Many of the hazards listed in this literature can be avoided through procedural changes. The 

engineer responsible for the AN unit should head the evaluation team. 

2. The investigation team recommends that engineers and operators establish safe process 

Recommendations Page 96 
operating parameters for the ammonium nitrate unit. Operating parameters should include 

at least pH, temperature and the presence of contaminants. This should include a program 

to monitor feed streams for the presence of known contaminants on a periodic basis as well 

as periodically reevaluating operating parameter ranges. 

3. The investigation team recommends that engineers and operators develop written, safe 

operating procedures for the ammonium nitrate unit. Procedures should be developed for 

activities conducted in the AN plant in all modes of operation, including periods when the unit 

is shut down and the vessels are charged. These procedures should require that process 

parameters be monitored and should specify actions to be taken when parameter values 

deviate from acceptable ranges. Management roles in this process include ensuring that these 

procedures are used and that procedures are appropriate for conducting tasks safely. 

4. The investigation team recommends that engineers and operators complete a management of 

change procedure for all operating parameter range changes. This should include approval 

to operate the unit outside of approved parameter ranges by the engineer responsible for the 

AN unit, and documentation of these activities. The engineer responsible for the AN unit 

should be included in all changes in the facility that could change feed streams or conditions 

in the AN unit. 

5. The investigation team recommends that maintenance, operations and engineering personnel 

develop a maintenance program that anticipates problems in the AN unit. This program 

should include predictive failure analyses and an aggressive preventive maintenance program. 

The maintenance program should be periodically evaluated by this group by at least utilizing 

maintenance records and equipment histories. 

6. The investigation team recommends that engineers, operators, and maintenance personnel 

develop and deliver training programs that include complete, written safe operating 

procedures. The training programs should contain specific performance and knowledge goals 

for participants and the procedures that will be utilized to evaluate participant competence. 

7. The investigation team recommends that plant management ensure that engineers, operators, 

and maintenance supervisors and managers develop lines of communication to ensure that 

these recommendations are implemented and maintained. Facility management should be 

responsible for conducting periodic audits of these programs to ensure that they are adequate. 

The investigation team further recommends that corporate managers conduct periodic audits 

of facilities to ensure that these programs are utilized and that communication between 

corporate facilities supports these programs at a corporate level. 

8. The investigation team recommends that information on the hazards of substances handled, 

the prevention measures in place or planned to prevent releases and the emergency response 

measures to be taken for the ammonium nitrate unit with the State Emergency Response 

Commission (SERC), Local Emergency Planning Committee (LEPC), first responders, and 

the public surrounding the facility. 

Recommendations Page 97 
The circumstances and procedures that contributed to the presence of the conditions that 

allowed the explosion to occur are illustrated in Figure 10-1. The investigation team was unable to 

develop one irrefutable scenario describing the origin of the explosion based on the evidence 

recovered and evaluated. The investigation team concluded, however, that the explosion could not 

have occurred without the factors contained in Section 9 of this report. The investigation team also 

concluded that implementation of the recommendations in this section will provide a strong and 

participatory management approach in addressing these issues. The recommendations include strong 

supervisory and management roles to ensure that programs developed to address these issues are 

effective and are evaluated and improved as necessary to meet the outlined chemical accident 

prevention goals. 

These recommendations reflect accidental release prevention requirements contained in the 

OSHA Process Safety management (PSM) regulations and in current industry guidelines and practices 

for prevention of chemical accidents and emergencies. EPA also intends to build on the OSHA PSM 

requirements and is currently considering how best to capture these recommended practices in the 

Risk Management Programs for Chemical Accidental Release Prevention rule to promulgated in 

March 1996. 

The recommendations given in this report are by no means all encompassing and should not 

be viewed as remedies for the fertilizer industry in general Individual facilities must customize these 

recommendations to fit their process, procedures, and management styles. 

Recommendations Page 98 
CONTRIBUTING FACTORS TO THE EXPLOSION 

AT THE TERRA PORT NEAL, IOWA FACILITY 

ON DECEMBER 13 - 1994 
SECTION 11: BIBLIOGRAPHY, WORKS CITED 

Publications and Articles 

1. Stability of Bubble-Sensitized Gel Slurry Explosives: K.F. Keirstead and D. De Kee: Ind. 

Eng. Chem. Prod. Res. Dev., Vol 22, No. 4, 1983; pp 603-607. 

2. Acid Catalysis in the Thermal Decomposition of Ammonium Nitrate: Bernard J. Wood 

and Henry Wise; The Journal of Chemical Physics, Volume 23, Number 4, April, 1955; pages 

693-696. 

