odsu

a r t i c l e i n f o

Received in revised form28 October 2013Accepted 29 October 2013

Keywords:Pool re exposureJet re exposureVessel failureTensile strength

a b s t r a c t

exposed to pool res. The API method is empirical based on tests performed in the 1940s. There is

regulators recommend and integrate modications into the Stan-dard. These modications involve lessons-learned from incidentsor near-misses, advances in engineering methodologies, and newguidance based on shared experiences of the members or inspiredby technical inquiries.

The new API 521 6th edition includes an analytical method toestablish relief loads for pressure relief devices and to design

mpirical method.1 and differentiatels and processingith ASME Sectiondesign codes). Ineric, and refriger-ith storage tankheat input equa-

tions in API 2000 are the same as those in NFPA 30. They wereestablished in a 1963 meeting between API and NFPA and are basedon a re test and experience with storage tank res. Because theorigin/basis of API 521 and API 2000 re equations are different andthe scope of the equipment design codes are different, the reexposure guidance API 521 and API 2000 can neither be inter-changed nor compared (i.e., use API 521 for pressure vessels andAPI 2000 for storage tanks). The subsequent discussion relates onlyto API 521.

Contents lists availab

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Journal of Loss Prevention in the Process Industries 27 (2014) 21e31E-mail address: edzamejc@ezreliefsystems.com.American Petroleum Institute (API) Standard 521 PressureRelieving and Depressuring Systems is an internationally recog-nized engineering standard used to design pressure relief systems,disposal systems (e.g., ares), and depressuring systems (ANSI/APIStandard 521, 2013). It is continually being reviewed, with neweditions published in about 5-year intervals. A technical committeeconsisting of industry representatives, engineering contractors, and

will complement, but not replace, the existing eIt is important to establish the scope of API 52

it from API 2000. API 521 covers pressure vesseequipment (e.g., vessels design in accordance wVIII, Division 1 and similar pressure vesselcontrast, API 2000 covers low pressure, atmosphated storage tanks designed in accordance wstandards such as API 650. The current pool re1. Introduction depressuring systems for the re scenario. The analytical methodTime to failureFire heat input0950-4230/$ e see front matter 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jlp.2013.10.016increasingly widespread interest in analytical methods based on heat transfer principles to model reheat input. The API committee agreed to include an analytical method in the 6th edition of API Standard521 to establish relief loads for pressure relief devices and to design depressuring systems for the rescenario. The analytical method provides more exibility than the empirical method but has limitations(e.g., too many permutations are possible leading to potential under-sizing of the pressure relief device).This paper discusses the basis for the empirical method in API Standard 521 and provides comparisons

of the empirical and analytical method with two more recent large-scale pool re tests. This pool re testdata indicates that the empirical method will provide a conservative estimate of pool re heat input formost applications and is still the method of choice when designing pressure relief systems. However,these recent tests indicate the empirical method needs to be modied when a vessel or equipment ispartially conned by adjacent embankments or walls equal or greater than the vessel height. In suchcases, the wetted area exponent should be 1.0 instead of 0.82.The analytical method is useful in determining time-versus-temperature proles for heating unwetted

vessels of varying wall thicknesses and materials of construction. These proles, which depend upon thetype of re (e.g., unconned pool re, jet re, etc.), can be combined with tensile strength and stress-rupture data to specify a depressuring systems pressure-versus-time prole. This will minimize fail-ure and/or mitigate the effects of failure due to overheating from re exposure.

2013 Elsevier Ltd. All rights reserved.Article history:Received 31 October 2012Since the 1950s, API Standards have provided guidance on determining relief loads for equipmentAPI Standard 521 new alternative methpressure relief device sizing and depres

Edward ZamejcEZ Relief Systems Consulting, Inc., 7905 Grady Circle, Castle Rock, CO 80108-6112, USA

Journal of Loss Preventio

journal homepage: wwAll rights reserved.to evaluate re relief forring system design

le at ScienceDirect

in the Process Industries

elsevier .com/locate/ j lp

Fig. 1. Pool re heat input versus wetted area exposure.Test data sources: a) API project test No. 1, b) API project test No. 2, c) Rubber ReserveCorporation test No. 17, d) Standard Oil Company of California, e) Underwriters Lab-oratories, Inc. (ANSI/API Standard 521, 2013 Table A.1).

Fig. 2. API 521 Table A.1 re tests e Wetted area versus re heat input.

in the Process Industries 27 (2014) 21e312. Fire scenario e API empirical method

2.1. Basis of the API empirical method to evaluate pool res

The re scenario generates the most technical inquiries of anytopic in API 521. The current method, given in Equations (1) and (2),is an empirical method based on re tests performed in the 1940s.

Pool re heat input with adequate drainage and promptreghting:

Q C1$F$Aws0:82 (1)Pool re heat input without adequate drainage and prompt

reghting:

Q C2$F$Aws0:82 (2)

where:Q is the total heat absorption (input) to the wetted surface,

expressed in W (Btu/h);C1 is a constant [ 43,200 in SI units (21,000 in USC units)];C2 is a constant [ 70,900 in SI units (34,500 in USC units)].F is an environment factor for reproong (F 1 for no

reproong);Aws is the total wetted surface, expressed in square meters

(square feet).Note 1: the SI equation constants include a conversion factor for

(Aws)0.82.Note 2: Inadequate drainage implies the burning liquid can

engulf the vessel resulting in increased heat input as comparedwith a non-engulng type pool rewhere the burning liquid drainsaway from the vessel.

Note 3: Fireghting reduces the re heat input to a vessel bywater spray cooling of vessel surfaces.

