By WAYNE KIRSNER, PE,*Kirsner, Pullin & Associates,Marietta, Ga.

t was 12:35 PM on a sunnyJune day. Jack had just re-turned from lunch and wasentering Steam Pit 1 to open

the steam valve. He had finishedrepairing the blown valve gasketin Steam Pit 2 the day before. Thesteam isolation valves in Pits 1and 3a had been closed two weeksearlier to facilitate the work at Pit2. The steam system formed a looparound the campus so steam couldreach almost every pit from twodirections (Fig. 1). Jack felt it wastime to put the cold 800-ft sectionof steam pipe between Pits 1 and3a back into service.

Jack opened the gatevalve at Pit 1, emitting 85psig steam to the cold pipesection. He probably heardthe hiss of steam as itrushed under the valvedisk to fill the relative voidin the 800-ft pipe section.Jack turned the handle ofthe gate valve until it wasfully opened. It took lessthan a minute. He climbedout of Pit 1 and headed forPit 3a to open that valve.He didn’t want to spendany more time in the hotsteam pits than necessary.

As Jack drove the shortdistance to Pit 3a and

parked his truck, newly admittedsteam was rapidly condensing inthe cold pipe section faster thanthe steam plant could supply it.Condensate coated the inside pipesurfaces and rolled off the pipewalls, forming a small rivulet atthe bottom of the pipe. About 600lb of condensate would form in thefirst minutes after steam enteredthe 800-ft pipe section.

The steam pipe was pitcheddownward from Pit 1 to Pit 2(Fig. 2). At Pit 2, the pipe jumpedup about 8 in. and then contin-ued its downward pitch to Pit 3.The rivulet of condensate form-ing at the bottom of the pipeflowed down toward the driplegs provided at the low points

at Pits 2 and 3.Under normal operation, steam

traps located in the drip legs ofPits 2 and 3 were supposed tocarry away condensate to keep the8-in. diameter steam main dryand safe. Why safe? Steam mainsare designed to carry steam at ve-locities upwards of 100 mph at fullload. With a 100 mph hurricane ofsteam blowing down a pipe, youdon’t want a lake of condensate atthe bottom of that pipe. Above 30mph, a steam “wind” can pick upwaves that, if there is enough con-densate, can lap up and block thepipe.1 Once the pipe is blocked,the steam is no longer blowingover the condensate; it is nowpushing a lethal slug of water

36 JULY 1995 ■ HEATING / PIPING / AIR CONDITIONING

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .STEAM SYSTEMS

. . . . . . .

I40

Boiler

plant

Site of accident

To laundry

VP-1

VP-2

VP-3a VP-3

VP-4VP-34

VP-5

VP-6

VP-7VP-9

VP-8

VP-33

VP-32

VP-10VP-11VP-12

VP-13

VP-14

VP-14a

VP-15

VP-16VP-35

VP-17

VP-18

VP-22

VP-23

VP-25

VP-28VP-29

VP-31

100

101

103

104204

206

300315 317

319

401403

400 402

404

406

408

410

AB-2

AB-4

1 Campus steam loop.

WHAT CAUSED THE STEAM SYSTEM A

*Mr. Kirsner performed theinitial investigation into thecause of the accident for theGeorgia Dept. of Human Re-sources and was later re-tained as a consultant and ex-pert witness to assist in thedefense of the engineeringfirm and contractor who re-designed the steam pits to im-prove their safety.

down the pipe with the same dif-ferential pressure that motivatesthe steam at high velocity. Thepressure will attempt to acceler-ate the slug to approach the flowvelocity of the steam—55 mph isachievable with 85 psid.

A slug of water just 8 in. long inan 8-in. diameter pipe weighs asmuch as a bowling ball. Steampiping systems and their supportsare not designed to withstand theimpact of 55-mph bowling ballsstriking their elbows, valves, andfittings. Removal of condensate insteam systems is paramount totheir safe operation.

There were no operable steamtraps, however, removing conden-sate from the 800-ft section of pipebetween Pits 1 and 3a that daywhen Jack opened the steamvalve at Pit 1. This was partly byaccident and partly by design. Thetrap located in Pit 3a was on thedownstream side of the closedgate valve in Pit 3a. Thus, thetrap was isolated from the coldpipe section. The design engineershad anticipated the problem ofcondensate pooling against theisolation valve when it was closed,however, and provided a 1-in. di-ameter “blowoff” pipe with a ballvalve shutoff immediately up-stream of the valve. Its functionwas to purge condensate that pud-dled upstream of the valve atstartup. Unfortunately, the 1-in.ball valve was closed.

As for the steam trap in Pit 2, itwas nonfunctional. Its dischargeline had been disconnected years

ago when the building containingthe condensate receiver to whichit had discharged had been torndown. The fact that the trap dis-charge had nowhere to go had ap-parently slipped through thecrack at the time of the buildingdemolition and had never beennoticed in subsequent years.

The lack of an operable trap at

Pit 2 allowed condensate to backup in the steam main and form aresident puddle starting at Pit 2and extending upstream (Fig. 3).During periods of low steam flow,as in summer, we surmise thatthe puddle would build almost toblock the 8-in. diameter pipe atthe rise at Pit 2. Only enoughspace between the puddle’s sur-

HEATING / PIPING / AIR CONDITIONING ■ JULY 1995 37

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

An investigation into the factors contributing to a rare and little understood class of steam system accident that likely will kill again

M ACCIDENT THAT KILLED JACK SMITH?

1Wallace and Dobbson predict a gasvelocity of less than 30 mph is suffi-cient to initiate slugging behavior inan 8-in. pipe (“Onset of Slugging inHorizontal Stratified Air-WaterFlow,” Int. J. Multiphase Flow, Vol. 1,1973, pp. 173-193).

