Nurse Tank Failure With Release of Hazardous Materials

Near Calamus, Iowa

April 15, 2003

Hazardous Materials

Accident Report

NTSB/HZM-04/01

PB2004-917001

Notation 7564A

National 

Transportation

Safety Board

Washington, D.C.

Hazardous Materials 

Accident Report

Nurse Tank Failure With Release of Hazardous Materials 

Near Calamus, Iowa

April 15, 2003

NTSB/HZM-04/01

PB2004-917001 National Transportation Safety Board

Notation 7564A 490 L’Enfant Plaza, S.W.

Adopted June 22, 2004 Washington, D.C. 20594
National Transportation Safety Board. 2004. Nurse Tank Failure With Release of Hazardous Materials

Near Calamus, Iowa, April 15, 2003. Hazardous Materials Accident Report NTSB/HZM-04/01.

Washington, DC.

Abstract: About 11:50 a.m. central daylight time on April 15, 2003, a nonspecification cargo tank used by

River Valley Cooperative (River Valley) exclusively for agricultural purposes as a nurse tank split open

after being filled with anhydrous ammonia at River Valley’s nurse tank filling facility near Calamus, Iowa.

About 1,300 gallons of the poisonous and corrosive gas escaped, seriously injuring two nurse tank loaders,

one of whom died from his injuries 9 days after the accident. Equipment repair and replacement costs

associated with the accident totaled about $3,100.

As a result of its investigation of the accident, the National Transportation Safety Board identified the

following major safety issues: the adequacy of standards for initial qualification and periodic testing of

nurse tanks, and the adequacy of River Valley’s emergency procedures for anhydrous ammonia nurse tank

loaders.

As a result of its investigation of this accident, the Safety Board makes safety recommendations to the

Research and Special Programs Administration and River Valley.

The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine,

pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board

Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study

transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board

makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and

statistical reviews.

Recent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also

may be obtained from the Web site or by contacting:

National Transportation Safety Board

Public Inquiries Section, RE-51

490 L’Enfant Plaza, S.W.

Washington, D.C. 20594

(800) 877-6799 or (202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To

purchase this publication, order report number PB2004-917001 from:

National Technical Information Service

5285 Port Royal Road

Springfield, Virginia 22161

(800) 553-6847 or (703) 605-6000

The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports

related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report. 
iii Hazardous Materials Accident Report

Contents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Factual Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Accident Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

The Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Emergency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Meteorological Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Nurse Tank Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Metallurgical Examination of the Nurse Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Other Postaccident Examinations and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Anhydrous Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

River Valley Cooperative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Nurse Tank Loading Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Nurse Tank Periodic Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Emergency Procedures and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Regulations and Standards Affecting Nurse Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

U.S. Department of Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

U.S. Department of Labor Occupational Safety and Health Administration . . . . . . . 16

American Society of Mechanical Engineers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Other Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Nurse Tanks in the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

New Trinity Manufacturing Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Full Examination of Nurse Tank Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Exclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

The Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Standards for Manufacturing and Testing Nurse Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Initial Qualification of Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Periodic Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Tank Shell Out-of-Roundness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

River Valley Emergency Preparedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Probable Cause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Appendix

A: Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
iv Hazardous Materials Accident Report

Executive Summary

About 11:50 a.m. central daylight time on April 15, 2003, a nonspecification cargo

tank used by River Valley Cooperative (River Valley) exclusively for agricultural

purposes as a nurse tank split open after being filled with anhydrous ammonia at River

Valley’s nurse tank filling facility near Calamus, Iowa. About 1,300 gallons of the

poisonous and corrosive gas escaped, seriously injuring two nurse tank loaders, one of

whom died from his injuries 9 days after the accident. Equipment repair and replacement

costs associated with the accident totaled about $3,100.

The National Transportation Safety Board determines that the probable cause of

the sudden failure of the nurse tank at the anhydrous ammonia filling facility near

Calamus, Iowa, on April 15, 2003, was inadequate welding and insufficient radiographic

inspection during the tank’s manufacture and lack of periodic testing during its service

life.

As a result of its investigation of the accident, the Safety Board identified two

major safety issues:

• The adequacy of standards for initial qualification and periodic testing of nurse

tanks.

• The adequacy of River Valley’s emergency procedures for anhydrous ammonia

nurse tank loaders.

As a result of its investigation of this accident, the Safety Board makes safety

recommendations to the Research and Special Programs Administration and River Valley.
1 Hazardous Materials Accident Report

Factual Information

Accident Synopsis

About 11:50 a.m. central daylight time1 on April 15, 2003, a nonspecification2

cargo tank used by River Valley Cooperative (River Valley) exclusively for agricultural

purposes as a nurse tank split open after being filled with anhydrous ammonia at River

Valley’s nurse tank filling facility near Calamus, Iowa. (See figures 1 and 2.) About 1,300

gallons of the poisonous and corrosive gas escaped, seriously injuring two nurse tank

loaders, one of whom died from his injuries 9 days after the accident. Equipment repair

and replacement costs associated with the accident totaled about $3,100.

1 Unless otherwise specified, the times used in this report are central daylight time.

2 The U.S. Department of Transportation publishes manufacturing specifications to which most tanks

to be used to transport hazardous materials must be built. However, tanks to be used to transport hazardous

materials for some specified activities, such as certain agricultural applications, do not have to meet these

specifications.

Figure 1. Accident nurse tank with shell fracture area circled.
Factual Information 2 Hazardous Materials Accident Report

The Accident

The two nurse tank loaders involved in this accident were working for River

Valley, a farmer’s cooperative in eastern Iowa. They loaded anhydrous ammonia, a

poisonous and corrosive liquefied gas, from a storage tank at the cooperative’s Calamus

filling facility into nurse tanks, which are nonspecification cargo tanks used for

agricultural purposes. Once filled, each nurse tank was taken to a cooperative member’s

farm, where the anhydrous ammonia was injected into the soil.3

 Spring is the primary

season for injecting anhydrous ammonia into farmers’ fields. During the peak period4

 for

this facility, about 150 nurse tanks per week were filled at the Calamus site. When this

accident took place, the Calamus filling facility was in its peak activity period.

On April 15, 2003, the two nurse tank loaders, who were the only people at the

facility at the time of the accident, had been filling nurse tanks from the storage tank,

which was their usual task. In the late morning, they finished filling the accident nurse

tank and disconnected the loading hoses and fittings. One loader was standing on the

Figure 2. Close-up view of accident nurse tank shell fracture area.

3 Farmers inject anhydrous ammonia into their fields before planting crops to increase the soil’s

nitrogen content. A nurse tank is typically pulled to the field using a pickup truck. In the field, the tank is

attached to an anhydrous ammonia injector. The injector and nurse tank are pulled through the field by a

farm tractor.

4 The peak period is a 7- to 10-day span each spring.
Factual Information 3 Hazardous Materials Accident Report

loading platform, while the other had backed a pickup truck up to the full nurse tank and

was standing at the back of the pickup truck hooking the nurse tank to the truck. Suddenly,

the nurse tank shell split open at the bottom of its front half. The pressure of the escaping

gas made several holes in the gravel lot surrounding the platform; the largest was about

7 feet long, 5 feet across, and 30 inches deep.

The force of the event threw the loader who had been standing by the pickup truck

against the back of the truck. He could recall nothing after this until he woke to find

emergency responders treating him.

The loader who had been on the loading platform could not tell investigators what

occurred immediately following the sudden release of anhydrous ammonia because the

injuries he sustained during the accident were so severe that he could not be interviewed by

investigators before he died 9 days after the accident. This loader did tell a witness that he

had activated the emergency shutoff button for the anhydrous ammonia pump to the other

three nurse tanks on the platform. (See figure 3 for layout of Calamus filling facility.)

Emergency Response

At 11:50 a.m., a woman who used this filling facility to provide anhydrous

ammonia for her farm was driving by the Calamus facility when she saw a large, white

vapor cloud emanating from the loading platform area. She pulled into the facility. She

Figure 3. Layout of Calamus anhydrous ammonia filling facility.

