NBSIR 88-3751

An Analysis of the Influence of

Piston Effect on Elevator Smoke

Control

John H. Klote

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

April 1988

75 Years Stimulating America's Pragmas

191 3-1 9W

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 88-3751

AN ANALYSIS OF THE INFLUENCE OF

PISTON EFFECT ON ELEVATOR SMOKE

CONTROL

John H. Klote

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

April 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
- • •• . ' . •; •
TABLE OF CONTENTS

Page

LIST OF FIGURES iv

LIST OF TABLES iv

Abstract 1

1. INTRODUCTION 1

2. SMOKE CONTROL SYSTEM • . 2

3. ANALYSIS . 4

3.1 Equations for Conservation of Mass 4

3.2 Equations for Mass Flow 5 3.3 Equations for P a and Pb 7

4. MOTION OF CAR 8

5. PRESSURE DIFFERENCE ACROSS LOBBY DOORS 9

6. HAZARD ANALYSIS . . 13

7. CRITICAL PRESSURIZATION RATE 14

8. FUTURE EFFORT 16

9. CONCLUSIONS 17

10. NOMENCLATURE 17

11. REFERENCES 18

iii
LIST OF FIGURES

Figure 1. Airflow due to shaft pressurization and car motion 3*

Figure 2. Calculated pressure differences across elevator lobby doors

due to piston effect of a single car moving in an eleven story shaft ... 12

LIST OF TABLES

Table 1. Flow Areas of Eleven Story Elevator Shaft for

Example Piston Effect Analysis 11

Table 2. Critical mass flow rates calculated from

equations (16) and (17) for example shafts 16

IV
AN ANALYSIS OF THE INFLUENCE OF PISTON EFFECT ON ELEVATOR SMOKE CONTROL

John H. Klote

Abstract

This paper is part of a joint project between the United

States and Canada to evaluate the feasibility of using elevators

for the evacuation of the handicapped during a fire. The

transient pressures produced when an elevator car moves in a shaft

are a potential problem for elevator smoke control. Such piston

effect can pull smoke into a normally pressurized elevator lobby.

This paper presents an analysis of an elevator smoke control

system emphasizing the influence of piston effect on system

performance. For most elevators the problem can be overcome by

designs that prevent smoke from being pulled into lobbies, and

equations for the amount of pressurization air to accomplish this

are developed for two arrangements of supply air outlets. Where

this approach is not feasible, the methods of analysis presented

in this paper can be used to determine smoke infiltration for a hazard analysis.

Key words: elevators (lifts), hazard analysis, piston effect,

pressurization, smoke, smoke control.

1 . INTRODUCTION

The problem of fire evacuation of the handicapped has become a topic of

concern within the fire protection community. One solution would be the use

of elevators. Logistics of evacuation, reliability of electrical power,

elevator door jamming, and fire and smoke protection are long-standing

obstacles to the use of elevators for fire evacuation. All of these obstacles

except smoke protection can be addressed by existing technology as discussed

by Klote [ 1 ]

.

1
The National Bureau of Standards (NBS) in the United States and the

National Research Council of Canada (NRCC) are engaged in a joint project to

develop smoke control technology for elevators. The initial report [2] of

this project was a concept study evaluating several elevator smoke control

systems by computer analysis using the NBS program for analysis of smoke

control systems[3]. The transient pressures due to 'piston effect' when an elevator car moves is a concern of building designers relative to elevator

smoke control. The second report [4] of this project developed an analysis of

the pressures due to piston effect in a building without smoke control and

evaluates piston effect in light of that analysis. This paper presents an analysis of piston effect incorporating elevator smoke control, and addresses

the problem based on this analysis.