3. Synergistic Catalysis of Ammonium Nitrate Decomposition: A. G. Keenan, K. Notz, and 

N. B. France; Journal of the American Chemical Society / 91: 12 / June 4, 1969; pages 3168­

3171. 

4. Isotopic Tracer Study of the Chloride-Catalyzed Thermal Decomposition of 

Ammonium Nitrate: C.I. Calvin, A. G. Keenan and J. B. Hunt; Journal of Chemical 

Physics; vol 38, 1963; pages 3033-3035. 

5 . Mechanism for the Chloride-Catalyzed Thermal Decomposition of Ammonium Nitrate: 

AG. Keenan and B. Dimitriadels; The Journal of Chemical Physics; Vo1 37, No. 8; October 

15, 1962; pages 1583-1586. 

6 . The Induction Period of the Chloride-Catalyzed Decomposition of Ammonium Nitrate: 

C.I. Colvin, P.W. Feamow, and AG. Keenan; Inorganic Chemistry; Vol 4; 1965; pages 173­

176. 

7. Differential Thermal Analysis of the Thermal Decomposition of Ammonium Nitrate: 

AG. Keenan; Journal of the American Chemical Society; Vol 77; 1955; pages 1379-1380. 

8. Self-Sustained Reaction in Liquid Mixtures of Ammonium Nitrate with Chromates and 

Dichromates: R.D. Smith; Trans Faraday Society; vol 50; 1959; pages 616-623. 

9. Decomposition of Liquid Ammonium Nitrate Catalyzed by Chromium Compounds: 

Willis A. Rosser, S. Henry Inami, and Henry Wise; Trans Faraday Society; Vol 60; 1964; 

pages 1618-1625. 

10. Oxygen and Nitrogen Isotope Effects in the Decomposition of Ammonium Nitrate: 

Lewis Friedman and Jacob Bigeleisen; The Journal of Chemical Physics; Vol 18 No. 10; 

October, 1950; pages 1325-1331. 

11. The Application of Kinetics to the Hazardous Behavior of Ammonium Nitrate; 

Propellant Burning: Source 5th Combustion Symposium; R.M. Hainer; 1954; pages 224­

Bibliography Page 100 
230.

12. Evidence for Homolytic Decomposition of Ammonium Nitrate at High Temperature: 

K.R Brower, Jimmie Oxley, and Mohan Tewari; The Journal of Chemical Physics; Vol 93; 

1989; pages 4029-4033. 

13. A Temperature-limiting Mechanism in the Thermal Decomposition of Ammonium 

Nitrate: George Feick and Raymond M. Hainer; Nature; Vol 173, No 4416; June 19, 1954. 

14. Ammonium Nitrate: Will H. Shearon, Jr., and W. B. Dunwoody; Industrial and Engineering 

Chemistry; Vol 45, No 3; March, 1953; pages 496-504. 

15. Nitric Acid and Fertilizer Nitrates: edited by Cornelius Keleti; Copyright 1985; page 265. 

16. Encyclopedia of Chemical Technology; Ammonium Nitrate: Edited ??; Vol 2; Year?; 

pages 698-708. 

17. Safety Considerations in the Processing, Handling, and Storage of Ammonium Nitrate: 

K.D. Shah and A.G. Roberts; Vol, source and Year??; pages 261-273. 

18. Procededings No. 110, The Fertilizer Society (London), The Production of Ammonium 

Nitrate Including Handling and Safety: Carter and Roberts; date?; pages 5-35. 

19. Ammonium Nitrate-Urea Explosion: Lowell G. Croysdale, W. Edmond Samuels, and Jack 

Wagner; Symposium on Air Separation and Ammonia Plants Safety at the 54th National 

Meeting of AIChE; September, 1964. 

20. What is Your Thermal Risk?: Francis Stoessel; Chemical Engineering Progress; October, 

1993; pages 68-75. 

21. Ammonium Nitrate Explosion Hazards: W.G. Sykes, R.H. Johnson, and R.M. Hainer 

(Arthur D. Little, Inc); Chemical Engineering Progress; Vol. 59, No. 1; January, 1963; pages 

66-71. 

22. Investigations on the Explosibility of Ammonium Nitrate: J.J. Bums, G.S. Scott, G.W. 

Jones, and Bernard Lewis; Report of Investigations 4994; US. Bureau of Mines; August, 

1953. 

