NUREG/CR-0636

SAND079-0263

RP

Nuclear Power Plant Fire Protection -

Ventilation (Subsystems Study Task 1)

John A. Milloy

Printed August 1 979

Sandia Laboratories

Prepared for

U.S. NUCLEAR REGULATORY COMMISSION
L11

NOTICE

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The views expressed in this report are not necessarily those

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Available from

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NUREG/CR-0636

SAND7 9-0263

RP

NUCLEAR POWER PLANT FIRE PROTECTION - VENTILATION

(SUBSYSTEMS STUDY TASK 1)

.John A. Milloy

Date Published: September 1979

Sandia Laboratories

Albuquerque, New Mexico 87185

operated by

Sandia Corporation

for the

U.S. Department of Energy

Prepared for

Engineering Methodology Standards Branch

Office of Standards Development

U.S. Nuclear Regulatory Commission

Washington, DC 20555

Under Interagency Agreement DOE 40-50-75

NRC FIN No. A-1080
14
ABSTRACT

This report examines the role of compartment ventilation from the standpoint of nuclear power plant fire

protection safety. Based on a review of numnerous

design standards which consider the influence of

controlled ventilation on reducing fire severity, the

report concludes that current standards and regulatory

guidelines inadequately define criteria for design of

ventilation systems and their operation under fire

emergencies. To resolve this deficiency, the report

evaluates four candidate design bases for ventilation

systems: smoke removal, smoke dispersion control,

fire spread control, and fire temperature control. It

is concluded that the lack of existing fire technology

precludes the implementation of all but one of these

criteria--fire temperature control. On this basis the

report presents an example design calculation for

applying the temperature control criterion.

5

CONTENTS

Page

Introduction 9

Bac kground 9

Task 1 Description 10

Technical Approach 10

Review of Existing and Proposed.Guidelines and Standards 11

Documents Considered 11

Guidance for Ventilation System Design 12

Candidate Technical Bases 14

Smoke Removal 14

Smoke Control 20

Fire Control. 22

Temperature Control 26

Relative Merits of Candidate Design Bases 29

Design of Heat Venting Stacks 31

Results and Recommendations 34

APPENDIX -- Sizing of Heat Venting Stacks for Natural Convection 37

References 52

7

NUCLEAR POWER PLANT FIRE PROTECTION - VENTILATION

(SUBSYSTEMS STUDY TASK 1)

Introduction

Background

An earlier Sandia Laboratories fire protection studyl surveyed the

guidelines and standards pertaining to nuclear power plant fire protection

and the investigative reports which followed in the aftermath of the

Brown's Ferry Nuclear Power Plant fire of March 22, 1975. The purpose of

that "survey was to establish a firm basis for future activities in

assessing the adequacy and development of improved design criteria for

nuclear power plant fire protection systems. One of the conclusions

reached in the study was that further work should be undertaken to develop

more compr ehensive guidanc~e for the design of ventilation systems.

Based on this and several other considerations, the NRC Office of

Standards Development funded a new program to carry out a more detailed

investigation. in particular, the following tasks were identified for

study:

Task 1 Ventilation Systems

Task 2 Fire Detection Systems

Task 3 Fire Barriers

Task 4 Fire Hazards Analysis

This report addresses the first of these subjects, Ventilation

Systems.

9
Task 1 Description

It was the objective of this task to examine the role of compartment

ventilation as it affects nuclear power plant fire protection safety. To

do this, the following general approach was used:

" Review and compare existing standards for ventilation

systems to evaluate the adequacy of the guidance provided;

" Develop technical bases for ventilation system functions and

performance in fire emergencies-and identify topics requiring

further investigation or testing; and

" Recommend changes or additions to existing guidance to clarify

intent and define design criteria.

Technical Approach

The existing guidelines and standards, as they apply to the effect

of ventilation systems on fire protection in nuclear power plants, were

reviewed from the point of view of a design organization attempting to

bring a facility into compliance. Specific criteria were listed and

examined to determine if they were adequate to evaluate and specify system

designs.

Current literature in the fire protection field was surveyed to

locate investigative research reports on ventilation-related aspects of

fire phenomenology. Particular attention was directed to reports dealing

with the effects of variable ventilation rates on the growth of

compartment fires and burning rates in fully developed compartment fires.

Information gathered from the review of the standards and the

literature search was used in formulating and evaluating four candidate

technical bases for ventilation system design. Each candidate basis was

examined to determine whether or not it fully met' the intent of the

guidelines and standards. The question of feasibility was also addressed

in each case, with regard to equipment design, plant layout implications,

10
and the availability of data upon which to base design parameters. From

the four candidates, a single design approach was selected and examples of

its application are given.

Review of Existing and Proposed Guidelines and Standards

Documents Considered

The following documents were considered representative of the

available guidance which exert substantial influence upon the nuclear

power industry in the area of fire protection. They' represent the

position of national and international regulatory agencies or are

statements of requirements for insurability:

1. Proposed Regulatory Guide 1.120, "Fire-Protection

Guidelines for Nuclear Power Plant," U.S. Nuclear

Regulatory Commission Office of Standards Development.

Several revisions of this document dating from June

1976, through November 1977, were included in the

review. (RG 1.120)

2. "International Guidelines for the Fire Protection of

Nuclear Power Plants," National Nuclear Risks Insurance

Pools and Associates, February 1974. (IGL)

3. "Property Loss Prevention Standards for Nuclear

Generating Stations," Nuclear Mutual Limited,

September 1975. (NML)

4. "Basic Fire Protection for Nuclear Power Plants,"

Nuclear Energy Liability Property Insurance Association,

April 1976. (NEL-PIA)

5. "Safety Guide on Fire Protection in Nuclear Power

Plants," International Atomic Energy Agency, January 1976.

(IAEA)

6. ANSI N 18.10, "Generic-Requirements for Nucl ear Power

Plant Fire Protection," American National Standards

Institute, February 1977. (ANSI N 18.10)

7. NFPA 803 "Standard for Fire Protection for Nuclear

Power Plants," National Fire Protection Association,

1977. (NFPA 803)

11
Guidance for Ventilation System Design

The review of the above documents indicates that the ventilation

system as a subsystem in the total fire protection program in a nuclear.

power plant should function:

" To assure accessiblity to fire-involved areas for manual

fire fighting operations

" To prevent the spread of smoke and toxic gases to other areas

of the plant

" To limit damage to electrical equipment and structural features

caused by corrosive products of combustion

" To ass ure continued habitability of areas normally occupied by

operating personnel

* To prevent smoke infiltration into routes of emergency egress

" To remove bieat which could result in serious structural damage

* To control the release of radioactive contaminants from fireinvolved controlled areas.

The IGL includes the most extensive section dealing with ventilation

system design. It not only states the underlying philosophy in each of

the areas of guidance, but, in most cases) gives specific minimum

standards for compliance with the stated requirements. In some areas in

which specific criteria are given, alternative courses of action are also

given.

En general, however, the existing guidance for ventilation systems is

brief, nonspecific, and lacking in sufficient detail to judge the adequacy

of a particular system or to specify system designs for compliance with

the standards. En those documents where specific criteria do appear, no

basis for the criteria is given nor is its source referenced. For

i nstance, RG 1.120 states that smoke and heat vents should be provided for

the control room, cable spreading rooms, diesel oil storage areas, switchgear rooms, and other areas where the potential exists for heavy smoke

12
conditions. RG 1.120 then references NFPA 204, "Guide for Smoke and Heat

Venting," for additional guidance. However, neither RG 1.120 nor NFPA 204

(see page 19) provides sufficient design information or design bases for

judging the adequacy of a particular smoke venting system design. Furthermore, no information is provided to identify "... areas where the potential exists for heavy smoke conditions.". Although some specific locations

are identified in RG 1.120, system-design guidance for these locations is

lacking.

A preference for separate smoke and heat venting facilities is

stated in several of the documents. if design criteria are stated for

these vents, they are generally given as a ratio of vent to floor area of

the compartment. Since the capacity to remove smoke and heat is the true

measure of adequacy of such a system, as opposed to the physical size of a

vent opening, it is conceivable that a system fully capable of functional

compliance could be rejected because of its physical measurements.

No consideration is given to the extent of modification that might be

required in existing facilities to bring them into compliance with newly

promulgated standards. Because of the capacities needed to meet the

stated ventilation objectives, the physical size of the equipment required

could preclude their installation in a plant which has many internal

compartments. In such cases, the only practical alternative is to waive

ve ntilation standards in favor of detection and suppression systems of a

specified reliability level.

En the following section, four candidate approaches for the design of

ventilation systems to meet the intent of existing guidance are presented.

The discussion is intended to develop an understanding of the significance

of ventilation in fire development in the light of current technology. A

design criterion is proposed which could aid both in system design and.

evaluation of compliance with the guidelines.

13
Candida .te Technical Bases

Smoke Removal

Specific reference is made in three of the guidelines (RG 1.120, IGL,

ANSI N 18.10) to the use of ventilation as a means of assuring access for

manual fire fighting. From a practical point of view, regardless of

whether .or not a fixed fire suppression system is provided, access to the

fire area must eventually be gained to allow mop up and damage assessment.