10 ft

9 ft

8 ft

7 ft

6 ft

5 ft

Pit 1

Pit 1

Pit 2

Pit 2

Pit 3

Pit 3

Pit 3a

Pit 3a

Pit 4

Pit 4

0 500 1000 1500 Pipe length (including expansion loops), ft

116 ft

495 ft 306 ft 95 ft 481 ft

θ

495 ft to Pit 1 (including expansion loops) 1.8 ft drop

at Pit 1

from rise From Pit 1

IL

7.98 in.

12 in.

To Pit 3a

Volume I = 0.5 × 0.3474 ft3/ft × L

(Not including drip leg or 45 deg elbow volume)

7.98 in. 1.8 ft drop from Pit 1495 ft to Pit 1*

tan θ

(7.98 in./12 ft)

(1.8/495)L =

where L = tan θ =

= 183 ft

∴Volume I = 0.5 × 0.3474 ft3/ft × 183 ≅ 32 cu ft or 240 gal *Assumes pipe pitch maintained in 95 ft of expansion loops

θ

2 Gradient profile of steam main between Pits 1 and 4.

3 Volume of condensate puddle at Pit 2.

face and the top of the pipe wouldremain for what little steam wasneeded by downstream buildingsto blow by. While this puddleposed a continual threat of waterhammer, if steam flow was lazyenough (as it apparently had beenin that section of pipe), the threatwould never turn to action. Thepuddle would just sit there burp-ing over condensate when toomuch accumulated. At its maxi-mum capacity where it would justblock the pipe, the puddle couldhold 1900 lb of water stretchingfor 183 ft up the 8-in. pipe fromPit 2 toward Pit 1. (The predictionof the existence of the puddle wasverified some months after the ac-cident when, during the course ofmaintenance at Pit 2, a largequantity of water was drainedfrom the drip leg.)

The puddle of condensate up-stream of Pit 2 was probably sit-ting there when Jack opened thesteam valve at Pit 1. Opening thevalve permitted a rush of steam toblow down the pipe section fromPit 1 over the puddle at Pit 2 to-ward the closed valve at Pit 3a.The initial shock of 85 psig steamrushing into the pipe probablyblew a substantial portion of thepuddle over the hump at Pit 2 anddown toward Pit 3a.

Did this create a water hammerat Pit 3a? I don’t think so. If a slugformed that did fill the entire pipeand was moving down the pipe to-ward the closed isolation valve inPit 3a, it would compress the airstill residing in the pipe as it spedtoward the dead end. This com-pressed air piling up in front ofthe slug would act as a shock ab-sorber to cushion the blow of theslug as it approached the closedvalve at Pit 3a. Thus, if there wasan initial water hammer incidentcaused by retained condensate atPit 2 being swept toward Pit 3a, Ibelieve it was a mild one. Proba-bly the primary result of the in-rushing steam on the puddleposted at Pit 2 was to push a por-tion of it to Pit 3a where the coldresidual condensate came to rest

against the valve disk at Pit 3a.It took Jack about a minute to

reach Pit 3a; he drove the 800 ft.Jack climbed down the ladder atPit 3a. We surmise that he did notnotice condensate or steam shoot-ing out of the downturned 1-in.blowoff pipe that emptied into thegrass above Pit 3a because the 1-in. ball valve in the line was notopen. Had he opened it, the coldresidual condensate from the pud-dle at Pit 2, which was now resid-ing upstream of the 8-in. gatevalve in Pit 3a, would have beenshooting out of the 1-in. line withgreat force. After the water wasemptied, hot flashing condensateand steam would have blown out the pipe. Jack and otherpassersby could hardly havefailed to notice the steam plumeand hear the discharge of water,steam, and air rushing out of thepipe.2 Interviews revealed that noone noticed a steam plume.

Once in the pit, Jack positionedhimself behind the 8-in. gatevalve so he could turn the handleto open it. The valve was a 125 lbClass, cast-iron, outside screw

and yoke valve requiring 14 turnsto open it fully. It had been about3 min since Jack had opened thevalve at Pit 1.

It was probably hard to turn thevalve handle. The pressures onthe two sides of the valve had notequalized yet, and the differentialpressure would be pressing thevalve disk hard against its seat.Jack must have tugged hard. Ittook a half turn for the disk to be-gin to lift off the valve seat. Jackturned the handle two more revo-lutions. He probably realized assoon as the disk cleared the seatthat there was trouble. He proba-bly heard the metallic “clang” andthen a roar as steam on one side ofthe valve met cold condensate onthe other side and violently col-lapsed, shaking the pipe andvalve with up to 1000 psi of “im-plosive” pressure. He may haveeven hurriedly tried to open the 1-in. diameter blowoff valve at this

Steam system accident

38 JULY 1995 ■ HEATING / PIPING / AIR CONDITIONING

2If there was no water upstream of thePit 3a valve and only air was rushingout, I calculate the air volume of the800-ft 8-in. diameter pipe sectioncould exit the system in about 40 sec.After air, steam and condensate wouldbe discharged.

4 Pit 3a 8-in. cast-iron gate valve.Note the broken gusset.

5 Pit 3a 8-in. cast-iron gate valve.Note the crack extending around cir-cumference of flange neck.

point, realizing that there wasstill condensate in the 8-in. diam-eter line.3 But it was too late.

The valve pit exploded as ifstruck by thunder. The valve Jackwas opening ruptured, spewingsteam and hot condensate in alldirections. Jack could not escapethe 300+ F steam and flashingcondensate spewing out into thepit through large fissures openedup in the body of the valve. Jackdied in the pit. His body wasfound wedged under the brokengate valve beneath 21/2 ft of hotcondensate that eventually emp-tied into the pit.

The initial theoryIt was not immediately clear

what caused the accident. Thefirst theory espoused by the con-sulting engineers hired to analyzethe cause of the accident was thata slug of condensate crashed intothe Pit 3a valve from the directionof Pit 2 and the steam plant. Themotive force for the slug was thedifferential between the pressureat the plant and the reducedsteam pressure on the other sideof the gate valve at Pit 3a after itwas opened. Recall from Fig. 1that the steam system is a loop.The steam on the other side of Pit3a had to travel approximately6300 ft around the entire loop toreach the back side of Pit 3a. Itwas postulated that the flowingfriction would have reduced thesteam pressure sufficiently by thetime the steam got to the backside of Pit 3a to create the pres-sure differential. The slug of con-densate that hit the valve waspresumed to have come from therapidly forming condensate in the800-ft section of cold pipe that wasbeing returned to service.