23. Stable Bubble Sensitized Gel Slurry Explosives: K. Kierstead and D. De Kee; Ind. Chem. 

Prod. Res. Dev.; Vol. 19, No. 1; 1980; pages 91-96. 

24. Effect of Bubble Dimensions on Shock Sensitivity in Gelled Slurry Explosives: K.F. 

Kierstead and D. De Kee; Ind. Eng. Chem. Prod. Res. Dev.; Vol. 24, No. 1; 1985; pages 134­

Bibliography Page 101 
139.

25. A Critical Review of the Thermal Decomposition Mechanisms of Ammonium Nitrate: 

A King and A. Bauer; Department of Mining Engineering, Queen’s University at Kingston, 

Ontario, Canada. 

26. Table 2, Nominal chemical composition of heat-resistant alloys: Chemical Engineering 

Progress; March, 1994; page 57. 

27. The Deflagration to Detonation Transition Characteristics of Molten Ammonium 

Nitrate, Part I: A. Bauer, A. King, and R. Heater; Department of Mining Engineering; 

Queen’s University at Kingston, Ontario, Canada; November, 1979. 

28. Critical Parameters for Low Amplitude Shock Initiation of Molten Ammonium Nitrate 

and Ammonium Nitrate Fertilizer Solutions: A. Bauer, A.W. King, R.D. Heater, and J.H. 

Paterson; Department of Mining Engineering; Queen’s University at Kingston, Ontario, 

Canada; August, 1982. 

29. The Sensitivity of Ammonium Nitrate Melts and Solutions to Projectile Impact: A. 

Bauer, R.D. Heater, and J.H. Paterson; Department of Mining Engineering; Queen’s 

University at Kingston, Ontario, Canada; October, 1981. 

30. The Explosion Hazards of Ammonium Nitrate and Ammonium Nitrate Based Fertilizer 

Compositions - A Summary: A.W. King, A Bauer, and RD. Heater; Department of Mining 

Engineering; Queen’s University at Kingston, Ontario, Canada; October, 1982. 

31. Inhibition of the Thermal Decomposition of Ammonium Nitrate: B. Yu. Rozman and L.I. 

Borodkina; Journal of Applied Chemistry; USSR No. 32; January - June, 1959. 

32. Thermal Stability of the System Urea-Ammonium Nitrate: B. Yu. Rozman and L.I. 

Borodkina; Journal of Applied Chemistry; USSR; No. 32, No. 11; November, 1959. 

33. Rational Selection of Inhibitors of Thermal Decomposition of Ammonium Nitrate: B. 

Yu. Rozman; Journal of Applied Chemistry; USSR; No. 33; April - June, 1960. 

34. Mechanism of Thermal Decomposition of Ammonium Nitrate: B. Yu. Roman; Journal 

of Applied Chemistry; USSR No. 33; April - June, 1960. 

35. The Thermal Stability of Fertilizers Containing Ammonium Nitrate: B. Perbal; Paper 

read before the Fertiliser Society of London, November 25, 1971. 

36. The Thermal Decomposition of Ammonium Nitrate in Fused Salt Solution and in the 

Presence of Added Saks: K.S. Barclay and J.M. Crewe; Journal of Applied Chemistry; Vol. 

Bibliography Page 102 
42 

17; January, 1967; pages 21-26. 

37. TNO Defence Research; Danger Aspects of Liquid Ammonium Nitrate, Part I, 

Detonation Properties: Ing. Th. M. Groothuizen; TNO-report M3038-1; November, 1979. 

38. TNO Defence Research; Danger Aspects of Liquid Ammonium Nitrate, Part II, 

Thermal Stability: Ir. J. Verhoeff; TNO-report M3038-II; 1979- 1983. 

39. Urea-ammonium Nitrate Solutions: Are They Safe?: L.G. Croysdale, W.E. Samuels, and 

J.A. Wagner; Chemical Engineering Progress; Vol. 61, No. 1; January, 1965; pages 49-54. 

40. Explosions and Safety Problems in Handling Urea-Ammonium Nitrate Solutions: John 

F. Anderson; ?????; ??????; pages 70-72. 

41. ASM Metals Handbook; 9th Edition; Vol 13; Corrosion; 1987; pp 677-679. 

Violent Decomposition of Ammonium Nitrate in Aqueous Solutions: Andrzej 

Kolaczkowski, OECD-IGUS-FCE Working Group Meeting; 19-20 June 1990, Berlin. 

Bibliography Page 103 
Drawings and prints 

D-l AN Neutralizer Scrubber, Equipment Location Plan: BECO Engineering; #9P3032-2, 

Revision 6; 04-19-93. 