N

Not to scale

Tank scale

Storage tank

Water immersion

tubs

Loading

platform

Nurse

tanks Accident

nurse tank

Pickup

truck

Office
Factual Information 4 Hazardous Materials Accident Report

saw one tank loader lying in the emergency water immersion tub5 on the platform and a

second loader standing outside the tub assisting the one in the water.6

 She went into the

facility’s office, called 911, and told the 911 operator that an accident had occurred. She

told the operator that anhydrous ammonia might still be leaking and that two people had

suffered injuries.

Over the next few minutes, several people arrived at the facility and, finding the

emergency situation, attempted to assist. One witness who arrived during this period

recalled seeing vapor coming from under the loading platform. No other witnesses

indicated to investigators that they had seen vapor in the vicinity at this time. With these

people on the scene, the second loader got into the immersion tub with the first.

Fire department equipment and an ambulance were en route to the scene by noon

and were on scene at 12:02 p.m. (By this time, the visible vapor cloud of anhydrous

ammonia vapor had largely dispersed from the platform area.) After arriving on the scene,

fire department representatives requested that a second ambulance and a helicopter be

dispatched. The injured loaders were decontaminated7

 and stabilized on scene. At

12:21 p.m., a second helicopter was requested.

The first helicopter arrived at 12:45 p.m. and departed at 1:00 p.m. with the loader

who had been in the immersion tub when the first passerby came on the scene. This loader

was first taken to Davenport Hospital and then transferred to the burn unit of University

Hospital in Iowa City, Iowa. The second helicopter landed after the first took off and

picked up the loader who had been standing beside the tub when the first passerby arrived.

This loader was taken directly to the University Hospital burn unit.

Injuries

Both nurse tank loaders suffered more than 50 percent body surface area chemical

burns, eye injuries, and inhalation injuries due to anhydrous ammonia exposure. The

loader who had been on the loading platform when the tank fracture occurred and who had

been assisting his colleague from outside the immersion tub when witnesses arrived died

as a result of inhalation injuries 9 days after the accident.

5 A water immersion tub is used to provide on-site emergency first aid for people who experience

injuries to the skin or eyes caused by exposure to corrosive materials, such as anhydrous ammonia. The tub

is used to immerse in water the part of the body exposed to the corrosive material to reduce the tissue

damage caused by the exposure.

6 The loader in the immersion tub ultimately survived the accident; the loader standing outside the tub

died as a result of his injuries.

7 In this case, decontamination would have consisted primarily of showering the affected person with

clean water and removing affected clothing.
Factual Information 5 Hazardous Materials Accident Report

Damage

As a result of the accident, the front half of the bottom of the nurse tank shell split

open. The 53.5-inch-long split was located about 6 inches to the right of center at the tank

bottom (viewed from the back). The split was along one side of a longitudinal weld in the

shell on the tank bottom. (See figure 4.) Equipment repair and replacement costs

associated with the accident totaled about $3,100.

Meteorological Information

The weather at the time of the accident was dry, the skies were clear, and the

temperature was about 81° F. Winds were gusting from the southwest about 19 to 24 mph.

Figure 4. Fracture running beside, to the left of, longitudinal weld (line of weld is marked 

with arrows).
Factual Information 6 Hazardous Materials Accident Report

Nurse Tank Information

The accident nurse tank, serial number 1019945, was a non-U.S. Department of

Transportation (DOT)-specification cargo tank built in 1976 to the requirements of the

American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code:

Section VIII, “Rules for Construction of Pressure Vessels,” by Trinity Industries, Inc.,

(Trinity) at Beardstown, Illinois,8 for use as an anhydrous ammonia nurse tank. Sometime

after its manufacture, the tank was acquired by S/M Service Company. River Valley

assumed ownership of the nurse tank during a 1999 merger with S/M Service Company.

Until April 11, 2003, the nurse tank had been stationed at River Valley’s Eldridge, Iowa,

filling location. On that day, a farmer who had been using the tank in his field after

obtaining it from the Eldridge facility dropped the tank off at the Calamus filling facility.

The nurse tank was made of SA-455 steel, which has a minimum specified tensile

strength of 75,000 pounds per square inch (psi). The nominal thickness of the shell was

0.375 inch, and the nominal thickness of the heads was 0.362 inch. The tank was about

47 inches in diameter and about 205 inches (about 17 feet) long. It had a water capacity of

1,415 gallons. The heads were hot-formed and ellipsoidal in shape (concave to pressure). The

maximum allowable working pressure of the nurse tank was 250 psi, gauge (psig) at 125° F.

The nurse tank was equipped with a 3/4-inch-diameter pressure relief device

manufactured by Continental NH3 Products, Inc., Dallas, Texas. This device was a

spring-loaded valve designed to open at 250 psig +10 percent (between 250 and 275 psig).

The nurse tank was also equipped with a float gauge manufactured by Rochester Gauges,

Inc., Dallas, Texas, and a dip tube.9 There were no openings on the nurse tank larger than

2 inches in diameter.

Tank shells are formed from two pieces of plate steel. Each plate is rolled into a

cylinder or tube, until the opposite edges of the plate meet. The location where the edges

meet is called a longitudinal seam. Each longitudinal seam is closed by welding both

inside and outside the shell. These two cylinders are welded together with a

circumferential weld to make a single cylinder, and the ellipsoidal steel heads are welded

on each end of the single cylinder to form the nurse tank. Before closing, baffles are

welded inside the tank to limit liquid surge during transportation.

The ASME Boiler and Pressure Vessel Code (to which the accident nurse tank was

built) permits the use of spot radiography to check the welds for all pressure vessels

constructed to the code and not containing “lethal substances.”10 Spot radiography consists

of performing a 6-inch-long radiograph for every 50 feet of welds. When manufactured,

the accident nurse tank’s longitudinal welds were subject to spot radiography, in

8 The Beardstown plant closed about 1986.

9 A fixed-length dip tube is a small diameter pipe that extends from the top of the tank into the tank to

the tank’s maximum permitted loading level. The tube is equipped with a valve at or near the tank jacket.

Loaders use the device to indicate when the liquid in the tank has reached the maximum permitted level.

10 ASME defines lethal substances as “poisonous gases or liquids of such a nature that a very small

amount of the gas or of the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled.”
Factual Information 7 Hazardous Materials Accident Report

accordance with the ASME code’s accepted spot radiography procedures. (See the

“Regulations and Standards Affecting Nurse Tanks” section of this report for additional

information about the ASME code and spot radiography.)

After manufacture, the accident nurse tank was hydrostatically proof

pressure-tested to 375 psig.

Metallurgical Examination of the Nurse Tank

The metallurgical examination of the tank indicated that, when manufactured, a

portion of the nurse tank’s interior longitudinal weld was not centered on the shell seam.

Examination of the nurse tank’s 53.5-inch-long fracture, which was along the longitudinal

weld, revealed four significant regions. (See figure 5.) They are described below:

• Unfused region - Where the inner weld bead11 was offset to one side of the

shell seam, an unfused region 3.25 inches long and up to 0.102 inch deep was

found. (See figure 6.)

• Oxidized region - Beneath and encompassing the unfused region, the fracture

surface contained a 12.75-inch-long region covered with black oxides, consistent

with oxidation in a low-oxygen environment over a considerable period. This

oxidized region had a smoothly curving boundary, consistent with the fracture’s

initiating from the vicinity of the unfused region. The oxidized region’s maximum

depth was 0.317 inch (including the 0.007-inch-wide “oxidized fatigue region”

described below), and it came to within 0.063 inch of the tank’s exterior surface.

Cleaning with a deoxidizer revealed that this region (except the outer edge

“oxidized fatigue region” described below) had relatively rough fracture features

consistent with overstress or low-cycle fatigue fracture.12

• Oxidized fatigue region - Cleaning also revealed a 0.007-inch-wide region at

the outer edge (in the direction of the tank exterior) of the oxidized region that

had features consistent with high-cycle fatigue cracking, which is caused by

repeated low-load events over time.