Access to a fire area could be precluded by three factors: toxicity

of smoke and gases, heat, and visibility. The hazard posed to fire fighting personnel by toxic products of combustion is acknowledged in all of

the guidelines reviewed by the requirement for self-contained breathing

apparatus and the training of the fire brigade in its proper use. The removal of heat from the fire area is discussed under Temperature Control

below. The impairment of visibility by dense smoke is the primary concern

of this section.

The reduction of visual range by dense smoke in a fire area presents

an obvious impairment to manual fire fighting efforts. In order to

effectively extinguish a fire, the suppression agent must be delivered to

the source of the fire. If smoke is so dense that it obscures the

location of visible flame, then efficient delivery of the suppressant

becomes impossible. The difficulty of directing the manual fire fighting

effort is further increased if the involved area possesses a high degree

of mechanical congestion, which is typical of many areas in nuclear power

plants. In such areas, the obstructing machinery makes it necessary to

approach the fire closely for direct application of the hose stream.

These obstructions also serve to mask the fire location and present

obstacles which impede the progress of the fire brigade. Since early.

arrest of fire development is an important factor in limiting the extent

of involvement, impaired visibility can severely limit fire fighting

effectiveness.

14
Besides acting as an impediment to effective manual fire fighting,

smoke and gases represent potential damage mechanisms which can cause

losses beyond the area of actual fire involvement. The deposition of

heavy soot in Adjacent areas obviously exacerbates postfire cleanup and

salvage problems. The presence of corrosive chemicals in the products of

combustion can also propagate equipment failures beyond the damaging range

of the flames. Electrical control equipment, such as relay racks and

switchgear, are particularly susceptible to damage caused by corrosive

chemicals evolved by cable fires. Therefore, the prompt removal of smoke

can serve to limit the extent of damage and the cost of salvage

operations.

It should also be recognized that smoke production is a time

dependent phenomenon. A practical smoke venting system might be designed

for a given fire area which would be capable of ensuring adequate

visibility in the early stages of fire development, but which would be

totally incapable of coping with a fully developed fire. Similarly, a

smoke venting system could be designed to satisfactorily vent smoke and

ga ses after successful operation of a fixed suppression system and allow.

manual mop up operations and damage assessment, but which would be totally

inadequate if the suppression system should failý: Thus, the design basis

for smoke removal is closely coupled with the speed and reliability of

detection, the expected response time of the fire brigade given detection

if manual suppression is the primary defense, or the effectiveness And

reliability of a fixed suppression system.

Smoke Production -- No definitive means of quantifying smoke

production from a fire is currently available. NFPA 258 sets forth a

standardized. test method and apparatus for the measurement of smoke

generated by solid materials.2 A. F. Robertson presents an analytical

approach for the application of data derived from this test to the

estimation of smoke production in fires within buildings.3 The latter

reference discusses in considerable detail the criticality of assumptions

made in smoke production analysis, and the uncertainties in the

applicability of laboratory smoke measurements to the prediction of smoke

production under full-scale fire conditions.

15
Several observations made by Robertson are pertinent to this

discussion. One of the most important uncertainties encountered in this

analysis was the relationship between smoke production and the rate and

extent of fire involvement. Although total smoke potential may be

indicated by the test performed per NFPA 258, the optical density within

the confined area as a function of time will be highly sensitive to how

fast the fire develops. The rate of developmnent is also influenced by the

ventilation rate which, in turn, fosters open flaming conditions under

which smoke can be consumed by the fire in the same fashion that an

afterburner functions to reduce air pollution. This type of combustion is

typical of cellulosic materials or simple hydrocarbons. However,

materials which contain inorganics such as fire retardants* often release

aerosols that are noncombustible and are, therefore, not consumed.

Finally, Robertson observes that even a small fire in a confined cell can

rapidly reduce the visual range to a few feet. This leads him to conclude

that the most effective means of limiting smoke production involve the

confinement, control, and early extinguishment of fires.

Some of th e questions rai sed by Robertson have been investigated in

the work of E. E. Smith.4-

6 The data derived from tests such as NFPA 258

for smoke production and ASTM E-84 for flame spread rating are intended to

be used for comparison purposes in arriving at appropriate choices of

construction and furnishing materials. The data are taken under carefully

prescribed conditions of thermal flux exposure, but generally at a single

exposure level. Smith suggests that the measurement of a single parameter

at a single exposure level does not provide sufficient information for

design purposes. Further, it is pointed out that cellulosic and noncellulosic materials exhibit very different hazard pr~oducing properties

when exposed at levels other than those specified in the standardized test

procedures. In Reference 4, Smith has proposed a test procedure in which

the rates of heat, smoke, and toxic gas release are measured over a range

of exposures. Smoke release rates at various exposures are reported for

both cellulosic and noncellulosic materials. These results show that the

smoke production rates from synthetic materials, in this case PVC and ABS

pipe, are much higher than those of red oak at the same exposure levels.

16
Since synthetics dominate the fuel loading in many power plant settings,

particularly those which include electrical cables, smoke could be

produced at rates which practically preclude the maintenance of

visibility.

Methods of Smoke Removal -- The means of smoke removal suggested by

the guidelines and standards include dedicated smoke vents (both natural

convection and power assisted), use of the normal HVAC system, and

portable smoke ejectors. The smoke and gases are to be removed to a"'safe

location," which generally means outdoors. However, in those cases where

the potential for radioactive contamination exists, the venting must be

monito red and, if permissible radiological maximums are exceeded, the

smoke must be directed to the gas treatment system or released at a

controlled rate. Of the three suggested options for smoke removal, the

dedicated or separate smoke venting system is listed as the preferred

approach by most of the documents, although normal HVAC systems may be

used provided appropriate design precautions are taken to ensure that:

1. HVAC equipment temperature limitations are not exceeded;

2. HVAC filters are demonstrated to be compatible with the

expected quantity and quality of vented smoke or are

appropriately bypassed during smoke venting operations;

3. HVAC ducting shared with other plant areas is isolated*

to prevent the spread of heat and smoke.

The generally accepted design basis for power plant HVAC systems is

the control of normal equipment and personnel heat loads. Consequently,

air-change rates range from one to perhaps four changes per hour in all

but the most heavily heat loaded areas of the plant. By comparison, for

smoke removal purposes, the IGL suggests air change rates of 5 to 10

changes per hour with a note that up to 50 changes may be required if the

start of smoke venting is delayed. it can be conc luded that these

criteria .and the observations made by Smith4 of smoke produced by burning

synthetics that normal HVAC systems: (1) could be capable of efficient

smoke 'removal from incipient fires; (2) would be of little use as a means

of smoke removal from well-established fires, or during active fire

17
suppression activities, when smoke production is at a maximum; and (3)

could be useful in conjunction with portable smoke ejection for clearing a

space of smoke-following fire extinguishment.

There are also requirements for fire area isolation which preclude

the use of normal ventilation systems for smoke venting. The duct systems

must be equipped with rated fire and smoke dampers to prevent the spread

of fire and smoke to other areas through the ducting. These dampers are

generally of the type which are actuated by excess heat. in the duct by

means of a fusible link. Once closed, these dampers must be manually

reset. Since closed dampers would normally be within the fire area, the

normal ventilation system would be unavailable for smoke venting of the

fire area.

In compartments equipped with gas flooding extinguishing systems, the

area must be sealed off before and during system actuation to allow the

required agent concentrations to be reached and maintained for adequate

soak times. 'Electrically closed datpers are usually installed in the

ductwork for this purpose with no provision for remote reopening.

Additional complications are encounteredein those plant areas where

radioactive materials may be entrained by smoke. For these areas a

r eliance on smoke venting to facilitate manual fire fighting or control

fire severity seems ill-advised. Venting from these areas would require

some means of preventing high-efficiency radiological filters from being

plugged with dense smoke and of monitoring radiological effluent

concentrations to prevent excessive discharges to the environment. In

addition, smoke leakage to other plant areas would need to be controlled

during times when fire fighters gain access to the contaminated area.

Under these conditions, a more reasonable fire protection philosophy

would emphasize the use of automatic detection and suppression systems,

while isolating and monitoring the burning area. Through remote mo'nitors

(e.g., video, radiologic-al, or temperature devices), the success of the

automatic extinguishing systems can be judged by the fire'brigade and

plant operators without entering the fire are a. Only after the fire has

18
been extinguished or the automatic extinguishing systems prove inadequate

would the fire area be entered by a fire brigade. These and similar

concerns are being considered in detail by a combined committee of the

American Nuclear Society (ANS .59.2) and the American National Standards

Institute (ANSI N189) in conjunction with drafting a new standard,

entitled "HVAC'Systems, Important to Safety, Located Outside Primary

Containment ."

The use of portable smoke ejectors will generally serve to reduce

smoke density in a fire area by spreading the smoke to other areas of the

building unless a duct system or direct access is provided to route the

products to outdoor areas. Since high concentrations of corrosive gases

can be attained in fire areas, ejection to other parts of the building

could result in extensive damage to equipment not involved in the fire.

