Foam System Design for AmmoniaStorage Tank Areas

Foam application is a successful method of reducing the releaseof vapors from anhydrous ammonia spills. It is a simple processalso proving to be the most economical.

Edward C. Norman and Thomas M. SwihartChubb National Foam, Lionville, PA 19353

General Considerations

It has been demonstrated that foam application is a viablemethod of mitigating the release of vapor from spills ofanhydrous ammonia.1'2'3 Reductions of over 80% in the rateof vapor evolution are possible. The authors are unaware ofany other practical technology which can yield similar reduc-tions.

The economy and practicality of the use of foam lie in the factthat the foam blanket used is approximately 98% air. Whencompared to water spray curtain systems, the water require-ments for a foam system are minuscule. One can argue thatwater is free, but the pumps and piping needed to deliver thewater are not. Also, a water spray curtain must be runcontinuously until the spill is cleaned up, possible presentingwater supply and /or run-off problems, whereas foam requiresonly periodic re-applications.

There are four basic elements to a foam system; the foamconcentrate supply, the water supply, the proportioner and thefoam making devices. The size, and therefore the cost ofthese elements, is dependent upon the size of the area to beprotected and the allowable time for coverage of the area.This area will normally be the area of the dike surrounding astorage tank minus the area of the tank itself. The allowableperiod of time for coverage is dependent upon the location ofthe tank. Obviously, spills in areas of concentrated populationmust be covered much more quickly than spills relatively un-populated areas. We consider 10 minutes to be a reasonabletime to cover an anhydrous ammonia spill in most settings inwhich an anhydrous ammonia tank is likely to be located.

Leakage from anhydrous ammonia tanks is most likely tooccur due to failure of the associated valves, pumps andpiping, which are usually located within the dike area. For thisreason, we recommend that an inner dike, lower than the tankdike, be constructed around the pipework in the dike area andthe tank. The foam system design could than permit veryrapid coverage of this area, without operating the much largerpart of the system which would cover the entire tank dikearea.

The larger part of the system would only be actuated if theinner dike overflowed. The advantages of this approach arethat:

1. The area of the most likely spills isdramatically reduced.

2. Response time to the most likely spills isshortened.

3. Use of foam concentrate is minimized.

The inner dike part of the system can be designed to betripped by gas detectors, with the outer dike system beingactuated either manually or by level detectors in the innerdike. If this approached is used, the cost of a false trip fromthe gas detectors is minimized.

84

System Components

1. Water SupplyThe operation of the foam generation equipment requires 200-400 KPa at the inlet to the equipment. By utilizing typicalhydraulic calculations, inlet pressure to the system and properpipe sizes may be calculated. See example.

2. Foam Concentrate SupplyThe foam concentrate supply is normally kept in a bulk tank,for which suitable materials of construction are high densitypolyethylene, glass reinforced plastic (epoxy ester resin) andstainless steel. Lined steel tanks are not recommended. Thesize of the foam concentrate supply is calculated based onthe flow and time operation of the system. See example.

3. ProportioningProportioning is accomplished by a balanced pressure propor-tioning system, the heart of which is a ratio flow controller,which is, in essence, a pipe "T" fitting with a water orifice anda foam concentrate orifice. The balance of the system isdevoted to maintaining foam concentrate pressure equal tothe water pressure, so that the ratio of foam concentrate flowto water flow is equal to the ratio of the areas of respectiveorifices.

4. Foam GeneratorsThe foam generators used are medium expansion foammakers in which a jet of the foam solution is projected againsta screen mounted in a tube. The air aspirated by the jetmixes with the solution and forms the foam on the screen.

System Design

Abbreviations used in this section.

HMN-60 Medium expansion foam generator withnominal solution flow of 227 1pm @ 414 KPa

HMN-120 Medium expansion foam generator withnominal solution flow of 454 1pm @ 414 KPa

IDA Inner dike area

RCF Ratio flow controller, used in the proportioningsystem to control percentage proportioning.

