Experimental Thermal and Fluid Science 34 (2010) 972–978

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Experimental Thermal and Fluid Science

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Experimental studies on the thermal stratification and its influence on BLEVEs

Wensheng Lin *, Yanwu Gong, Ting Gao, Anzhong Gu, Xuesheng LuInstitute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China

a r t i c l e i n f o

Article history:Received 7 February 2010Accepted 9 March 2010

Keywords:StratificationBLEVELPGDepressurization

0894-1777/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.expthermflusci.2010.03.001

* Corresponding author. Tel.: +86 21 34206533; faxE-mail address: linwsh@sjtu.edu.cn (W. Lin).

a b s t r a c t

The thermal stratification of Liquefied Petroleum Gas (LPG) and its effect on the occurrence of the boilingliquid expanding vapor explosion (BLEVE) have been investigated experimentally. Stratifications in liquidand vapor occur when the LPG tank is heated. The degree of the liquid stratification b increases with anincreasing heat flux and decreasing filling ratio. The effect of stratification on the BLEVE has been exam-ined with depressurization tests of LPG. The results show that the pressure recovery for the stratified LPG(b = 1.4) upon sudden depressurization is much lower than that for the isothermal LPG (b = 1). It can beconcluded that the liquid stratification decreases the liquid energy and the occurrence of the BLEVE.

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1. Introduction

The boiling liquid expanding vapor explosion (BLEVE) is a cata-strophic accident resulting from explosive flash of pressure-lique-fied gas (PLG) out of the tank induced by fire. When a tank carryingPLG is subjected to fire impingement, the liquid near the wall heatsup and rises to the top. The pressure in the tank will be higher thanthat calculated by the average liquid temperature because the topliquid dominates the pressure. When the pressure reaches the setvalue, as shown in Fig. 1 (t0–t1), the safety relief valve opens to re-lease the liquid. The upper warmest liquid flows off till the liquid inthe tank becomes isothermal. The relief valve is venting at its max-imum rate to maintain the tank pressure at or near the reliefvalve’s full-open pressure, as illustrated in Fig. 1 (t1–t2). At thesame time, the strength of the tank wall heated by fire decreases.When the wall strength cannot withstand the internal pressure,the tank fails and PLG flashes explosively, which may lead to theBLEVE as depicted by line 1 in Fig. 1 (t2–t3) [1,2]. However, some-times the tank wall is so thin that a BLEVE takes place before thesafety relief valve opens to destratify liquid, as depicted by line 2in Fig. 1.

The energy of PLG is one of the significant parameters for theoccurrence of the BLEVE. There are a lot of studies on the thermalresponse of PLG in tanks impinged by fire. Specifically, Chen et al.[3] developed a computer model to determine the thermal re-sponse of horizontal LPG tanks involved in fire engulfment acci-dents. Yu et al. [4] used an integral approach to investigate thedevelopment of the free convection boundary layer on a heatedconcave surface. Hadjisophocleous et al. [5] employed field and

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zone modeling techniques to study the behaviour of LPG tankand to predict the time for valve opening. Van den Berg et al.[6,7] presented a method to calculate the blast effects originatingfrom an exploding vessel of liquefied gas. Pinhasi et al. [8] devel-oped a 1D plane numerical model to estimate the thermodynamicand the dynamic state of the boiling liquid during a BLEVE event.Abbasi and Abbasi [9] described an attempt to develop a frame-work with which superheat limit temperature (SLT) of new sub-stances can be theoretically determined with fair degree ofconfidence. Their results also showed that the tank response wassignificantly affected by the degree of fire engulfment.