As mentioned earlier, separate venting systems are preferred by the

guidelines. The IAEA document states that "fire venting shall predominantly be of the underpressure-venting type where a separate smoke/gas fan

is provided to evacuate the smoke." IGL, NML, NEL-PIA, and NFPA 803 give

vent-to-floor-area ratios for various areas of the plant which range from

0.005 to 0.04, depending upon combustible loading. NFPA 204 is also

referenced as a source of smoke and heat venting design criteria. This

document seems to be the source of the vent area ratios specified, but the

criteria by which the ratios were determined are neither stated nor

referenced in the text. It is interesting to note in the introductory.

paragraph of NFPA 204 that the document was inspired by a trend toward

construction of large single-area, one-story buildings of light construction. In the section entitled "Application and Scope," it is further

stated that many of the features suggested by the guide would be difficult

or impractical if applied to multistory buildings.- This is understandable

since the proposed vent configurations are intended, at least in part, to

provide access for hose streams to fight the fire from above.

Since a range of vent area ratios is stated by the standards, some

criterion of adequacy is implied. The IGL publication alone provides

insight into what the design basis might be by associating volume change

19
rates with the venting process. The stated venting capacity is to be

determined by the smoke production of the materials within the fire area

and the number of air changes per unit time needed to sweep the smoke from

the volume enclosed. Rather than a given ratio of vent-to-floor area, the

basic measure of venting adequacy is clearly stated to be volumetric

capacity of the system that can be brought to bear upon the fire area.

G. T. Tamura and C. Y. Shaw have proposed a design basis for smoke

venting shafts in multistory buildings.7 Their paper describes the

mechanism of "stack action" in tall buildings by which smoke is spread

throughout the upper floors through ex~isting service shafts. A method is

developed for the design of smoke venting shafts to prevent this from

occurring. The calculations presented are based on the venting of a low

temperature fire with a temperature difference of 75*F between interior

and outside air providing the stack draft. Losses due to friction,

velocity pressure, and leakage are considered. A table of minimum smoke.

shaft sizes for a range of floor areas and building heights at several

leakage rates is also given. The minimum sizes are indicative of volume

change rates of two to four per hour at the bottom floor of the building.

The work of Tamura and Shaw provides insight into the design of

natural convection smoke vents for multistory buildings, presumably of

residential or office occupancy. The selection of a low temperature fire

and a relatively low air change rate would be indicative of this type of

combustible loading. To be applicable to nuclear power plants, the shaft

sizes would have to be increased to provide much higher flow rates to

accommodate greater smoke production from synthetic materials.

Smoke Control

As used here, smoke control is defined as the prevention of smoke

migration from the involved fire area to other parts of the building. The

IGL lists the confinement of smoke and gases as a specific point for

considerat-ion in the design of enclosures for fire areas. Several of the

standards specifically list the stairwells as areas for which special

measures should be taken to prevent smoke infiltration, generally by means

20
of overpressure ventilation, to assure a positive pressure inside the

escape routes relative to other building areas.

The movement of combustion products toward the upper floors of high

rise buildings-has been recognized as cause for concern because these

.gases can result in serious visibility and toxicity hazards long before

fire reaches the area. The use of differential pressure to control

infiltration has been applied successfully as a solution to the problem.

Fung has reported the results of full-scale smoke movement experiments

conducted in the 36 story Seattle Federal Building and the 42 story

Chicago Federal Building.8 Both facilities employ the "systematic

pressurization" concept in which the structures are divided into vertical

zones, of several floors each, served by separate air handling systems.

In a fire emergency, the systems can be switched to smoke control mode by

simultaneously placing the fire zone in full exhaust condition and the

adjacent zones in full supply operation. Thus air is forced to flow

toward the fire from all directions.

An approach such as this could conceivably be adapted for use in a

nuclear power plant, but only at the expense of severely complicating HVAC

system design. In the high rise situation, the fire zones are arranged

vertically with entire floors being considered single fire areas. To

.consider a single floor or group of floors as a separate air distribution

zone follows naturally from the high rise configuration. In the power

plant, howeve r, the arrangement is primarily one of horizontal expanse

with much subdivision of a single level. An arrangement of ductwork and

remotely controlled dampers which would allow any given area to be aligned

for exhaust and all surrounding compartments to supply 'Mode would pose

substantial problems both in design and operational reliability.

Typically nuclear power p lanit layouts incorporate a great deal of

totally internal compartmentalization with very few or no exterior

windows. Large open areas are also included in buildings, such as the

turbine hall, which have considerable vertical extent, unsealed floor

openings, and extensive-6pen grating. The combustible loading and smoke

production potential of many plant areas is also much higher than the

21
typical office and residential occupancies of the buildings in which this

system has been successfully implemented.

The air handling facilities for p ower plants generally incorporate

both supply and exhaust fans. It might, therefore, be opted to seal off

the involved fire area with fire dampers, and shut down the exhaust fans

but continue operating the supply fans of adjacent compartments.. This

approach may provide sufficient differential pressure to confine smoke

during the very early stages of fire developmnent. But unless some means

of venting the fire's combustion products to'the outside air is provided,

pressure will rise very rapidly in the heated compartment and soon overcome the differential.

it is therefore concluded that for differential pressurization to be

effectively and economically used as a means of smoke control in power

plants, it must be accompanied by a properly designed smoke venting system

to provide pressure relief. The option to independently control supply

and exhaust fans could, however, prove to be an asset during fire

emergencies.

Fire Control

Intuition and Fire Phenomenology -- The concept of "control" over a

phenomenon can have many different meanings depending-largely upon the

nature and intended application of the event being observed. For example,

to many motorists, the term "controlled skid" may seem to be a contradictory use of words. Yet when viewed from the premise that a skid has

occurred and the problem is now that of limiting undesirable consequences,

the fact that an effective degree of control can be exerted on the

situation can be significant to the eventual outcome.

The term "fire control" can be viewed in a similar way. Given that

fire, in the context of this work, is an undesirable event, one "control"

might be the elimination of fire as a possiblity. Another course might be

to provide for the immediate and decisive extinguishment of any fire

occurrence so that no fire can get "out of'control." The difficulty

22
inherent in these approaches is that neither can be applied with absolute

certainty. one is therefore forced to assume that a potentially destructive fire has occurred and to investigate practical means by which the extent of damage might be limited. More specifically, in this section, the

discussion is concerned with the role of ventilation in the development of

the fire and its influence upon the eventual confinement of damage.

Most people have acquired some familiarity with fire phenomena.

However, because of the potential for destruction, most first-hand

knowledge is based on observation of fire in controlled circumstances.

If, for instance, a fire is not physically confined, we have learned that

its size c~an be controlled by regulating the amount and geometry of the

fuel. For confined fires, say in a wood stove, the amount and arrangement

becomes less important since the supply of oxygen can be regulated to

control the burning rate. In eith er case, it is necessary to deal with

the question of ventilation, particularly in the initial stages of

development, since we become intuitively aware of the difficulties of

starting a fire in a strong breeze. on the other hand, we have also

encountered the situation in which, because of the arrangement or moisture

content of the fuel, it is necessary to direct a stream of air to the fire

in order to sustain burning.

It is perfectly natural to extrapolate acquired intuition from these

controlled small-scale encounters and apply the same principles to largescale destructive fires. Thus, a forest fire could be viewed as a largescale campfire, and a fully developed compartment fire as a larger version

of one in ,a wood stove. The fact is, however, that for all of our longtime association with fire, the actual phenomenology is, at best, poorly

understood. The complexity of burning and the multiplicity of parameters

involved preclude all but a cursory understanding of the events which

occur during a fire. Because of this, the use of various scaling theories

which have been developed to predict large-scale fire phenomena from smallscale experiments appear to be of questionable value and, therefore, the

study of fire phenomenology becomes a complex and expensive endeavor

involving large-scale experiments whose results, often, violate intuition.

23
Confinement of Compartment Fires -- The work of T. Z. Harmathy is

noteworthy, not only for reporting the effect of ventilation rates on

compartment fires but also as a source of detailed description of the

processes and parameters influencing the course of destructive fires.91

A basic premise of this work is that if a fire can be confined to the

compartment of origin, then the goal of effective fi re safety design has

been met. Thus, if buildings could be designed so that a fire cannot

propagate beyond the first set of barriers, even in the absence of active

intervention, then the ultimate limits of destruct ion have been set.

Active intervention could then be expected to further limit fire loss and

provide an extra measure of protection.

A potential application of this philosophy would be in nuclear

facilities in which redundant safety systems are physically separated from

each other by fire barriers. If this were the case, and if compartment

barriers could be shown to be the ultimate limit of propagation, then fire

fighting, whether manual or automatic, could be directed toward limiting

the extent of damage within the cell of origin. Fire would therefore pose

no threat to the ultimate safety of the reactor system.

It must, however, be pointed out that Harmathy' s work applies to

relatively light fire loadings, as would be encountered in office and

residential occupancies. The compartments modeled are typically limited

in depth and ventilated through windows on at least one side. Further,

the expressions developed are based on results of tests employing only

cellulosic materials. These qualifications preclude the direct

application of the results to the power plant setting. The theoretical

framework and the method of analysis should, nevertheless, be valid for

any compartment fire, given an appropriate experimental data base. In

some regards, the analysis of the power plant problem by this technique

may be more tractable than the one addressed in the referenced material.