The origin of the empirical method can be traced back to the1950s, when the API Pressure Relief Systems (PRS) technicalcommittee analyzed the available pool re test data and developedempirical equations to determine the pool re heat input to a vessel(Heller, 1983). This heat input could then be used to calculate there relief load by dividing by the heat of vaporization. Theseempirical equations include the:

Maximum re heat input (i.e., maximum heat ux absorbed bythe vessel and its contents)

Effect of wetted surface area of the vessel or equipment (i.e., areaof the equipment in contact with liquid or below liquid level) onre heat input

Effect of burning liquid drainage (i.e., whether the pool reengulfs the vessel) on re heat input

The maximum re heat input into a vessel is sometimesconfused with the ame surface heat ux (i.e., pool re heat dutydivided by ame surface area) and the incident heat ux at a vesselexposed to the re. The incident heat ux excludes reduction inheat ux due to the absorptivity of the vessel and re-radiation fromthe vessel. Based on plotting hydrocarbon pool re test data (seeFig. 1), the maximum heat input into the vessel was determined bythe API committee to be 34,500 BTU/h ft2 of wetted surface (seeconstant C2 in Equation (2)). This maximum heat input would occurwhen the wetted surfaces of the vessel are completely andcontinuously exposed to ame.

Outdoor pool res are easily inuenced by even relatively calmwind conditions; wind causes ames to move around, thereby onlyintermittently exposing surfaces of larger vessels to the highest

E. Zamejc / Journal of Loss Prevention22incident heat ux. To determine the effect of vessel wetted area, theAPI Committee plotted the total heat input versus the wetted areafrom several pool re tests and an actual pool re (see API 521Table A.1). The results, shown in Fig. 2, indicate the heat inputcorrelates with a 0.82 exponent on the wetted area (Aws). It shouldbe noted that, per convention, the heat input across only thewettedsurfaces of vessels containing a liquid that can boil is consideredwhen designing pressure relief systems for the re scenario usingthe empirical method. The effect of heating unwetted surfaces andgas-lled vessels is discussed later in this article.

The effect of drainage was determined from Hottels pool retest data (ANSI/API Standard 521, 2013, Table A.1, Test 1 and Test 2).Test 1 actually consists of the average of 31 tests without eitherdrainage or reghting. Test 2 consists of the average of 8 tests withdrainage and 5 tests with both drainage and reghting. The ratiofor the pool re heat input from Test 2 to Test 1 (no drainage norreghting) is 17,400/30,500 0.6. Hence, the maximum pool reheat input to vessels with adequate drainage and reghting is34,500*0.6 21,000 BTU/h ft2 (see constant C1 in Equation (1)).Hottels test data from Personal correspondence (1950), shown inTable 1, suggest that drainage alone (no reghting) has a com-parable reduction in heat input.

The remaining parameter of the empirical method, designatedF, is the environment factor which credits for adequate re-proong (See ANSI/API Standard 521, 2013). Fireproong thatmeets the requirements of API 521 reduces the pool re heat inputto a vessel thereby reducing the relief requirements. Fireproongwill also reduce the vessel wall heatup rate thereby increasing theData sources: all tests shown in ANSI/API Standard 521 (2013) Table A.1 as well as theactual plant re involving a 380 butane sphere.

because of the erosive power of the momentum jet. API 521 does

in thnot allow credit for re protection systems because of reliabilityaspects.

2.2. Limitations of the API empirical method to evaluate res

There are two types of res relevant to pressure relief anddepressuring system design e pool re and jet re. A pool re istime to failure of gas-lled vessels. The 6th edition has an expandedexpression to account for multi-layer reproong. It is important tonote that the API 521 criteria for determining adequacy of re-proong correspond to typical pool re exposure, but not jet res

Table 1Hottel pool re test data showing effect of drainage with and without reghting(Personal correspondence, 1950).

Run Drainage Fireghting Average heat input, BTU/h ft2a

10 Yes None 22,86211 Yes None 30,08112 Yes None 981913 Yes None 19,51714 Yes None 21,70715 Yes None 26,95317 Yes None 216619 Yes None 16,43618 Yes Chemical foam for 30 s

then mechanical foam6594

22 Yes Chemical foam for 30 sthen mechanical foam

9337

23 Yes Chemical foam for 30 sthen mechanical foam

14,535

24 Yes Chemical foam for 30 sthen mechanical foam

25,028

25 Yes Chemical foam for 30 sthen mechanical foam

20,672

Average for all tests 17,362Average for tests withdrainage, but no reghting

18,693

Average for tests with bothdrainage and reghting

15,233

a Heat input into wetted surface area.

E. Zamejc / Journal of Loss Preventiondened as a burning pool of liquid. A jet re is a re createdwhen a leak from a pressurized system ignites and forms a burningjet. A pool re can be classied as an open pool re, a conned poolre, or somewhere in between. A conned pool re is dened as are inside a building or a compact process modulewhere thewallsand/or surrounding equipment can reradiate and preheat thecombustion air causing higher heat uxes than an unconned (i.e.,open) re. Generally only pool res are considered whendesigning pressure relief systems while both pool res and jet resare often considered when designing depressuring systems.

Typical ranges for peak re heat intensity (i.e., incident heatux) are (Energy Institute, 2003):

. Open pool re e 16,000e48,000 Btu/h ft2 (50e150 kW/m2). Conned pool ree 32,000e79,000 Btu/h ft2 (100e250 kW/m2). Jet re e 32,000e127,000 Btu/h ft2 (100e400 kW/m2)

These peak re intensities generally correspond to locationswithin the re where the stoichiometric fuel-to-air ratio is equal toone. Because of the effects of ventilation (e.g., wind effects andconnement), fuel type, fuel-air stoichiometry and other factors,the peak re heat intensity is generally only observed in localizedparts of the ame volume. Most of the ame volume has signi-cantly lower re intensities than the peak. Note that API 521 doesnot provide specic guidance on completely conned res becauseof their complexity (e.g., sensitivity to ventilation effects) whichgenerally requires case-by-case evaluations.Pressure relief systems designed for re exposure require a totalheat input into the relevant surfaces (e.g., wetted areas). Becausethe re heat intensity will vary with time and location, the totalheat input should be determined using a surface average heat uxobtained by averaging the re heat intensity across the entire amevolume. In contrast, when designing depressuring systems, thepeak re heat intensity (designated as the local peat heat ux) isimportant because localized overheating in a small area can resultin equipment failure due to overheating.

