1995: Ammonia Release from Conservation Vent during Barge ...

Ammonia Release from ConservationVent during Barge Unloading

Discharging liquid into the vapor space of an 18,000-ton storage tank during barge unloadingcreated a vacuum in the tank. As a result, a leak occurred when one of the tank's conservation vents

failed to properly reseat. The mechanisms involved in the vacuumcreation and in the leakage from the conservation vent are explained.

George M. VolkerE.L DuPont de Nemours & Co., Belle, WV 25015

Kenneth W. FariasE.L Du Pont de Nemours & Co., Wilmington, DE 19898

Introduction

The Du Pont Belle, WV plant stores ammonia inan 18,000-tonne cryogenic storage tank, initial-ly designed and installed along with a 900

tonne per day Kellogg ammonia production plant. In1978, DuPont shut down the ammonia productionplant, discontinuing production of ammonia at the site.However, since on-site processes still required rawmaterial ammonia, DuPont converted the tank into aterminal operation supplied with ammonia via barge.

The facilities in support of this converted operationinclude:

• A barge unloading dock and piping• The 18,000-tonne storage tank• Tank deinventory pumps• A vapor compressor and condenser system• A natural gas assisted flare stack• A liquid distribution system including pumps,

heat exchangers, and header.The storage tank is 33.5 m diameter by 36.5 m over-

all height. It contains the ammonia as a cryogenic liq-

uid at -33.3°C at a tank gauge pressure of 0.021-0.034bar. Two 10-in. (254-mm) conservation vents locatedon the roof of the tank provide protection against pres-sure and vacuum excursions. DuPont maintains theinventory in the tank between 3,300 and 9,000 tonne,so that the liquid elevation varies betwen 5.5 and 14.5m. All of the supporting facilities (i.e., flare stack,compressor, distribution pumps, etc.) are located with-in 15 m of the tank.

A distribution pump causes ammonia to flow to plantconsumers through a header pipeline that travels atotal distance of 700 m. Continuous delivery of liquidammonia to the consumers at high utility and withminimum flow interruption is critical to their opera-tions.

The tank inventory is replenished with ammoniaabout once every two months by unloading ammoniafrom barges. Barge pumps transfer the liquid ammoniathrough an unloading hose into an insulated 10-in.(254-mm) unloading pipeline that transfers it to thetank base area 110 m away. At the base of the tank,this unloading pipeline tees into the section of distrib-

AMMONIA TECHNICAL MANUAL 286 1996

ution header that transfers ammonia between the tanklower outlet nozzle and the distribution pump suction(see Figure 1). As a result, during barge unloading,ammonia from the barge flows to the tank and to theconsumers at the same time.

Standard Unloading Conditions andProcedure

When DuPont unloads a barge into the DuPont Bellestorage tank, there are some unique constraints in thefacility design that influence the procedure used:

• Barge unloading is infrequent enough so that theliquid remaining in the unloading pipeline inventorywould have vaporized back into the tank, leaving itwarm and full of vapor.

• Ammonia flows from the barge into the tankthrough the same nozzle that provides material to thesuction of the terminal's distribution system pumps.

• Gas injection into the suction line severely impactsdistribution pump performance, and the ammonia con-sumers cannot tolerate the consequence when thathappens.

To meet these constraints, DuPont developed a pro-cedure that slowly cools and fills the unloading linewith liquid ammonia. This procedure de-inventoriesthe gasses to the top of the storage tank and preventsthem from entering the distribution pump. Once theline is full of liquid and cooled, normal unloading pro-ceeds through the distribution pump suction line.DuPont developed the following steps to accomplishthis:

(1) Open the 2-in. (50-mm) unloading line vaporvent valve to the storage tank.

(2) Slowly open the 10-in. (254-mm) unloadingline/suction line isolation valve.

(3) Start up the barge pump in bypass so that liquidfrom the pump discharge is recirculated back into thebarge.

(4) Very slowly, begin adding liquid from the bargeinto the unloading line by closing back on the bargerecirculation valve. Do this slowly to permit ammoniavapors to pass into the tank through the vapor ventvalve rather than into the distribution pump suction.

(5) Monitor the unloading line check valve for chat-tering, indicating vapor is passing through the valve.

If chattering occurs reduce or shut off the flow fromthe barge pumps.

(6) Monitor an uninsulated low-point drain valve onthe unloading line (near the check valve) for the for-mation of heavy frosting, indicating the presence ofliquid ammonia to this point.