Since the ventilation rate is of primary importance in the analysis, a

closed compartment with a well defined ventilation system should be more

easily modeled. The class of combustibles available for involvement in a

power plant is much more limited and more clearly defined than those in

residential and office applications. Therefore a more coherent set of

24
experimental data should be obtainable for evaluation of' the various

parameters. Finally, a power plant is designed as a single-use facility,

and by its very nature is a more carefully controlled environment during

operation. The uncertainties inherent in the analysis of human

occupancies would thus be minimized.

The main thrust of Harmathy's work is toward the development of a set

of expressions, which are sufficiently comprehensive to match theoretical

predictions to existing experimental data. Since the objective was to

arrive at a measure of fire severity that could be applied in the

evaluation of the barriers which form the boundaries of the compartment,

only fully developed fires. were considered. The burning rate was found to

be a function of the flow rate of air and the free surface area of the

combustible materials. A critical value of air flow rate is defined below

in which the rate of burning is determined by the air flow rate and the

fire is said to be "ventilation controlled." At air flows above this

value, the rate of burning is determined by the'available free surface of

the involved fuel and is termed "fuel surface controlled." Conclusions

reached for cellulosics indicate that fires in the ventilationý-controlled

regime (i.e., low ventilation rates) tend to be of longer duration and

reach higher peak compartment temperatures than those which are surfacecontrolled. This can be attributed, in part, to the cooling effect of the

excess air at high ventilation rates. The ventilation rate, therefore,

not only governs the rate of burning of the involved combustibles., but it

also determines the duration and peak temperature attained in fully

developed compartment fires.

H1armathy' s approach to the analysis of compartment fires involves

consideration of the fire and its confining barriers as elements of a

total fire system. By formally accounting for the differences between

unconfined and confined burn test results and fitting the expressions

developed to compartments of various geometries, a method of interpreting

burn test data in the context of compartment hazard analysis is provided.

The analysis also provides insight into the relative importance of the

various parameters-in the total fire cell chronology. This information
could therefore be used to structure a fire testing program for the

acquisition of the requisite data in specific applications.

Unfortunately, many of the conclusions reached by Harmathy apply to

cellulosic-fueled fires in relatively light combustible load settings.

These are unlike the conditions typically occurring in nuclear power

plants and, therefore, a program of carefully designed confirmatory

compartment burn tests would probably be required. It should also be

noted that this approach to fire safety requires that, as a last resort

once a compartment fire has developed, the fire is allowed to run its

course, thus consuming the entire contents of the compartment. In many

existing plants where spatial separation is used as a means of limiting

the extent of fire damage, such a premise could not be tolerated.

Temperature Control

Where separate venting is discussed in the guidelines, the

terminology used is generally "smoke and heat venting." The design bases

for smoke venting anSI heat venting are quite different, so the two

functions are considered separately in this report. This section deals

with temperature control, which can be equated with "heat venting."

The work of Tamura and Shaw,7 which was discussed under Smoke

Removal, is concerned with smoke removal by means of separate shafts. The

motivation for considering separate smoke venting systems is at least

twofold: (1) The capacity of normal HVAC systems is no t sufficient to

support the required flow rates, and (2) the high particulate concentration of smoke rapidly degrades the capacity of the air handling system

because it clogs the exhaust filters.

As was the case for smoke venting, the normal HVAC system is

gemer-ally of insufficient-ca-pacity to handle the flow rates required for

heat venting. In this case, however, the design criteria imposed on the

heat exhaust system are much more demanding than those for the normal air

handling function. The temperatures that could be attained by the

effluent from a compartment on fire ar e very much higher than those of

26
normal exhaust air. The ductwork must therefore not only be noncombustible, but must be capable of retaining structural integrity at elevated

temperatures. The high-temperature gases also threaten the integrity of

exhaust filter systems and exhaust fans. Indeed, the charcoal filters

used. to filter radioactive effluent are a secondary fire hazard when

exposed to high temperature.

Harmathy has proposed the use of "fire drainage systems" in connection with his studies of fully developed compartment fires.10 These

systems are actually heat vents designed to remove combustion products by

natural convection at flow rates compatible with the fuel surface controlled fire regime. This method of ventilation was proposed to contend

with large areas or deep compartments in which ventilation through broken

peripheral windows could not be relied upon.

A compartment fire model has been presented by J. Quintiere which

deals with the early stages of fire development, namely that period from

ignition up to room flashover.12 Flashover can be defined as the critical

point in fire growth, within a cell which marks the difference between a

fire that stays relatively small and confined to its initial surroundings

and a fire that reaches its fully developed state, i.e., total room

involvement. Quintiere' s work is therefore complementary to Harmathy' s in

that the two models taken together address the entire history of a compartment fire. The two models are also functionally complementary since

the same course of corrective action is indicated by both models,

specifically the design of effective heat venting systems.

Quintiere has modeled conditions within a door-ventilated compa rtment

with a centrally located fire source. The model is quasi steady state in

nature, primarily because the history of turbulent mixing within the cell

is not well defined. Because of this limitation, the predicted behavior

is probably more accurate in the early stages of fire development when

mixing has less influence on room conditions.. The ventilation rate is

determined by the size and proportions of the door opening in a fashion

similar to Harmathy's use of exterior windows. The features considered

which are involved in the rate of fire spread are the fire plume itself

27
and a layer of hot gases confined in the upper portion of the room. The

inclusion of this gas layer was inspired by the observations of Waterman

from a series of experiments performed to determine the conditions supporting room flashover.13 These experiments led him to observe that the

ability of fire to jump the intervening space between widely separated

fuel packages is traceable to the heating of the remote combustibles by

convection and radiation from the upper portion of the room, not propagation directly from the source fire. J. B. Fung has observed average

temperatures in this upper gas layer in excess of 600%C (1110*F) resulting

from confined wood crib fires.'4 The model suggests that compartment

flashover and the onset of fully developed fire could be significantly

delayed, or perhaps precluded, if the maximum temperature of this hot gas

layer could be limited. It can also be observed that the rate of fire

growth in the early stages following ignition is dependent upon thermal

feedback to the source fuel. Thus, if the immediate environment of the

source fire is cooled by ventilation, radiative and convective losses from

the source fire increase, thereby slowing its growth.'

It should be noted that, if the combustible loading is spatially

continuous, the fire can spread along the material directly. Even in this

case, however, the growth rate can be slowed by ventilation since the

materials are not preheated to the same extent by convection. In fuel

configurations which have a vertical arrangement, materials near the

ceiling can be heated to their ignition temperatures by envelopment in the

hot gas layer. The extent of this envelopment may also be limited by

removal of the hot gases.

In summary, by venting heat from a burning room to reduce compartment

temperatures several benefits can be realized. These include:

e Slowing the growth rate of a fire in early stages of

deve lo pment

e Limiting the extent o~f fire involvement by delaying or

preventing flashover to remote fuel packages by radiative

or convective heating from the upper portion of the room

28
*Reducing peak temperatures within the compartment with

consequent reduction of threat to the integrity of structural members and fire barriers

* Controlling the spread of combustion products to other

areas by reduction of pressure within the fire cell,

whether by natural convection or power assisted ejection

* Improving accessibility for manual fire fighting by reduction

of both compartment temperature and smoke concentrations.

Relative Merits of Candidate Design Bases

The preceding sections have discussed the role of ventilation as a

subsystem of fire protection in nuclear power plants from four different

points of view. The objective of this discussion has been to consider

each of these viewpoints in the light of existing guidance and current

fire protection technology to determine which approach or combination of

approaches could be sufficiently quantified to generate design criteria.

Taken in the order of appearance in the above discussion, the relative

merits of the candidates are addressed in this section.

Smoke Removal -- Considered as a separate design goal, the~removal of

smoke to facilitate manual fire fighting is poorly defined. Minimum

requirements for visibility and maximum allowable temperature in the

involved compartment are not known. Even if they were, no definitive data

is available to predict either the total amount or the rate of smoke

production. Therefore, no specific design criteria can be identified to

establish firm design bases.

Smoke Control -- The use of a systematic pressurization technique to

confi ne smoke and gases to the involved fire area by realignment of the

normal HVAC system is not a- practical design basis unless some means of

pressure relief is available in the fire cell. When considered in con--

junction with a separate venting system, however, the option to continue

the operation of supply fans to surrounding compartments following closure

of fire dampers in the ductwork serving the fire area could be used to

advantage to assure flow of air toward the fire source.

29
Fire Control -- This concept of allowing a compartment to reach full

fire development while supplying excess air to limit temperatures and insure survival of compartment barriers is currently developed only for

cellulosic combustibles in relatively light fire loading occupancies.

Since many nuclear power plant areas contain high concentrations of noncellulosic combustibles, it is questionable whether sufficient air can be

supplied to a room to achieve a surface-controlled, fully developed.fire.