2. SMOKE CONTROL SYSTEM

The term 'smoke control' is used in this report to mean the limiting of

smoke movement by pressurization produced by mechanical fans. This meaning has

attained some level of acceptance in North America. Ideally, an elevator

smoke control system should protect the elevator shaft and the elevator

lobbies such that smoke contamination in these areas does not present a hazard

Most elevator doors have large gaps around them [5]. Such large leakage

areas around the doors result in lobby and shaft pressures that are nearly

equal under most conditions. Thus if pressurization air is supplied to the

elevator shaft, the lobbies will be pressurized indirectly to almost the same pressure as the shaft. A concern with such systems is that a few open doors

might result in significant loss of pressurization. The first paper [2] of

this project demonstrates that this problem can be overcome by use of a system

with feedback control. The flow rate of air into the shaft is controlled by a differential pressure sensor to maintain a constant pressure difference across

the elevator lobby door on the fire floor. One method of varying the flow

rate is a fan bypass system. It may be possible to develop other systems that

can also solve the pressure loss problem. Because the elevator smoke control

2
Fan Elevator

Machine Room

Figure 1. Airflow due to shaft pressurization and car motion

3
system discussed above can deal with this problem, it is the basis of the

analysis and discussions that follow.

3. ANALYSIS

The direction of airflow illustrated in figure 1 is that which would

result from shaft pressurization. When an elevator car moves downward, piston

effect tends to increase the pressure below the car and to reduce the pressure

above the car. In extreme cases the reduction of pressure could overcome a shaft pressurization system and result in smoke infiltration of the elevator

lobby or shaft. For the sake of simplicity, buoyancy, wind, stack effect, and

the heating and ventilating system have been omitted from this analysis.

Omitting stack effect is equivalent to stipulating that the building air

temperature and the outside air temperature are equal. Because these

temperatures are the same, the gravity effects on air density and pressure are negligible. For this analysis elevator car motion is limited to a single car moving in a single car shaft or in a multiple car shaft. The analysis is for

a downward -moving elevator car (as illustrated in figure 1) , however the

problem of an upward-moving one is same mathematically. Thus the equations

developed can be extended for an upward-moving car by reversing subscripts a (above the car) and b (below the car)

3.1 Equations for Conservation of Mass

The law of conservation of mass can be written for the volume, Qa , above

the car

' Net mass flow Rate of mass change

into volume Qa within volume Qa

j j — ( P Qa

) dt

( 1 )

4
where

nipg = mass flow rate of pressurization air to the shaft space above the car mba = mass flow rate from below the elevator car, Qb , to volume, Qa mao = mass flow rate from volume, Qa

, to the outside

p = air density within the shaft.

For a downward moving car velocity, V, and a cross-sectional area of the

shaft, As

, the derivative of the volume, Qa , can be expressed as

d Qa = AS V (2)

dt

The air density is essentially constant within the shaft. Therefore,

Substituting equation (2) into equation (1) yields

m.pa m,

'b a nr P AS V

The conservation of mass equation for the entire shaft is

(la)

®pa + “pb ‘ ®ao * mbo = 0 ( 3 )

Where m^ is the mass flow rate of pressurization air to the shaft below the car

3.2 Equations for Mass Flow

To expedite the analysis, the flow areas are chosen such that they are

the same for each floor of the building and that the only vertical airflow in

the building is within the elevator shaft. The flow from Qa to the outside is

i :

ma o Na C Ae S

a

( 4 )

5
where

Na = number of floors above the car

C flow coefficient

Ae = effective flow area per floor between the shaft and the outside

P a = pressure of the air in Qa relative to the outside

S

a = the sign of P a

.

The absolute value signs and S

a are included in equation (4) to allow for

pressurization failure above the downward moving car. The outside pressure is

not explicitly incorporated in equation (4) , because Pa is a gage pressure

which is the difference between the absolute pressure and the atmospheric

pressure outside the building. The effective flow area, Ae

, is the area that

results in the same flow as the system of flow areas from the building to the

outside when Ae is subjected to the same pressure difference as the system.

The system of flow areas can consist of areas in parallel with one another, in

series, or a combination of both parallel and series. The ASHRAE smoke

control manual [6] presents a detailed discussion of effective flow areas, and

an example evaluation of Ae for a system of flow paths is presented later in

this paper.

The flow rate from Qb to the outside is

where

Nb = number of floors below the car

P

b = pressure of the air in Qb relative to the outside.