The mechanism of the BLEVE has been studied experimentallyby some researchers as well. For example, Birk and Cunningham[10] investigated the reason why certain ruptures lead to a BLEVErather than a jet-type release. The effects of the liquid temperatureand the filling ratio on the BLEVE process were also analyzed bythem. The influence of vent device on the prevention of BLEVEswas discussed by Sheboko et al. [11]. The failure mode of the vesseland the effect of its thermo-hydraulic state on the BLEVE processwere demonstrated experimentally by Venart et al. [12]. Detaileddata on the thermal response of two 500 gal ASME code propanetanks which were partly (25%) engulfed in a hydrocarbon fire werepresented by Birk et al. [13]. Stawczyk et al. [14] described exper-iments with explosions of small LPG tanks, by which the dynamicsof the process was learned and the hazard factors were deter-mined. Birk et al. [15] claimed that the liquid energy content didnot contribute to the shock overpressures in the near or far fieldby medium scale BLEVE tests. Chen et al. [16] developed a small-scale experiment to investigate the possible processes that couldlead to a BLEVE, and the results suggested that the swelling ofthe two-phase layer was the possible reason for the first over-pres-sure at the top and bottom of the vessel.

Nomenclature

A cross area of the tank (m2)c specific heat (kJ kg�1 K�1)D diameter of orifice (m)f the fraction of liquid flash (–)h height (m)k specific heat ratio (–)m mass (kg)P pressure (Pa)Q energy (kJ)t time (s)T temperature (K)V volume (m3)W equivalent of TNT (kg)

x distance (m)

Greek lettersb the degree of liquid stratification (–)q density (kg m�3)

Subscriptsatm atmosphereL liquidp pressuresat saturationV vaporw wall

W. Lin et al. / Experimental Thermal and Fluid Science 34 (2010) 972–978 973

However, the influence of the liquid stratification on BLEVEs hasseldom been studied so far. The main purpose of this work is toinvestigate experimentally the liquid stratification and its effecton the pressure recovery that may lead to BLEVEs. First, the strat-ification of LPG heated by an electric heater is examined. Next, theinfluence of the depressurization on the temperature stratificationis discussed. Finally, the depressurization tests for the stratifiedand the isothermal LPGs have been carried out to reveal the influ-encing principle of the stratification on the pressure recovery.

2. Liquid energy

An increase in the liquid energy per unit volume of a tank mayincrease the occurrence probability of BLEVEs (see [10]). The liquidenergy can be expressed as follows [17]:

W ¼ 0:024ðP=PatmÞV�

k� 11� Patm

P

� �k�1k

" #; ð1Þ

where W is the equivalent of TNT (kg); Patm and P are the pressuresin the atmosphere and in the tank, respectively; V� can be calculatedas

V� ¼ VV þ V�L; ð2Þ

where VV is the volume of vapor in the tank; V�L is the volume of li-quid that can flash just after the moment of rupture described as:

V�L ¼ VLðfqLo=qVTÞ; ð3Þ

with VL being the actual volume of liquid and qLo and qVT being thedensities of liquid and vapor, respectively. The fraction of liquid thatflashes after depressurization, f, will be [18]:

t / min

P / M

Pa

2 1

t0 t1 t2 t3

Fig. 1. Pressure history of BLEVE.

f ¼ 1� e�CpðT0�TB Þ

Dhv; ð4Þ

where T0 and TB represent the initial and the boiling temperaturesof the liquid, respectively; Cp is the specific heat; and Dhv is the la-tent heat of vaporization.

Eq. (2) is reasonable only to the isothermal PLG. However, theliquid stratification plays an important role in the tank failure pro-cess and the resulted hazards [19]. For the stratified liquid, V�Lshould be:

V�L ¼Z hL

0AðxÞf qLo

qVT

� �dx; ð5Þ

where hL is the height of the liquid, and A(x) is the cross section areaof the tank.

Substituting Eq. (4) into Eq. (5), one can get:

V�L ¼Z hL

0AðxÞ 1� e�

CpðTðxÞ�TB ÞDhv

� �qLo

qVT

� �dx: ð6Þ

Here, T(x) is the liquid temperature depending on the liquidstratification. The degree of the liquid stratification, b, can be de-fined as [19]:

b ¼ PPsat

; ð7Þ

where Psat is the saturation pressure based on the mass averaged li-quid temperature.

From the above discussion, it is clear that the liquid energy de-pends on the degree of the liquid stratification. Hence, the compar-ison of depressurization processes of PLG with different degrees ofthe liquid stratification (b = 1 and b > 1) is useful for studying theinfluence of the stratification on the occurrence of the BLEVE.