The physical evidenceThe engineers felt that the

physical evidence supported theirtheory. Fig. 4 shows the valve andpit after the accident. The gusset

connecting the outlet flange of thevalve to the steam pipe headingtoward Pits 3 and 4 (downstreamof Pit 3a from the steam plant) isbroken, and a crack extendsaround the front of the valve upthrough the valve body bonnetflange. Fig. 5 shows that the crackadjacent to the downstream outletflange extends around the entireneck of the valve body connectedto the flange. In fact, when thevalve was removed from its con-necting piping, the entire flangefell off, carrying with it a largechunk of the upper valve body atthe gusset. The upstream side ofthe valve (toward Pit 2 and thesteam plant) sustained a crack onthe back side of the valve body(Fig. 6), but in general, the up-stream side of the valve was notas severely damaged as the down-stream side.

What does the physical evi-dence tell you? Does it support thetheory that a slug of condensatestruck the valve disk from the up-stream side—i.e., from the direc-tion of Pit 2 and the steam plant—as the consulting engineerspostulated? If so, the slug wouldhave struck the disk inside thevalve head-on, which would havetransferred the impact to the in-ternal valve guides inside thevalve body (Fig. 7). This wouldhave put the downstream half ofthe valve body in compression andthe upstream half in tension,tending to split the valve in half

down the middle.Cast iron is roughly three

times as strong in compression asit is in tension. If the impact hadcome from the upstream side, wewould expect most of the damageto be on this side of the valvesince it would have been placedin tension. Instead, the damageis concentrated on the down-stream side of the valve, wherethe gusset was broken and theflange neck completely separated

from the valve body.It is evident then, from the ori-

entation of the damage, that theslug that struck the valve did notcome from the upstream side ofthe valve but from the “back side”of the steam system loop.

Take a look at how the drip legin Pit 3a is supported in Fig. 7.The drip leg actually is the termi-nation of a downturned elbow inthe pipe coming from Pit 3. Ofcourse, the original design engi-neers planned on steam and con-densate “going to” Pit 3 from Pit3a. This is the normal direction ofsteam flow from the steam plantout into the system. Steam or con-densate was not supposed to flowfrom Pit 3 back to 3a. The elbowand drip leg are supported “offi-cially” from an anchor in the pitwall at the top of Fig. 7, a few feetfrom the drip leg. “Unofficially,”however, the drip leg and elbow

HEATING / PIPING / AIR CONDITIONING ■ JULY 1995 39

3The 1-in. quarter-turn blowoff valvewas found half open.

7 Pit 3a after the accident. To theright are Pit 2 and the steam plant;toward the top are Pits 3 and 4.

6 Back side of valve at Pit 3a.

are also supported by the cast-iron gate valve via its connectionto the drip leg elbow. The unoffi-cial valve support cantilevers outfrom the kitty-corner pit wall.There is no support under the dripleg termination to transfer itsload to the concrete floor of thepit.

The stiffest connection closestto the load will carry the load im-posed on the drip leg. The supportcontaining the cast-iron valve hasthe shortest cantilevered distanceand is very stiff. It is this “unoffi-cial” support that carries most ofthe drip leg’s load.

Now imagine if the slug of con-densate that broke the valve camefrom the direction of Pit 3—i.e.,from the backside of the steamloop. It would have entered Pit 3afrom the top, as shown in Fig. 8,rounded the full radius elbow, andthen crashed into the dead end atthe bottom of the drip leg, wrench-ing the drip leg and its elbow vio-lently down.

Fig. 9 shows the torque the slugwould have applied to the valveand how I imagine the valvewould have deformed if the valvebody were made of some imagi-nary ductile material. Cast iron,of course, is not ductile. It will notaccept any appreciable strain. De-formation as shown in Fig. 9would severely strain the gusset,tend to rip the flange out of thevalve body, and if the gusset frac-tured, probably break off the en-tire flange. (It does not take muchenergy to propagate a crack incast iron once it is started.) Theright side of the valve body mightsustain a crack as well, wideningtoward the top of the valve body(which was in tension) as the rootof the cantilever (i.e., the valve)tried to resist bending.

Judging by the damage to thevalve, shown again in Fig. 10, thisappears to be what happened. Butif the slug came through the “backdoor,” where did the motive forceto accelerate it come from, andwhere did the slug of condensatecome from?

Initial theory’s motive forceThe initial theory presumed

that the steam pressure washigher on the steam plant side ofthe Pit 3a valve than on the backside of the valve toward Pits 3 and4. Let’s examine this assertion.

For there to have been a signifi-cant pressure drop in the steampipe as steam circumnavigatedthe campus to the back side of thePit 3a valve, there would havehad to have been significantsteam flow in the pipe. But steamflow in the system was low. It wasa warm day, the campus laundryhad shut down for the day, andonly one boiler was providingsteam. Operators had placed theboiler controls on manual opera-tion. Gas consumption charts in-dicate that the steam plant wasonly putting out about 5200 lb perhr or about 87 lb per min.

An 8-in. diameter steam line isroutinely sized to carry 30,000 lbof steam per hr, so 5200 lb per hrwould not create much pressuredrop in the 8-in. main. Around theback side of the loop, however, the8-in. main reduces to 6 in. andthen 4 in. I estimate (roughly)that the pressure drop of steamflowing in 7000 equivalent ft ofpipe around the back side of theloop to Pit 3a, before the valve inPit 3a was opened, could havebeen in the neighborhood of 7 psi.

Is a 7 psi pressure differentialenough to motivate a sufficientvelocity of steam flow to pick up aslug of water, say from the puddlelying at Pit 2, and send it cannon-balling toward the barely openedvalve at Pit 3a? I don’t think so.