• Overstress region - The remainder of the fracture surface (to the breach in the

exterior tank wall) had features consistent with overstress failure, such as

failure caused by a single load event exceeding the load-bearing capacity of the

cracked tank wall.

11 The weld bead in the area of the unfused region was observed to make a large angle with the shell

wall (as shown in figure 6). Such a feature is known to significantly increase the local operating stresses in a

structure. See John F. Harvey, Theory and Design of Modern Pressure Vessels, Second Edition (New York,

NY: Van Nostrand Reinhold Company, 1974) 348–349.

12 Both overstress and low-cycle fatigue generate rough fracture features on a crack surface, as opposed

to the smooth features generated by high-cycle fatigue. A tank that has experienced low-cycle fatigue has

been exposed to infrequent high-load events, such as overpressurization or accident damage, rather than the

single high-load event of an overstress fracture. Over time, corrosion may reduce or eliminate the evidence

of the occurrence of multiple events, leaving what appears to be evidence of an overstress fracture.
Factual Information 8 Hazardous Materials Accident Report

Figure 5. Diagram showing the four regions of the fracture surface. This diagram is not to scale 

and is intended only for use as a visual aid to the reader. [*Note: The oxidized fatigue region indicated above was actually part of the total oxidized region found during the postaccident examination. Because the fatigue region was metallurgically different from the rest of the oxidized 

region, it is indicated on this graphic by a different shade from the rest of the oxidized region.]

Figure 6. Cross section view of nurse tank longitudinal weld, showing left steel shell plate 

A and right steel shell plate B welded together. Fracture through tank shell is indicated by 

arrow. Circle marks area of lack of fusion.
Factual Information 9 Hazardous Materials Accident Report

Postaccident examination and matching of the characteristics on each side of the

longitudinal fracture in the shell of the accident nurse tank revealed that the two sides of

the shell could not be aligned to form a circular arc. Where they had been welded together,

the two sides were at about a 30-degree angle relative to each other.

Other Postaccident Examinations and Testing

After the accident, the nurse tank’s pressure relief device was found on the tank

with a dust cover still in place over the device opening.13 The device was tested at an

independent laboratory using argon gas. The device activated between 260 and 261 psig,

within the required tolerances.

The nurse tank’s float gauge was examined at the manufacturer’s facility under

Safety Board supervision. The float gauge dimensions were compared to, and determined

to meet, the manufacturer’s design specifications. The moving parts (gears, shafts,

magnet, and indicator needle) were examined and tested; they were operating within

design specifications.

The nurse tank’s dip tube was measured by the Iowa Department of Agriculture

and Land Stewardship and found to have a length of 9 3/4 inches. (According to Trinity

drawings, the standard dip tube length was 9 1/2 inches with the tank 85 percent full.)

Anhydrous Ammonia

According to the material safety data sheet used by CF Industries, Inc., which was

River Valley’s anhydrous ammonia supplier, anhydrous ammonia (NH3) is a poisonous

and corrosive gas with a boiling temperature of -28° F. Anhydrous ammonia is transported

as a liquefied compressed gas in pressure vessels (tank cars, cargo tanks, nurse tanks, and

cylinders). Pressurization keeps the anhydrous ammonia liquid while in the pressure

vessel. If it is released from the pressure vessel, anhydrous ammonia will immediately

return to a gaseous state and expand rapidly. At 80° F, about the reported temperature at

the time of the Calamus accident, the vapor pressure of anhydrous ammonia would result

in an internal tank pressure of 142 psig.14

Under DOT regulations (49 Code of Federal Regulations [CFR] Parts 171–180),

anhydrous ammonia is classified and regulated for domestic shipment as a nonflammable

gas but is required to be identified as an “inhalation hazard.” For international shipment,

13 A dust cover is a soft rubber cap that fits over the opening of the pressure relief device to keep out

dust, debris, and water. It is held in place by friction. The dust cover is designed to blow off the opening if

internal tank pressure causes the pressure relief device to activate.

14 The vapor pressure of a liquefied compressed gas is the pressure exerted by vapors in equilibrium

over the liquefied form in a closed container. Vapor pressure thereby provides a measure of internal tank

pressure when the liquefied gas is at a given temperature.
Factual Information 10 Hazardous Materials Accident Report

the DOT classifies anhydrous ammonia as a poisonous gas, with a subsidiary hazard

classification of class 8, corrosive.

According to Medical Management Guidelines for Ammonia, issued by the U.S.

Department of Health and Human Services’ Agency for Toxic Substances and Disease

Registry,15 anhydrous ammonia has the following health effects:

Ammonia is highly irritating to the eyes and respiratory tract. Swelling and

narrowing of the throat and bronchi, coughing, and an accumulation of fluid in the

lungs can occur.

Ammonia causes rapid onset of a burning sensation in the eyes, nose, and throat,

accompanied by lacrimation [discharge of tears], rhinorrhea [runny nose], and

coughing. Upper airway swelling and pulmonary edema may lead to airway

obstruction.

Prolonged skin contact (more than a few minutes) can cause pain and corrosive

injury.

According to the National Institute for Occupational Safety and Health (NIOSH),16

the “low lethal” concentration (LCLO)

17 of anhydrous ammonia for humans is 5,000 parts

per million (ppm) for a period of 5 minutes. NIOSH also stipulates that the “immediately

dangerous to life or health” (IDLH) concentration18 of anhydrous ammonia is 300 ppm.

Humans can detect the odor of anhydrous ammonia at 3 to 5 ppm.

River Valley Cooperative

River Valley is a farmer-owned cooperative with about 1,800 members/owners. It

maintains 11 active anhydrous ammonia filling facilities, all in eastern Iowa. It owns about

600 nurse tanks used in its anhydrous ammonia service. The cooperative provides its

members propane, gasoline, diesel, livestock feed, crop nutrients, herbicides, and

insecticides. In addition, the cooperative provides grain storage and conducts marketing

for its members.

15 See <http://www.atsdr.cdc.gov/MHMI/mmg126.html> for the complete medical management

guidelines.

16 NIOSH is a Federal agency responsible for conducting research and making recommendations for the

prevention of work-related injury and illness. It is part of the Centers for Disease Control and Prevention

within the U.S. Department of Health and Human Services.

17 The LCLO is the lowest concentration of a substance that has been reported to cause death in humans.

18 In its hazardous waste operations and emergency response regulation (29 CFR 1910.120), the

Occupational Safety and Health Administration defines an IDLH concentration as “An atmospheric

concentration of any toxic, corrosive or asphyxiant substance that poses an immediate threat to life or would

cause irreversible or delayed adverse health effects or would interfere with an individual’s ability to escape

from a dangerous atmosphere.” NIOSH, in its “Respirator Decision Logic,” defines IDLH exposure

condition as a condition that poses a threat of exposure to airborne contaminants when that exposure is likely

to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an

environment.
Factual Information 11 Hazardous Materials Accident Report

The River Valley Calamus facility was used solely for anhydrous ammonia filling

service. Farmers using the Calamus facility for anhydrous ammonia service typically

dropped off an empty nurse tank and picked up a full one.

Nurse Tank Loading Procedures

River Valley’s Calamus facility manager indicated that loaders generally used

uniform procedures when filling a tank with anhydrous ammonia at the loading platform.

He said the loaders would:

• Weigh the empty nurse tank. (Using the tank scale beside the facility office.)

• Pull the tank to the loading platform using a pickup truck.

• Unhook the pickup truck and pull it away.

• Hook up the hoses. (The loaders were to attach the red fitting to the red valve

for the liquid line and the yellow fitting to the yellow valve for the vapor return

line.) (See figure 7.)

• Open both valves on the top of the nurse tank.

• Open both valves on the hose ends.

• Open both valves on the platform. (Loaders were told that any leak would

cause an odor and, if they detected an odor, they should shut the valves down.)