If the car were standing still, mbo would be positive. A downward -moving

car only increases the positive pressurization below the car. Thus the

analysis only accounts for positive pressurization of the shaft below the car

as can be observed from equation (5). Neglecting hydrostatic pressure

(5)

6
difference, the mass flow rate from below the car to above it can be expressed

as

( 6 )

where

Af = free flow area in the shaft around the car

C c = flow coefficient for flow around the car

S

b a “ si Sn ° f ( Pb

- P a>-

Equation (6) includes the absolute value signs and Sba to allow for flow

from above the car to below it. This can occur when the pressurization air,

twelve -story elevator shaft at the NBS administration building [4]

For one car traveling in a two car shaft, the flow coefficient was .94, and for two cars traveling side-by-side together the flow coefficient was .83. The case

of the two cars moving together was measured to obtain an approximation of a car moving in a single car shaft.

Substituting equations (4) and (5) into equation (3), and solving for Pb yields

mpa , above the car is very large. Tests were conducted to evaluate C c on a

3.3 Equations for Pa and Pb

< c - b J I P,| ) 2 (7)

where

c = ( mpa + mpb )/Kbo

b = S

a Na/Nb

K,kb o

Combining equations (la) , (4) , (5) , (6) and (7) yields

7
(8)

m

P< Kb a S

b a J17T ]*-

- Ka o P AS V = 0

where

Kba - Af Cc J 2 p

K.„ " N.CA, J 2 p

As might be expected, for m^ = - 0 , equation (8) reduces to

equation (7) from the earlier paper [3] on piston effect without shaft

pressurization (note that the sign convention for Pa is opposite in the two

papers)

4. MOTION OF CAR

For this paper three phases of elevator car motion are considered:

constant acceleration, transitional and constant velocity motion. A car starting from rest accelerates at a constant rate, a, until the transitional

velocity, Vt , is reached. The time, tt , to reach this velocity is

a

The distance, Xt , the car travels in this time is

2 a

(9)

( 10 )

8
During the transitional phase the acceleration decreases until full

operational velocity, V , is reached. Strakosch [7] uses the following

approximate relations for this phase:

and

V - Vt

2

2 a Vt

3 a V

V*

X.

( 11 )

( 12 )

where X^ is the distance the car travels before it reaches full operational

velocity at time t

p from the start of motion. These equations leave the

motion between Xt and X^ undefined. Motion in this region is not necessary

for evaluation of piston effect in the context of this paper as is

demonstrated by the example in the following section.

5. PRESSURE DIFFERENCE ACROSS LOBBY DOORS

For fire evacuation by elevators, the pressure difference, AP i

, across

the elevator lobby doors is of major importance. If APi

is positive, the

resulting airflow from the lobby to the building will act to prevent smoke

infiltration of the lobby. This pressure difference can be evaluated by

examination of the effective flow area. For the system of three series flow

paths from the shaft to the outside illustrated in Figure 1, the effective

flow area per floor is

Ae

l

A

2 A1 s

where

Als = leakage area between the lobby and the shaft

( 13 )

9
Ai L = leakage area between the building and the lobby

Aoi = leakage area between the outside and the building.

For paths in series the pressure difference across one path equals the

pressure difference across the system times the square of the ratio of the

effective area of the system to the flow area of the path in question. Thus

for flows above the elevator car, APi

can be expressed as

Apil - (14)

This equation is general in that it applies to any system of flow paths

not just those shown in figure 1, provided that Ae is evaluated for that

particular system. This analysis does not include the effects of other shafts

such as stairwells and dumbwaiters. Provided that the leakage of these other

shafts is relatively small compared to A0

, equation (13) is appropriate for

evaluation of Ae for buildings with open floor plans. The configuration of

figure 1 was selected because many buildings are constructed with open floor

plans and because evaluation of this system may provide some understanding of more complicated systems with interior partitions. The complicated flow path

systems probably require case by case evaluation which can be done by using

the effective area techniques presented in the ASHRAE smoke control manual

[ 6 ].