It should be noted that API 521 5th and prior editions providedesign guidance for pressure relief and depressuring systems onlyfor open pool res. Also, the 34,500 Btu/h ft2 maximum re heatinput used in the API empirical method includes the vessel ab-sorptivity, which can reduce the incident heat ux by 20e70%.

It is important to note that in the case of jet re exposure ofvessels, a pressure relief device will usually not provide signicantprotection/mitigation benet because the vessel walls can beheated by an impinging jet re to a temperaturewhere thematerialloses strength resulting in vessel failure. In these cases, a properlydesigned depressuring system is one potential mitigation alterna-tive that can be used.

3. Analytical method to evaluate res

Several authors recommended using an analytical method tomodel the re scenario to overcome limitations with the empiricalmethod and to provide more exibility in modeling (e.g. EnergyInstitute, 2003; Roberts, Medonos, & Shirvill, 2000; Salater;Salater, Overa, & Kjensjord, 2002; SCANDPOWER, 2004; Shirvill).The analytical method is based on theoretical heat transfer equa-tions that include radiative and convective heat transfer terms. Incontrast, the API 521 empirical method determines the re heatinput based on a correlation derived from pool re test data whereall of the heat transfer terms and effects are lumped together asshown in Equations (1) and (2). Because the analytical methods arebecoming more widely used, the API 521 committee agreed toincorporate the analytical method as an alternative to the empiricalmethod and to include guidance on its application. The empiricalmethod will still be recommended as the preferred method toevaluate most re scenarios involving pressure relief system designwith the analytical method preferred for special cases and resoutside the scope of the empirical method.

The analytical method to evaluate res is a basic heat transferequation as shown in Equation (3). The method determines the reheat input to a vessel and conservatively ignores internal heattransfer limitations. It can be applied to all types of res includingopen pool res, conned pool res, and jet res. The proposedtypical ranges in the parameters are given in Table 2 for the surfaceaverage heat ux for pool res. Recommended values, whichshould be used where data or other resources are unavailable, aregiven in ANSI/API Standard 521, 2013. ANSI/API Standard 521, 2013also provides typical values of parameters for the local peak heatux of pool res and for the local peak and surface average heatuxes for jet res.

qabsorbed s$asurface$fire$T

4fire surface$T4surface

h$Tgas Tsurface

(3)

where:qabsorbed is the absorbed heat ux from the re, expressed in Btu/

h ft2 (W/m2);s is the StefaneBoltzmann constant 0.1713 108 Btu/

h ft2 R4 (5.67 108 W/m2 K4);a is the equipment absorptivity, dimensionless;

e Process Industries 27 (2014) 21e31 23surfacere is the re emissivity, dimensionless;

gallon (125 m3) LPG tank car was exposed to a semi-enclosed poolre where the pool re and tank car were located in a pit, withembankments on all sides exceeding the tank car height (no roof).Fire and wall temperatures versus time at the top of the front andrear walls of the tank car are shown in Fig. 3a and b, respectively.

The analytical method (Equation (3)) was used by the author inan attempt to reproduce the wall temperature versus time. Thereported values in Anderson et al., 1974 were used for the param-eters where given (e.g., re temperature, re emissivity, initialtemperature). There is insufcient technical basis to theoreticallypredict the value of the unknown parameters that would apply tothis specic re due to the re variability. Hence, values for theunspecied parameters were adjusted by trial-and-error until thecalculated timeetemperature prole approximated the wall tem-perature data from the re test shown in Fig. 3b. The values ofparameters selected tomodel tank car rear wall temperature versustime at several locations were:

re 0.62 (determined by BRL)surface 0.5asurface 0.5h 1.76 Btu/h ft2 R (10 W/m2 K)Tgas 2112 R (1652 F) 1173 K (900 C)Tre 2112 R (1652 F) 1173 K (900 C) e see Fig. 3aTsurface Initially @ 529 R (69 F) 294 K (21 C)

asurface Equipment absorptivity 0.3e0.8

in the Process Industries 27 (2014) 21e31surface is the equipment emissivity, dimensionless;Tre is the re temperature, expressed in R (K);Tsurface is the equipment temperature, expressed in R (K);Tgas is the temperature of air/re in contact with the equipment

surface, expressed in R (K);h is the convection heat transfer coefcient of air/re in contact

the equipment, Btu/h ft2 R (W/m2 K);s$asurface$re$T

4re is the radiative heat ux to the equipment;

s$surface$T4surface is the re-radiation from the equipment;

h$(Tgas Tsurface) is the convection heat transfer between thecombustion gases and the equipments surface.

h is the convection heat transfer coefcient of air/re in contactthe equipment, Btu/h ft2 R (W/m2 K);

s$asurface$re$T4re is the radiative heat ux to the equipment;

s$surface$T4surface is the re-radiation from the equipment;

h$(Tgas Tsurface) is the convection heat transfer between thecombustion gases and the equipments surface.

Note that published heat transfer coefcients and emissivitiesare often empirically determined but the test conditions may notaccurately represent the conditions associated with a particularre. Caution must be taken when specifying the parametersbecause a wide range in re heat inputs can result. When applyingthe analytical method to sizing pressure relief devices, the total

h Convective heat transfercoefcient between equipmentand surrounding air

1.76e5.28 Btu/h ft2 R(10e30 W/m2 K)

Tgas Temperature of combustiongases owing over the surface

1392e2112 R (932e1652 F)773e1173 K (500e900 C)

Tre Fire temperature 1572e2292 R (1112e1832 F)873e1273 K (600e1000 C)

Tsurface Equipment temperature Increases as surface heats ups StefaneBoltzmann constant 0.1713 108 Btu/h ft2 R4

(5.67 108 W/m2$K4)qre Fire heat ux e a wider range

is possible9510e31,700 Btu/h ft2

(30e100 kW/m2)qabsorbed Absorbed heat ux at start

of the re7925e23,775 Btu/h ft2

(25e75 kW/m2)Table 2Typical range in analytical method (Equation (3)) parameters for an open pool resurface average heat ux.