D-2 AN Neutralizer Scrubber, General Elevation: BECO Engineering; #9P3032-3, Revision 

5204-01-93. 

D-3 AN Neutralizer Scrubber, P&I Diagram: BECO Engineering; #9P3032-6; 01-28-94. 

D-4 AN Neutralizer Scrubber, Piping Elevation Facing South: BECO Engineering; #9P3032­

16, Revision 5; 02/02/94. 

D-5 AN Neutralizer Scrubber, Piping Elevation Facing West: BECO Engineering; #9P3032­

15; 02-01-94. 

D-6 AN Unit P&ID: C&I Girdler; #1147A0601-1 (2), Revision ??; ??-??-66. 

D-7 AN Unit P&ID, Maxwell Updates: C&I Girdler; #1147A0601-1, Revision ??; Date=??. 

Maxwell drawing, Revision B 

D-8 Terra General Facilities PFD. The M.W. Kellogg Company; #P2445-D; 11-30-65. 

D-9 Terra 600 TPSD Ammonia Plant (all drawings, 18 pages): The M.W. Kellogg Company; 

#P2423-D (et al), 09-27-66. 

D-10 Terra Nitric Acid Plant P&ID (2 pages): C&I Girdler; #1147A0101-1; 04-19-66. 

D-11 Terra Plant Utilities and Offsite (14 pages): The M.W. Kellogg Company; 06-10-66. 

D-12 Terra Urea Plant(s) Drawings (11 pages): C.F. Braun & Co.; #2682-100-KD- et al; 12-16­

65. 

D-13 Terra Plant Plot Plans/Drawings (7 pages): Various sources; 1966-1994. 

D-14 Terra Plant Plot Plan: Industrial Risk Insurers; #772002/856997; 09-03-92. 

D-15 Terra Steam Distribution System: ???; ???; ???. 

D-16 Fragment Debris Maps, Baker Engineering. 

D-17 Explosion Site Sketch, EPA. 

Bibliography Page 104 
D-18 Terra Sketches, AN Plant Piping & NA Spargers: Sketches hand-drawn; ~ 1980. 

Bibliography Page 105
Interviews and Depositions 

ID-1 Mick Ashley: 07-14-95. 

ID-2 BECO Engineering: 04-10-95. 

ID-3 Kevin Braun: 04-05-95. 

ID-4 Larry Chapman: 06-29-95. 

ID-5 Nick DeRoos: 04- 19-95. 

ID-6 Jimmie Edwards: 04-06-95. 

ID-7 R Guy Hopkins: 06-0l-95. 

ID-8 Bert Huot: 04-05-95. 

ID-9 Doug Jessom: 07-13-95. 

ID- 10 Gary Kaiser: 02-02-95. 

ID- 11 Ken Kinyon: 0l-20-95. 

ID-12 Stan Lilly: 04-19-95. 

ID-13 New Jac: 07-06-95. 

ID- 14 Dorothy Maxwell: 0l-25-95,04-06-95, 07- 14-95. 

ID-15 Bernie McBride: 07-13-95. 

ID-16 Ron McIntosh: 02-02-95,04-06-95. 

ID-17 Kevin Peterson: 02-02-95. 

ID-18 Bob Rolfes: 06-29-95. 

ID-19 Dennis Rush: 07-13-95. 

ID-20 Bill Sandeson: 07-13-95. 

ID-21 Dick Skelton: 06-29-95. 

Bibliography Page 106 
ID-22 Verti Mechanical: 07-06-95.

ID-23 Dan Walters: 02-02-95, 04-05-95, 07-14-95.

Bibliography Page 107
Data Packages and Data Bases 

DP-1 Fragment/Debris Data Base, Baker Engineering.

DP-2 Meteorological Data, U.S. Weather Service, Sioux City.

DP-3 Southwest Research Institute, Sample Analysis Data Sheets with Custody Record.

DP-4 TOC Analytical Results with Custody Record.

DP-5 Quality Analytical Labs, Inc.; Data for Samples 2, 3, 4, and 5. No custody records

recognizable. 

DP-6 USPCI Analytical Services; Data for Samples 1, 2, 3, and 5. No custody records 

recognizable. 

DP-7 AN Sensitivity Test Results, Source Unknown. No custody records recognizable. 

DP-8 AN Runaway Reaction Curves, Source Unknown 

DP-9 Sample Chain of Custody, Baker Engineering 

Bibliography Page 108