Without first reaching surface-controlled conditions, additional air will

increase, instead of limit, fire temperatures. However, since additional

air will reduce a fire's duration, it may be possible to balance increased

room temperatures and reduced fire durations in a manner compatible with

compartment barrier survival.

Temperature Control -- The concept of heat removal from a burning

compartment embodies desirable features of all of the proposed technical

bases. Heat removal obviously will remove smoke as well as heat and will

lower the pressure within the involved compartment, thereby controlling

the spread of smoke and gases by differential pressurization. In addition, a properly designed path for heat removal will ensure fire control

by balancing peak room temperatures and fire durations to the capabilities

of installed fire barriers.

it is therefore concluded that temperature control is the most

reasonable and effective technical basis for designing a ventilation fire

protection subsystem, because existing fire technology insufficiently

quantifies design criteria for smoke removal, smoke control, and fire

control. Although temperature control appears to represent the best basis

for integrating controlled ventilation into an overall fire protection

scheme, no evaluation has been made in this or any other study of the

importance of controlled ventilation relative to other fire protection measures (e.g., automatic suppression, automatic detection, or se paration).

Such a relative evaluation needs to be made on the basis of both the

benefits and detriments resulting from the use of a temperature control

ventilation system in nuclear power plants. Further consideration of a

controlled ventilation scheme in this report presupposes that the relative

merit of a temperature control system will be demonstrated before any

30
serious consideration is given to its implementation. The task of demonstrating the relative merit of fire protection options lies outside the

scope of this present study.

Design of Heat Venting Stacks

On the basis of the conclusion reached in the preceding section, that

temperature control represents the best technical basis for designing a

ventilation fire protection subsystem, it was decided to outline here one

possible design approach for accomplishing temperature control. It should.

be recognized, as stated earlier, that the relative merit of a temperature

control ventilation system to overall fire safety should be demonstrated

before any serious consideration is given to implementing the design technique presented in this section.

The venting of heat from a fire can be accomplished by the use of a

heat venting stack designed specifically to channel the gases directly

from the compartment involved to the atmosphere. Since gases flowing in

this duct could reach high temperatures, material used in its fabrication

must be capable of withstanding a high temperature environment for extended periods of.time and maintaining its structural integrity for the duration of the fire. For the reasons discussed earlier, it is unlikely that

the normal HVAC ductwork, filters, and fans would have the capacity or the

high temperature capability required. Therefore, a vent stack dedicated

to this purpose should be considered.

The driving force to carry the gases to the outdoors could be either

natural or power-assisted convection. The use of natural convection is

attractive in that no external power need be supplied during the fire

emergency. The disadvantages of the passive system are that a temperature

differential must exist for the stack to function and, since the velocities created by the buoyant forces are relatively low, fairly large crosssectional areas are required to provide the necessary flow rates. Since

the purpose of the stack is the removal of heat, the temperature differential will exist when the system is called upon. However, if the fire is a

31
relatively low-temperature, smoldering fire with heavy smoke production,

.the smoke removal function could be too slow to maintain good visibility.

This problem could be overcome by the attachment of exhaust blowers at the

roof level.

The size of the stack must be determined by the maximum flow rate

that it must provide to fulfill its intended function. The starting point

for the analysis would be to determine the maximum heat generation rate of

the compartment served-by the stack. The maximum permissible temperature

must then be specified for the compartment. The flow of air necessary to

carry away sufficient heat to limit the temperature to the acceptable

value can then be calculated. The required flow up the stack would then

be the calculated air flow rate at that temperature. Provision, of

course, must be made to assure that an equivalent mass flow of air at

ambient temperature can be supplied to the compartment during the venting.

Applying this calculational sequence, a design equation was developed

as described in the Appendix. This equation relates the stack's flow

rate, diameter, and height to the temperatures in a burning compartment.

By solving this equation for a variety of conditions (see Appendix) it was

found that, for specified venting rates and temperature limits, the required stack diameter varies considerably with the availability of make-up

air entering the burning compartment or with the extent of air leakage

along t-he stack length. If room openings are too few or too small, makeup air to the burning room will be restricted. Also,. if any air leaks

into a stack through dampers from other rooms, the natural draft characteristics of the stack will be reduced. Either of these shortcomings

would require an increase in stack diameter to ensure adequate flow rates

for temperature control._

In addition to these findings, it was shown for an example case (a

5 ft2 gasoline fire in a 37 500 ft3 compartment) that the stack size

required to limit peak temperature to a relatively low value is indeed

reasonable: a 3-ft diameter stack would limit the peak to 400 0F. It was

also observed in this same example that the required airflow rates were

quite high compared to those normally handled by an IIVAC system. This

32
supports the statement made earlier that normal plant air handling systems

lack the capacity to provide adequate heat removal. Finally, it was found

that the compartment air change. rates, particularly for the lowtemperature cases, are high enough to remove appreciable amounts of smoke

from the fire area. Therefore, such a system would also serve to facilitate manual fire fighting.

Although a number of factors which could require that vents be made

larger were neglected in the calculations, including resistances to makeup airflow occurring upstream of the compartment opening, other factors

should be mentioned that could have the opposite effect: no heat losses,

to the contents or w~alls of the compartment were included. These losses

would have the effect of reducing the required air flows. No mention has

been made of the role of suppression systems in the removal of heat. A

fixed water suppression system is capable of delivering water at a rate of

about 0.25 gal/ft2-min or 2.0 lb/mmn-ft2 . Given the heat of vaporization

at atmospheric pressure of 1150 Btu/lb, the heat removal capability of a

sprinkler system could be 2300'Btu/ft 2 -min if all water were vaporized.

In the 2500 ft 2 compartment used in the examples, this represents a heat

removal capability of 5.75 3t 106 Btu/min. In actuality, total vaporization of the water is neither achieved nor desirable in sprinkler system

design. But the heat removal capability of a sprinkler system could

substantially reduce the required capacity of the heat venting system.

Another point which must be considered in the design of heat venting

facilities is the handling of radioactive effluent. Smoke and gases from

areas of the plant which have potential for radioactive contamination must

be monitored to make sure nuclide concentrat Iions do not exceed permissible

limits. If limiting concentrations are detected, the effluent must be

directed to the normal plant gas treatment system or the rate of release

controlled. This suggests that heat venting and normal RVAC system dampers in the ductwork servicing radiologically controlled areas must be

capable of continued remote operation. If both types of venting capability were available, it could be possible to limit the heat and smoke

load imposed on the gas treatment-system.

33
Results and Recommendations

As stated in the section titled Technical Approach, the technical

position taken in this study was that of the designer whose goal is to

bring the ventilation system of a nuclear power plant into compliance with

the referenced standards. By virtue of the current state of the art in

fire protection technology, this approach leads to the conclusion that the

technical design basis for the fire protection design of ventilation

systems should be that of heat removal from the involved fire area for

purposes of controlling fire temperatures. Examination of the ventilation

system design parameters generated by this approach reveals that all but

one of the roles listed under Guidance for Ventilation System Design would

be fulfilled. The single exception is control of radioactive release from

controlled areas of the plant.

The control of entrained radioactive substances presents a ventilation design problem in that the normal HVAC system, which is equipped to

remove these substances, is generally incapable of handling the particulate concentrations and temperatures associated with fire generated

effluent. NFPA 803 states that, for radioactive substances, the options

are either confinement or release under controlled conditions. The IGL

suggests using the normal gas-treatment system, provided that filters are

suitably protected by the installation of prefilters or scrubbers and

demisters to cope with smoke, heat, and corrosive gases. Presumiably, the

IGL suggestion meets the "controlled conditions" requirement of NFPA 803;

however, another option might be the regulation of the removal rate

through the vent which could lower the entrainment rate if the radioactive

substances are being picked up by high-velocity flow patterns within the

compartment. The confinement of smoke and gases, however, is clearly not

a viable option unless the fire is arrested in its early stages of growth.

If a system of prefilters were added to the normal filter banks

serving the controlled areas, the particulate concentration of contaminated smoke could be reduced to manageable levels. The addition of an

34
upstream water curtain and demister, as suggested by IGL, would reduce

temperatures and corrosive properties before filtration. The design or

backfitting of all of the associated exhaust ductwork to withstand- hightemperature gas flows remains a formidable problem.

As previously mentioned, the extinguishment of a fire in its early

stages and the cooling ef fect of fixed water suppression systems greatly

reduces the smoke and heat load on the ventilation system. Therefore the

practical approach to fire protection in plant areas with high potential

for radioactive contamination may be increased emphasis on early detection

and automatic fixed suppression systems with attendant relaxation-of

ventilation requirements. Plant areas in this category might include the

reactor building,.radiation waste treatment areas, fuel handling facilities, and some emergency pump rooms. A potential-problem with this

approach arises in the case of the BWR, for which the turbine building and

main steam tunnels are included as controlled areas. It is most probable,

even in this case, that radioactive concentrations in these areas would be

within acceptable limits for direct venting during fire emergencies in the

absence of the simultaneous occurrence of a pipe break.