A computer program was developed which solved equation (8) for Pa by the

method of bisection [8] using the car velocities and displacements of

equations (9) through (12). Equations (13) and (14) were used to obtain APi:L . In the preceding analysis, the number of floors, Na

, above the car might be

thought of as an integer, however, a real number value for Na was used for the

computer program to allow calculations when the car is not located exactly at

a floor. This occurs at the points where transitional acceleration begins and

ends. The real number approach assumes that at each floor the leakage area,

Ae

, is uniformly distributed over the floor height. Even though it is obvious

10
that leakage areas in buildings are not uniform, it is believed that the errors due to this assumption are insignificant.

Table 1. Flow Areas of Eleven Story Elevator Shaft for

Example Piston Effect Analysis

ft2

For Single Car Shaft

A1

s

, area between lobby and shaft

Ai

x

, area between building and lobby

Ao

i

, area between outside and building

As

, cross-sectional area of shaft

Af

, free flow area around car

0.167 1.80

0.0390 0.42

0.0502 0.54

5.61 60.4

1.80 19.4

For Double Car Shaft

A. ls , area between lobby and shaft

Ai ± , area between building and lobby

Ao

i

, area between outside and building

A

s , cross-sectional area of shaft

Af , free flow area around car

0.0836 0.90

0.0390 0.42

0.0502 0.54

11.22 120.8

7.41 79.8

Note: For the single car shaft a value of C c = 0.83 was used, and for the

double car shaft a value of C c = 0.94 was used. The flow coefficient

was C = 0.65. Pressurization air was m^ a = 0 and n^ b = 2.160 kg/s (3810

standard cfm at 68 °F and one atmosphere). Car acceleration was 1.22

m/s 2 (4 ft/sec2 ), and Vt at 60 % of V .

Figure 2. shows computer calculated values of AP i l for a single and a double car shaft for two values of V

p

. The flow areas for these examples are listed in table 1. These flow areas are based on the measured values of As and Af from tests of the NBS administration building [4] and average leakage

values from Appendix C of the ASHRAE smoke control manual for a building with

a floor size of 14.0 m x 67.7 m (46.0 ft x 222 ft) and 3.099 m (10.17 ft)

between floors. Pressurization air was supplied below the car and at a rate

such that APt

L

was 25 Pa (0.10 in H2 0) when the cars were still.

11
Figure 2. Calculated pressure differences across elevator lobby doors

due to piston effect of a single car moving in an eleven story shaft

12
As expected figure 2 illustrates that piston effect is more pronounced

for greater car velocities and for single car shafts. For the double car shaft of this example, piston effect does not adversely effect lobby

pressurization. This is because the free flow area, Af

, around the car is so large that there is little resistance to air flow around the car. Because Af is large for all multiple car shafts, piston effect in these shafts will

probably not have an adverse effect on lobby pressurization except at high car velocities

.

For the example single car shaft at V

p = 3.5 m/s (700 fpm) , piston

effect caused a flow from the building to the lobby for a duration of about 3 seconds. In a fire situation this could result in smoke being pulled into the

lobby. The severity of such a situation would depend on the speed of the car,

the size of the lobby, the toxicity of the smoke and the number of times the car passes the floor. Modern elevators operate at speeds up to 10 m/s (2000

fpm). At these high speeds piston effect is much more significant.

Fortunately such high speeds are not common for single car shafts.

6. HAZARD ANALYSIS

One approach to the piston effect problem is an analysis determining the

quantity of toxic gases pulled into an elevator lobby by piston action and

determining the resulting hazard to life. The mass, m, of smoke laden air

pulled into the elevator lobby during the decent of a car is

where the smoke and air mixture is pulled into the lobby during the time

interval from t. to t

2

. The mass flow rate can be obtained from equation (4)

using the pressures as defined by equation (8). The mass of gases pulled into

a lobby due to an upward moving car can be determined by a similar approach.

13
An evaluation of the resulting hazard must include considerations of mixing of

air and smoke within the lobby, an estimate of the number of times a car passes the floor of concern and an analysis of the effects of the toxic gases on people waiting in the lobby. Such an analysis is beyond the scope of this

paper. An alternate approach is to design elevator systems such that piston

effect does not cause any smoke infiltration of elevator lobbies. This

approach is developed in the following section.