• Watch the float gauge on top of the nurse tank and shut down the valves when

the gauge reached 85 percent. (The nurse tank loader who survived the

accident said that in addition to watching the float gauge, he routinely checked

the level of anhydrous ammonia in each tank using the dip tube to ensure the

tank was not overfilled.)

• Shut down the valves in reverse order. (Platform to hose to nurse tank.)

• Hook the full tank to the pickup truck and reweigh it to determine the amount

of anhydrous ammonia that had been loaded into the tank.
Factual Information 12 Hazardous Materials Accident Report

The facility manager stated that loaders typically loaded four nurse tanks

simultaneously. When loading multiple tanks, loaders were to walk around and check the

gauges.

The nurse tanks were placarded “Nonflammable Gas.” No shipping papers were

prepared, and none were required.

Nurse Tank Periodic Inspection

River Valley told investigators that it carried out external visual inspections of its

nurse tanks, including the accident tank, annually before each spring planting season.

There is no record that River Valley had conducted any internal inspections, periodic or

random, on the nurse tank since its manufacture. There is no Federal or State requirement

that periodic inspections be conducted on nurse tanks. (See the “Regulations and

Standards Affecting Nurse Tanks” section of this report for additional information on

inspection requirements.)

Emergency Procedures and Equipment

River Valley. The River Valley Calamus facility had no written procedures

detailing what loaders should do in case of a release of anhydrous ammonia. The facility

manager said that he told facility loaders that if an anhydrous ammonia release occurred,

they were to hit an emergency shutoff button on the loading platform (which turns off the

Figure 7. A nurse tank hooked to loading hoses. 
Factual Information 13 Hazardous Materials Accident Report

anhydrous ammonia pump) and get into the water immersion tub. (See figure 8.) Loaders

were to remain in the tub until emergency responders arrived and told them it was safe to

get out. Water immersion tubs are designed to provide on-site emergency first aid for

injuries caused by exposure to corrosive materials by enabling a person whose skin or soft

tissues are exposed to the material to flush the area with water. They are not intended to

provide protection from, or first aid for, inhalation injuries.

The tub held 200 gallons of water and was large enough for two individuals to sit

in it, but it would have been difficult for two occupants to fully immerse themselves. A

second immersion tub was located at one end of the facility’s storage tank, about 40 feet

from the loading platform.

River Valley officials told Safety Board investigators that, as of May 2004, the

organization had developed no new procedures for its employees to follow when facing a

major release of anhydrous ammonia.

Other. The CF Industries’ November 2002 material safety data sheet for

anhydrous ammonia states that in response to inhalation and eye exposure to anhydrous

ammonia, responders should “Immediately move victim away from exposure and into

Figure 8. Immersion tub in the center of loading platform. Arrow indicates emergency 

shutoff button location.
Factual Information 14 Hazardous Materials Accident Report

fresh air.” It further states that when skin is exposed to anhydrous ammonia, responders

should “Immediately flush affected area(s) with large amounts of water ....”

A NIOSH study entitled HAZOP of Anhydrous Ammonia Use in Agriculture19

recommends that:

Untrained personnel are not to attempt to stop a release in the event of a broken

nurse tank hose. Untrained personnel are instead to: 1) Immediately vacate the

area by heading upwind ....

According to the 2000 Emergency Response Guidebook,

20 in case of a small spill

of anhydrous ammonia “from a small package or a small leak from a large package,”

people should evacuate to a distance at least 100 feet (in all directions) from the spill/leak

source. In case of a large spill (from a “large package or many small packages”), people

should withdraw at least 200 feet from the source.

Regulations and Standards Affecting Nurse Tanks

The DOT and the U.S. Department of Labor’s Occupational Safety and Health

Administration (OSHA) both have regulations covering nurse tanks. Both sets of

regulations reference the requirements of the ASME Boiler and Pressure Vessel Code as

they apply to nurse tanks.21 The ASME requirements are detailed later in this section.

U.S. Department of Transportation

The DOT’s Research and Special Programs Administration (RSPA) promulgates

the Hazardous Materials Regulations (49 CFR Parts 171–185).

Regulations Covering Initial Manufacture and Qualification of Cargo Tanks.

Under the Hazardous Materials Regulations, the manufacturing specifications for

DOT-specification cargo tanks used to transport anhydrous ammonia (MC 331 tanks)

incorporate the requirements of the ASME Boiler and Pressure Vessel Code. Also, these

19 Thomas McKelvey, HAZOP of Anhydrous Ammonia Use in Agriculture, prepared for NIOSH by

Technica, Inc., Columbus, Ohio, May 1991.

20 The Emergency Response Guidebook was developed jointly by the DOT, Transport Canada, and the

Secretariat of Communications and Transportation of Mexico for use by firefighters, police, and other

emergency services personnel who may be the first to arrive at the scene of a transportation incident

involving a hazardous material. It is primarily a guide to assist first responders in (1) quickly identifying the

specific or generic classification of the material(s) involved in the incident and (2) protecting themselves and

the public during this initial response phase. The guidebook is updated every 3 to 4 years to accommodate

new products and technology. The next update is scheduled for 2004.

21 The OSHA regulations specifically cite Section VIII of the ASME Boiler and Pressure Vessel Code,

entitled “Rules for Construction of Pressure Vessels,” which concerns pressure vessels, including nurse

tanks. The DOT regulations for nurse tanks do not specifically cite this section of the ASME code, but they

reference Section VIII (and several other sections unrelated to nurse tank manufacture). DOT

representatives told Safety Board investigators that the part of the ASME code applicable to the DOT

regulations is Section VIII.
Factual Information 15 Hazardous Materials Accident Report

cargo tanks must meet numerous additional requirements established by the DOT,22

including weld testing and inspection requirements. Under the weld requirements, the

MC 331 tank welds must be subjected to one of a series of nondestructive inspections and

tests to identify weld defects. The acceptable inspection and testing methods include wet

fluorescent magnetic particle inspection, radiography, liquid dye penetrant testing, and

ultrasonic testing.

The nonspecification cargo tanks known as nurse tanks must also be manufactured

in accordance with the ASME Boiler and Pressure Vessel Code. However, as long as nurse

tanks meet the general criteria set forth in section 173.315(m) of the Hazardous Materials

Regulations,23 they are not required to meet any additional DOT-specified requirements.

Regulations Covering Periodic Inspections. Under the DOT Hazardous

Materials Regulations, all DOT-specification bulk containers for liquids and liquefied

compressed gases that carry hazardous materials and meet DOT specifications—including

railroad tank cars, cargo tanks, and intermodal bulk containers—must be periodically

inspected and tested to ensure they are structurally sound. DOT-specification cargo tanks

must adhere to a schedule of periodic inspections. DOT-specification MC 331 cargo tanks,

which carry anhydrous ammonia, are inspected on the following schedule:

Until 1981, the DOT allowed nonspecification cargo tanks with the same

configuration as nurse tanks to be manufactured for the transportation of liquefied

petroleum gas.24 In 1981, the regulations were amended to prohibit new manufacture of

nonspecification tanks for liquefied petroleum gas service. Title 49 CFR 173.315(k)

allowed the continued use of nonspecification cargo tanks in liquefied petroleum gas

service in intrastate commerce. Section 173.315(k)(5) required that these tanks in

liquefied petroleum gas service be periodically inspected and hydrostatically tested in

22 Specification MC 331 is found at 49 CFR 178.337. This section addresses cargo tank motor vehicles

used for transportation of compressed gases. It includes sections on materials specifications and on impact,

pressure, and stress testing procedures.

23 This section states that a nurse tank does not have to meet the requirements of DOT-specification

cargo tanks if it: (1) Has a minimum design pressure of 250 psig and meets the requirements of the edition of

the ASME code in effect at the time it was manufactured and is marked accordingly; (2) Is equipped with

safety relief valves meeting the requirements of Compressed Gas Association pamphlet S1.2; (3) Is painted

white or aluminum; (4) Has capacity of 3,000 gallons or less; (5) Is loaded to a filling density no greater than

56 percent; (6) Is securely mounted on a farm wagon; and (7) Is in conformance with the requirements of

part 172 of this subchapter except that shipping papers are not required; and it need not be marked or

placarded on one end if that end contains valves, fittings, regulators or gauges when those appurtenances

prevent the markings and placard from being properly placed and visible.