3. Experimental apparatus and procedure

The experimental equipment is illustrated in Fig. 2. The testfacility consists of a 0.06 m3 volume tank with venting system, dataacquisition system including internal pressure transducer andthermocouples, external heating and experimental control system.The tank is made of stainless steel with a thickness of 7 mm and adesign pressure of 3.5 MPa. The diameter and the height of the tankare 0.4 m and 0.6 m, respectively. LPG is used as the working fluid.An electric heater that wrapped around the tank is employed as theouter heat impingements (see Fig. 2a). Comparing with the directexposure to fire impingement, the use of the electric heater hastwo main advantages: (1) safer because of less probability of theoccurrence of fire and explosion, (2) controllable and uniform heatflux. The insulation outside the electric heater ensures the efficient

No.5 6(a)

4

No.4

No.3

No.2

No.1

5

7(a)

98 1110

3

2

1(a)

6(b)

12

7(b)

(b)

Fig. 2. Schematic diagram of test facility (1) fast action valve; (2) orifice plate; (3) pressure transducer; (4) pressure gauge; (5) thermocouples; (6a) electric heater (b) heatingrods; (7a) insulation (b) water; (8) FLUKE; (9) computer; (10) data acquisition system; (11) charge amplifier; (12) liquid level meter.

974 W. Lin et al. / Experimental Thermal and Fluid Science 34 (2010) 972–978

energy transfer to the loading. When the heater heats the tank, theliquid near the wall warms up first. Then it becomes less dense andrises up to generate the stratification region. For the stratified LPG,b is greater than 1. Tests at different heat fluxes (4 kW m�2 and10 kW m�2) and filling ratios (85% and 45%) have been conducted.

When the tank pressure reached the set value, the depressuriza-tion tests were initiated by activating a fast action ball valve con-necting to the tank through a vertical vent line. The vent linewas a 50 mm diameter stainless steel tube. The valve closed after2 s. Orifice plates in diameters of 10 mm and 20 mm were placedbetween two flanges connected to the fast action valve and were0.75 m away from the vessel.

In order to study the effect of b on the BLEVE, the depressuriza-tion tests for the isothermal liquid (b = 1) were also performed. Tokeep the LPG at its saturation temperature, a water bath was usedto heat the LPG. As shown in Fig. 2b, the water surrounding the

tank was heated to the set temperature by heating rods, whichare controlled by a temperature controller. After reaching the settemperature for 5 h, the LPG within tank is considered to be in auniform temperature that equals to the water temperature.

Temperatures of the LPG were measured by five T-type thermo-couples with the maximum uncertainty of 0.1 K. These thermocou-ples were located uniformly in an axial distance of 100 mm. Thesignals of temperatures were recorded by a data acquisition system(FLUKE 2620T). The pressure history during the depressurizationprocess was indicated by a piezoelectric medium pressure trans-ducer (CY-YD-205), whose full scale is 30 MPa.

4. Results and discussion

Tests were carried out in three steps. First, the thermal stratifi-cation tests, i.e. the process from t0 to t1 shown in Fig. 1, were

W. Lin et al. / Experimental Thermal and Fluid Science 34 (2010) 972–978 975

conducted. Secondly, the effect of the valve opening (t1–t2) on thestratification was analyzed. Finally, the depressurization tests (t2–t3) for the stratified and the isothermal LPGs were performed toinvestigate the influence of the stratification on the BLEVE.

4.1. Stratification tests (t0–t1)

Stratification tests have been performed at different liquid lev-els (85% and 45%) and heat fluxes (4 kW m�2 and 10 kW m�2).Fig. 3 shows variations of the temperatures at the different loca-tions. T1 to T5 are the temperatures recorded by thermocouplesfrom No. 1 to No. 5, respectively (see Fig. 2). The interface betweenliquid and vapor is located between thermocouples No. 4 and No. 5at 85% filling ratio, while the interface is between No. 2 and No. 3 at45% filling ratio. It can be observed from Fig. 3a that the tempera-ture at the top of the liquid (T4) rises first and T1 rises last. Theseindicate that the liquid stratification forms firstly at the top regionand grows gradually to the bottom. The temperature gradients aredifferent at different positions. When the pressure reaches2.3 MPa, the temperature differences between T1 and T2, T2 andT3, and T3 and T4 are 26.1 K, 9.8 K, and 4.3 K, respectively. The ther-mal stratification can also be seen in Fig. 3b, i.e., the slope of T2 isgreater than that of T1. The stratification is more obvious in the va-por region due to the higher boundary velocity and the largerboundary thickness.

