HEAT TRANSFER:

Protecting Cooling Towers fromOverpressure

A vent installed on the top of the riser was found sufficientto relieve a large surge condition caused by failure of anexchanger when a second accident occurred three monthslater without damaging the tower.

J.A. Veazey, Monsanto Agricultural Products Co., Luling, La.

Monsanto operates three ammonia plants in Luling, La.; adual-reform train unit built by Chemico in 1954, a 600ton/day (544 metric t/d) facility erected by Kellogg in 1965,and a 1,040 metric t/d complex put up by Kellogg in1975-1976.

This last facility suffered damage to its cooling tower inOctober, 1977, while it was onstream when a heat exchangerfailed. At approximately 2:00 a.m. on the thirty-first of themonth, the control board operator noticed a reduction inpressure exit of both the high and low cases of the synthesisgas compressor. The outside senior technician was directedto investigate. While checking, he observed water flowingfrom the hot water distribution basin on the west end of thecooling tower and saw water blowing from the south returnline vent on the east end of the tower. The techniciannotified the control room of his findings and the boardoperator, who at that moment was having trouble control-ling pressures in the syn gas loop, proceeded to shut theplant down.

A gas test of the cooling tower indicated the presence ofsyn gas and confirmed suspicions of a ruptured tube bundlesomewhere in the synthesis gas loop. The cooling towerwater headers were isolated and purged, and the tower wasfreed of gas. Upon inspection the following damage wasrevealed:

1. The last 3 m of the south cooling tower return distri-

0360-7275/79/2410-0073 $01.00® 1979 AIChE

bution header had separated—gas or water pressure hadsimply blown it apart at a mechanical joint—and the lastfew meters of the header section was protruding throughthe west end of the tower. Figures 1 and 2 illustrate.

2. The section of header had shattered a large portion ofthe tower's Transite siding and, in its travels, snapped anumber of 4 x 4 supports.

3. Close inspection of Figure 3 reveals that probably theonly thing that kept the 3 m of distribution header in thetower was a vertical run of electrical conduit anchored tothe west end of the tower. We feel extremely fortunate thatthis conduit was there, since just below the tower and 7.6 maway is the unit's electrical substation. The consequences of3 m of 61-cm diameter header, filled with water, falling on a1,000-kVA substation about 12 m below, is to say the least,alarming.

A water-cooled exchanger, item 116-C in Kellogg termi-nology, is the second of the three heat exchangers used tocool the syn gas exit the low-pressure case of the syn gascompressor. This unit is a shell and U-tube exchanger withsynthesis gas at approximately 940 Ib/sq in gauge (6,481kPa) at 177°C flowing in the tube side, and cooling towerwater at 40 Ib/sq in gauge (276 kPa) on the shell side.

Even though the rupture disc on the shell-side (water-side) of the exchanger was not blown, a pressure testindicated a leaking tube or tubes in the exchanger bundle.The head was pulled and the following results observed:

Figure 1.The separated

section ofheader.

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Figure 3. The con-duit was the onlything that kept theheader section inthe tower. V

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A Figure 2.Sidingdamage.

UP-

1. One leaking U-tube on the very inside of the bundlehad failed rather completely, Figure 4.

2. No other tubes were found to be leaking, so the singletube was plugged, the exchanger pressure tested andreturned to service (after repairs to the cooling tower).

Rupture of a tube in 116-C allowed synthesis gas at 940+Ib/sq in gauge (6,481 kPa) to enter the 40-lb/sq in gauge(276-kPa) return water header of the cooling tower. Theescaping gas caused a water hammer effect in the returndistribution header in the tower, and the end section of theheader was blown apart at a mechanical joint by the exces-sive force.

Potential consequencesInvestigation of this incident indicated a rather high

potential existed for this type of failure occurring again.

This led to a review of possible consequences that couldoccur if this or a similar failure were to recur:

1. The cooling tower is susceptible to fire or explosiondue to the presence of synthesis gas in the tower following atube rupture of this type.

2. The possibility exists that injury to personnel couldoccur due to falling debris from the tower if the header isagain blown apart.

3. In conjunction with above, damage to adjacent equip-ment is a distinct possibility, i.e., pumps, electrical substa-tions, etc., if the header is blown out of the tower.

4. Downtime to repair the tower or adjacent equipmentcould certainly be significant and would result in substan-tial cost and production outages.