To get moving, the steam had toflow under the partially openedgate valve in Pit 3a once it wasopened. At 21/2 turns (of the 14 toopen the valve fully), I estimatethe valve disk would have raisedabout 1 in., exposing a crescentmoon opening under the valvedisk equal in area to about 10 sqin.

The steam flowing under thisopening would have consumed at least one “velocity head”—

Steam system accident

40 JULY 1995 ■ HEATING / PIPING / AIR CONDITIONING

����

Pit wall

Cond

ensa

te s

lug

8 Inside of Pit 3a valve. Pencil pointsto valve disk guides.

10 Disassembled Pit 3a valve.

9 Imaginary performance of Pit 3a“ductile” valve as slug struck (viewfrom the direction of Pit 3).

continued on page 45

V2/2gc—in pressure potential en-ergy, which would have been con-verted to kinetic energy and notregained. If the entire 7 psi pres-sure differential was consumed inonly overcoming one velocity headof static pressure drop, thensteam velocity through the 10 sqin. valve opening would havebeen:

Volumetric flow through theopening, then, would have beenno more than:

A volumetric flow rate of 2.3 cuft per sec through the full cross-sectional area of an 8-in. pipeequates to a velocity in the pipe of:

6.6 fps is about 5 mph. The ve-locity would be greater whilepassing over the puddle at Pit 2but would subside again down-stream of the puddle. I do not be-lieve that a 5 mph steam velocitycaused this accident.

But if not, where did the motiveforce for this accident come from?Let’s look closely at what was go-ing on inside the 800-ft section ofcold pipe in the 3 to 4 min beforethe valve at Pit 3a was opened.

Revised accident theoryI mentioned earlier that as

steam entered the 800-ft sectionof cold pipe, it began to condenserapidly. This was a fast-movingtransient heat transfer process.

Steam rushed into the 800-ftpipe section, compressing, mixingwith, and warming the 247 cu ft ofair that would have occupied thecold pipe section. The sudden ex-pansion of the steam systemcaused steam pressure to drop inthe entire system and steam toflash off from the surface of the ac-tive boiler. I estimate that steampressure at the boiler plant

dropped from around 85 psig toabout 79 psig (94 psia) immedi-ately. Because the operator hadapparently not opened the 1-in.vent valve at Pit 3a to vent air andcondensate, the air in the 800-ftpipe section could not escape. Theresulting steam-air mixture con-sisted of approximately 79 per-cent steam and 21 percent air bymole content. This furtherdropped the steam pressure in the“mix” to its partial pressure—i.e.,79 percent of the total pressure.Thus, steam partial pressure =0.79 3 94 psia = 74.2 psia or 60psig.

The drop in steam pressurefrom 85 to 60 psig caused thesteam in the steam-air mixture tobe slightly superheated. The su-perheat would tend to slow heattransfer until the superheat wasgiven up and condensing heattransfer was initiated. Since theair in the pipe section had nomeans of escape, each new chargeof steam that replaced that beingcondensed would initially be su-perheated.

Nevertheless, steam beganrapidly condensing on the insideof the 8-in. steel pipe. The rate atwhich the condensing heat trans-fer proceeded was controlled bythe connective heat transfer coef-ficient, h, for the steam-air mix-ture.

With no air in the system, Icompute the initial value of h tohave been about 500 Btuh per sqft per deg F. At this rate, heattransfer would have proceeded ata lightning-quick pace, going to 99percent completion within 1 minand thereby quickly stabilizingthe system. Air in a steam system,however, drastically impedes heattransfer. As steam condenses outof the steam-air mixture at a heatexchange surface, the air is leftbehind. Very quickly, a layer of in-sulating air will blanket the heatexchange surface, slowing therate of condensation to a fractionof its pure saturated steam value.For a mole fraction of air equal to21 percent of the steam-air mix-

ture, h is reduced to one-twelfth ofits pure steam value.4

Fig. 11 shows the progression ofheat transfer between the steam-air mixture and the 800-ft pipesection in terms of the number ofpounds of steam condensed towarm the pipe section. It showsthat it took over 6 min for the heattransfer to go to completion—i.e.,for the pipe to reach steam tem-perature.

The timing here is important.Let’s focus on what was happen-ing in the first 3 min after Jackopened the 8-in. valve at Pit 1, ad-mitting steam to the 800-ft pipesection.

In the first minute, 230 lb ofsteam would have been consumedin heating the pipe, according tothe table in Fig. 11. That is morethan the one on-line boiler couldproduce. Remember, its controlswere set in “manual fire” position,and it was only outputting about5200 lb per hr or 87 lb per min ofsteam. In fact, Column 3 in thetable shows that during each ofthe first 3 min, the rate of steamconsumption due to heat transferin the pipe exceeded the rate atwhich steam was produced in theboiler. Thus, instead of pressurebeing higher in the 800-ft pipesection compared to the rest of thesystem, as was postulated in theinitial theory, it was lower!

Did the lack of steam genera-tion from the steam plant starvethe heat transfer process in the800-ft pipe section? No, althoughit might have slowed it downslightly in the first few minutes.Steam would have been suppliedby other sources. First, the on-lineboiler would have contributedroughly 48 lb of steam due to in-ternal flashing of boiler water tosteam as steam pressure fell(from 79 to about 52 psig). The re-maining shortfall of steam wouldhave backflowed into the 800-ft

Steam system accident

HEATING / PIPING / AIR CONDITIONING ■ JULY 1995 45

V gc= × ×

=

2 7

32 3

psi 2.31 ft/psi

ft/sec.

Q = ×=

32 32 3

..

ft/sec 10 sq in./144 cu ft/sec

Vpipe = =2 3

6 6.