External visual inspection Annual

Internal visual inspection Every 5 years

Leakage test Annual

Pressure test Every 5 years

24 Propane is the liquefied petroleum gas most commonly transported in these tanks.
Factual Information 16 Hazardous Materials Accident Report

accordance with DOT specifications for MC 331 cargo tanks. However, the DOT

Hazardous Materials Regulations do not require periodic inspection or testing of

nonspecification cargo tanks used for anhydrous ammonia service (nurse tanks).

U.S. Department of Labor Occupational Safety and Health Administration

OSHA regulations (29 CFR Part 1910–General Industry) provide, as stated under

section 1910.111, general standards for the construction of tanks used to hold anhydrous

ammonia. Subsection 1910.111(b)(2) provides the requirements for the construction,

original testing, and requalification of various types of nonrefrigerated containers,

including farm vehicles used in the application of ammonia, such as nurse tanks. These

standards set minimum manufacturing requirements and state that all such tanks shall be

constructed and qualified in accordance with the ASME Boiler and Pressure Vessel Code,

Section VIII, “Rules for Construction of Pressure Vessels.”

Nothing in the OSHA regulations requires periodic inspection or testing of nurse

tanks.

OSHA regulations also require that facilities containing stationary storage tanks

for anhydrous ammonia, such as the large storage tank at the River Valley Calamus

facility, have an easily available shower or 50-gallon drum of water.25

American Society of Mechanical Engineers

The ASME Boiler and Pressure Vessel Code, Section VIII, “Rules for

Construction of Pressure Vessels,” establishes manufacturing and qualification testing

requirements for some types of pressure vessels, including nurse tanks. This part of the

ASME code covers a wide range of vessels used in a variety of industrial applications,

from large fixed storage vessels to portable compressed gas cylinders.

The ASME code provides a formula for design and fabrication of pressure tanks

that includes such factors as tank wall thickness, tank service pressure, tank radius, stress

values for steel, and joint efficiency of the tank welds. The joint efficiency is determined

by the scope of the examination performed on the welds to ensure their quality.

Manufacturers may choose to use full radiography, spot radiography, or no radiography to

examine the welds.26 Typically, one of two options—full radiography or spot

radiography—is selected. Full radiography means that longitudinal welded joints must be

radiographically examined along their full lengths. When conducting spot radiography, a

6-inch-long radiograph must be taken for every 50 feet of longitudinal weld. Spot

radiography does not require a radiograph to be taken on the longitudinal weld of each

25 Subsection 1910.111(b)(10)(iii).

26 Technically, under the ASME code, a manufacturer may choose not to conduct any type of

radiographic test of the weld, provided the other elements in the manufacturing formula (tank thickness,

etc.) are so adjusted as to compensate for the lack of weld examination. According to Trinity, in practical

terms, if a manufacturer did not use either full or spot radiography to ensure joint efficiency, it would not be

economically feasible for the manufacturer to produce a competitive tank.
Factual Information 17 Hazardous Materials Accident Report

nurse tank manufactured. (On nurse tanks manufactured by Trinity, the longitudinal welds

typically range in length from 12 to 15 feet.)

Under the ASME code, if manufacturers choose to use spot radiography instead of

full radiography to ensure the quality of tank welds, their tank design strength calculations

are affected in such a way that for a given design pressure, tanks must either be

constructed of thicker materials, constructed of higher strength steels, or constructed with

some combination of both thicker and higher strength steels to compensate for the less

rigorous weld inspection provided by spot radiography.

The ASME code requires that full radiography be used to qualify the welds on

tanks intended to contain “lethal substances.” ASME defines lethal substances as

“poisonous gases or liquids of such a nature that a very small amount of the gas or of the

vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled.” ASME

does not identify any specific materials as lethal substances. The tank user is required to

tell the designer or manufacturer of the tank if the tank is to be used to hold a lethal

substance. Historically, tank users have not identified anhydrous ammonia as a lethal

substance.

The ASME code states that any weld containing an area of incomplete fusion or

penetration is unacceptable. It also states that other indications27 shall not exceed

two-thirds the thickness of the weld, with a maximum acceptable length of 3/4 inch.

The ASME code also limits the permissible out-of-roundness of a tank shell to

1 percent of the nominal diameter.

The ASME code in effect at the time of the accident nurse tank’s manufacture

required, as the final step in the manufacturing qualification process, that each tank be

hydrostatically pressure-tested after all fabrication was completed and all examinations

were performed.28 The required test pressure was 1.5 times the maximum allowable

working pressure (375-psig test pressure for a 250-psig tank).29

Neither the ASME code in effect at the time of the accident nurse tank’s

manufacture nor the current code requires periodic inspection or testing of pressure

vessels, such as the nurse tank.

27 The term “other indications” refers to other areas of a weld identified by the radiograph as having

problems or flaws.

28 Including radiographic examinations of the welds.

29 The current edition of the ASME code requires each newly manufactured tank to be hydrostatically

pressure-tested to 1.3 times the maximum allowable working pressure (325-psig test pressure for a 250-psig

tank).
Factual Information 18 Hazardous Materials Accident Report

Other Information

Nurse Tanks in the United States

General. A director of Government Relations for The Fertilizer Institute told

investigators that the organization estimates that about 200,000 nurse tanks are currently

in use in the United States. The institute also estimates that about one to five nurse tanks

are retired annually and considers it probable that some tanks that were manufactured in

the 1950s and 1960s remain in service today. The institute estimates that the tanks in

service deliver about 3.9 million short tons (in the range of 1.0 million to 1.5 million nurse

tank loads) of anhydrous ammonia to farmers’ fields annually.

Nurse tanks have no established regulatory limit to their service lives. Provided the

tanks continue to perform satisfactorily and have no obvious external defects, the tanks

likely will remain in service.

RSPA maintains data on nurse tank accidents occurring in transportation. Upon

reviewing this data, investigators found no recorded instance of a nurse tank accident

resulting from a longitudinal weld or other structural weld failure. Most nurse tank

accidents recorded by RSPA involved hose breaks, component failures, and rollovers.

However, nurse tanks are used in farm environments, and accidents may have taken place

when the tanks were not in transportation (that is, being transported over public roads).

Such accidents might not have been reported to RSPA. For example, the Calamus

accident, which took place at a filling facility, did not appear in RSPA’s accident database.

The Fertilizer Institute does not collect information on nurse tank accidents, and

nurse tank manufacturers could provide no reliable information to investigators regarding

nurse tank accident rates. Neither Iowa nor Minnesota (both significant farming States)

could provide data on nurse tank accidents in their States beyond some anecdotal

information.

Minnesota Department of Agriculture Information. In September 1996, the

Minnesota Department of Agriculture began a program to address a statewide problem of

illegible or missing nameplates on nurse tanks used for handling anhydrous ammonia. The

State allowed the tanks to be certified and re-nameplated, provided they passed certain

nondestructive tests. The testing for each nurse tank, performed by ASME-certified

pressure vessel repair shops, had three successive stages: first, a thorough visual

examination to detect obvious defects in the tank; then, metal thickness testing on the

heads and shell to determine that the calculated maximum allowable working pressure

(MAWP) equaled or exceeded the Federal MAWP of 250 psig; and, finally, a hydrostatic

pressure test to 375 psig. As each nurse tank passed a test stage, it moved on to the next. If

a tank failed a given test stage, the State permitted it to undergo ASME-authorized repairs

to correct problems, after which the tank was retested. If a tank could not be repaired, it

was removed from service.