As mentioned, the rupture disc on the exchanger was not

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Figure 4.The exchangerthat failed. V

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;fVf l'. i fff FYäf%T« .• /fif, • «*»,Al^'*"-" "»":'S|"* A

A Figure 5.Cutaway ofbell-and-spigotjoint on header.

blown. The rupture disc on 116-C was designed to provideprotection for the exchanger shell. Rating on the rupturedisc was 107 Ib/sq in gauge (738 kPa) at 24° C or 105 Ib/sq ingauge (724 kPa) at 43°C with the shell designed for 150Ib/sq in gauge (1,034 kPa).

It was obvious after this failure that this disc would notprotect the cooling tower in the event of a tube failure andresulting overpressure on the shell side. The conclusionreached following this investigation was that the tube hadruptured, allowing syn gas to enter the return water headercausing a water hammer that, in turn, ruptured the distri-bution header in the tower. Failure of the rupture disc torelieve indicates that the overpressure condition in the shellwas dissipated through the water header rather than thedisc. Rupture pressure of the disc was later verified, as thedisc failed at its rated pressure while the vessel wasundergoing a pressure test.

The return water distribution headers in the tower are

fabricated of polyester resin mortar, reinforced with fiber-glass. The joints are bell-and-spigot design, sealed with around rubber gasket. A. cutaway of the joint is shown inFigure 5.

The header rests on redwood saddles above the towerfloor and is restrained from moving by metal straps thatcircumvent the pipe and anchor it to the saddles and, inturn, to the floor. Installation of the header is shown inFigure 6.

The tower manufacturer states that "the water distribu-tion system on this tower is designed to operate at 3 Ib/sq in(21 kPa). Pressures exceeding 5.0 Ib/sq in (34 kPa) or 3.5 mof water must not be applied to this system.

It is apparent that due to thé type of restraints andbracing incorporated in this design, any pressure over andabove the stated maximum will cause the bell-and-spigotjoints to separate and, if the pressure is great enough, tearthe header loose from its restraints. Obviously, this is just

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rn.12" VENT(305 MM)

36"x 24"(914 x 610 MM)

MECHANICAL JOINT

SADDLE AND \ FLOW DISTRIBUTIONTIE-DOWN STRAPS J TO TOWER

36"

TIE-DOWN STRAPS-

HEADED

A Figure 6. Original distributionheader installation.

<-NEW VENT36" 0 PIPE

Figure 7. . . .

12" VENT(305 MM)

36" x 24"914 x 610 MM)

/

MECHANICAL JOIN

SADDLE ANDTIE-DOWN STRAPS

\FLOW DISTRIBUTION

TO TOWER

what happened.The existing vents on the cooling tower inlet header were

designed to relieve small amounts of entrained gases andnot as a relief for the tower. A 30.5-cm vent is located at apoint on the horizontal section of the header that renders itpractically useless as a relief of force due to water hammer.

The momentum of the flowing water due to the introduc-tion of the 940-lb/sq in gauge (6,481-kPa) syn gas hadalready been changed from the vertical to the horizontal atthe elbow, and it was impossible for the header to containthis unusually large force. The saddles with the bandsencircling the header were literally torn from the floor ofthe tower, and the end section of the header separatedcausing most of the damage.

Actions to prevent recurrence

As is often the case following a crash shutdown, repairtime had to be kept to a minimum, so any modification tothe tower to prevent recurrence of this nature of failurewould have to fit into a two-to-three day turnaround time(repair time for the tower). The following two solutionswere proposed:

1. Restrain the distribution header by installing addi-tional bracing, straps, etc. This would allow the header tosurvive additional pressure surges over and above that for

which it was designed. Also, a pressure relief device (rup-ture disc, etc.) would be installed in the end of the header torelieve any surges.

This option was rejected for several reasons. First, therewas the problem of an anchor point. The tower floor andside could not be used for this purpose because the struc-ture was not designed for this additional force. The risercouldn't be used either, for the same reason. Also, a cumber-some guy support system would have to be installed tooffset the extra force created by the water surge. Finally,any surge in pressure may have ruptured the disc on the endof the header sending 30,000 gallons of water cascadingagainst, and probably out of, the side of the tower.

2. Install a larger vent on the vertical riser, one capableof relieving the flow caused by failure of any of the heatexchangers in the syn loop, or any other for that matter.This seemed to be the best option of the two proposed, forseveral reasons: a. it was relatively inexpensive; b. installa-tion fit the time constraints; and c. it involved no supportproblems.