. cu ft/sec

0.3474 sq ft ft/sec

4Henderson, Heat Transfer DuringVapor Condensation in the Presenceof Noncondensible Gas, 1967, doctoraldissertation, University of Maryland.

continued from page 40

pipe section from the rest of the7300 ft of steam-filled pipe. Thesteam accounting ledger duringthe first 3 min sums like this: Thecumulative pounds of steam con-sumed was 482 lb, per the table inFig. 11. The boiler generated 261lb of steam in 3 min plus another41 lb due to flashing in the boiler.This left a shortfall of: 482 – 261 –48 = 172 lb of steam to be made bythe rest of the system. Removing172 lb of steam from the entiresteam system would have droppedits pressure to 54 psig.

The situation, then, at Pit 3awhen Jack climbed down to openthe 8-in. valve was most likelythis: Relatively cold water wasresting against the valve disk onthe upstream side of the valve to-ward Pit 2, and steam was push-ing to enter the low-pressure 800-

ft pipe section from the down-stream side of the disk. The watermostly consisted of the residualcondensate that had been sweptover from Pit 2 with the initial vi-olent inrush of steam. Its bulktemperature would still be coolsince its relatively small exposedsurface near the top of the pipewould have allowed little steam tocontact the bulk of the water, es-pecially that water restingagainst the valve disk where thewater almost filled the pipe.

As the valve was opened, steamwould have flowed underneaththe valve disk into the cold con-densate, forming an expandingsteam bubble surrounded by coldwater. Heat transfer between the300+ F steam and the subcooledwater would have been extremelyhigh.

The bubble of steam would havealmost instantaneously implodedas the steam transferred its heatto the cold water and collapsed tocondensate occupying 1/350 of itsformer volume. Had the steamflow been restricted, the sur-rounding water would haveslapped into the void, creating aloud and potentially destructivepressure pulse on the order ofmagnitude of 1000 psi. But with aready steam supply from the 7200ft “reservoir” of steam pipe in thelooped system, the collapsing voidwould instead have drawn inmore steam underneath the valvedisk to refill the bubble and re-peat the collapse. (In a race to fillthe void, steam will beat water be-cause of its lower inertia.) The fre-quency of bubble collapse andsteam replenishment would have

Steam system accident

46 JULY 1995 ■ HEATING / PIPING / AIR CONDITIONING

Area beneath curve proportional to net heat transfer

82 percent of net heat transfered in 3 min

Minutes after steam admitted

0 0

1

2

3 4 5 6 7 8 9

250

200

150

100

50Tem

pera

ture

diff

eren

ce,

stea

m-p

ipe

wal

l, F

Solve for Θ(t ):1. Heat given up equals heat absorbed ˙ q heat transfer = ˙ q heat absorbed

2. Newtonion cooling hA(T sat −T steel) = mc dT steeldt

3. Let Θ be (T sat −T steel ) h(Θ)AΘ = −mc dΘdt

4. Note h is a function of Θ 1h(Θ)

dΘΘ

=

Adtmc

5. From Rohsenow, h(Θ) = H /Θ 0.25 whereΘ 0.25

HdΘΘ

=

Adtmc

h = 0.555r 1g(r 1 − r v)h fg

*k13

m1g cD(T sat −T steel )

0.25

×F airdΘ

Θ0.75= HAdt

mc

where

h fg* = h fg + 3

8 c p1(T sat −T steel)dΘ

Θ0.75230

Θ(t)

∫ = HAdtmc

0

t

∫to account for heat transfer from condensate 4Θ(t )0.25 −2300.25 = HAt

mct

F air = 0.83 to account for reduced h due to air Θ(t ) = 2300.25 + HAt4mc

t

4

Steam Cumulativeconsumed steam Heat

Temperature during consumed, transfer,Minutes difference, F minute, lb lb percent

0 229 0 0 01 139 230 230 392 79 154 384 653 41 98 482 824 18 57 539 925 7 29 568 976 2 13 581 997 0 4 585 998 0 1 586 100

11 Progression of heat transfer to exposed 8-in. pipe.

been so rapid as to appear as a“condensate flame” blowing intothe pool of water from underneaththe valve disk. (This phenomenonis described in a number of exper-iments on rapid condensation car-ried out at Creare, Inc.5)

The rate of steam consumptionwould have been very high in thefirst few seconds after the valve atPit 3a was opened. The condensa-tion rate would have only beenlimited by the rate at which steamwould flow underneath the valvedisk. This limit was the sonicmass flow rate—i.e., the rate atwhich steam flow underneath thevalve disk became “choked.”

An approximate formula for thechoked mass flow of steamthrough the area, A, beneath thevalve disk is:

where r and T are the inlet condi-tions.6 Thus, at inlet steam condi-tions of 54 psig (r = 1/6.38 lb percu ft, T = 327 F with 24 F super-heat):

This is a large steam flow. Inthe full cross-section of an 8-in.pipe, this steam flow would movethrough the pipe with a velocity of134 mph.

If we imagine steam was beingemptied from the 8-in. steammain extending back toward Pits3 and 4, roughly 200 ft of steampipe would have been evacuatedper second due to the steam con-sumption of 10.7 lbm per sec in

the condensate flame at Pit 3a. Ibelieve this rapid steam move-ment was the motive force thatpicked up a slug of condensatethat struck the drip leg at ValvePit 3a.

But where did the slug of con-densate come from? We don’tknow for sure, but I suspect thatthe slug of condensate was sittingin the system between Pits 3 and4. 125 cu ft of water filled Pit 3a toa depth of 21/2 ft after the valveruptured. Only 50 cu ft of thisamount can be accounted for ascoming from the residual conden-sate that had been resident up-stream of Pit 3a prior to the acci-dent plus condensing steam thatflowed into the cold pipe sectionbefore Pit 3a was isolated. There-fore, the other 75 cu ft of waterthat filled the pit plus 10 percentto account for flash steam lost tothe atmosphere must have comefrom the pipe section downstreamof Pit 3a toward Pits 3 and 4. 150cu ft of condensate could have ac-cumulated there if the trap failedclosed (the F&T trap at Pit 4 wasreplaced and discarded after theaccident; hence, we were neverable to examine it).