0 5 10 15 20 25290

300

310

320

330

340

350

(a) 85%,10kWm-2 T1

T2

T3

T4

T5

T / K

t / min

0 5 10 15 20 25 30280

300

320

340

360

380

400

(b) 45%,10kWm-2 T1

T2

T3

T4

T5

T / K

t / min

Fig. 3. Temperature history.

In Fig. 3, the temperature records show that there are stratifica-tions in both liquid and vapor zones when the tank is heated byouter heating. On trace T4 in Fig. 3a there is a spike whent = 27 min. This can be interpreted as that the vapor flows moreviolently than the liquid. As can also be seen, the liquid tempera-tures (T1 and T2) in Figs. 3b and 4b increase smoothly while the va-por temperatures (T3, T4 and T5) fluctuate although the tank isheated under uniform and unchanged heat flux by the electric hea-ter. If the tank is heated by fire rather than by the electric heater,the temperatures of the vapor will change more greatly and thatwill make the study more difficult.

Fig. 4a depicts variations of the saturation pressures based onthe measured temperatures at 85% filling ratio. psat(T1) representsthe saturation pressure based on Ti (i = 1, 2, 3, 4, 5) and p is themeasured tank pressure. Clearly, p-curve is close to those frompsat(T3) to psat(T4), and its slope is greater than that of psat(T2) andmuch greater than that of psat(T1). These indicate that the upperwarmer liquid dominates the tank pressure.

The effects of the filling ratio and the heat flux on the pressureare shown in Fig. 5. An increase in the heat flux increases the pres-sure-rise-rate because the higher heat flux enhances the naturalconvection and in return brings about the quick increase of the li-quid temperature. Higher filling ratios result in higher rise rates ofthe pressure. This is because the energy through the vapor phase is

0 5 10 15 20 25

0.8

1.2

1.6

2.0

2.4

2.8

(a) 85%,10kWm-2 psat(T1) psat(T2) psat(T3) psat(T4) psat(T5) pp

/ MPa

t / min

0 5 10 15 20 25 300

1

2

3

4

5

(b) 45%,10kWm-2 psat(T1) psat(T2) psat(T3) psat(T4) psat(T5) p

t / min

p / M

Pa

Fig. 4. Comparison between pressures measured and pressures based on thesaturation temperatures.

0 10 20 30 40 50 600.5

1.0

1.5

2.0

85%,4kWm-2

85%,10kWm-2

45%,4kWm-2

45%,10kWm-2

p / M

Pa

t / min

Fig. 5. Pressure history.

0 5 10 15 20 25 30280

290

300

310

320

330

340

350

(a) 60%, 1.70MPa

t / min

T / K

T1

T2

T3

T4

T5

time to open the valve

300

320

340

360

380

400

420

(b) 45%, 2.96MPa

T / K

T1

T2

T3

T4

T5

time to open

976 W. Lin et al. / Experimental Thermal and Fluid Science 34 (2010) 972–978

lower than that through the liquid phase and the heat transport tothe tank at lower filling ratio is lower.

The effects of the heat flux and the filling ratio on the degree ofthe liquid stratification are shown in Fig. 6. As supposed, the higherthe heat flux is, the greater the degree of stratification will be.According to Ref. [20], the heat amount Q and the mass m of theliquid that flows into the stratification region are proportional tox and x8/7, respectively, where x is the height of the liquid. It canbe seen that when the filling ratio increases, the mass in the strat-ification region rises faster than that of the heat amount added. Asa result, the degree of the liquid stratification decreases.

0 10 20 30 40 50280

t / min

the valve

Fig. 7. Temperature history during venting.