7. CRITICAL PRESSURIZATION RATE

For an unpressurized shaft Klote and Tamura [4] developed an expression

for the limit of the extent of piston effect, and this limit was obtained for

the conditions of the elevator car being at the top (Na = 0) of the shaft and

a car velocity of V . Similarly, a limit or minimum value of Pa can be

obtained for a pressurized shaft using the same conditions. Observation of

figure 2 reveals that once the car reaches V , AP iincreases with time or with distance from the top of the shaft. If the car were traveling at V

p over

all its decent, AP iwould increase with distance traveled from the top of the

shaft. Obviously for the above conditions, equation (8) yields a value of P a which is less than that resulting from realistic car motion starting at rest.

For a pressurized shaft, setting Na to zero results in equation (4) becoming

mao = 0 and by definition b = 0. Equation (8) becomes

1 "Va + 2 P AsVp

- mpa 2

CM ^ N. .. C A tot e ^ AfCc

where

(P ) i = minimum level of pressurization above a downward moving elevator car

N*. = total number of floors. tot

A negative value of (Pa

) min indicates a failure of pressurization due to

piston effect. The velocity V

p was used in equation (15) because this is the

14
maximum speed at which the car can travel making the resulting pressure a limit below which the pressure Pa would not fall.

A value of = 0 means that elevator pressurization remains

positive throughout the car's descent. The total mass flow rate to achieve

this condition is referred to as the critical pressurization rate, mcrit . A

common situation is to supply all of the air at one point. If all the air is

supplied below the car (mpa = 0), the critical mass flow rate is

mcrit = P As Vr

N.tott AC© Af Cc

(16)

If pressurization air is supplied at this rate or greater to the space

below a downward moving elevator car, piston effect will not result in loss of

shaft pressurization. It is obvious that equation (16) also applies to an upward-moving car with all the air supplied above the car. Thus it can be

stated in general that equation (16) defines the critical mass flow rate for a shaft with an air pressurization inlet at only one location.

For pressurization air supplied evenly at the top and bottom of the

shaft (mpa = mpb = mcrit /2), the critical mass flow rate is

2 Pr A Vs p mc r i t

1 +

2 Af Cc

N, . A C tot ©

(17)

If at least half this amount of pressurization air is supplied above the car and an equal amount below it, piston effect will not result in loss of shaft

pressurization. Equations (16) and (17) can be used to check during smoke

control design to assure that piston effect does not result in loss of shaft

pressurization. Table 2 lists critical mass flow rates calculated from these

equations for shafts of the previous example. An elevator smoke control

system may need a much greater supply rate of pressurization air in order to

15
produce the pressure differences desired for smoke control as discussed in

previous reports [1,2]. It can be observed from table 2 that mc r i

is larger

when air is supplied at only one location as opposed to being supplied both at

the top and bottom of the shaft. Obviously, injecting air into the shaft

above the car reduces piston effect for a downward moving car. It can also be

observed from table 2 that a double car shaft has a much lower mc r ithan a similar single car shaft. This supports the belief that generally piston

effect would not be a problem for multiple car shafts.

Table 2. Critical mass flow rates calculated

equations (16) and (17) for example shafts

from

Single Car Shafts Double Car Shafts

kg/s

Standard

cfm kg/s

Standard

cfm

FOR V

p = 2.5 m/s (500 ft/min)

Pressurization air supplied at one point 2.44 4310 1.00 1770

Pressurization air evenly divided

between top and bottom of shaft 2.27 4010 0.96 1700

FOR V

p = 3.5 m/s (700 ft/min)

Pressurization air supplied at one point 3.42 6040 1.40 2470

Pressurization air evenly divided

between top and bottom of shaft 3.18 5620 1.35 2380

Note: For areas and flow coefficients see table 1. Standard cfm is at 68

°F and one atmosphere.

16
8 . FUTURE EFFORT

Elevator smoke control tests are being conducted by the NRCC at the Fire

Research Tower near Ottawa to evaluate various system concepts under full

scale fire conditions. Tests will be conducted by the NRCC on an existing

building with a pressurized elevator shaft in an attempt to verify the

analysis presented in this paper. The final effort planned for this project

will be for NRCC and NBS to jointly develop practical engineering design

information for elevator smoke control based on this research.