ODA Outer dike area

The information needed for system design consists of the sizeof the area(s) to be protected, the maximum time allowable forcomplete coverage of a spill and the desired method ofsystem activation. We have prepared an example whichillustrates how this information is used.

Example

Figure 1 shows the layout of a hypothetical anhydrous am-monia storage facility. Figure 2 shows the same facility withthe addition of an inner dike around the tank and pipework.

The total area of the inner dike is 1770 m2; the total areainside the outer dike is 10,000 m2. The tank is 40 m diameter.

We have assumed that we wish to cover the inner dike in 2minutes or less, and the total area in 10 minutes or less.Actuation of the inner dike system will be by gas detectors.The outer dike system is actuated by liquid level detectorswhen the liquid level is 0.1 m below the top of the inner dike.Both detector systems have manual overrides.

Foam Requirements and Calculations

The first step is to calculate the volume that a 0.305 m blanketof foam will occupy in the Outer Dike Area and the Inner DikeArea.

Volume of ODA = Area of Dike - Area of Tank - Area ofInner Dike, multiplied by the height ofthe foam blanket.

Volume of IDA = Area of Inner Dike - Area of Tank plusArea of the pipe rack multiplied by theheight of the foam blanket.

Volume of ODA = (10,000 m2 -1,257 m2 -1,662 m2)(0.305 m)

= (7,081 m2) (0.305 m) = 2,160 m3

Volume of IDA = (1,662 m2 -1,257 m2 + 108 m2)(0.305 m)

= (513 m2) (0.305 m) = 156.5 m2

Next, the liters of expanded foam (50:1 expansion) is calcu-lated by converting the volume the foam occupies into liters.

Liters of expanded foam in ODA=2,160 m3 10001 = 2.160 x10 6 1

Liters of expanded foam in I.DA=156.5 m3

Im3

10001 = 1.565 x 10s 1

1 m3

The liters of foam solution is then calculated from the liters ofexpanded foam by dividing the liters of expanded foam by theexpansion ratio (50:1).

Liters of foam solution in ODA = 2.160 x 1061 = 4.320 x 1041

50

Liters of foam solution in IDA = 1.565 x 10s 1 = 3,130 1

50

85

The flowrate required to cover the respective dike areas isnow calculated by dividing the liters of foam solution by thetime requested or required to cover the area with expandedfoam.

Flowrate required to cover ODA in 10 minutes:

Flowrate (QR) ODA = 4,320 x1041 = 4,320 1

10 mins.

Flowrate (QR) IDA = 3,1301 = 1,565 1pm

2 mins.

The number of nozzles required to cover the respective areasis calculated by dividing the flowrate required to cover the areaby the rated flowrate of the nozzle.

# of nozzles for ODA = 4,320 1 pm = 9.51 or 104541pm HMN - 120's

# of nozzles for IDA = 1,565 1 pm = 3.45 or 4454 1 pm HMN-120's

To get better flow around the tank, HMN - 60's are substitutedfor HMN - 120's. Therefore, for IDA we will use 7 HMN 60nozzles.

Where the flowrates of the nozzles are:

HMN - 60 = 227 1 pm and HMN -120 = 454 1 pm@414KPa @414KPa

The amount of foam concentrates required for the first applica-tion is calculated by multiplying the flowrate, by the time re-quirement, by the percent concentration being used. ForHazmat NF Foams, we use 6% concentration.

Amount of foam concentrate required for first application for

ODA = QR (Time) (% cone.)= (4,320 1 pm) (10 min.) (0.06)= 2,592 liters of foam cone.

For IDA = (1,565 1 pm) (2 min.) (0.06)= 187.8 liters of foam cone.

For Anhydrous Ammonia, 3 applications of Hazmat NF Num-ber 1 will be needed to achieve vapors suppression and control.Therefore, the amount of foam concentrate required to achievesuppression is equal to the number of applications, multipliedby the amount of concentrate required for the first application.