Additional damage found at Pit3 after the initial accident sup-ports the theory that the slugcame from this direction. At Pit 3,a crack was found in the flangeneck of the cast-iron gate valve,and the valve’s connecting gasketwas blown out. But if the slug that

slammed into Pit 3a traversed Pit3, why didn’t it fracture the cast-iron valve at Pit 3 as well, insteadof just cracking it? This wouldhave saved Jack’s life.

Examine the schematic dia-gram of the Pit 3 layout in Fig. 12,taken from the design engineer’sdrawings. The layout is very dif-ferent from that of Pit 3a. There isno dead-ended drip leg, as is thecase at Pit 3a. The most abruptchange of direction in the high-pressure steam (HPS) main is twolong radius 45-deg elbows. The in-ertial force imposed on the valvewhen the condensate slug hit thischange in direction would bemuch less than that exerted at Pit3a. Nonetheless, the reactionforce was apparently enough tocrack the valve but not rupture it.The location of the crack, asshown in the figure, is consistentwith the location of maximumtensile bending stress placed onthe valve body by a slug travelingfrom the direction of Pit 4 throughPit 3 and heading to Pit 3a.

Shock to valve at Pit 3aThe question remaining is:

Could the force of the shockcaused by the slug of condensatedescribed above have fracturedthe valve at Pit 3a? Water ham-mer theory predicts the overpres-sure due to the collision of a fluidto be r times c times V. In thiscase:

● r is the density of the conden-

HEATING / PIPING / AIR CONDITIONING ■ JULY 1995 47

m T A* = 35ρ

m* / .

.

= × ×

+ ×

===

35 1 6 38

327 46010

10 7

lbm/cu ft

F in.

144 in. /ft lbm/sec

641 lb/min38,475 lb/hr

2

2 2

6Lindeburg, Mechanical EngineeringReview Manual, 6th edition, Equation26.69.

5Block, James A., “CondensationDriven Fluid Motions,” Int. J. Multi-phase Flow, Vol. 6, 1980, pp. 113-129.

PR

To Pit 3aBV-6

Drip leg

HPS

Trap line

GV-10

5 in. diameter

Crack

8 in. diameter

Sump pit

Slug from Pit 4

1 in.

diameter

BV-7

GV-9W6 × 8.5

Ladder

6

3

12 Valve Pit 3plan view. Insetshows locationof crack in valveneck (lookingfrom the direc-tion of Pit 4).

continued on page 50

sate in the slug.● c is the speed of sound in 300

F water—3825 ft/sec.● V is the velocity at which the

slug was traveling when it struckthe end of the drip leg. The maxi-mum theoretical velocity the slugcould attain is:

∆P is the pressure differentialaccelerating the slug of conden-sate.

If we assume that the steampressure on the back side of thevalve had a value intermediatebetween the pressure when thevalve opened and the eventual re-duced pressure and also assumethat the steam downstream of theslug had essentially been evacu-ated by the rapid condensationflame taking place beneath thevalve, the ∆P across the slug con-ceivably approached 71 psia.

Thus, the theoretical velocityof the condensate slug from theformula and assumptions abovecould have been as high as 52mph. This is a rough “higherbound” on the velocity of the slugsince it assumes the pressure dif-ferential across the valve equaledthe full steam pressure, and itdoes not account for any resis-

tance to flow.Another way to estimate the

velocity of the slug is to assumethat it attained a velocity atwhich the flowing friction alongits path equaled the pressure dif-ferential pushing it. If the slugwas picked up near Pit 4, the av-erage flowing distance is about375 ft to Pit 3a. There are 12 el-bows between Pits 4 and 3awhere flow energy would begiven up. If the velocity of theslug increased until the flowingpressure resistance, includingthe dynamic losses at elbow andfittings, equaled the ∆P motivat-ing the slug, the velocity of theslug would have been 30.3 ft persec or 20.7 mph.7 We considerthis figure to be a “lower bound”on the slug’s velocity. The over-pressurization that would resultfrom the slug of condensate trav-eling at the lower bound velocityand slamming into the dead endat Pit 3a would be:

At 52 mph (the higher bound onslug velocity), the overpressurewould be 21/2 times as much. Howmuch damage could 1430 psi, thelower bound of overpressuriza-tion, do?

Stress on the valveThe valve in Pit 3a was a Class

150 cast-iron valve built to ASTMA 126 Class B specifications. Thismeans that the ultimate tensilestrength (UTS) of the cast ironfrom which the valve body waspoured was tested to a strength ofat least 31,000 psi (since strengthtests are conducted on separatelypoured test bars in accordancewith ASTM A 126, actual valvebody strength can, and is ex-pected to, vary from 31,000 psi).

Sample coupons cut from thevalve body after the accidentwere tested in a materials lab forUTS. The coupons pulled apartat UTSs from 16,000 to 22,000psi.

Fig. 8 showed that the cast-iron valve was the closest andmost rigid support for the dripleg that absorbed the force of thewater hammer. The water ham-mer force hitting the bottom ofthe drip leg would have created amoment at the valve to resist the

Steam system accident

50 JULY 1995 ■ HEATING / PIPING / AIR CONDITIONING

( ./ )

57 2

1430

lb/ft )(3825 fps)(30.3 fps)(144 in. ft

psi

3

2 2 gc

=

7Psi loss = 65 psi = r/f(L/D + fittings) =375 ft/0.66 ft + 1.975. Solve for Vwhere f is a function of V.

7 in.

7/√ 2 in.

5/√ 2 in.

√8.52 + 102 = 13 in. = 1.08 ft

8.5 in. Side elevation

8.5 in.

8.5 in.

10 in.10 in.

Top view

Horizontal distance centerline of drip leg to gusset

Front elevation

~10 in.

~0.5 in.

B

A

2 in.

4.56 in.

0.5625 in.