Preventive measures

Rough calculations indicated that a 91.4-cm vent locatedon the top of the riser would be sufficient to relieve any

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Revised distribution headerinstallation.

TOWER FLOOR

7 TIE-DOWN SIR APS-

HEÂDER-

SADDLE

Figure 8. Tower riserwith new vent installed.^

arge volume of water or surge condition caused by failure ofmy exchanger in the process. Even when figured at theirorst possible case, i.e., 2,200-lb/sq in gauge (15,170-kPa)yn gas discharging unrestrained into the return waterleader and all of the force and flow reaching the tower, theonditions in the horizontal section of header would still benaintained within design conditions.

The work was completed within two days, and the towertas been in operation since with no apparent ill effects oniperation. Figures 7 and 8 show the modifications that wereaade.

It was not the intent of this article to present a long andaborious calculation as to how we determined the size andocation of this revised cooling tower vent. We are sure thathere are many methods that would achieve the samelegree of protection, and we acknowledge this. The decisiono add this vent to the tower system was a joint decision ofhe plant contractor, the tower manufacturer, and Monsan-o.

We feel rather confident that our solution is a viableIternate that protects the tower from overpressure due toailures of the type described here. It was not our intentiono test these modifications. However, approximately threeaonths after the first failure, another failure occurred in16-C and the tower suffered no damage whatsoever, whichaakes us feel secure that this equipment is now adequatelyirotected. #

J. A. Veazy earned his B.S. degree in electricalengineering at the Univ. of Southwestern Louisianaand is with Monsanto's Agricultural Products Co.,at Luling, La. The maintenance supervisor of athree-plant ammonia complex, he has also beeninvolved with design, process control and utilitiesassignments.

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TOM TERRY, Kaiser Agricultural Chemicals (ret.):We've had two or three problems like this in the lastfew months in the Savannah plant, and we've goneto practically the same, thing, to put the riser pipeabout the same size as the cooling water return line,and we've had very good luck with it. But I would liketo ask what was the pressure set on the rupturediscs?J.Â. VEAZEY, Monsanto Agricultural Products: Theset pressure on the rupture disc on 116-C is 150lb.The disc did not blow, however, when we did pres-sure test the exchanger, it blew at 150 Ib.TERRY: Well we have the set pressure about 120 to125 Ib., and we feel that is much too high. The setpressure ought to be maybe 10-15 Ibhigherthan thereturn pressure in the cooling water line. In ourplant, the pressure on the cooling water pumps issomewhere in the range of 50 Ib. I think that if thepressure set on the rupture disc is too high, it proba-bly will not blow.VEAZEY: I think I would still have to question thefact that the relief line on the disc is only 3 in, asopposed to a 10-in return line. Would it be capable ofrelieving the entire volume of gas without overpres-suring the return header?TERRY: Well, you would have to calculate if that canbe done, but a 3-in disc sounds a little on the smallside to me. I still think the weak point in the wholeline at the cooling tower, so really the best is to havethe riser on the cooling tower do the job and have itthe same size as the return line.JAN BLANKEN, UKF, Holland: :l would like to askwhere you route the discharge line of your burstingdisc. The reason why I ask is that we have a burstingdisc with a short discharge line on the boiler feedwater heater downstream of our MEA reboiler, andwe have a deaerator with a high pressure drop. Thismeans that if we generate steam in the BFW heaterduring an emergency shutdown, we blow the rup-ture disc. There is a block valve under the disc, butwe cannSt close it after a rupture because of the hotwater coming from the discharge pipe.

From you slide, I understand that you also have abursting disc with a block valve and a short dis-charge line. This would mean that in the case of atube failure and rupture of the bursting disc, youwould get a hydrogen cloud at low level and a lot ofwater coming down, which would make it difficult toclose the block valve.