The selection of "heat removal for temperature control" as the fire

ventilation system design basis implies a close systems relationship with

other phases of the fire protection program. Sizing of venting facilities

is determined by potential heat production rates identified for fire areas

in the fire hazards analysis. The cooling capabilities of fixed suppression systems can also enter into the ventilation requirements as well as

the performance capabilities of the fire detection system. The maximum

permissible temperatures for which the venting system must be designed are

governed by the ignition temperatures of the materials confined Within the

vented area, and by the thermal and structural characteristics of the fire

barriers which form the boundaries of the compartment.

The results of this investigation can be summarized in the following

conclusions and recommendations:

35
" An evaluation needs to be made of the benefits and detriments

of a heat removal fire ventilation system relative to other

available fire protection measures (e.g., automatic suppression,

automatic detection, or separation). This effort should be

completed before serious consideration is given to implementing a

temperature control ventilation scheme in nuclear power plants.

" If the use of a heat removal fire ventilation system is evaluated

as worthwhile, the technical design basis for the fire venting

system should be the required rate of heat removal from involved

fire areas.

" Existing guidelines and standards are generally lacking in sufficient detail to function as criteria for the design of ventilation

systems as an integral part of the fire protection system.

" Current fire protection research activities are directed primarily

toward the solution of light fire loadings, which are not typical

of all areas of a nuclear power plant setting, Experimental

programs should be proposed to provide basic fire performance

data on combustibles normally found in critical areas of power

plants.

" In areas of the plant which involve high probability for

entrainment of radioactive containments in the smoke and gases

and in backfitting of existing facilities, emphasis should be

placed on the design and reliability analysis of fire detection

and suppression systems with accompanying deemphasis on venting

requirements.

* To allow sufficient flexibility of operation during fire emergencies to adequately control the spread of smoke and provide makeup

air for fire vented compartments, the fans, isolation dampers,

and their associated power supply and control cables should be

protected from fire damage. Manual remote operation capability

should be provided so that regulation and realignment of the

systems can be accomplished as the particular-fire situation

demands.

36.
APPENDIX

Sizing of Heat Venting 'Stacks for Natural Convection

The design of heat venting stacks can be approached by considering

the stack operation as a steady flow process. A general energy balance

for the system between state 1 at the stack entrance and state 2 at the

stack exit then takes the form15

(2- PI) + (K2 -K 1) + (Wf2 -Wfl) + (U2 -U 1) Q-W, (1

where

(2- P1.) =potential energy change

(K2 - K1) =kinetic energy change

(Wf2 - Wf1) system flow work

(U2 -U 1) internal energy change

Q =heat input to the fluid

W =work done by the fluid.

In order to evaluate this energy expression, it is necessary to cast

each term in a farm appropriate to the physical. system being considered.

Accordingly, it can be observed that the gases flowing in a stack produce

no useful work, thus eliminating the work term. The potential energy

change is defined in terms of the force required to overcome the gravitational field in changing the elevation of the fluid from points 1 to 2.

This requires accounting for the buoyancy of the flue gases. For a given

volumetric flow rate F, a unit volume of air is displaced by each unit

volume of flue gas. The bouyant mass flow then can be written as

W

w=F (Y - Ya) - 19(Y 9- ya)

g a y g

37
where

Ygl Ya =specific weight of flue gas and air, lb/ft3

w, wg =mass flow rate, lb/win

F =volumetric flow rate, ft3/min.

The potential energy term then becomes

w

(2- PI) -I}, (Yg - Ya (Z2 - Z1)

where

Zl'2 =elevation of points 1 and 2, in feet.

The change in kinetic energy in the stack is then written in terms of the

velocities

where

go= standard gravitational acceleration, ft/sec2

=lv velocity at points 1 and 2, ft/sec.

Flow work is a consequence of the movement of the gases against pressure

at the two reference points in the system. It is expressed as

2 (Wf -2( Wf 1 w g\ pY2

2 T1,

38
where

PP2=pressure at points 1 and 2, lb/ft2 .

Substitution in Eq (1) gives

Wg Y - Y) (z2 - z) + V 2 Y

+ Wg (U2 7 *-U 1) Q .(2)

This is the expression for the general energy balance of the stack under

steady flow..

Some further conditions can be imposed on Eq (2) to put it in a form

that will be useful for stack design. The expression is for steady state

operation of the stack, which implies that stable flow has been established and that initial transients have died out. Thus, it is reasonable

to assume that the wall temperature of the stack has approached that of

the flue gases. It can also be assumed that the stack would not be exceedingly high and that the'time required for the gases to pass through the

stack is therefore relatively short. The heat loss from the flue gases to

the walls should consequently be small compared to the total heat carried

by the gases. Accordingly, the heat transfer term, Q, can be set to zero.

The same arguments applied to the third term lead to the assumption that

the density change of the gases between points 1 and 2 is minimal, Thus,

Y can be substituted for Yand Y2. This is equivalent to an assumption

g

of incompressible flow in the stack. Equation (2) can then be recast in

this form:

(Y ,Y ) (Z2 - Z) +- -- (v2 _ v2) +(p-p

g a 2 1 2g0 \2 1/ (2 1l

+y Y U2 -U 1 ) =0. (3)

39
The change in internal energy can be attributed to friction losses

within the stack. In order to obtain a practical expression for friction

losses as well as to include consideration of other configuration-related

losses of the stack system, it is appropriate to introduce some of the

techniques used in the design of duct systems into Eq (3). 16 In duct

design, friction losses are usually expressed in terms of the velocity of

the flowing fluid and the Moody friction factor:

Pf 'g")

where

Pf = pressure loss due to duct friction, lb/ft2

f = Moody friction factor, dimensionless

L = length of duct, ft

D = diameter of duct, ft

pv=velocity pressure, lb/ft2

The Moody friction factor is a function of the re-lative roughness of the

duct and the Reynolds number indicative of the degree of turbulence of the

flow. its value can be found by reference to a generalized flow resistance diagram.17

Duct configuration losses, such as bends and entrance and exit

losses, are accounted for by inclusion of an empirical loss coefficient

for each fitting. The product of this coefficient and the local velocity

pressure gives the pressure loss due to flow through the feature being

considered. Chapter 31 of the ASHRAE 1977 'Fundamentals Handbook18

tabulates a number of fitting loss coefficients, including those applicable to the stack design problem. In applying the loss coefficients it

is necessary to carefully observe the velocity to which each given

coefficient is referred. For the purposes of this work, it is assumed

that the stack is one of constant cross section, which, in turn, implies

40
Lhat a constant gas velocity applies from entrance to exit. The applicable ASHRAE coefficients are such that the reference velocity is that of

the stack gases. Consequently, a combined friction and system configuration pressure loss expression can be written as

= v : + f ,(4)

where

PL combined pressure loss, lb/ft2

Ci individual configuration loss coefficients.

The second term of Eq (3) represents the change in kinetic energy

during stack transit. If the velocity of the gases in the vented compartment at some point near the entrance to the stack is assumed to be

small compared to the velocity in the stack, then v, can be taken as zero

and v2 becomes the gas velocity in the stack. Thus, the term represents

the exit pressure loss with an implied coefficient of unity which is

appropriate for a stack exit without a tapered diffuser section.

.Combining the kinetic and internal energy terms of Eq (3) in the form of

Eq (4), and noting that.(Z2 -zl) and L are equal to the stack height,

yields

.2

H gY y 2g (3C (5)

-P --- 2gy + (2 -l 1 05

where

H =stack height above entrance, ft.

T'his result is more conveniently applied to the present problem if the

stack velocity is stated in terms of the volumetric flow, rate, using

41
g 1517D2

where

vg

Q g

flue gas velocity, ft/sec

stack flow rate, ft3/min

D = stack diameter, ft~.

Substitution in Eq (5) and rearranging gives

2 -p I{( - Yg -4O17 2g0D5 \''~ + /H

Solving this expression for the volumetric flow rate results in:

(6)

40r2 90D5 1/2

Qg(y = [( Yg (P2 - P

(7)

Evaluation of this equation is inconvenient because of the presence

of the gas densities. These can be expressed in terms of temperature if

the air and flue gases are considered ideal gases by invoking the Ideal

Gas Law in the form' 5

Y=p

=RT

.where

p = gas pressure, lb/ft2

R = gas constant, ft-lb/lb-*R

T = absolute gas temperature, *R.

42
The gas pressures throughout the system are essentially atmospheric. It

is further noted that the equivalent molecular weights of air and flue gas

are very nearly identical,19 which allows the use of a single gas constant

for both constituents. Making the substitutions for Ya and Y 9in Eq (7)

gives the following:

t45O r

2 gOD 5 rR(T Lg(ZC T a 

- T)a RT Pa A( 2 -. 1, / 9 8

where

Pa = atmospheric pressure, lb/ft2 .

Now examine the significance of the second term in the bracketed

expression. Reference to Eq (2) shows that p1 and P2 are the pressures at

the inlet and outlet of the stack, respectively, during steady flow

conditions. If, by some means, the fire compartment were to be supplied

with air by a completely unrestricted path, then both the inlet and outlet

of the stack would be at atmospheric pressure and the term would vanish.

This could be done, for example, by supplying air at exactly the right

flow rate through a fan powered input duct system. A more likely

scenario, however, would involve drawing the air into the compartment friom

an adjacent plant area through an open door or some other opening. In

this case, the pressure in the compartment would be subatmospheric, i.e.,

Pl< P2 , and the system flow rate would be reduced by this term.