According to State officials, between 1,500 and 2,000 nurse tanks were tested

under this program. State officials told investigators that some of the tanks tested did not
Factual Information 19 Hazardous Materials Accident Report

pass the initial testing. The primary defects found with the tanks included external

corrosion, tank dents, and small leaks. The State did not record the number of nurse tanks

that failed the initial testing and were repaired, nor did the State require the repair shops to

forward reports on nurse tanks that were removed from service as a result of the retesting.

However, the State located records showing that at least 10 nurse tanks failed the retesting

and were removed from service.

Of these 10 tanks removed from service, 3 were removed as a result of defects

noted during the visual examination: 1 had a severe dent in the rear head; 1 had a bulge in

the rear head; and 1 had an attachment welded to the head that was not part of the tank’s

original design. Six tanks were removed from service because the metal thickness in the

shell, heads, or both was too thin, and the tank could not be certified to a MAWP of

250 psig. One tank, having passed the first two stages of testing, failed the hydrostatic

pressure test when an unrepairable leak was discovered in the rear head.

In addition, Minnesota State officials told investigators that in 1995 two nurse

tanks in the State had catastrophic failures. The two tanks had been manufactured by

Trinity in 1973 and had proximate serial numbers (832660 and 832656), indicating that

they likely were produced in the same manufacturing lot. State officials said the running

gear on the two nurse tanks was unusual in that the wheel sets were mounted directly to

each of the tank ends, without any cross support between them. Both tanks split open at

the middle circumferential weld, one in a storage yard and one as it was being transported

on the road. Although no failure analysis was performed on these tanks, the State

considered that a manufacturing defect and/or the unusual design (which may have

allowed the middle of the tank to bend under load) may have been factors in their failures.

RSPA’s database contained no record of either of these two accidents.

New Trinity Manufacturing Procedures

Trinity had no record of the specific quality control procedures its Beardstown

plant used to ensure tank roundness in 1976 when the accident nurse tank was

manufactured. However, at that time, Trinity’s general policy was to test each nurse tank

for roundness after welding. A rod appropriately sized for the tank’s internal diameter was

placed (held horizontally) inside each tank following welding. The rod was rotated and, if

it did not rotate freely, Trinity knew that an out-of-roundness condition existed. Trinity

then conducted the necessary calculations to determine whether the degree of the tank’s

out-of-roundness was greater than the ASME-accepted level of 1 percent of the tank’s

nominal diameter. If it was, the tank was rejected. Trinity told investigators that it

discarded this testing procedure some years ago because it was not finding any incidence

of tanks being rejected for out-of-roundness.

For some years, Trinity has been requiring that its plants check tank roundness

with a roundness template before the tank’s longitudinal seam is welded. The procedure is

used to ensure that each tank will meet the ASME standard limiting the out-of-roundness

of a tank shell to 1 percent of the nominal diameter.
Factual Information 20 Hazardous Materials Accident Report

Full Examination of Nurse Tank Welds

Information from the two manufacturers of nurse tanks indicates that in the mid1980s, a technology known as “radioscopy,” which permits direct observation of objects

opaque to light by means of some other form of radiant energy, such as x rays, was

developed for use in the industry. This technology allows real-time examination of the

entire longitudinal weld on nurse tanks. Radioscopy is, by definition, a radiographic

method of examining welds. The industry is using radioscopy to conduct full radiographic

examinations of the longitudinal welds on nurse tanks.

Adopting radioscopy as the means of checking the full longitudinal weld enabled

manufacturers to use thinner materials in the manufacture of nurse tanks. This provided an

economic incentive for the use of radioscopy in lieu of spot radiography. According to the

two companies, all nurse tanks manufactured since the mid-1980s have received

100-percent radioscopic examination of their longitudinal welds at the time of

manufacture.
21 Hazardous Materials Accident Report

Analysis

This analysis is presented in three main parts. First, the Safety Board identifies

factors that can be readily excluded as causal or contributory to the accident. In the second

part, the Board analyzes the causes and factors contributing to the accident. In the

remainder of the analysis, the Board addresses the safety issues arising from the

investigation, which are as follows:

• The adequacy of standards for initial qualification and periodic testing of nurse

tanks.

• The adequacy of River Valley’s emergency procedures for anhydrous ammonia

nurse tank loaders.

Exclusions

The Safety Board considered whether the nurse tank might have been

overpressurized on the day of the accident. Although investigators were unable to

determine the exact procedures the loaders used to fill the nurse tank involved in the

accident, the surviving loader stated that he typically used the dip tubes installed on the

tanks to ensure that the tanks were not overfilled. The dip tube on the accident nurse tank

was examined after the accident and determined to be an appropriate size for the tank. The

nurse tank had been filled and disconnected from the filling apparatus without any

difficulty before the tank failure occurred.

Further, the dust cover was found still covering the opening of the nurse tank’s

pressure relief device following the accident. If the device had activated due to excess

pressure within the tank, the dust cover would have been blown off the device. The device

was tested at an independent laboratory and found to operate within specification.

Therefore, the Safety Board concludes that the nurse tank was not overpressurized.

The effectiveness of the emergency response was also examined. Representatives

of the local emergency response agencies were on site and providing assistance within

about 10 minutes of their notification. Medical evacuation helicopters arrived at the

accident site soon after being called and quickly transported the injured to hospitals.

The Accident

This accident was caused by a fracture in the shell of the nurse tank at the

longitudinal weld, which resulted in a complete release of the anhydrous ammonia from

the tank.
Analysis 22 Hazardous Materials Accident Report

Postaccident metallurgical examination indicated that a portion of the nurse tank’s

interior longitudinal weld was not centered on the shell seam. Further, in one area where

the inner weld was offset to one side of the shell seam, there was a 3.25-inch-long and up

to 0.102-inch-deep region of incomplete fusion. This area would have increased the local

stresses experienced by the tank shell when the tank was pressurized.

In addition, the failed section of the nurse tank had some indications of an

out-of-roundness condition. Postaccident examination indicated that, during the nurse

tank’s manufacture, the two sides of its shell that formed the longitudinal seam may have

been improperly aligned. When examined after the accident, the two sides could not be

aligned to form a circular arc and appeared to be aligned at about a 30-degree angle

relative to each other at the weld. This indicates that there was some misalignment of the

shell surfaces when the tank was manufactured. A misalignment would have produced an

area of localized stress concentration and increased tensile stress on the inner surface of

the shell when the tank was pressurized. The Safety Board concludes that the unfused area

and offset portion of the longitudinal weld significantly weakened the nurse tank shell;

misalignment of the shell surfaces and the large angle weld bead increased the operating

stresses in the weakened area.

Examination of the area of black oxidation below the unfused area of the weld

showed that a crack initiated from the unfused area, moving from the interior surface of

the tank toward the outside surface, as indicated by its curved shape encompassing the

unfused area. This 12.75-inch-long oxidized area, in combination with the unfused area,

penetrated 83 percent of the tank shell, leaving only about 0.063 inch of steel to maintain

the tank’s integrity. Because the process of oxidation in a low-oxygen environment takes

place over time, the black oxidation covering the fracture surface of this part of the crack

indicates that the crack was present for some time before the final fracture occurred on

April 15, 2003. Also, the relatively rough surface of the bulk of the oxidized area30 means

this part of the crack likely formed as a result either of a single overstress event or of

low-cycle fatigue.

The proof pressure test that took place at the conclusion of the 1976 manufacture

of the accident nurse tank would have provided an elevated stress event. Given the

identified weaknesses of the tank shell in the region containing the unfused area and the

offset weld, the stress caused by the 375-psig proof pressure test would likely have been

sufficient to initiate the crack. It is also likely that some portion of the fracture surface

found covered with black oxidation had opened during the initial proof pressure test and

had been within the tank shell since the tank’s manufacture in 1976.