Parameter Description Pool re surface averageheat ux parameter range

re Hydrocarbon ame emissivity 0.6e1.0surface Equipment emissivity 0.3e0.8

E. Zamejc / Journal of Loss Prevention24heat input into the vessel shall use the wetted area to the 1.0exponent, not the 0.82 exponent used in the API empirical methodas shown in Equations (1) and (2). The 0.82 exponent is empiricallyderived from re test data as shown in Fig. 2 and has no theoreticalbasis. It is important to note that API 521 provides methods todetermine the re heat input to equipment while it is up to the userto determine the vessel area exposed to a re based on the type,size, conguration and location of their postulated re. Conse-quence modeling is outside the scope of API 521.

Application of the analytical model to pool res is discussed inSection 4.0 Application of the analytical method to modeling jetres is given in Salater and Overa, (2004).

4. Application of the analytical method to model recent poolre tests

4.1. Use of the analytical method to model the Ballistics ResearchLaboratory (BRL) pool re test wall temperature versus time

In 1974, a pool re test was performed by the Ballistics ResearchLaboratory (Anderson, Townsend, Zook, & Cowgill, 1974). A 33,000Fig. 3. (a). Ballistics Research Laboratory pool re test data illustrating re tempera-ture versus time at the top of the front and rear walls of a rail tank car. (b). Ballistics

Research Laboratory pool re test data illustrating rail tank car wall temperatureversus time at the top of the front and rear walls.

qre Maximum of 16,700 Btu/h ft2 (52.7 kW/m2) (calculated)qabsorbedMaximumof 9560 Btu/h ft2 (30.2 kW/m2) (calculated)

qre is calculated by setting surface 0, asurface 1. The maximumabsorbed heat ux is predicted by the analytical model to occur atthe start of re when the equipment is at ambient temperature. Asimilar set of values was used by the author to predict the front walltemperature versus time with the exception that the re and gastemperature was set at 1472 F (800 C). Other combinations ofvalues in the analytical model can be used that may provide anequal or better t to the test data. Note that, because the vessel wasengulfed in the pool re, the gas temperature should be set equal tothe re temperature. The gas temperature will be lower than there temperature for non-engulng pool res. Also, the re and gastemperatures were assumed to be constant throughout the poolre.

A comparison of the analytical model with the wall tempera-tures recorded during the test is shown in Fig. 4. The analyticalmethod provides a reasonable approximation to the observed rearwall temperature versus time. The leveling off of the front walltemperature at about 800 F (425 C) as observed in the test (see

Figs. 10 and 11, respectively. It is important to note that failureoccurred before the pressure reached the pressure relief deviceopening pressure. A discussion of this failure as it relates todepressuring system design is given in Section 6.3.

Because of the wide pool re and wall temperature rangesshown in Figs. 10 and 11, a single set of values for the parameters inthe analytical method would not predict all variations. Only tem-perature data was given in the report. As in the BRL comparisons,

Fig. 5. BAM pool re test setup involving propane rail tank car (Balke et al., 1999;Ludwig & Heller, 1999).

Embankment dimensions: 60 50 6 m (197 164 20 ft) Tank car capacity 12,000 gallons (45.36 m3) TeT

E. Zamejc / Journal of Loss Prevention in the Process Industries 27 (2014) 21e31 25Fig. 3b) cannot be approximated with a single set of parameters,indicating that one or more parameters changed during the courseof the re. Once the temperature versus time prole is approxi-mated by the analytical method, then the resultant vessel heatinput (i.e., qabsorbed) can be determined. The analytical method willindicate the maximum heat input is at the start of the pool rewhere the vessel wall temperature is the lowest unless parameterschange during the course of the re or there was an instrumentfault.

4.2. Use of the analytical method to model the Federal Institute forMaterials Research and Testing (BAM) pool re test (Balke, Heller,Konersmann, & Ludwig, 1999; Ludwig & Heller, 1999) walltemperature versus time

In 1999, a full scale pool re test was performed by the FederalInstitute for Materials Research and Testing (BAM or Bundesanstaltfr Materialforschung und -prfung) in Germany (Balke et al., 1999;Ludwig & Heller, 1999). The test evaluated and compared reexposure effects on a rail tank car containing propane and a Castorcar used to transport radioactive material. The test setup is shownFig. 4. Comparison of rail tank car wall temperature versus time between theanalytical model and Ballistics Research Laboratory pool re test data.in Figs. 5 and 6. Although the tank car was semi-conned by em-bankments on 3 sides, a light to calm northerly wind was still ableto signicantly affect the pool re exposure of the tank car asshown in Fig. 7. The tank car maximum pressure reached 25 bar(362 psig) about 15 min after the start of the pool re at which timethe tank car ruptured, resulting in a boiling liquid expanding vaporexplosion (BLEVE). The BLEVE aftermath is shown in Fig. 8. Pool reame/gas and tank car wall temperatures versus time at the variouslocations around the tank car, as shown in Fig. 9, are illustrated in

length 5.95 m; (19.5 ft) Diameter 2.9 m (9.5 ft) Tank car test pressure 28 bar (406 psia) Tank car contained liquid propane Fuel oil pool re in troughs under the tank car and Castorcontainer

Castor container is used to store and transport radioactive ma-terials and was tested along with the tank car.Fig. 6. BAM pool re test setup involving propane rail tank car (Balke et al., 1999;Ludwig & Heller, 1999).

test data. The rail tank car was lled with about 2650 gallons(10 m3) of 95% liquid propane, resulting in an initial wetted surfacearea of about 249 ft2 (23.16 m2). The pool re heat input deter-mined by the author for the empirical method without adequatedrainage (see Equation (2)), and the analytical method for severallocations around the tank car are given in Table 4. There are twolocations in the rear of the tank car where the analytical methodindicated higher heat inputs than the empirical method. However,when averaged across the entire tank car, as one should do if sizinga pressure relief device, the empirical method resulted in about 30%more heat input than the analytical method.