(7) Slowly increase the flow of ammonia from thebarge, continuing to monitor the check valve for chat-tering. If no chattering occurs, close the unloading linevapor vent valve and slowly increase the unloadingrate to normal rate.

Description of Incident

On the morning of April 29, 1994, an ammoniabarge arrived for unloading into the storage tank. Theliquid level in the storage tank was 5 m. The bargecrew that delivered this barge was not the crew thatnormally delivered barges to the plant site, and theywere not completely knowledgeable about the specialprocedure for cool down of the unloading line. TheDuPont operators reviewed the procedure and require-ments with the barge personnel, arranged for them toput their pumps in bypass, and then began introducingammonia into the unloading line.

After 5-10 min, the operators noticed that the linewas cool and full of liquid. This was much quickerthan a normal cool down that required 30-45 min.Coincidentally, the control room received a low-pres-sure alarm on the storage tank. The field operator wentto the local panel and noticed the tank pressure gaugewas indicating 0 bar (the minimum gauge reading). Inresponse, he opened the unloading line/suction lineisolation valve and began unloading ammonia fromthe barge into the storage tank. He did this because herecognized that as the tank began to fill, its pressurewould return to normal.

5-10 min later, 2 area ammonia sensor alarmstripped. They were indicating 30-40 ppm ammonia 25m from the storage tank. The operator immediatelystopped unloading the barge and responded to the areato determine the source of the leak. By using sulfurdioxide (SO2) as a tracer gas, ammonia vapors werefound at a blind flange on an abandoned overflow line.During efforts to stop leakage at this flange, plant per-sonnel located as far away as 600 m from the storage


tank noticed ammonia odors. Additional checks weremade to look for other potential sources of leakage,and as a result we found one of the two conservationvents on top of the storage tank leaking. In response,operators reduced the tank pressure below 0.007 bar tofacilitate repairs of the flange and the conservationvent.

Upon inspection/repair of the flange gasket, no posi-tive, identifiable means of leakage were found on theremoved gasket (such as damage, misalignment, etc.).Upon inspection/repair of the conservation vent, wedetermined that the vacuum vent portion of the conser-vation vent had operated to relieve a negative pressureexcursion in the tank but did not properly reseat whenthe excursion ended (see Figure 2). The mechanicsrepaired the vent, allowing the vacuum pallet to prop-erly rest on the valve seat. After completing therepairs, we believed leaks from the conservation ventwere stopped.

Later that afternoon, the area ammonia sensors againwent into alarm. When the two previous repair loca-tions were checked, a leak was found in the accesscover to the vacuum vent pallet area (see Figure 3).The access cover leak was repaired, this time with noadditional leakage.

Root Cause Identification

Now, to identify the reason for the leak so that wecan prevent recurrence and determine the effectivenessof response capability. We will consider these threeelements:

(1) What caused pressure in the tank to drop belowatmospheric pressure?

(2) Why did the vacuum relief device fail to reseat?(3) Were the equipment, systems, and procedures

adequate to control the hazard?

What Caused Tank Pressure to DropBelow Atmospheric Pressure?

To understand what caused the tank pressure to dropwhen barge unloading began, one must determinewhat the differences were in this particular unloadingoperation (since many previous unloading operationsoccurred without this effect), the condition of the

vapor space in the tank, and the impact barge unload-ing had on that space.

First lets consider the differences in this particularoperation. Observations made during preparations forunloading indicated that the unloading line cooleddown much faster than normal. Investigation deter-mined this was a result of two actions:

(1) Prior to the start of cool down, the operator didnot open the unloading line/suction line isolation valveas specified in the procedure.

(2) The new barge unloading crew did not thorough-ly understand our need to slowly cool the unloadingline. Therefore, they discharged ammonia into the lineduring the initial cool-down period at a much higherrate than normal.

Because of these two differences, liquid ammoniaentered the top of the storage tank through the unload-ing line vapor vent when normally only ammoniavaporized from the cooling line entered. Normal pro-cedures exclude liquid because pressure differentialbetween the unloading line and the open valve to thedistribution pump suction is insufficient to overcomevent line elevation head.