Assuming a single opening into the compartment and considering it a

square-edged orifice, the pressure drop acros.s the opening can be written

as

(2- - go y(v)

43
where

C0 = orifice coefficient referred to the velocity through

the orifice

Va = velocity of air through the orifice, ft/sec.

Since the underlying premise of the heat venting system is that sufficient

excess air is provided to the fire compartment to limit peak temperatures,

it will be assumed that the flow rate of combustion air into the fire is

small compared to the total air flow. The mass flow rate into the compartment must then equal the outflow and

Qa QTa Qg (10)

ga Qg

Now expressing the orifice velocity in terms of the stack flow rate and

the orifice area gives:

C p TQ2

(p 2 -PI1 2 92' 7200A g 0 RT

where

A0 area of air supply opening, ft.

Substituting this result in Eq (8) and solving again for the volumetric

flow rate of the stack gases yields the final expression:

450v2 90 D 5H(T 9.- T a)/

Qg{T[(D7 a 2 (1T

44
where

H = stack height, ft

Qg = stack flow rate, ft 3 /min

go= standard gravitational acceleration, ft/sec2

D =stack diameter, ft

TaTg =absolute air and flue gas temperatures, OR

f =Moody frictio n factor, dimensionless

Co orifice coefficient of compartment air supply

opening, dimensionless

C. individual configuration loss coefficients,

1dimensionless

A= area of compartment air supply opening, ft.

Evaluation of Eq (11) for a range of stack heights and diameters,

stack configuration, compartment air supply conditions, and ambient and

peak temperatures produces a corresponding range of stack flow rates.

Estimates of the potential heat production in a fire area can then be used

to determine the stack flow rate required to limit peak temperatures to

acceptable value-s. The appropriate combination of stack parameters can

then be selected, based on the required flow rate. The presence of the

Moody friction factor in the equation requires that some degree of conservatism. be introduced by selection of a suitably high value for this

parameter. A check on the validity of the assumed value can be made as a

secondary calculation once the velocity of the stack gases is determined

for a given design situation.

Tables 1 and 2 give the stack flow rates resulting from evaluation of

Eq (11) for stack diameters from 1 to 10 ft, stack heights from 50 to

200 ft, and gas temperatures from 2000 to 800*F. Table I represents the

case in which make-up air is somehow supplied to the compartment at a rate

which maintains the compartment at atmospheric pressure. As such, the

flow rates are the maxima attainable by natural stack action. The values

in Table 2 include the assumption that air is being drawn into the room b y

45
stack action through an opening of 20 ft2 , approximately that of a fully

opened door. The expression is quite sensitive to the inlet area assumption so the tabulated results should be viewed only as indicative of

passive air supply. A constant friction factor, ,f = 0.025, was assumed

for both sets of calculations. over the range of stack flows obtained,

this is a conservatively'high value representative of a stack liner of

relatively rough concrete. The assumed configuration is that of a centrally located stack with dampered openings of the same cross-sectional

area as the stack into each of the compartments served. The loss coefficients were taken from Reference 16 and the orifice coefficient for

the doorway is from Reference 18.

It should be noted that no leakage losses were included in the

calculations. Tamura and Shaw have shown that leakage can reduce the flow

rates by a factor of two if the total leakage fraction is as little as 3%

of the stack flow.7 If a central stack were to be designed to vent

several compartments, only-the damper in the fire cell would-be actuated,

while all others would remain in their normally closed condition. All of

these dampers must be very carefully designed to assure minimum leakage.

46
Table 1

Predicted Smoke Vent Flow Rates*

(Unlimited Inlet Area)

Stack Height = 50 ft Stack Height = 100 ft

Di am

(ft)

1

2

3

4

5

6

7

8

9

10

Flaw

2000 F

Rate (cubic

4ooOF

feet/minute) at

600OF 800OF

Di am

(ft)

Flow Rate (cubic

200OF 400OF

feet/minute) at

600OF 800*F

2

6

11

18

26

36

47

60

74

634

765

426

620

348

609

405

735

600

998

1

4

10

18

29

43

59

77

98

122

035

515

494

975

962

453

450

952

959

472

1

5

13

24

38

55

75

99

126

156

320

756

377

189

194

392

784

370

149

122

1

6

15

28

44

65

89

116

148

183

553

773

741

463

943

180

175

928

439

708

1

2

3

4

5

6

7

8

9

10

3

8

15

24

36

49

65

83

103

781

587

535

642

913

352

959

734

679

7.92

1

5

13

25

40

59

81

107

136

169

276

859

938

543

684

363

583

344

647

492

1. 627

7 468

17 768

32 562

51 862

75 673

103 998

136 837

174 191

216 -061

2

10

24

44

71

105

145

192

245

304

105

001

218

866

98.2

585

683

280

379

983

Stack Height = 150 ft Stack Height = 200 ft

Di am

(ft)

1

2

3

4

6

7

8

9

10

Flow

2000 F

Rate (cubic

400OF

feet/minute) at

600OF 800OF

Dian

(ft)

Flow Rate (cubic

200OF 400*F

feet/minute)- at

600OF 800OF

4

9

18,

29

43

59

78

100

124

859

083

887

316

386

105

474

498

175

509

1

6

16

29

47

70

97

128

163

203

403

667

145

910

988

390

122

186

586

322

1

8

20

38

61

89

123

163

208

259

789

499

581

128

173

730

807

406

532

186

2

10

24

44

71

105

145

192

245

304

105

001

218

866

982

585

683

280

379

983

1

2

3

4

5

6

7

8

9

10

908

4 422

10 1U358

20 296

32 766

48 283

66 855

88 486

113 178

140 934

1

7

17

33

53

78

109

144

184

230

483

221

731

.143

507

847

175

497

820

145

1

9

22

42

68

100

139

184

235

293

.891

205

603

249

209

171

199

600

378

2

10

26

49

80

118

163

216

277

345

225

832

597

715

261

271

76.2

746

230

218

*Amnbient Ai-rTemperature = 80*F; Friction Factor = 0.025

Loss Coefficients: Entrance = 0.5; Exit = 1.0; Stack Tee' = 1.2

47
Table 2

Predicted Smoke Vent Flow Rates*

(Limited Inlet Area)

Stack Height = 50 ft Stack Height = 100 ft

Diam

(ft)

1

2

3

4

5

6

7

8

9

10

Flow

2000 F

Rate (cubic

4000 F

feet/minute) at

600OF 800OF

Di am

(ft)

Flow Rate (cubic

200*F, 400OF

feet/minute) at

600OF 800OF

2

6

10

14

17

19

20

21

21

634

744

178

319

205

156

105

311

047

502

1

4

10

17

24

30

34

36

38

39

035

489

178

281

359

138

214

863

535

591

1

5

13

22

32

40

46

50

53

55

319

729

048

393

096

451

658

865

600

362

1

6

15

26

38

49

57

63

67

70

553

746

413

654

680

466

826

700

622

197

1

2

3

4

5

6

7

8

9

10

3

8

14

19

23

26

28

29

30

781

565

242

031

597

909

791

583

677

353

1

5

13

23

33

41

47

51

54

55

276

830

567

451

523

906

897

817

296

'861

1

7

17

30

44

56

65

71

75

78

627

439

381

348

090

144

222

424

469

077

2

9

23

42

63

82

97

108

115

120

105

972

818

464

149

347

643

583

953

807

Stack Height = 150 ft Stack Height = 200 ft

Di am

(ft)

1

2

3

4

.5

6

7

8

9

10

Flow

2000 F

Rate (cubic

400OF

feet/minute) at

600OF '8000F

Diam

(ft)

Flow Rate (cubic

200OF 400OF

feet/minute) at

600OF 800OF

4

9

16

23

28

32

34

36

37

859

060

582

569

442

868

540

836

241

108

1

6

15,

27

40

50

58

63

66

68

403

639

,759

648

010

491

080

083

255

259

1

8

20

35

52

67

78

86

92

95

789

470

179

738

537

531

978

865

028

362

2

9

23

42

63

82

97

108

115

120

105

972

818

464

149

347

643

583

953

807

1

2

3

4

5

6

7

8

9

10

4

10

18

26

32

37

40

41

42

908

401

553

493

465

875

267

032

726

771

1

7

17

30

45

57

66

72

76

78

483

194

346

814

078

384

412

411

227

639

1

9

22

39

59

76

90

99

105

109

891

178

202

793

105

630

189

611

807

812

2

10

26

47

70

93

ill

124

133

139

225

805

198

249

966

325

376

406

230

054

*Ambient Air Temperature = 80'F; Friction Factor = 0.025

Loss Coefficients: Entrance = 0.5; Exit = 1.0; Stack Tee = 1.2

Compartment Air Inlet: Area = 20 ft; Orifice Coefficient = 2.5

48
Examples of Vent Stack Applications

To illustrate the use of Tables 1 and 2 and to investigate the feas~ibility of heat removal by stack action, some sample calculations were made

for a postulated fire, the results of which are presented here. The heat

removal capability of air can be expressed as

q wc p (T2 -Tl)

where

q = heat removal capability, Btu/min

cp= specific heat of air at constant pressure, Btu/lb-*R

Tl, T2 =initial and final air temperatures, *R

w =mass flow rate, lb/min.