5.2. Comparison with pool re heat input based on BAM re testliquid sensible heating

Test data on the sensible heating of the propane liquid wasobtained during the BAM test. This data can be used as an inde-pendent means to determine pool re heat input during the BAM

E. Zamejc / Journal of Loss Prevention in the Process Industries 27 (2014) 21e3126unspecied parameters had to be determined by the author bytrial-and-error whereby values were selected so the calculatedtime-versus-temperature prole matched the re test data as closeas possible. Table 3 illustrates the values of parameters selected tomodel tank car wall temperature versus time at two locations.Other combinations of values in the analytical model can be usedthat may provide an equal or better t to the test data. It should benoted that a transient approach to the analytical method, where there and gas temperatures were varied with time based on the testdata shown in Fig. 10, was evaluated; however, did not appear tosignicantly improve the t with the test data.

A comparison of the analytical model with the wall tempera-tures recorded during the test, shown in Fig. 12, indicates theanalytical method can provide reasonable approximations to walltemperatures versus time if parameter values are empiricallydetermined by selecting those that would provide a temperature-time prole similar to the test data.

5. Comparison of the pool re heat inputs between theempirical method, the analytical method and pool re testdata

5.1. Comparison with pool re heat input based on BAM time-versus-temperature test data

Fig. 7. BAM pool re test (Balke et al., 1999; Ludwig & Heller, 1999). Note: taken nearend of test (calm wind speed).The pool re heat input determined by the empirical methodand the analytical method can be compared using the BAMpool re

Fig. 8. BAM pool re test e BLEVE aftermath (Balke et al., 1999; Ludwig & Heller, 1999).test. Note that the pressure did not reach the pressure relief deviceopening pressure prior to rail tank car failure during the BAM test.The test data indicated an average temperature rise of 8.06 F/min(4.48 C/min). Hence, the calculated total heat input due to sensibleheating of the liquid is about 3.805 106 BTU/h (1115 kW). Forcomparison, the empirical method (assuming inadequate drainage)predicted a total heat input of 3.18 106 BTU/h (933 kW) perTable 4. This is roughly the same as the analytical approach usingonly the averaged tank car rear temperature data. A possible reasonfor these differences is discussed below.

Liquid swelling as the liquid heats up would increase wettedsurface area; however, the temperature did not increase enoughduring the test for it to explain the difference between the test dataand the empirical and analytical methods. A likely explanation isthat the embankment on three sides of the tank car heated upduring the re and caused higher heat uxes due to re-radiation,preheating of combustion air, and enhanced heat transfer. Indeed,the re should be classied as semi-conned because the height ofthe embankment walls exceeded the height of the tank car. In suchcases, the API 521 empirical method (Equations (1) and (2)) doesnot directly apply. However, the equations can bemodied by usinga wetted surface area (Aw) exponent of 1.0 instead of 0.82. Thiswould be appropriate in scenarios where the pool re amesdirectly and continuously contact all of the wetted surfaces.Applying this to the BAM test rail tank car assuming 50% of the railtank car is partially conned due to the embankments on threesides results in the Equation (4):Fig. 9. BAM pool re test e Temperatures measurement locations (Balke et al., 1999;Ludwig & Heller, 1999).

E. Zamejc / Journal of Loss Prevention in thQ API modified empirical method

70;900*hAw confined1:0 Aw open0:82

i(4)

Q API 70;900*h11:581:0 11:580:82

i

1:349 106Watts 4:604 106BTU=hThis is a conservative estimateof the totalheat input as compared

with the 3.805 106 BTU/h (1115 kW) determined from liquidsensible heating. Based on the test data, the analytical methodshould use the rear averaged heat input predicted by the analyticalmodel (i.e., 3.75 106 BTU/h (1098 kW)) to obtain a reasonableapproximation. Where validating data is unavailable, the highestheat input obtained from the analytical model should be used.

Fig. 10. BAM pool re test e Fire temperatures versus time (Balke et al., 1999; Ludwig& Heller, 1999). Note 1: Time 0 is when gasoline starter uid in a small plasticcontainer was ignited. The main pool re started about 100 s later when the plasticcontainer failed and spilled burning gasoline into the fuel oil pool. Note 2: The un-marked temperature curves were primarily in the front of the tank car (upwindlocation and without an adjacent embankment).The re relief load can be determined by dividing the re heatinput by the heat of vaporization of the uid at relieving pressure.

These results indicate that the API empirical method can beapplied to some semi-conned congurations, where adjacentembankments exceed the vessel height, by using a wetted areaexponent of 1.0 instead of 0.82 for the portion of the vessel adjacentto the embankment. This would not apply to completely conned

Fig. 11. BAM pool re test e Tank car wall temperatures versus time (Balke et al., 1999;Ludwig & Heller, 1999).situations (e.g., enclosed buildings or structures with a roof) whichwould require special modeling. Note that the empirical method isbased on full-scale test data, not theory. Hence, this adjustment ofthe wetted exponent for partially conned vessels based on testdata is consistent with the origin of the empirical method.

5.3. Comparison with pool re heat input based on the BRL test

TheBRLtestobtaineddataon the relievingrateversus time,whichwas compared with that obtained with the empirical and analyticalmethods. A transient approach was used by the author in thesemethodswhereby the relief rate was variedwith time to correspondto the decrease inwetted area asuid is relieved.A comparison of theactual relief rate and that predicted by the empirical and analyticalmethods is given inFig.13. Both theempirical andanalyticalmethodspredicted a decrease in relief rate with time because thewetted areais decreasingasuid is relieved.However, the test indicated the reliefrate actually increased with time.

One explanation that can increase the relief rate versus time isthat there was an increase in heat ux with time due to heating ofthe surroundings. Adjustments to the analytical methodweremadeto account for enhanced heat transfer due to heat-up of the sur-rounding embankments during the test. Fig. 13 illustrates a modi-ed analytical method where the convective heat transfercoefcient and the vessel absorptivity were increased by 20% every2.5 min with a limit of 1.0 for the absorptivity. These were empir-ically determined by trial-and-error so predicted relief rate versustime matched the re test data as close as possible. This should beconsidered an example of adjustments that can be made to adjustthe model to t test data, but they may not represent actual con-ditions nor be applicable to other res. Note that the re and gastemperature were not adjusted because the data showed the retemperature to slightly decrease during the test.