Amount of foam concentrates = (2,592 1) (3) requiredto achieve suppression for ODA = 7.776 liters

For IDA = (187.8 1) (3) = 563.5 liters

The amount of foam concentrate required for vapor suppres-sion up to 24 hours is equal to 20 times the amount needed forthe first application. This number is approximate and varieswith spill size, depth, and climatic conditions.

For 24 hour suppression: ODA = 20 (2,592 1 ) = 51,840 1:IDA = 20 (1881) = 3,7601

Water requirements are also of concern for this system. Wewill calculate the water flowrates for both areas, the waterrequirement for each application, and the amount of waterrequired to sustain suppression for up to 24 hours.

For flowrates, the water required is 94% of the foam solutionflowrate. Therefore:

Water (ODA) = 0.94 (4,320 1pm) = 4,061 1pm

Water (IDA) = 0.94 (1,565 1pm) = 1,471 1pm

For each application, the water requirement is the amount offoam solution multiplied by 94%.

Water (ODA) = 4,320 x 1041 (0.94 = 4.061 x 10" 1

Water (IDA) - 3,130 1 (0.94) = 2,942 1

And total water required to sustain suppression for up to 24hours is equal to the number of applications, multiplied by94%, and by the liters of foam solution per application.

ODA = 20 (0.94) (4.320 x 104) = 8.122 x 10s 1

IDA = 20 (0.94) (3,130 1) = 5.88 x 1041

Hydraulic requirements complete the calculations for thesystem. In this problem, we have used 10 HMN-120 nozzlesto protect the ODA, and 7 HMN-60 nozzles to protect the IDA.The hydraulic calculations are completed from Figure 2, layoutof nozzles, piping and placement.

Hydraulic requirements for the IDA nozzles are 701.4 KPa atthe inlet of the Balanced Pressure Unit. Hydraulic require-ments for the ODA nozzles are 780.8 KPa at the inlet of theBalanced Pressure Unit. This creates a problem because theproportioners on the Balanced Pressure Unit will be parallel.Therefore, each proportioner will have approximately thesame inlet pressure. This will be either 701.4 KPa or 780.8KPa. In either case, this will cause less than 414 KPa at thenozzles because one or the other set of nozzles will notreceive adequate pressure. There are 3 possible solutions tothis problem:

1. Resize the piping to the HMN-60 nozzles to in-crease the Motional pressure loss by approximately 79.4 KPa.Or resize the piping to the HMN-120 nozzles to decrease thefrictional pressure loss by approximately 79.4 KPa.

2. Consider placing a restricting orifice in the waterinlet to the 0.10 m RCF. This may cause a rich mixture offoam to be sent to the IDA nozzles if both systems areactivated simultaneously.

3. Pace an In-line Balanced Pressure Unit after the0.10 RCF to control the proper pressure to the nozzles.

For cost and simplicity, a change of pipe size to eitherincrease or decrease the frictional pressure loss/gain isrecommended.

86

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87

Benefits of System Summary

In dense gas dispersion models, the area enclosed by anyisopleth is directly proportional to the source term, regardlessof what the other conditions are. The use of foam reduces thesource term by 80% or more, thereby reducing the are af-fected by the spill by 80% or more. This means that in manycases, vapor concentration levels of over 25 ppm can belimited to the plant or terminal property or its immediatevicinity, where residential structures are likely to be located.