4.2. De-stratification tests (t1–t2)

After the pressure reaches the set value, the valve is opened toexamine the effect of venting on the liquid stratification. The ori-fice diameter is 10 mm and the heat flux is 10 kW m�2. The tem-perature histories with the filling ratio of 60% and 45% areplotted in Fig. 7a and b, respectively. When the filling ratio is60%, the interface between liquid and vapor is located at a certainposition between thermocouples No. 3 and No. 4. It can be seen

0 10 20 30 40 50 600.9

1.0

1.1

1.2

1.3

1.4

t / min

85%,4kWm-2

85%,10kWm-2

45%,4kWm-2

45%,10kWm-2

Fig. 6. The degree of stratification, b.

from Fig. 7a that after the valve operates, T3 and T2 deceases by23 K and by 16 K, respectively, while T1 increases 3.5 K. Fig. 7bshows that upon depressurization T2 drops by 28 K and T1 in-creases by 7 K. The reason may be that the upper warmer liquidboils earlier than the lower liquid and the orifice is too small to re-lease the sufficient amount of vapor; therefore the tank pressurerises rapidly and further prevents the boiling of the lower liquid.At the same time, the violent boiling of the upper liquid and thecrash of bubbles resulting from the pressure rebound will promotethe mixing of the LPG liquid. So the stratification layer developsdown and the temperature of the lower liquid may accordingly in-crease during the depressurization. Only when the valve is openedfor a long enough period, the lower liquid is cooled due to theevaporation cooling effect. But in our experiments, the valve isclosed 2 s later when the tank pressure has dropped down to thesafety value. So the temperature of the lower liquid increasesand the degree of the liquid stratification b decreases. When thepressure rises again to the set value, the average temperature willbecome higher because the temperature difference between theupper and lower liquids decreases. The liquid energy in the tankis related to both the temperature and mass. As a result, althoughthe liquid mass decreases, the liquid energy may increase if theaverage temperature increases. It is expected that temperaturesbecome uniform if the valve opens for a long time and results ina decreasing b.

0.6 0.8 1.0 1.2 1.4 1.61.0

1.5

2.0

2.5

3.0

3.5

4.0

(a) D=10mm =1.4 =1.0

p / M

Pa

t / s

1.0 1.2 1.4 1.6 1.8 2.01.0

1.5

2.0

2.5

3.0

3.5

4.0

(b) D=20mm

t / s

p / M

Pa

=1.3 =1.0

Fig. 8. Pressure history during depressurization.

W. Lin et al. / Experimental Thermal and Fluid Science 34 (2010) 972–978 977

It can also be seen from Fig. 7 that after the valve closes againthe temperatures of vapor rise faster than those of liquid. This isbecause the flow of vapor and liquid enhances the convection heattransfer between the LPG and the wall and the specific heat of va-por is lower than that of liquid.

4.3. Depressurization tests (t2–t3)

The above results show that the liquid temperature may be-come uniform (b � 1) if the valve keeps open before the occurrenceof a BLEVE. The following two groups of comparative tests wereconducted to study the effects of the liquid stratification and de-stratification on the pressure variation when LPG was suddenlydepressurized.

The stratified and isothermal LPGs are obtained by the appara-tus shown in Fig. 2a and b, respectively. When the pressure reachesthe set value, the depressurization test is initiated via opening thefast action valve. The initial conditions are listed in Table 1. It canbe seen that the energy of the isothermal LPG is greater than that ofthe stratified LPG.

Fig. 8a illustrates the pressure history for the stratified LPG. Thedepressurization orifice is 10 mm in diameter. The associated pres-sure and temperature histories for the stratified LPG before depres-surization are shown in Figs. 3b and 4b. When the stratified liquid(b = 1.4) is depressurized suddenly, the pressure drops down at0.48 MPa s�1 without obvious pressure recovery. But for the uni-form LPG (b = 1.0) whose liquid energy is greater than the stratifi-cation liquid, the pressure drops from 2.20 MPa to 2.05 MPa, andwill rebound 0.05 s later. The maximum pressure recovery(2.55 MPa) exceeds the initial pressure about 0.35 MPa, whichcan lead to the development of the crack and the failure of the tank.