Instead of modifying the analytical method, the empiricalmethod can be adjusted to account for the apparent increased heatinput with time by increasing the exponent on the wetted areaversus time. This effect can be illustrated by inserting the heat inputdetermined from the actual relief rate and the wetted area in theempirical method for inadequate drainage; the equation is thensolved for the wetted area exponent versus time. The results,shown in Fig. 14, indicate the wetted area exponent approaches 1.0toward the end of the test. This suggests that ame contact with theentire vessel surfaces increases with time. Using a wetted areaexponent of 1.0 for the entire vessel (located in a pit with em-bankments exceeding the vessel height on all sides), with theempirical method, would provide a conservative pressure reliefsystem design.

5.4. Application to pressure relief device sizing

The analytical model can be used to estimate the time-versus-temperature proles of unwetted walls of vessel exposed to poolres. API 521 provides recommended values for the parameters inthis model that should provide a reasonable approximation for awide range of pool res. However, the comparisons with full-scalere tests indicate the model parameters would need empiricaladjustment based on actual full-scale test data in order to moreclosely predict a specic pool re behavior. Unfortunately, thestate-of-the art is insufcient to allow a validated theoretical basisfor adjustment that would apply to pool res in general. Hence, theuser is cautioned when applying the analytical model to pressurerelief device sizing for the re exposure scenario because improperselection of the parameter values can result in underpredicting there heat input to a vessel and consequent undersizing of the

e Process Industries 27 (2014) 21e31 27pressure relief device. Where validating re test data is unavailable,

the user should use the API 521 empirical method for sizing pres-sure relief devices for the pool re scenario.

Table 3Analytical method parameters used to model tank car wall temperature versus timeat two locations of the BAM pool re test tank car.

Parameter Rear center Front right

Fire temperature, F (C) 1832 (1000) 482 (250)Gas temperature, F (C) 1832 (1000) 482 (250)Convective heat transferCoefcient, BTU/h ft2 R (W/m2 K)

3.52 (20) 3.52 (20)

Fire emissivity 0.6 0.6Metal emissivity 0.5 0.4Metal absorptivity 0.5 0.4The following were calculated using the values above:Calculated initial incident heat ux,

BTU/h ft2 (kW/m2)34,570 (109.0) 2260 (7.1)

Calculated maximum absorbed heat ux,BTU/h ft2 (kW/m2)

20,321 (64.1) 1721 (5.4)

E. Zamejc / Journal of Loss Prevention in th28The comparisons in the previous sections indicate the 50 yearold empiricalmethod inAPI 521given inEquations (1) and (2)wouldprovide a reasonable approximation of unconned pool re heatinput to a vessel but would need adjustment for cases where thevessel is partially conned. If the vessel is partially conned, the BRLfull-scale test indicates a conservative size for a pressure relief de-vice can be obtained using the heat input fromEquation (2) butwitha wetted area exponent of 1.0 instead of the 0.82 exponent valid forunconned pool res as shown in Equation (4).

API 521 does not provide any recommendations regardingcompletely conned pool res. In the case of completely connedpool res, there is no full scale test data available so it cannot bedetermined if theAPI 521 empiricalmethodswould be conservative.Although the analytical methodmay provide the potential tomodelcompletely conned pool res, selection of the values for the modelparameters would need to consider their variationwith time as there heats the surroundings. This can only be done on a case-by-casebasis. Further, validation tests to support selection of appropriatevalues for the model parameters would be recommended.

6. Application of the analytical method to depressuringsystem design

6.1. Effect of overheating unwetted metal plates

Unwetted metal surfaces are not cooled by boiling liquid insidethe vessel. Hence, the metal temperature can get high enough suchFig. 12. Comparison of rail tank car wall temperature versus time between theanalytical model and BAM test data.that metals such as carbon steel lose signicant strength. Table 5illustrates the effect of high temperatures on the tensile strengthof carbon steel and 304 stainless steel. The loss of strength due topool or jet re exposure could exceed the safety factor used in thedesign of the vessels, thereby resulting in vessel rupture due tooverheating, rather than overpressure.

The specic pressure vessel design code and material used willdetermine the appropriate safety factor to use in the vessel design.For example, the current edition of ASME Section VIII, Division 1Pressure Vessel Design Code (ASME Section VIII) includes a safetyfactor (now termed design margin) of 3.5 between the tensilestrength of the vessel and the allowable stress at room temperaturefor materials in which the tensile strength governs (e.g., carbonsteel). For carbon steel, the safety factor implies the design pressureis a minimum of 3.5 times the burst pressure (assuming the weaklink in the vessel is the wall plate, there are no imperfections in thewall, etc.). It should be noted that carbon steel vessels constructedto pre-1999 versions of the ASME Section VIII, Div. 1 code used asafety factor of 4.

ASME Section VIII, Division 1, UG-27 provides equations thatrelate the allowable stress, vessel design pressure and wall thick-ness. In the case of circumferential stress for a cylindrical shell,Equation (5) applies in many cases:

P S*E*t=R 0:6*t (5)

where:P must be < 0.385 *S*E.

P internal design pressure, psi (MPa)E joint efciency 1.0 for full X-ray.S maximum allowable stress value, psi (MPa)t minimum thickness of the shell, inches (mm)R inside radius of the shell, inches (mm)

For example, at room temperature, ASTM A515 Grade 70 carbonsteel plate has a tensile strength of 70,000 psi (482.6 MPa) (ASMESection II, 2007) therefore, the allowable stress will beS 70,000/3.5 20,000 psi (137.9 MPa). If a vessel fabricated fromthis material and designed to this allowable stress is heated to1200 F (650 C), the tensile strength will decrease to 20,000 psi(137.9 MPa), as shown in Table 5. In other words, the materialstrength is reduced to the equivalent of a zero safety factor. Vesselrupture would be a certainty if the pressure then exceeded thedesign pressure because the loads on the vessel would exceed thetensile strength. Rupture would occur at even lower internalpressures if there are other coincidental loadings on the vessel(such as the weight of the vessel and attached equipment, tem-perature gradients, static head, internals, etc.) or defects in thevessel. In all these cases, a pressure relief valve would not provideprotection because it is typically designed to reseat (i.e., close)when the pressure decreases to about 93% of its set pressure forvapor trim valves. This would thereby maintain the vessel pressurenear its design pressure. Instead of a pressure relief valve, adepressuring system can be used to provide vessel protection, or atleast mitigation of the effects of failure.