Although the proof pressure test probably initiated the rough-featured part of the

oxidized portion of the crack, this test may not have been the only event that contributed to

its extent. Some of this portion of the crack may have been formed by low-cycle fatigue

events occurring during the service life of the tank, and the evidence of that fatigue may

subsequently have been obliterated by corrosion. The Safety Board was unable to develop

30 A 0.007-inch-wide region on the outer edge of the oxidized area showed evidence of high-cycle

fatigue.
Analysis 23 Hazardous Materials Accident Report

a full and accurate history of this nurse tank, but it is possible that some low-cycle fatigue

event, such as a high-force impact accident or an additional proof pressure test, could have

further extended the blackened, oxidized portion of the crack from its extent following the

1976 proof pressure test to its eventual size. However, an accident of sufficient magnitude

to extend the crack would likely have left physical evidence of damage and repair on the

tank, and none was found. Also, proof pressure tests are rarely conducted on nurse tanks

after the initial proof pressure test, and no record was found indicating that such a test was

performed on this tank after its manufacture in 1976.

The crack continued to grow by fatigue as the tank continued in service over 27

years, being repeatedly loaded to about 140 psig31 and unloaded. This process is evidenced

by the relatively smooth fracture features with curving boundaries typical of high-cycle

fatigue in the 0.007-inch-wide region on the outer edge of the oxidized area of the crack.

Then, on April 15, 2003, the nurse tank failed when the crack became unstable and

propagated through the remaining thickness of the shell under the pressure from the

routine loading of the tank with anhydrous ammonia on that day. This is evidenced by the

fact that the final region in the fracture shows indications of overstress failure.

Based on the foregoing analysis of the postaccident metallurgical findings, the

Safety Board concludes that a crack initiated from the unfused area of the longitudinal

weld in the nurse tank, most likely during the manufacturing proof pressure test, and grew

through the tank shell by fatigue until the tank shell failed under normal operating

conditions.

As a result of these findings, the Safety Board examined the adequacy of

regulations and standards for the manufacturing and testing of nurse tanks used for the

transportation of anhydrous ammonia. The Safety Board also examined River Valley’s

procedures for responding to an anhydrous ammonia release.

Standards for Manufacturing and Testing Nurse Tanks

Initial Qualification of Welds

Trinity stated that the accident nurse tank was manufactured in 1976, in adherence

with the standards in the ASME Boiler and Pressure Vessel Code, Section VIII. As

permitted by the code, Trinity used spot radiography to qualify the longitudinal welds on

its nurse tanks. The use of spot radiography meant that only one representative

6-inch-long radiograph was used to ensure the quality of every 50 feet of Trinity’s nurse

tanks’ longitudinal welds. In other words, this method tested only 1 percent of the length

of the longitudinal welds on Trinity’s nurse tanks. Further, because the code does not

specify that a spot radiograph must be taken from each nurse tank and because Trinity’s

31 The loading pressure would have been about 140 psig on a day such as the one on which the accident

occurred, with the temperature around 80° F. Somewhat lower pressures would have been experienced on

days with lower temperatures, and somewhat higher pressures would have occurred on warmer days.
Analysis 24 Hazardous Materials Accident Report

tank shell lengths range from 12 to 15 feet, a single 6-inch-long radiograph could have

been used to qualify the longitudinal welds on three or four nurse tanks. Consequently,

most of the tanks produced by Trinity during this period received no individual

radiographic testing of their welds.

Use of the spot radiography method to qualify the longitudinal welds on nurse

tanks allows the majority of these welds to go uninspected and significantly reduces the

likelihood that critical defects and flaws that would result in the rejection of the welds will

be detected. Had a full radiographic examination been made on the accident nurse tank

during its manufacture, the 3.25-inch-long weld flaw it contained likely would have been

detected and the tank repaired or rejected. The Safety Board concludes that using spot

radiography to qualify longitudinal welds in nurse tanks manufactured to transport

anhydrous ammonia, a poisonous and corrosive gas, is not a sufficiently reliable method

of detecting critical flaws that can result in tank failure.

Since the mid-1980s, as the result of the development of radioscopy, a

radiographic technology that allows real-time examination of the longitudinal weld, the

two manufacturers of nurse tanks have chosen to examine the full lengths of the

longitudinal welds on all their nurse tanks using radioscopy. Thus, this full radiography

method of qualifying longitudinal welds has become general practice in the nurse tank

manufacturing industry. However, under existing regulations, the manufacturers of nurse

tanks can, if they choose, return to using spot radiography to verify longitudinal tank

welds. Although it does not appear likely that nurse tank manufacturers will return to

using spot radiography to qualify longitudinal welds, if they chose to do so it would

increase the risk that they would fail to identify weld defects during the manufacturing

qualification process. Given the serious consequences of a major anhydrous ammonia

release, which could be caused by a weld defect, the Safety Board urges RSPA to monitor

nurse tank manufacturers to ensure that they continue to use a full radiography method of

qualifying the longitudinal welds of their nurse tanks.

Periodic Testing

After being manufactured and proof pressure-tested, the accident nurse tank was

never required to undergo periodic testing of any kind to ensure its safety during its

service life. As was noted in a previous section of this analysis, the initial crack in the

nurse tank shell was likely introduced during the nurse tank’s original proof pressure test

in the manufacturing process in 1976.32 Because the crack was not visible on the outside

surface of the tank, exterior examination during or after manufacture would not have

detected the crack. Because the nurse tank had no openings larger than about 2 inches in

diameter and internal baffles blocked portions of the interior shell from visual examination

through those openings, attempts at normal internal visual inspection would also have

been unsuccessful. However, given the crack’s considerable size and the fact that it

penetrated 83 percent of the tank shell thickness, it could have been detected during the

32 Because ASME requires proof pressure testing to be performed after all manufacturing and testing

has been completed, the crack did not exist when the initial manufacture spot radiography qualification of

the accident nurse tank’s welds took place.
Analysis 25 Hazardous Materials Accident Report

service life of the tank by a variety of nondestructive testing methods. The Safety Board

concludes that periodic nondestructive testing could have detected the weld defect and

internal crack in the nurse tank during its service life, and the tank could have been

repaired or removed from service before it failed.

Neither the DOT, OSHA, nor ASME has requirements for periodic inspection and

testing of nurse tanks. Although the DOT Hazardous Materials Regulations establish

periodic testing requirements for specification cargo tanks and all other specification bulk

containers used to transport hazardous materials (including anhydrous ammonia), nurse

tanks are excepted from these requirements. In fact, with the single exception of nurse

tanks, all nonspecification cargo tanks built to the same configuration as nurse tanks are

required to have periodic inspection and testing.

Of bulk containers that are used to transport hazardous materials, only nurse tanks

are allowed to transport anhydrous ammonia without being required by the DOT to

undergo some type of periodic inspection and/or testing during their service lives to ensure

tank integrity. Nurse tanks, like any other cargo tank in pressurized service, experience

stress and wear as they undergo repeated pressure cycles over months and years of service.

They also are often transported over back roads, where pavement surfaces may be rough,

as well as into fields. Such transport environments could put additional stress on the tanks.

Some deterioration of nurse tank condition after years of service under these conditions

seems inevitable.

According to The Fertilizer Institute, an estimated 200,000 nurse tanks are in

service today. These apply between 1.0 million and 1.5 million loads of anhydrous

ammonia to fields annually. The institute also estimates that only one to five nurse tanks

are removed from service each year. Because they are not required to be removed from

service at a given age, many nurse tanks that have received no effective safety inspections

for several decades likely remain in use on farms and at filling facilities. In addition, based

on the practices used by Trinity and other nurse tank manufacturers, it appears that many

of the tanks that were manufactured before the mid-1980s did not receive full radiographic

examination of their longitudinal welds upon manufacture. As shown by this accident, an

undetected flaw in a longitudinal weld can cause a serious accident many years after a

nurse tank’s manufacture.