The orifice diameter shown in Fig. 8b is 20 mm. The depressur-ization test for the stratified LPG shows that upon the depressur-ization there is a little oscillation and then the pressuredecreases smoothly from 2.96 MPa to 1.73 MPa in 0.767 s. Thedepressurization test starts for the isothermal LPG (b = 1.0) whenthe pressure reaches 3.05 MPa. During the test, the pressure dropsdown to 2.93 MPa in 0.04 s and then increases. The maximumrecovery pressure can reach 3.45 MPa, which is 0.40 MPa higherthan the initial pressure. This means that the recovery pressureafter the sudden depressurization can be a great threat to the tankbecause its strength has decreased under fire.

From the above two figures, it can be seen that the pressure his-tories upon rapid depressurization for the stratified LPG are differ-ent from those for the isothermal LPG. The explanation can begiven as follows:

(1) The stratification of vapor reduces the rate of depressuriza-tion. When the LPG is suddenly exposed to the atmosphere,the pressure decreases first. Before the liquid reaches a cer-tain superheat degree, the depressurization rate is dedicatedby the hot vapor. Fig. 3b shows that the stratification occursboth in the liquid and in the vapor. The vapor is superheatedand its maximum temperature will be 150 K higher thanthat of liquid. Under the same pressure and orifice diameter,an increase in the vapor temperature reduces the depressur-ization rate. In the first group, the depressurization rates for

Table 1The initial test conditions.

D (mm) b Ti (K) P0 (MPa) Hi (mm) WL (kg)

(a) 10 1.4 297.3–343.2 2.26 265 (45%) 0.01631 333.7 2.20 264 0.0192

(b) 20 1.3 312.0–360.5 2.96 235 (40%) 0.01991 353.2 3.05 217 0.0271

the stratified and isothermal LPGs are 1.75 MPa s�1 and8 MPa s�1, respectively. The experiments conducted by Ven-art and Ramier [21] showed that the liquid–vapor interfacerose up and a rapidly rising two-phase swell developeddue to the boiling after the liquid was depressurized sud-denly. Rapid nucleation and growth of bubbles within theincreasingly superheat liquid further accelerate the risingof the two-phase swell. The fluid in the two-phase swellbegins to exit from the vessel only after it has impactedthe top of the vessel and causes an impulse, which is like awater hammer, to the tank head. Reid [22] suggested thatthe higher depressurization rate causes higher superheatdegree for the liquid. Therefore, the liquid at high depressur-ization rate will reach a high degree of superheat and itspressure rebounds more violently.

(2) Comparing with the isothermal liquid, the stratified liquid atthe same pressure contains low energy WL (as shown inTable 1) because the upper liquid at higher temperature dic-tates the pressure. Low liquid energy cannot induce pro-nounced pressure rebound. In Fig. 8a, two groups of testshave near pressures (2.26 MPa and 2.20 MPa) and fillingratio (45%) but the different stratification degrees (b = 1.4and b = 1.0) and the different liquid energy (0.0163 kg TNTand 0.0192 kg TNT). That results in weaker pressure reboundof the former test than that of the later.

978 W. Lin et al. / Experimental Thermal and Fluid Science 34 (2010) 972–978

It can be concluded that an increase in the degree of the liquidstratification reduces the liquid energy and the pressure recoveryand will cause the possibility for the occurrence of the BLEVE tobe decreased.

5. Conclusion

A series of tests related to the stratification of LPGs in the tankheated by the outer heating have been carried out in this study.The effects of the heat flux and the filling ratio on the stratificationare analyzed. The depressurization tests were also conducted tostudy the influence of the stratification on the pressure recovery.The following conclusions can be drawn from the experimental re-sults: (1) The greater of the heat flux is, the greater the degree ofthe liquid stratification will be. The higher the filling ratio is, thesmaller the degree of the liquid stratification will be. (2) The open-ing of the valve can result in the temperature rise of the lower li-quid and the decrease of the degree of the liquid stratification.(3) The liquid energy of the LPG with a uniform temperature isgreater than that of the stratified LPG. Higher energy may causehigh pressure recovery and increases the probability of the occur-rence of BLEVE.

Acknowledgement

The authors are grateful to the support of China’s National Nat-ural Science Fund (No. 50076024).

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