6.2. Depressuring criteria

In order to be effective, the depressuring system needs todepressure at a high enough rate to compensate for the loss ofstrength as the vessel heats up. The vessel heat up rate is dependenton the type of re, materials of construction, and wall thickness.

API Std. 521 Fig. 1 illustrates the heat-up of carbon steel plates of

e Process Industries 27 (2014) 21e31several thicknesses in an open pool re (ANSI/API Standard 521,

2013). One curve (Plate 2) was obtained from open pool re testdata while the others were extrapolated based on the test data.Combining these temperature-versus-time curves along with thetensile strength data shown in Table 5 will allow determination of aminimum depressuring rate to keep the pressure below the tensilestrength of the vessel. An appropriate safety factor should beconsidered given the uncertainties. Results obtained by the author,applying a 25% safety factor (i.e., Table 5 tensile strengths weremultiplied by 0.75), are shown in Fig. 15. The depressuring prolefor a specic wall thickness needs to stay to the left of the speciccurve shown in Fig. 15. Failure will occur if the depressuring proleeither intersects or is on the right side of the curve for the thickness

6.3. Application of the analytical method to depressuring systemdesign

The analytical method can be used to extend the curves in Fig.15to other wall thicknesses. The analytical method along with the

Table 4Pool re heat inputs using the empirical and analytical method along with BAM re test data.

Location Max re heat ux,BTU/ft2 h (kW/m2)

Max absorbed heat ux,BTU/ft2 h (kW/m2)

Aw exponent Total heat input, BTU/h (kW) % Of API

Analytical e Rear center 34,560 (109) 20,330 (64.1) 1 5.07 106 (1485) 159%Analytical e Front right 2260 (7.1) 1720 (5.4) 1 0.43 106 (126) 14%Analytical e Rear average (Note 1) 24,350 (76.79) 15,040 (47.41) 1 3.75 106 (1098) 118%Analytical e Front average (Note 1) 6650 (20.97) 4620 (14.57) 1 1.15 106 (337) 36%Analytical e Total average 15,500 (48.88) 9830 (30.99) 1 2.45 106 (718) 77%Empirical method N/A 34,500 (70.9) (Note 2) 0.82 3.18 106 (933) 100%

Note 1: Average of left, center and right locations.Note 2: The API maximum absorbed heat ux has units of BTU/h/[(ft2)0.82] or kW/[(m2)0.82].

E. Zamejc / Journal of Loss Prevention in the Process Industries 27 (2014) 21e31 29in question. As noted in the previous section, failure can occur ateven lower pressures, depending upon the amount of additionalloads on the vessel.

An often used criteria for depressuring is to depressure to 50% ofthe design pressure in 15 min. As shown in Fig. 15, this would beappropriate for open pool re exposure of vessels whose wallthickness is 1 inch or greater. A second criteria often used is todepressure to 100 psig (6.90 barg) in 15 min. This is generally moreconservative given the high pressures involved in most depres-suring applications. The more stringent criteria, (almost always thelatter) would be preferred when protecting against jet reexposure.Fig. 13. Empirical and analytical method calculated relief rates versus BRL test data.parameters determined in Section 6.2, for example, can be set up ina spreadsheet as a transient model in which the wall temperaturechangewith time is calculated. At each time interval, the metal wallmass can be conservatively assumed to absorb all of the heat input,thereby increasing the wall temperature. The effect of wall thick-ness is accounted by the metal mass. This temperature-versus timeprole is then combinedwith tensile strength data as in Section 6.2.

For example, the BAM pool re test data indicated failure of therail tank car occurred at rear center wall (in unwetted zone) (Balkeet al., 1999; Ludwig & Heller, 1999). Test data further indicated thewall temperature ranged from 1020 to 1200 F (550e650 C), but itis possible that local temperatures got even higher because tem-perature was recorded only at a few locations. Failure occurred15 min after the start of the pool re, or about 10 min after the retemperature reached about 1832 F (1000 C). The rail tank car wallthickness was 0.59 inches (14.9 mm) and the material of con-structionwas assumed to be comparable to carbon steel. The failurepressure of 362 psig (25 bar) was slightly lower than the testpressure of 406 psig (28 bar). The Rear Center parameters wereused in the analytical model to predict the time-versus-temperature prole. This was combined with the tensile strengthand stress rupture data by the author to obtain the depressuringprole shown in Fig. 16. In order to minimize the potential forrupture due to overheating, a depressuring system would need tostay to the left of the curve shown in Fig. 16. Because the pressure atfailurewas slightly lower than the test pressure, Fig.16 predicts thatFig. 14. Empirical method wetted area exponent versus time using BRL test data.

a pool or jet re exposure.

Table 5High temperature tensile strength of carbon steel and 18-8 stainless steel (Wharton,1946).

TemperatureF

TemperatureC

18-8 Stainlesssteel (304, 304L)

Carbon steel(SA-515, SA-516)

Tensilestrengthpsi

TensilestrengthMPa

Tensilestrengthpsi

TensilestrengthMPa

900 482 45,500 313.71000 538 53,000 365.4 36,500 251.71100 593 48,500 334.4 27,200 187.51200 649 43,000 296.5 20,000 137.91300 704 35,000 241.3 13,500 93.11400 760 27,000 186.2 9025 62.21500 816 20,500 141.31600 871 17,650 121.7

Fig. 16. Depressuring prole to minimize failure potential of the rail tank car due tooverheating in the BAM pool re test.

E. Zamejc / Journal of Loss Prevention in the Process Industries 27 (2014) 21e3130failure would occur about 14 min after the start of the pool re,which is a reasonable approximation as failure actually occurredabout 15 min after the main pool re started (see Figs. 10 and 11).Note the rst 2 min of the pool re test is not considered becausethe re was localized to a small igniter assembly that did not causeany signicant increase in rail tank temperatures.