14 Moment of inertia at flange neck.13 Pit 3a drip leg.

V P= ∆ / ρ

I = IA +IB + ABd 2

I A = pr3t = p(4.28 in.)3 × 0.5625 = 138.5 in. 4

IB = 112 (0.5 in.)(2 in.)3 = 0.33 in.4

ABd 2 = (0.5 in. × 2 in.)× (5.56 in.)2 = 30.9 in.4

∴ I = 170 in. 4

continued from page 47

movement of the drip leg in re-sponse to the shock. The momentwould equal the moment arm, r,times the impact force. From Fig.13, r = 13 in. or 1.08 ft. F equalsthe overpressurization calcu-lated above times the cross-sec-tional area of the drip leg. Thus,for the lower bound estimate ofoverpressurization, F = 1430 psi3 50.03 sq in. = 71,543 lb. Themoment, M , equals 1.08 ft 371,543 lb, or 77,000 ft-lb (lowerbound estimate).

The valve acted as a can-tilevered beam in supporting thedrip leg. Bending stress, s, for acantilevered beam is: s = My/I,where M is the moment just cal-culated, y is the distance from theneutral axis, and I is the momentof inertia of the beam.

If, as a first cut estimate, wemodel the valve at the flangeneck where it fractured as an an-nular section of 8-in. pipe with a2-in. gusset on top,8 as shown inFig. 14, the moment of inertia is170 in.4, y is 6.5 in. from the neu-tral axis to the point where thegusset broke, and thus, bendingstress is:

The valve was also placed intorsion by the impact of the slugof water, as shown in Fig. 13. Thetorsional stress tending to twistthe neck of the valve flange offcombined with the bending stressincreased the overall stress feltby the gusset and valve neck atthe point of fracture.

In an analogous calculation tothat of the bending stress, I esti-mate the torsional stress on the

valve at the point of fracture toequal roughly 10,000 psi. Thus,the maximum stress due to com-bined effects of bending and tor-sion at the point where the gus-set fractured, combined bymeans of Mohr’s circle equations,would be over 37,000 psi.

The combined stress of 37,000psi is greater than the UTS of thematerial from which the valvewas cast and much higher thanthe tensile strengths exhibited bythe samples cut from the valvebody. While the computation ofthis number is only a first cutbased on some rough assump-tions, it does demonstrate thatthe torque exerted by the lowerbound estimate of overpressur-ization generated by a slug of wa-ter hitting the drip leg couldhave, and likely would have, bro-ken the valve at its gusset andflange neck.

Summarizing . . .By opening the gate valve at

Pit 1 wide open, the operator al-lowed steam to rush into the 800-ft cold piping section and begincondensing at a rate greater thansteam could be supplied by thesteam plant’s single operatingboiler, which was set to manuallow fire. This put the cold pipingsection between Pits 1 and 3a ata negative pressure with respectto the rest of the system. Becausethe system was a loop, steamwanted to flow into the cold pip-ing section from the direction ofboth the steam plant and theback side of the steam systemloop.

A more-or-less permanent pud-dle of 1900 lb of cold residual con-densate resided upstream of Pit 2due to an unintentionally discon-nected trap. Much of this waterwas swept down to Pit 3a by theinitial inrush of 85 psig steamwhen the valve at Pit 1 wasopened. This water, as well asthat being formed by condensa-tion, was not purged from thesystem by the operator using the1-in. blowoff line provided for

that purpose directly upstream ofthe gate valve at Pit 3a, nor didthe operator allow the cold pipingto warm to equilibrium tempera-ture with the rest of the systemand thereby stabilize. Instead, heimmediately traveled to Pit 3a,climbed down the pit, and at-tempted to open the 8-in. gatevalve.

Cold water sat on one side ofthe valve disk, steam on theother side. When the valve’s diskwas raised off its seat, 300+ Fsteam was sucked underneaththe valve disk into the cold pipingsection and was enveloped bycold water. Extremely rapid heattransfer between the steam andcold water instantaneously col-lapsed the steam, creating a voidthat voraciously demanded moresteam to fill it. The rapid conden-sation took the form of a “conden-sate flame.” The only factor limit-ing the rate of condensation andsteam consumption was the flowrate at which steam could feedthe flame. The flow rate was lim-ited by the sonic choked flow ofthe steam beneath the valve disk.The magnitude of the chokedflow was sufficient to evacuate200 ft of the steam main per sec-ond and generate a steam flowvelocity in the 8-in. main of 134mph.

About 400 ft downstream of Pit3a, between Pits 3 and 4, a 75 cuft slug of condensate was drawnto Pit 3a by the sudden evacua-tion of steam. The slug crashedthrough Pit 3, where it crackedthe pit’s cast-iron gate valve andblew out its gasket. The slug con-tinued on to Pit 3a, where itslammed into the pit ’s dead-ended drip leg with a force of over70,000 lb. The drip leg’s closestand stiffest support was the 8-in.cast-iron valve that was can-tilevered off the pit wall. Thebending moment and torsioncaused by the impact force com-bined to stress the valve at itsflange neck and gusset beyondthe ultimate tensile strength ofthe material from which the

HEATING / PIPING / AIR CONDITIONING ■ JULY 1995 51

s =

≅

( ,

,

77 000170

35 000

ft - lb)(12 in./lb)(6.5 in.) in.

psi

4

8This is the point at which the gussetfractured. Since bending tension ismaximum farthest from the neutralaxis, our first cut assumption is thatonly the neck of the valve and 2 in. ofthe gusset participated in resistingbending at the cross-section of thevalve that first gave way.

before attempting to open it. Theresulting explosion severelycracked the cast-iron disk of thegate valve and blew out an ex-pansion joint, killing the workersin the pit as well as a resident ofa nearby apartment building andinjuring 24 others. An implosivecollapse due to extremely rapidcondensation of the inflowingsteam was blamed for the over-pressurization that blew out theexpansion joint and cracked thevalve.9

So what was learned as a re-sult of the 1989 Gramercy Parkaccident, and what was learnedabout this accident, which oc-curred in 1991? Not much, I’mafraid, by those who are in theline of fire.