9. CONCLUSIONS

1. For most elevators, especially those in multiple car shafts, it is

feasible to deal with the piston effect problem by designing so as

to prevent smoke from being pulled into the elevator lobby by

piston effect.

2. For an elevator shaft with only one pressurization air inlet,

piston effect will not result in loss of shaft pressurization

provided the mass flow rate of pressurization air is at least as great as the critical mass flow rate determined from equation

(16).

3. For an elevator shaft with equal amounts of pressurization air

supplied evenly at the top and bottom of the shaft, piston effect

will not result in loss of shaft pressurization provided the mass

flow rate of pressurization air is at least as great as the

critical mass flow rate determined from equation (17).

4. For single car shafts with high velocities or multiple car shafts

with very high velocities the approach of 1 above may not be

feasible. For such cases a hazard analysis may provide useful

information, and the methods of analysis presented in this paper

can be used for the fluid flow portion of such a hazard analysis.

17
10 . NOMENCLATURE

A area

a acceleration

b S

a Na /Nb C flow coefficient

C ( mpa + )/Kbo

K coefficient

m mass

m mass flow rate

N number of floors

P pressure

Q volume

t time

V elevator car velocity

X distance of car travel

p density

AP pressure difference

Subscripts

a above elevator car

b below elevator car

c elevator car crit critical

e effective

f free flow around

i building

1 lobby

min minimum

o outside

s shaft

p full operational

t transitional

tot total

18
11. REFERENCES

1 J. H. Klote, Smoke control for elevators, ASHRAE J., 26 (4) ( 1984) 23 - 33

.

2 J. H. Klote and G. T. Tamura, Smoke control and fire evacuation by

elevators, ASHRAE Trans . , 92(1A) (1986)231-245

.

3 J. H. Klote, A computer program for analysis of smoke control systems,

Nat. Bur. Stand. (U. S.), NBSIR 82-2512, June 1982.

4 J. H. Klote and G. T. Tamura, Elevator piston effect and the smoke

problem, Fire Safety J., to be published.

5 G. T. Tamura and C. Y. Shaw, Air leakage data for the design of elevator

and stair shaft pressurization systems, ASHRAE Trans., 83(2) (1976)

179-190.

6 J. H. Klote and J. W. Fothergill, Design of smoke control systems for

buildings, American Society of Heating, Refrigerating and AirConditioning Engineers, Atlanta, GA, 1983.

7 G. R. Strakosch, Vertical transportation: Elevators and Escalators,

Wiley & Sons, NY, 2nd Ed., 1983, pp 122-127.

8 R. L. Burden, J. D. Faires and A. C. Reynolds, Numerical analysis,

Prindle, Weber & Schmidt, Boston, 2nd Ed., 1981, pp 21-25.

19
NBS-1T4A (rev. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR-88/3751 April 1988

An Analysis of the Influence of Piston Effect on Elevator Smoke Control

5o AUTHOR(S) John H, Klote

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL bureau of standards

DEPARTMENT OF COMMERCE

Gaithersburg, MD 20899

8. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here) This paper is part of a joint project between the United States and Canada to evaluate

the feasibility of using elevators for the evacuation of the handicapped during a fire.

The transient pressures produced when an elevator car move in a shaft are a potential

problem for elevator smoke control. Such piston effect can pull smoke into a normally

pressurized elevator lobby. This paper presents an analysis of an elevator smoke

control system emphasizing the influence of piston effect on system performance. For

most elevators the problem can be overcome by designs that prevent smoke from being

pulled into lobbies, and equations for the amount of pressurization air to accomplish

this are developed for two arrangements of supply air outlets. Where this approach

is not feasible, the methods of anlaysis presented in this paper can be used to

determine smoke infiltration for a hazard analysis.

12. KEY WORDS (Six to twelve entries ; alphabetical order; capitalize only proper names; and separate key words by semicolons)

elevators (lifts); hazard analysis; piston effect; pressurization; smoke; smoke

control

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