6.4. Effect of material of construction

The material of construction can signicantly affect thedepressuring requirements. The preceding sections discussed car-bon steel vessels. As shown in Table 5, 304 stainless steel is superiorto carbon steel regarding high temperature effects on tensilestrength. A comparison of the depressuring proles to minimizethe potential for failure of a inch wall thickness carbon steelvessel and a inchwall thickness stainless steel vessel is illustratedin Fig. 17. The depressuring system pressure-versus time prolewould need to stay to the left of the applicable curve. These resultsindicate that the depressuring system for the stainless steel vesselwould require a signicantly lower depressuring rate than for thecarbon steel vessel of comparable wall thickness. This method canbe extended to other materials provided tensile strength data athigh temperature is available.

7. Conclusions

The increasingly widespread use of analytical methods toevaluate re exposure of equipment prompted the API Std. 521Fig. 15. Reduction of carbon steel plate tensile strength versus time due to open poolre exposure.committee to include an analytical method in the 6th edition asan alternative to the existing empirical method. The analyticalmethod provides more exibility than the empirical method buthas limitations (e.g., too many permutations are possible). API521 provides recommended values that would apply to manyopen pool res. However, the comparisons with full-scale poolre tests indicate that caution needs to be taken when selectingthe values; otherwise, the re heat input can be underestimatedresulting in an undersized pressure relief device. Where uncer-tain, the values selected in the analytical model should be vali-dated with testing.

More recent pool re test data indicates the 50 year old API521 empirical method will provide a conservative estimate of poolre heat input for most applications and is still the method ofchoice when designing pressure relief systems for an open pool rescenario. However, these recent tests indicate the empiricalmethod needs to be modied when a vessel or equipment ispartially conned by adjacent embankments or walls equal orgreater than the vessel height. In such cases, the wetted areaexponent should be 1.0 instead of 0.82.

The analytical method is useful in determining time-versus-temperature proles for heating unwetted vessels of varying wallthicknesses and materials of construction. These proles can becombined with tensile strength and stress-rupture data to specify adepressuring systems pressure-versus-time prole to minimizefailure and/ormitigate the effects of failure due to overheating fromFig. 17. Depressuring proles to minimize failure potential of a 0.5 inch wall thicknesscarbon steel and stainless steel vessel due to overheating.

References

Anderson, C., Townsend, W., Zook, J., & Cowgill, G. (September 1974). The effects of are environment on a rail tank car lled with LPG. FRA-OR&D Report Number 75e31, PB-241358.

ANSI/API Standard 2000. (November 2009). Venting atmospheric and low-pressurestorage tanks (6th ed.).

ANSI/API Standard 521. (2013). Pressure-relieving and depressuring systems (6th ed.).API Standard 650. (2013). Welded tanks for oil storage (12th ed.).ASME Section II, Part D, materials e Properties. (2007).ASME Section VIII, Division 1, Pressure Vessel Code, 2007 with 2008a Addenda.Balke, C., Heller, W., Konersmann, R., & Ludwig, J. (September 13, 1999). Study of the

failure limits of a railway tank car lled with liqueed petroleum gas subjected toan open pool re test. BAM Final Report.

Energy Institute. (March 2003). Guidelines for the design and protection of pressuresystems to withstand severe res, ISBN 0 85293 279 0.

Heller, F. J. (1983). Safety relief valve sizing: API versus CGA requirements plus anew concept for tank cars. API Rening Proceedings, 82, 123e140.

Ludwig, J., & Heller, W. (1999). Fire test with a propane tank car. BAM Test ReportIII.2/9907.

National Fire Protection Association NFPA 30 (2008): Flammable and CombustibleLiquids Code.

Personal correspondence from H.C. Hottel to L.W.T. Cummings December 12, 1950.Roberts, T. A., Medonos, S., & Shirvill, L. C. (June 2000). Review of the response of

pressurised process vessels and equipment to re attack. Offshore Technologyreport, OTO 2000-051.

Salater, P. Proposed changes to the next revision of API 521. 2006 Presentation to APIPressure Relief Systems Committee.

Salater, P., & Overa, S. J. (March 2004). Pipes exposed to medium sized jet res eRupture conditions and models for predicting time to rupture. Paper presented atFire and Blast Information Group (FABIG), London and Aberdeen, January 2004and Houston.

Salater, P., Overa, S. J., & Kjensjord, E. (September 2002). Size depressurization andrelief devices for pressurized segments exposed to re. Chemical EngineeringProgress, 38.

SCANDPOWER. (March 31, 2004). Guidelines for the protection of pressurised systemsexposed to re. Report no. 27.207.291/R1-Version 2.

Shirvill, L. C. Heat Fluxes in Severe Fires. 2002 Presentation to API Pressure ReliefSystems Committee.

Wharton, H. R. (1946). Digest of steels for high-temperature service. Timken Steel.

E. Zamejc / Journal of Loss Prevention in the Process Industries 27 (2014) 21e31 31

API Standard 521 new alternative method to evaluate fire relief for pressure relief device sizing and depressuring system d ...1 Introduction2 Fire scenario API empirical method2.1 Basis of the API empirical method to evaluate pool fires2.2 Limitations of the API empirical method to evaluate fires

3 Analytical method to evaluate fires4 Application of the analytical method to model recent pool fire tests4.1 Use of the analytical method to model the Ballistics Research Laboratory (BRL) pool fire test wall temperature versus time4.2 Use of the analytical method to model the Federal Institute for Materials Research and Testing (BAM) pool fire test (Ba ...

5 Comparison of the pool fire heat inputs between the empirical method, the analytical method and pool fire test data5.1 Comparison with pool fire heat input based on BAM time-versus-temperature test data5.2 Comparison with pool fire heat input based on BAM fire test liquid sensible heating5.3 Comparison with pool fire heat input based on the BRL test5.4 Application to pressure relief device sizing

6 Application of the analytical method to depressuring system design6.1 Effect of overheating unwetted metal plates6.2 Depressuring criteria6.3 Application of the analytical method to depressuring system design6.4 Effect of material of construction

7 ConclusionsReferences