Steam system operators, ingeneral, are not aware of thistype of water hammer event orthe magnitude of destructivepower that can be released by it.Neither are engineers. Most ofthe scientific and engineeringreferences I found were directedat the nuclear power industry.The operator of our story was al-most certainly unaware of theprevious accident at GramercyPark, as were most people out-side the Con Ed system.10 Therewas no “Notice to Steam SystemOperators” as there would be toairmen after a serious aircraftaccident. The operator of ourstory was probably lulled intocomplacency on that summer dayafter lunch by the laziness of thesteam flow in the mains, theknowledge that the boiler wasfixed at low fire, and the absenceof any eventful episode in thethree years he had worked there.Only a few academicians andsome very gun-shy Con Ed em-

ployees could have predictedwhat might have happened whenhe opened the valve at Pit 3a.

So can’t training get the wordout on accidents of this nature?As of this writing, there is no cer-tification or continuing educationrequired of steam system opera-tors in Georgia, where this acci-dent occurred. That is the case inall but eight states, according tothe National Institute for theUniform Licensing of Power En-gineers. The State of Georgia,which owns the hospital wherethis accident took place, did ad-minister a nine-question test onsteam plant operation to the op-erator subsequent to his employ-ment, and there were records oftraining. But the test, while per-haps adequate to determine em-ployability, was not sufficient toindicate that the operator wasfully trained, and the ongoingtraining was in-house and some-times conducted by the operatorhimself. Still, I imagine this is abetter program than most insti-tutions can probably demon-strate.

More ominous is the fact thatthis accident was never thor-oughly investigated by the State.Once excused from the lawsuitthat was brought by the widow ofthe operator, the State’s interestin finding out what caused theaccident noticeably waned. (Theexcusal was not based on a lackof culpability but on worker com-pensation laws that shield theState from liability.) Effectively,the State left it up to the lawyersand courts to sort out what hap-pened.

The lawyers ’ object ive, ofcourse, was not to get at the ul-timate cause of the accident—especially if aspects of the causewere unflattering to theirclients’ interests. They wantedto win an attractive settlementfor their clients. Their focus wason simple arguments that wouldappeal to the emotions of thejury rather than on the salientfacts of what caused the acci-

Steam system accident

52 JULY 1995 ■ HEATING / PIPING / AIR CONDITIONING

10Training in rapid condensation typewater hammer was recommended forCon Ed personnel as a result of the ac-cident.

9Con Edison, Report of GramercyPark Steam Explosion: August 19,1989, December 1989.

valve was cast. The valve frac-tured, spewing hot condensate,flashing steam, and live steaminto the pit.

Was accident preventable?This accident would not have

happened if:● The operator had allowed the

800-ft pipe section to warm tosteam temperature before at-tempting to open the gate valve atPit 3a.

● The operator had blown offthe residual condensate restingupstream of the valve at Pit 3abefore attempting to open thevalve.

● The trap at Pit 2 had been op-erational, thereby preventing for-mation of the large puddle of coldcondensate that resided upstreamof Pit 2.

● The drip leg at Pit 3a had notbeen inadvertently cantileveredoff the cast-iron gate valve (allthis required was a 3-in. pipe sup-port beneath the drip leg to thefloor of the pit).

● The gate valve had been steelrather than cast iron.

● The steam system had notbeen a loop.

● The boiler at the steam planthad not been on manual low fire.

● The valve at Pit 3 had bro-ken, relieving the pressure beforethe slug reached Pit 3a.

Could it happen again?Yes. A very similar accident

with almost identical causes oc-curred two years earlier in theCon Edison system that servesNew York City. Below the streetin a manhole in the GramercyPark region of the city, two ConEd employees were killed whenthey opened a 24-in. gate valve,allowing 150 psig steam on oneside of the valve to backflow intocold condensate resting on theother side of the valve. Just as inour accident, the workers werereactivating a cold portion of alooped steam network and didnot drain the cold condensate di-rectly upstream of the gate valve

dent. To them, the pictures ofthe scalded body of the deceasedoperator were of much greatersignificance than the photos ofthe broken valve.

The engineers who came for-ward as expert witnesses shouldhave helped keep the focus onwhat really happened and whatcaused it. They did not. The opin-ions of the consulting engineers,in too many cases, seemed to con-form plastically to the interestsof the side by which they werebeing paid. In the plaintiff ’scase, I believe this extended ef-fectively to egging on the attor-ney with exaggerated stories ofmalfeasance by the engineer andcontractor who were the defen-dants—e.g., “No competent engi-neer would specify cast-ironvalves in a high pressure steamsystem”; “Steam pits are totally

unsafe, and no operator in hisright mind would go into one ofthose steam pits”; “The operatordid nothing wrong—he just hap-pened to be in the wrong place atthe wrong time”; and “The con-tractor who retrofitted the pitsdid not provide proper trainingfor the steam plant operators.”The lawyers, of course, rewardedthose who told them what theywanted to hear with handsomehourly fees to give depositionsand such. Unfavorable informa-tion or doubt as to the origin ofthe accident was not well re-ceived.

As might be expected, thisturned out to be a lousy way toget at the truth. Three years af-ter the accident, the case settled.No statement was ever releasedas to who or what was to blame.The State has never undertaken

to get the word out on whatcaused the accident or to trainits steam system workers atother institutions on precautionsto take to avoid a similar acci-dent. In light of the fact that sev-eral of the State’s other large in-stitutions were designed by thesame engineering firm that de-signed this steam system (andthat has long been out of busi-ness), I believe the State hasbeen less than vigilant in fulfill-ing its responsibility to protectthe safety of its workers.

This accident was a tragedy forthe operator who died in the pitand the wife and four children heleft behind. More tragic will bethe next steam system operatorwho dies a horrible death in asteam pit because of a failure tolearn from this accident and dis-seminate what was learned. V

HEATING / PIPING / AIR CONDITIONING ■ JULY 1995 53