The specific heat is defined by

h 2-h1

-2 T2 1,

where

=~h enthalpy at T2 and TI, Btu/lb.

Making this substitution and applying the previously stated form of the

Ideal Gas Law, the following expression for the required flow in the stack

can be written as follows:

Qf + RT (12

Qg P a(h 2- h)1(2

49,
where

Qg required stack flow rate, ft /min

Qf = heat evolved by postulated fire, Btu/min

R = gas constant, ft-lb/lb-*R

TF= assumed allowable maximum temperature, OR

Pa =atmospheric pressure, lb/ft 2

h2l= enthalpy of air at maximum and ambient te mperatures,

Btu/ lb.

For purposes of estimating stack size requirements, Eq (12) can be simplified by assuming standard atmospheric pressure and R = 53.3 which gives

Qg = (2.5186 x 102) q fT (13)

The values for enthalpy of air at various temperatures can be found in

standard gas tbe.0Equation (10) can be used to estimate the required

make-up airflow rate. Equation (9) will give the value of negative

differential pressure in the compartment unde?"atrl tckatin we

air is supplied through an assumed opening for a given supply air velocity

through the inlet opening. Again using the standard air conditions stated

above and an orifice coefficient of 2.5, Eq (9) become s

2

Ap -(8.2334 x 10 ) _! (14) T

a

where

Va =inlet air velocity, ft/mmn.

Using these expressions, consider a 5 ft2 gasoline fire in a

compartment 50 ft square by 15 ft high. NFPA 204 gives a pool-burning

50
heat evolution rate of 10 000 Btu/ft2-min for gasoline, which represents a

heat input to the compartment of 50 000 Btu/min. A stack height of 100 ft

was assumed. Stack sizes were taken from Table 2 which implies an ambient

temperature of 800 F and an air inlet opening of 20 ft2. The results of

these calculations are shown in Table 3.

Table 3

2Example Heat Venting Results

(5ft2 gasoline fire in 37 500 ft3 compartment)

Assumed maximum permissible 200 400 600

temperature, OF

Enthalpy change, Btu/lb 29 76 126

Required gas flow in vent 28 660 14 250 10 594

stack, ft3/min

Required air flow into 23 450 8 950 5 397

compartment, ft3/min

Minimum stack diameter 8.0 3.1 2.5

from Table 2, ft

Velocity in stack, ft/mmn 570 1 888 2 158

Velocity through 20 ft2 1172 447 269

compartment air inlet,

ft/mmn

Air change rate at 80*F, 37.5 14.3 8.6

vol/hr

Compartment differential -0.210 -0.031 -0.011

pressure, in; H20

As might be expected, very large flow rates, and hence large stacks,

would be required to maintain low peak temperatures. In this example, a

peak permissible temperature of 400OF results in a reasonable stack

diameter and flow rates. If a stack height of 50 ft had been assumed,

Table 2 suggests that the 200*F temperature could not be maintained.

Reference to the flow rates in Table 1 for the 50-ft stacks shows that

this latter case is governed by the limitations of the compartment air

inlet.

51
Re ferences

1. E. A. Bernard and G. L. Cano, Report on Task I - Fire Protection

Study, Sandia Laboratories, SAND76-0630 (NUREG 766516), February

1977.

2. Standard Test Method-for Measuring the Smoke Generated by

Solid Materials, National Fire Protection Association, NFPA 258,

1976.

3. A. F. Robertson, "Estimating Smoke Production During Building

Fires," Fire Technology, Vol. 11, No. 2 (pp. 80-93) May 1975.

4. E. E. Smith, "Measuring Rate of Heat, Smoke and Toxic Gas

Release," Fire Technology, Vol. 8, No. 3 (pp. 237-245), August

1972.

5. E. E. Smith, "Application of Release Data to Hazard Load

Calculations," Fire Technology, Vol. 10, No. 3 (pp. 181-186),

August 1974.

6. E. E. Smith, "Relation of Performance Tests to Actual Fires,"

Fire Technology, Vol. 12, No. 1 (pp. 49-54), February 1976.

7. G. T. Tamura and C. Y. Shaw, "Basis for the Design of Smoke

Shafts," Fire Technology, Vol. 9, No. 3 (pp. 209-222), August

1973.

8. F. C. W. Fung, "Smoke Coutrol by Systematic Pressurization,"

Fire Technology, Vol. 11, No. 4 (pp. 261-268), November 1975.

9. T. Z. Harmathy, " A New Look at Compartment Fires, Parts I and

II," Fire Technology, Vol. 8, No. 3, (pp. 196-217), August 1972

and Vol. 8,No. 4 pp. 326-351), November, 1972.

10. T. Z. Harmathy, "Design of Buildings for Fire Safety, Parts I

and II," Fire Technology, Vol. 12, No. 2, (pp. 95-108), May 1976,

and Vol. 12, No.3 (pp. 219-236 , August 1976.

11. T. Z. Harmathy, "Fire Resistance versus Flame Spread Resistance,"

Fire Technology, Vol. 12, No. 4 (pp. 290-302), November 1976.

12. J. Quintiere, "Gro wt h of Fire in Building Compartments," Fire

Standards and Safety, ASTM STP 614, A. F. Robertson, Ed.,

American Society for Testing and Materials (pp. 131-167), 1977.

13. T. E. Waterman, "Room Flashover--Criteria and Synthesis,"~

Fire Technology, Vol. 4, No. 1 (pp. 25-31), February 1968.

52
14. 3. B. Fung, "Fire Buildup in a Room and the Role of Interior

Finish Materials" NBS Technical Note 879, National Bureau of

Standards, Washington, DC, June 1975.

15. V. M. Faires, Thermodynamics, Fifth Edition, MacMillan Co.,

London, 1970.

16. ASHRAE Handbook of Fundamentals, 1977 Edition, American

Society of Heating, Refrigerating, and Air-Conditioning

Engineers, Inc., New York, Chapter 31.

17. Marks' Standard Handbook for Mechanical Engineers, Seventh

Edition, T. Baumeister, Editor, McGraw-Hill Book Co.., New

York, p 3-60, 1967.

18. ASHRAE Handbook of Fundamentals, 1972 Edition, American

Society of Heating, Refrigerating, and Air-Conditioning

Engineers, Inc., New York, Chapter 25.

19. Steam, Its Generation and Use, 38th Edition, Babcock &

Wilcox Co., New York, p 17-2, 1975.

20. P. J. Potter, Power Plant Theory and Design, Second

Edition, The Ronald Press Co., New York, 1959.

53
DISTRIBUTION:

U. S. Nuclear Regulatory Commission (RP')

David P. Notley, USNRC (50)

Sandia Laboratories

P. 0. Box 5800

Albuquerque, NM 87185

Attn: J. E. Christy (50)

54
NRC FORM 3W3 .. NCERRGUAOYCMISO 1. REPORT N UMBER (Asigned by DDCj

(7-77) US ULA EUAOYCMISO

BIBLIOGRAPHIC DATA SHEET NUREG/CR-0636.

4. TITLE AND SUBTITLE (Add Volumne No., if appropriare) 2 (Leave blank)

Nuclear Power Plant Fire Protection - Ventilation

(Subsystems Study Task 1) 3 RECIPIENT'S ACCESSION NO

7. AUTHOR (S) 5 DATE REPORT COMPLETED

John A. Milloy MNHAugust YER1979

9. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Code) DATE REPORT ISSUED

Sandia Laboratories MO979IEA

Albuquerque, New Mexico 87185 September YEAR

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12 SPONSORING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Code) 1 RJC/AKWR NTN

Engineering Methodology Standards Branch PRET/AKWRUTN

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U.S. Nuclear Regulatory Conmmission

Washington, D. C. 20555 FIN No. A-1080

13 TYPE OF REPORT PERIOD COVERED (Inclusive dates)

Final TechnicalI

16 ABSTRACT 000O Words Or less)

This report examines the role of compartment ventilation from the standpoint of nuclear

power plant fire protection safety. Based on a review of numerous design standards -which consider the influence of controlled ventilation on reducing fire severity, the

report concludes that current standards and regulatory guidelines inadequately define

criteria for design of ventilation systems and their operation under fire emergencies.

To resolve this deficiency, the report evaluates four candidate design bases for

ventilation systems: smoke removal, smoke dispersion control, fire spread control,

and fire temperature control. It is concluded that the lack of existing fire technology

precludes the implementation of all but one of these criteria--fire temperature control.

On this basis the report presents an example design calculation for applying the

temperature control criterion.

17 KEY WORDS AND DOCUMENT ANALYSIS 17a DESCRIPTORS

Fire

Ventilation

17b IDENTIFIERSIOPEN-ENDED TERMS

18. AVAILABILITY STATEMENT 19 SECURITY CLASS (This report) 21 NO OF PAGES

Unlmied20 SECURITY CLASS ('This page) 22 PRICE

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