Second, consider the condition of the vapor spaceinside the tank. The liquid level in the tank immediate-ly before the barge unloading process began was fairlylow at 5 m elevation, so the vapor space was large. Atthe liquid and vapor interface, the vapor was at theammonia dew point temperature (-33.3°C). However,there was no circulation or agitation of the tank con-tent, causing the vapor above the liquid surface toremain stagnant. As a result, the vapor space above theliquid surface warmed due to heat transfer through thetank wall above the liquid surface, and due to heattransfer through the tank roof. Therefore, the majorityof vapor in the tank was superheated ammonia vapor.See Appendix 1 for a summary of the initial gas condi-tions.

Third, consider the result of the difference in thisparticular unloading operation on the tank vaporspace. By adding saturated liquid ammonia to the topof the vapor space and dispersing it by free fall, thevapor in the tank changed from superheated vapor tosaturated vapor with a corresponding reduction in it'sspecific volume. This caused a net loss of vapor vol-ume that required replenishment by some combination


of the following items:(1) Vaporization of liquid ammonia added to the top

of the tank (heat supplied by the superheat of thevapor in the top of the tank).

(2) Increased vaporization of liquid at the vapor/liq-uid interface, i.e., top layer of liquid in the tank.

(3) Reduction in the system pressure.(4) Addition of additional vapor from outside into

the tank, such as a vaporizer system or inlet of airthrough a vacuum relief vent.

(5) Collapse of the tank.Calculations show that the volume supplied from the

first item is not enough to offset the decreased specificvolume of the vapors. The second item happens veryslowly due to limited heat- and mass-transfer rates atthe vapor/liquid interface and can therefore beignored. Since the vaporizer system originallydesigned as part of the tank was eliminated when thetank was converted to a terminal operation, ammoniavapor is not available to displace the decreased vol-ume. This means that adding cold liquid ammonia tothe top of the storage tank can result in a combinationof system pressure reduction, air inclusion and/or tankcollapse. (See Appendix 2 for a summary of the calcu-lations that show the impact of liquid addition on thesuperheated vapor in the tank.)

Why Did Vacuum Relief Device Fail toReseat?

During the repair and inspection of the vent, the vac-uum pallet was found in a position where it was notproperly sitting on the valve seat. Also, after therepairs, the vacuum vent access cover plate beganleaking. Both these items were due to poor design ofthe conservation vent itself.

The two 254-mm conservation vents utilized to pro-tect the storage tank from pressure and vacuum excur-sions were the original vents supplied when the tankwas manufactured in 1966. These vents were Shandand Jurs model 4017 (see Figures 2 and 3). The partic-ular design flaw with these vents was the pivot pindesign of the vacuum pallet (see Figure 2). When thevacuum pallet operated, it opened but then becamedisengaged from the pivot pin. Then there was nolonger a mechanism to cause the pallet to return to its

normal position centered on the valve seat. Therefore,it came down off-set from the valve seat and did notadequately seal when the tank pressure returned tonormal.

For inspection/repair of the vacuum vent pallet andseat area, access is through the vacuum vent areacover plate. During this incident, when the conserva-tion vent was first repaired, this cover plate was usedto enter the vacuum vent area and center the vacuumpallet correctly onto the valve seat. In the design of thecover plate only one hold-down bolt and wing nut islocated at the center of the plate (see Figure 3). It isvery difficult to positively assure that the gasket seal-ing surface is properly aligned with the gasket, andthen the single hold-down point in the center of theplate does not provide a sufficient positive sealingforce against the entire gasket. When the mechanicsreplaced the cover plate, it was not perfectly centeredover the gasket surface, providing a source for leak-age. However, it did not leak immediately followingthe repairs since the operators had reduced the tankpressure. Therefore, unknown to the mechanics, therewas not sufficient driving force to force ammoniathrough the off-set cover plate at that time. The coverplate did not begin to leak until much later when nor-mal operating pressure returned to the storage tank.

These vents have been replaced with Protectosealmodel 8540 conservation breather vents (see Figure4). The vacuum pallet rises in a guide and also hasfour posts positioned on the outside of the pallet tohelp keep the pallet centered should it be required torelieve. The vacuum vent access cover plate is sealedwith an O-ring and is held down by three hold-downbolts located at 120° angles on the outside edge of thecover plate assuring a more consistent and positivesealing force at the O-ring location.

Were Equipment, Systems, andProcedures Adequate to Control theHazard?

A primary concern was to determine the capacity ofthe vacuum breaker, since inadequate capacity mayhave resulted in unidentified damage to the inner tankwall.