Thanks to the invaluable assistance of Safer Emergencysystems we have been able to model the effect of foamapplication using their TRACE !! model. In each se, weassumed the following conditions:

Air Temperature

Suppression Due toFoam Application

3 m/sec

80%

Figures 4 through 7 show the results. We believe that, in thecases where foam has been applied, the model tends to over-predict lateral dispersion and underpredict vertical dispersion.We believe this because the ammonia gas which penetratesthe foam blanket is warmed by contact with the water (satu-rated with ammonia) in the foam. It will therefore be at asomewhat higher temperature than would be the case if itwere released from the surface of a pool of liquid, and will actas a buoyant cloud. We unfortunately have no experimentalevidence of this, but the principle has been demonstrated withLNG by Welker et ai.4'5

In the tests which we conducted at Pueblo, CO., we were notable to detect any odor of ammonia downwind of the test panafter foam application, despite the fact that ammonia wasbeing released. This suggests that vertical dispersion was, infact, taking place. The advantages of being able to presentsuch information to your local Emergency Response Commit-tee hardly need to be elaborated, to say nothing of the valueof minimizing the impact of any actual spill on the public.

Costs of System

The system shown in the example would cost in the area of$100 K for the foam hardware, proportioning system and foamconcentrate. Additional costs would include piping, installa-tion, construction of the inner dike and the control system.We are unable to estimate piping, installation and constructioncosts. The cost of the control system could range $5 K to$25 K depending upon the degree of sophistication desired.

The effects of spills of anhydrous ammonia from refrigeratedstorage tanks may be minimized by the use of fixes foamsystems installed in the diked area around the tank. Thesesystems are relatively simple to design and install. Foamsystems are the most economical method of mitigating vaporrelease from spills of anhydrous ammonia.

References

1. Clark, W.D."use of Fire Fighting Foam On Ammonia Spills"AICHE Meeting Boston, MA (Sept. 1975)

2. Norman, E.G. and Dowell, H.A."Using Aqueous Foams to Lessen Vaporizationfrom Hazardous Chemical Spills"Loss Prevention, Vol. 13, 1980 AICHE

3. DiMaio, L.R. and Norman, E.G."Performance of Aqueous Hazmat Foams onSelected Hazardous Materials"Plant/Operations Progress Vol. 7, No. 3 July 1988

4. "LNG Fire Control, Fire Extinguishment, and VaporDispersion Tests,"Report to American Gas Association, Project IS-3-1,University Engineers, Inc. (July 1972).

5. "Vapor Dispersion, Fire Control, and FireExtinguishment of High Evaporation RateLNG Spills,"Wesson, H.R., L.E. Brown, and J.R. Welker, AGADistribution conference, Minneapolis (May 6,1974).

Thomas M. Swihart

88

-100m

Figure 1. Layout of ammonia storage facility.

HMN - 120NOZZLES

INNER DIKE

-100m Figure 3. Typical nozzle installation.

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Figure 2. Ammonia storage facility with addition of Figure 4. Spill-inner dike area-no foam,inner dike and foam system.

89

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DISCUSSIONBILL SWITZER, UNOCAL: How long does this materialstay there after being foamed? How long does it stay beforeit starts breaking down?E.C. NORMAN: According to our actual experience inthe field, it depends on the atmospheric conditions. Thelife of the foam blanket will be anywhere from 30 minutesto \l/2 hours, depending on the humidity, wind conditions,the amount of sun on it, and the temperature. Obviously,on a cold, damp, cloudy day, it lasts a lot longer thanon a bright, sunny, dry day. We noted that when severalreapplications are made to the ammonia after a certainperiod of time, the foam blanket seems to last longer.

We don't know yet whether it is caused by dilution

of the ammonia by drainage from the foam or the ammoniaunder the foam blanket is just getting colder and colder,which it does. The foam blanket shuts off all sources ofheat other than that from the ground, and the pool ofammonia under the foam blanket tends to become very,very cold. We tried to measure the temperature at Pueblo,but unfortunately our thermometer only measured as faras -100°C, even though it got colder than that. We foundthat an ice hydrate layer has been forming under the foamlayer, almost as an interstitial barrier between where thefoam is and where pure ammonia will tend to pool. Thereis an iced hydrate layer which helps the blockage ofadditional vapor release off a spill.

90