The alternative is to have a long discharge line to ahigh point remote from the block valve, but this, atleast under our climatic conditions, could introducethe risk of freezing up. Could I have your opinionabout this problem?VEAZEY: I can't disagree with your analysis. Alonger discharge line would create support prob-

lems, but nothing that couldn't be worked out. It's agood suggestion, and we will consider it.INDERJST OHRS, Indian Farmers Fertilizer Cooper-ative, Ltd.: In this 116-C leak, was it noticed before,or was it a sudden rupture?VEAZEY: It was not noticed at all. We had totallynormal operation on the 116-C before it leaked. Wedid notice some gas in the tower the second time.We shut the process down and it was almost an iden-tical rupture—in the same row, the inside row, on116-C, which is of course the sharpest U bend in thebundle. We f eel that we did get some thinning duringfabrication when the tubes were made. This is ourbest guess. We do not yet know that this is in fact,because 116-C is still operating.OHRI: We are having leakage in our syngas com-pressor discharge cooler 124-C. We are runningwith it leaking for the last four or five months, wait-ing for the annual shutdown. What you suggest forthe high pressure cooler if there is a sudden leak? Inthat eventuality will it be able to take it up?VEAZEY: I would be quite concerned with the possi-bility of fire or explosion in your tower.KEN WRIGHT, Cominco American: We had a leak inour 124-C and it ruptured the stave header on top ofthe cooling tower, pouring water all overthe substa-tion. We had a sudden power failure. I suggest youshut that down as soon as possible.BERNEY GROTZ, C.F. Braun & Co.: In 1964, in the360-ton per day ammonia plant, we had a problemsimilar to yours. We added a vent stack the same asyou did, only it was half the diameter of the mainriser and on the same centerline as the riser. Theoperators tell me it worked very well.DALLAS ROBINSON, Agrico: We also have an inter-mediate sized vent of 12 in on our inlet header thathas not prevented several blowouts. We found thatthere was not enough area to vent the pressureagainst the main header, just like your case. We triedtensioning cables on each side of the header to holdit firmly, but these too, failed to hold the header fromblowing. A 2-in standpipe, about six to eight feet tall,was installed so that we can observe venting of slowleaks, by noting the pulsing of this small header.When pulsing occurs, we check the discharge with acombustible meter for the presence of syngas.

We have come back and put additional risers onour headers , but on the dead head end. When a largeleak occurs, water and gas will vent at both endsrather than separating the header.VEAZEY: Is this 12-in full size at your vertical riser?ROBINSON: No, our inlet is 36 in.VEAZEY: We ran some very rough calculations andwe feel that 36-in full size vent should keep thatpressure below 3 Ib at the bell and spigot joint.KEES VAN GRIEKEN, UKF: What makes the heat

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exchanger fail? Is it carbon steel, and if so, is it dueto vibration of outside baffles with the water side?VEAZEY: We don't know, it was an inside tube, andwe really cannot see the failure area. What we havedone as a preventive measure is to plug all of ourinside tubes. Both our failures havç occurred on thevery inside tube, which is, of course, the smallestradius bend. We feel if that is indeed the problem,hopefully this will prevent it from happening again,until we do replace this exchanger.BARNEY STROM, CF Industries, Inc.: What I reallywanted to know is why 116-C leakage is more of aproblem that the cooling towers. What are we doingto prevent leakage of these tubes?VEAZEY: I really can't say we are doing anything.The position we took on it is to plug every tube.Granted, that does not solve the problem.STROM: No, it's not solving the problem. You canput vents all over the place, but if you have a leakingtube, you have to go down anyhow. That costsmoney.VEAZEY: We know that, but we don't blow up thetower.STROM: No, but you cost money for operation, andyou have two leaks now. What are you doing to pre-vent more leaks? You can't say that the tubes are notbuilt properly. We have had no leaks, zero, at CF. Wehave six plants of your plant type. We have never hadleaks in 116-C.

VEAZEY: Who fabricated your 116-C?STROM: I wouldn't know. They are all different.There are six plants, built from 12 years ago to twoor three years ago. First of all, we have a problemtoday. EPA demands no chromate, and all the rest.Now seriously, what are we doing? We are not look-ing at the water side which is putting pits etc. to corrodethe tubes.VEAZEY: I don't think our problem was externalwater corrosion of the tubes. We pulled the bundleout in August. We could not see the tu bes that failed,but judging from the tubes we could see, we saw nocorrosion.STROM: Why did they fail? I'm not so sure that it wasthe short bend of tubes, because every U-tube bun-dle has short bends.VEAZEY: I agree. Give us another year or two, may-bee we'll have the answer.KEN KITTELSTAD, Air Products: I'd like to com-ment on our cooling tower unit. We have a30-in ventwhich is in the same location as your 12-in was, andwe've never had a syngas leak but we have had a 3,000Ib nitrogen leak into that header and it did blow water outthe vent, with no damage to the cooling tower.VEAZEY: Do you have the same type joints we do?KITTLESTAD: No we had steel joints, not fiberglas.That's probably the reason.

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