Manufacturer information for the vacuum vents doc-


UNLOADING LINE VAPOR

DUPONT BELLE PLANTAMMONIA STORAGE TANK

LIQUID LEVEL AT 5 M ELEVATION

f-S L

AMMONIA TOCUSTOMERS

VENT VALVE TO STORAGE TANK

UNLOADING

10 IN UNLOADING PIPELINE

j ßARGE PUMP BYPASS VALVE UNLOADING LINE I SUCTIONLINE CHECK VALVE ANDISOLATION VALVE

0X5:0

DISTRIBUTION SYSTEM PUMPS

Figure 1. DuPont Belle plant ammonia terminal.

ATMOSPHERICPRESSURE

its

Figure 2. Original tank conservation vent design. Figure 3. Shand and Jurs model 4017 vent.


MUCTMWT

Figure 4. Protectoseal model 8540 vent.

uments that each has the capacity to vent 9,900air when subjected to a negative pressure of 1.0 psi(0.069 bar). Since there were 2 identical vacuumbreakers on the tank, they would each be required tovent one-half of the total maximum venting rate calcu-lated in Appendix 2 (or 5,000 m3/h air for each).Vendor flow curves for these devices indicate a tanknegative pressure equal to 0.40 psi (0.027 bar) wouldexist at that flow rate. The design negative pressure ofthe tank is 1.155 psi (0.080 bar), so we confirmed thevacuum vent was sufficient to protect the tank in thissituation.

Of secondary concern was the ability to quicklylocate the leak. One item that was clear from theinvestigation was that the area ammonia sensors locat-ed at various locations around the storage tank provideexcellent detection of ammonia. DuPont utilizes theDrager Polytron ammonia measuring head sensor.There are eight sensors located at ground level at vari-ous locations around the perimeter area of the tank andat barge unloading. Following this incident, we addedtwo additional sensors at the top of the storage tank.The sensors export a 4-20 ma signal to a HoneywellDistributed Control System (DCS) where they aremonitored and alarmed. The alarm level is set at 5ppm. Experience has indicated that the sensors are

very reliable and accurate with moderate routine main-tenance and calibration. However, ammonia sensorsonly tell you that there is a concentration of ammoniaat a specific location where the sensor is positioned; itdoes not tell you where the ammonia is coming from.

Personnel at this site also utilize sulfur dioxide (SO2)canisters to locate leak sources. A valve is opened onthe canister discharging SO2 vapor from a section of 6mm tubing. When the SO2 vapor comes in contactwith ammonia vapor, it reacts, forming a milky whitecloud. As the concentration of ammonia vapor increas-es, the cloud will become more pronounced, but ifthere is a lot of ammonia present in the leak testingarea or if there are complicated air disturbances, it canbe difficult to pinpoint the exact source of the leak.

In this incident, it is theorized that the ammonia leakdetected by the local area sensors was from the leak onthe conservation vent at the top of the tank. Althoughthere may have also been a slight leak on the overflowpipe flange gasket, we think that we were mislead inthe selection of that source due to false indications ofthe SO2 testing due to turbulent air movement aroundthe tank. In either case, everyone assumed the leakwas from the flange gasket and there was no effort totry to locate any other sources of leakage until odorswere noticed elsewhere in the plant.

Due to the height of the storage tank, there is poten-tial for a leak to occur at the top that would then carryaway with the prevailing wind, never being picked upby the area ammonia sensors located at ground level.Therefore, during the incident investigation, a recom-mendation was made (and completed) to installammonia sensors at the top of the storage tank to morequickly identify a leak source from this area if anyshould ever occur in the future.

Summary and Recommendations

A large volume anhydrous ammonia atmosphericpressure tank can contain vapors warmer than theammonia dew point when the liquid level is low, andwhen there is no circulation and minimum disturbanceto the vapor space to remove heat gain from the wallsand roof above the liquid. In this situation, admissionof ambient pressure saturated liquid to the top of thetank may cause rapid pressure reduction. Avoid this by


maintaining the vapor space at the ammonia dew pointtemperature, or by preventing admission of liquid tothe top of the tank, or by installing methods of han-dling the reduced pressure.

A conservation vacuum vent with poor control of thepallet position caused the pallet to reseat improperlyfollowing operation, creating an ammonia leak whenthe tank returned to normal operating pressure. Inaddition, due to poor design, a second leak occurredwhen the vacuum vent's access cover plate was not re-positioned properly following initial repairs. Avoidthese problems by replacing conservation vents thatutilize the pivot pin design of the vacuum pallet withone that has a vertical guided pallet and with a designthat properly seals maintenance access ports.