Further, the information provided by the Minnesota Department of Agriculture’s

unique 1996 program indicates that some nurse tanks in use may have corrosion, denting,

or leaking problems requiring repair. Unless they are detected by an inspection and testing

program, however, these defects might go unnoticed and continue to grow until a tank

failure and anhydrous ammonia release occurs. The Minnesota program information also

indicates that some of the defects found in the State’s nurse tanks in the course of the 1996

program were so extensive the tanks could not be successfully repaired. At least 10 of the

tanks tested eventually had to be removed from service because they could not be made

sufficiently sound. Had Minnesota not detected these defective tanks through its own

program, these tanks—containing serious defects—might still be in service today. Nurse

tanks in other States may have integrity problems similar to those discovered in the

Minnesota program.
Analysis 26 Hazardous Materials Accident Report

Although failures of nurse tanks may be rare, when they occur, as in the case of the

Calamus accident, they can be catastrophic, given the extremely hazardous nature of the

anhydrous ammonia they contain. Therefore, the Safety Board believes that RSPA should

require periodic nondestructive testing to be conducted on nurse tanks to identify material

flaws that could develop and grow during a tank’s service and result in a tank failure.

Tank Shell Out-of-Roundness

The ASME Boiler and Pressure Vessel Code limits the permissible

out-of-roundness of a tank shell to 1 percent of the nominal diameter. Although the

original geometry and roundness of the tank after welding is unknown, the Safety Board is

concerned that, given the findings of the postaccident examination, the tank could have

had a geometry with a significant out-of-roundness. Trinity is now using a roundness

template on all its tank shells to ensure that each tank is not misaligned or out-of-round

before the tank’s longitudinal seam is welded. Trinity’s current procedures require that any

tank found to be out-of-round be sent back, reformed, and then checked again before that

section of the tank is welded.

River Valley Emergency Preparedness

The sudden and complete release of anhydrous ammonia from a single nurse tank

at the River Valley Calamus facility affected the safety only of the employees in the

immediate vicinity of the accident—the two loaders. No one else was present, and the

facility was in a remote location.

When the nurse tank split open at the Calamus facility, it quickly lost the bulk of its

liquid contents. The spilled anhydrous ammonia rapidly vaporized, and for some minutes

the vapor cloud probably enveloped the platform, the immersion tub, and the two loaders.

When an anhydrous ammonia release occurs, it puts those in the vicinity at risk of two

types of exposure—inhalation and skin/soft tissue exposure. The loaders evidently tried to

follow the procedure that River Valley had told them to use in case of an anhydrous

ammonia release—they tried to immerse themselves in the nearest water-filled immersion

tub, which was on the loading platform. Immersion tubs enable workers to wash corrosive

liquid from their skin and soft tissues. They are not designed to provide protection from or

treatment for inhalation of chemical vapor. Although the loader immersed in the tub

received some protection from skin/tissue exposure, both loaders were unable to avoid

inhaling the ammonia vapor. The loader who was not initially immersed in the tub (and

who subsequently died from his injuries) remained at risk not only to the inhalation

hazards but also to the damaging effect of the gas on his skin.

Hazardous materials authorities have indicated that evacuation is the appropriate

response to minimize inhalation exposure from significant anhydrous ammonia releases.

The material safety data sheet for anhydrous ammonia provided by River Valley’s

anhydrous ammonia supplier states that when an inhalation exposure to anhydrous

ammonia occurs, the victim should immediately be moved away from the exposure site
Analysis 27 Hazardous Materials Accident Report

and into fresh air. According to NIOSH, in its study HAZOP of Anhydrous Ammonia Use

in Agriculture, workers should “Immediately vacate the area by heading upwind” when an

anhydrous ammonia release occurs. The 2000 Emergency Response Guidebook

recommends that when a large spill of anhydrous ammonia takes place, people should

move at least 200 feet away from the source. By contrast, the only procedure that River

Valley told its loading employees to follow in the event of an anhydrous ammonia release

directed immersion in the nearest water-filled tub, which was in the immediate proximity

of the release from the nurse tank, leaving the loaders at risk of inhalation exposure. This

was contrary to the guidance in the material safety data sheet, the NIOSH study, and the

2000 Emergency Response Guidebook. On this basis, River Valley’s instruction was

ineffective as a response to a significant release such as can result from the failure of either

a nurse or storage tank. Instead, River Valley should have directed its loaders, when faced

with such a significant release, to evacuate the release area before taking steps to flush

affected skin and tissue with water.

The Safety Board concludes that River Valley’s emergency procedures were

ineffective because they did not direct the nurse tank loaders to evacuate the area when an

anhydrous ammonia release posed an inhalation hazard. To date, River Valley has not

developed new procedures for its employees to follow in the event of a significant

anhydrous ammonia release. Therefore, the Safety Board believes that River Valley

should review manufacturers’ material safety data sheets for anhydrous ammonia,

NIOSH’s HAZOP of Anhydrous Ammonia Use in Agriculture, and the Emergency

Response Guidebook and establish written emergency procedures for employees to follow

when an anhydrous ammonia release poses an inhalation hazard.
28 Hazardous Materials Accident Report

Conclusions

Findings

1. The nurse tank was not overpressurized.

2. The unfused area and offset portion of the longitudinal weld significantly weakened

the nurse tank shell; misalignment of the shell surfaces and the large angle weld bead

increased the operating stresses in the weakened area.

3. A crack initiated from the unfused area of the longitudinal weld in the nurse tank,

most likely during the manufacturing proof pressure test, and grew through the tank

shell by fatigue until the tank shell failed under normal operating conditions.

4. Using spot radiography to qualify longitudinal welds in nurse tanks manufactured to

transport anhydrous ammonia, a poisonous and corrosive gas, is not a sufficiently

reliable method of detecting critical flaws that can result in tank failure.

5. Periodic nondestructive testing could have detected the weld defect and internal crack

in the nurse tank during its service life, and the tank could have been repaired or

removed from service before it failed.

6. River Valley Cooperative’s emergency procedures were ineffective because they did

not direct the nurse tank loaders to evacuate the area when an anhydrous ammonia

release posed an inhalation hazard.

Probable Cause

The National Transportation Safety Board determines that the probable cause of

the sudden failure of the nurse tank at the anhydrous ammonia filling facility near

Calamus, Iowa, on April 15, 2003, was inadequate welding and insufficient radiographic

inspection during the tank’s manufacture and lack of periodic testing during its service

life.
29 Hazardous Materials Accident Report

Recommendations

As a result of its investigation of the April 15, 2003, hazardous materials accident

near Calamus, Iowa, the National Transportation Safety Board makes the following safety

recommendations:

To the Research and Special Programs Administration:

Require periodic nondestructive testing to be conducted on nurse tanks to

identify material flaws that could develop and grow during a tank’s service

and result in a tank failure. (H-04-23)

To the River Valley Cooperative:

Review manufacturers’ material safety data sheets for anhydrous ammonia,

the National Institute for Occupational Safety and Health’s HAZOP of

Anhydrous Ammonia Use in Agriculture, and the Emergency Response

Guidebook and establish written emergency procedures for employees to

follow when an anhydrous ammonia release poses an inhalation hazard.

(H-04-24)

BY THE NATIONAL TRANSPORTATION SAFETY BOARD 

ELLEN ENGLEMAN CONNERS MARK V. ROSENKER

Chairman Vice Chairman

CAROL J. CARMODY

Member

RICHARD F. HEALING

Member

DEBORAH A. P. HERSMAN

Member

Adopted: June 22, 2004

31 Hazardous Materials Accident Report

Appendix A

Investigation

The National Transportation Safety Board’s Communications Center intercepted

an Internet news story about the accident about 11:30 a.m. eastern daylight time on

April 17, 2003. An investigator was dispatched from Washington, D.C., to Calamus, Iowa.

No Board Member went with the investigator. Investigative groups were established for

hazardous materials and metallurgy.

Parties to the investigation were the Federal Motor Carrier Safety Administration;

the Research and Special Programs Administration; the River Valley Cooperative; the

Iowa Department of Agriculture and Land Stewardship; Trinity Industries, Inc.; and

Continental NH3 Products.

The Safety Board did not conduct a public hearing during this investigation.