Although the conservation vacuum vents on theBelle anhydrous ammonia storage tank leaked afteroperating, they were adequate to prevent damage tothe tank. Consider this failure mode when sizing vacu-um breakers for similar service tanks.

Ammonia leak detection is fairly simple, but locat-ing a source location is difficult, complicated by airdisturbances, sensor positioning, and operator skills.Consider installing multiple sensors in positions whereleaks are most likely, consider using portable detectionequipment such as SO2 lecture bombs, and considertraining personnel in the use of these devices.

Appendix 1 : Initial Gas ConditionCalculation

Vapor height (not including roof)= Tank height (straight wall only) - Liquid

height= 30 m-5 m= 25 m

Area of Tank= rir2= H (16.75 m)2= 880 m2

= 880 m2 * 25 m= 22,000 m3

Vapor Volume of Roof= (4/3 O r2 h) / 2

(where h = height of roof above tank wall)= (4/3 O (16.75 m)2 (6 m))/2= 3,500 m3

Total Original Vapor Volume in Storage Tank= 22,000 m3 + 3,500 nv*= 25,500 m3

Vapor Space Intensive Properties Before Liquid isAdded:

Estimated average temp of the vapor space= 0°C

Absolute Tank Pressure= 1.013 bar + 0.021 bar tank gauge= 1.047 bar

Specific Volume= 1.30m3/kg

Unit enthalpy (relative to saturated liquid at-40°C)

= 1,470 J/g

Vapor Space Extensive Properties Before Liquid isAdded:

Mass= (Vapor Volume/Specific Volume)= (25,500 m3/1.300 mVkg)= 19,600 kg

Enthalpy= Specific Enthalpy * Mass= 1,470 J/g* 19,600 kg= 2.88 x 107 u

Vapor Volume (not including roof)= Area * Height


Appendix 2: impact of Liquid onSuperheated Vapor Calculations

Ammonia Intensive Properties at 1 atm

Saturated Liquid Unit Enthalpy= 29J/g

Saturated Vapor Unit Enthalpy= l,400J/g

Show the consequence of adding a certain amount ofsaturated liquid to the vapor with properties describedin Appendix 1. The quantity selected is not important,as long as it is enough to remove the superheat fromthe tank vapor phase. Start with 2,500 Kg.

Extensive Properties of Saturated Liquid Added:

Enthalpy= Specific Enthalpy * Mass= 29 J/g * 2,500 kg= 72,500 kJ

Adiabatically mix the vapor and liquid together and,since there is now good mixing between the liquid andvapor, cause the mixture to come to equilibrium.

Let X = Ib of saturated liquid after mixingLet Y = Ib of saturated vapor after mixing

Mass Balance:X + Y = 2,500 kg + 19,600 kg

Energy Balance:[X (29 J/g)]+ [Y (1,400 J/g)]= 2.88 x!07kJ +72,500 kJ

Solving the Mass and Energy Balance formulaetogether for X and Y yields:

X = 1,480 kg liquid, which will drop to the liquid pool at the bottom of the tank

Y = 20,600 kg of saturated vapor in the vaporspace of the storage tank

Therefore, the mass of ammonia added to the vaporphase is:

(20,600 kg -19,600 kg) = 1,000 kg

This shows that 1,000 kg of saturated liquid is allthat is required to be added to the top of this tank totake all the superheat out of the vapor.

Since we now know the vapor in the tank becamesaturated, determine the resulting change in volume.

Saturated Vapor Intensive Properties at 1 atm

Specific Volume= 1.133 itf/kg

Saturated Vapor Extensive Properties at 1 atm

Volume= Mass * Specific Volume= 20,600 kg* 1.135 m3/kg= 23,400 m3

Show difference between the extensive volume of thesaturated ammonia vapor and the tank:

= Tank volume - Extensive volume of ammonia= 25,500 m3 - 23,400 m3= 2,100m3

How is the volume difference reconciled?

By Reduction in pressure from 1.047 bar to1.013 bar:(VI/V2) = (Pl/P2),SoV2 = V1*(P2/P1)V2 = 25,500 m3 * (1.013 bar /1.047 bar)V2 = 24,700 m3

Volume difference= Tank volume - reduced volume= 25,500 - 24,700= 800 m3

By vaporization from the liquid inventory inside thetank:


Since the event happened very quickly, and thereare known mass- and heat-transfer limitations atthe liquid level, this quantity was determined tobe minimal, and the value approximated at zero.

By Air Addition to the Storage Tank Through theVacuum Breaker.

= Volume to be reconciled - Volume due to pres-sure reduction.

= 2,100m3-800m3= l,300m3

Determine if vacuum breaker capacity is sufficientto admit the required air.

Based on the investigation of this incident, theestimated flow rate of liquid through the 2 in. (50mm) vapor vent line into the top of the storagetank:

= 190L/min.

Saturated Liquid Ammonia Density:= 0.68kg/L

Air inclusion rate required for vacuum breaker:= (Addition rate) (Volume to be reconciled)/= (Minimum quantity to saturate vapor)

= (190 L/min) (0.68 kg/L) (1,300 irf) /1,000

= 168 m3/m= 10,000 m3/h.Paper 5-C

DISCUSSIONS. Ahsenuddin, Engro Chemical, Pakistan: You men-tioned the vacuum breaker. I would consider that to belike a safety relief valve. Do you have any frequencyfor checkup for this system?Volker: Yes, we do inspect the vacuum breaker on aroutine basis.Ahsenuddin: As a permanent solution, you shouldhave some sort of an isolation scheme of this valve,and then you can have a more reliable calibrationschedule on this.Volker: Yes, we do have an isolation valve for thevacuum breaker and isolation vent with a system toassure that the isolation valve is open when the vacu-um breaker is in service. The isolation valve is used sothat we can remove the conservation vent from thetank every 2 years for inspection and calibration in ashop controlled environment.Stalin, SPIC: About the capacity control system of thecompressor for the storage tank, I am not sure of thetype of system that you have. I guess, when the tankpressure is coming down, the compressor is initiallyunloading . Once the compressor is fully unloaded and

if the tank pressure continues to fall, you may have asystem of sending the vapors back to the tank withoutcondensing. Thus, the tank can be protected fromentering into vacuum. This system is normally fol-lowed in our ammonia storage terminal.Volker: Are you considering that the negative pressuremay be made more severe by the action of the com-pressor?Stalin: No. What I am saying is as the tank pressure iscoming down, normally, the refrigeration compressorwill be gradually unloading. Once the compressor isfully unloaded and if the pressure is coming down fur-ther, there could be a system to send the vapors fromthe compressor back to the tank instead of condensingthe vapors. That would protect the tank from develop-ing vacuum. There is a vapor line upstream of the con-denser to send the vapors back to the tank in case offalling tank pressure.Volker: I guess it's possible that you could design asystem to do that. In our case...Stalin: We have a system like that.Volker: Where does it get the air to make up for it to


go backward through the unloading line?Stalin: In our case, the capacities are different.Unloading is too huge from a ship. We are unloadingsomething like 600 ton. However, the pumping rate isonly 20 or 25 ton. The figures might not exactlymatch. However, we do have a system like that. Onfalling pressure, vapors are bypassed directly back tothe tank.Stalin: My second comment is about the tank pressuregauge. The minimum value in the pressure gauge iszero. I believe it can be a compound gauge so that incase of falling pressure, you can read vacuum.Volker: You are right. It would have been good tohave that, but we didn't at that time.Larry Mackie, Mackie International, Inc.: If you'reunloading warmer liquid to the tank from a barge orship, and you're loading it to the bottom of the tank,aren't you concerned about rollover of the liquid?Volker: We are certainly concerned about rollover. Tobe concerned about rollover, I think there's a specifictemperature difference between what you have in yourtank and what you unload. I know that has been dis-cussed in this committee various times. At the Belle

plant, we've done a lot of that work on rollover. WhatI was referring to is lower than that differential tem-perature. We actually have temperature measurementdevices on the unloading line to make sure that thematerial that we're unloading is not warm enough tocreate a rollover situation.Mackie: Is it then recommended to unload a barge inthe bottom nozzle near the floor or into the top into thevapor space?Volker: If in fact you normally have the vapor spacein the tank stagnant, the only thing I would recom-mend is that you consider in the design basis for yourtank that when you begin to spray cold saturated liquidinto the top of the tank, you can increase the vaporspace density and create a negative pressure. I'm notsaying that can't be done, as I heard before; there areways you can handle that. What I am saying is thatyou need to design for that.Paul Khouri, Coastal Chemical: You mentioned thatyou found a vacuum breaker that you like. What wasthe name of the breaker and the manufacturer?Volker: It's a Protect-A-Seal Model 8540 Protect-A-Seal.


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