Condensation Induced Water Hammer

8/4/2019 Condensation Induced Water Hammer

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Steam Condensation Induced WaterhammerJan '98 HPAC Article by Wayne Kirsner, P.E.

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The steam pipe started to vibrate and shake. Don yelled at Clyde--"Let's getthe Hell out of here...this thing's going to blow!" Clyde stuck his head outfrom beneath the steam pipe from where he was removing insulation. Heheard a loud roar rumbling down the steam line like a freight train comingfrom the direction of the C-4 Manhole. Don was already clamoring up the

exitbehind Don. Don was trying to break through the Visqueen plastic sheetthat covered the manhole. It was sealed tight to prevent asbestos fibers fromescaping. A white steam cloud rolled down the utilidor fr ladder. Clyde slidfrom beneath the maze of pipes and scrambled up the ladder om the directionof C-4 and began to flood the manhole. Another worker fleeing theencroaching steam crawled up behind them. Together they desperately torethrough the stubborn Visqueen seal until it finally gave way, shoved open thesteel hatch above, and tumbled out into the fresh air above. The swelling heatfrom the utilidor rose around them. Up top there was pandemonium. Steamwas billowing out the C-4 manhole as well as the manhole they'd just exited.Fire engines were arriving. Men were shouting trying to figure out who was

still down in the utilidors. Bobby and Wayne had not gotten out.

Moments earlier, before the Accident, Bobby had opened the 10" gate valveat Manhole C-4 a second incremental turn. He thought "this is strange, thevalve's handwheel spun freely". Just fifteen minutes earlier, he'd "crackedopen" the 10" cast iron valve to admit steam into the 2,200 foot steam line tobegin warming it up. For 3 weeks now, he'd been energizing the G-line at theend of the asbestos worker's shift and had never had the system warm up thisquickly. It usually took from 30 to 45 minutes. When the handwheel spunfreely, he understood the lack of friction to mean that steam pressure on eitherside of the valve had equalized, so the warm-up was complete and he could

open the valve the rest of the way. This seemed too quick though. He'd bettercheck with his supervisor before spinning the valve open the rest of the way.

Bobby nudged past his co-worker, Wayne, as he made his way over to thematerial passout and yelled up through the plastic flaps to his boss --"She'sspinnin' freely, is it okay to open her up all the way?". The supervisor waspuzzled too. "No", finally came the muffled response, "better continue toopen her a little at a time like we were told to do". About a minute hadelapsed since Bobby had opened the valve enough to lift it off its seat. As heturned back to the valve, a "pop" was heard. Then a moment later,"KABOOM"! Hot water and steam exploded from the 10" valve. A whitecloud of flashing condensate and steam engulfed the utilidor with a palpablewave of heat. Wayne was knocked down and stunned by the scalding waterspraying from the valve. Egress via the manhole exit was cut off by steamspraying from the valve. The only way out appeared to be through the

Page 1 of 14Steam Condensation Induced Water Hammer by Wayne Kirsner, P.E.

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material passouts constructed into the roof of the utilidor. Bobby clamberedup on top of the pipes and jumped up catching his armpits above the opening.From there he was able to hoist himself through the plastic covered opening.He emerged with second and third degree burns but otherwise okay.

Wayne stumbled through the piping to the other material passout. His firstump was too weak and he fell back onto the piping which by now was

becoming slippery with condensing steam. Air temperature in the utilidor wasapproaching 200 degreeF. Wayne desperately collected himself. He knew thatthis might be his final chance. He groped his way back up onto the slipperypipes, took a breath of the searing air, and leapt up again into the plasticcovered opening. This time he was able to hook one elbow above the rim and,with his life on the line, kick up through the opening.

Clyde and Don saw Wayne crawl out thru the plastic flaps of the materialpassout. He rose to his feet and started screaming for help. His protectiveclothing was shredded, loose skin was sloughing off his exposed arms andlegs. He was badly burned. Clyde yelled at passers-by to call an ambulance asthey ushered Wayne away from the steaming manholes. Soldiers with aknowledge of first aid rushed him to a barracks across the street and started toapply cold packs to his burns and give him cold drinks. Wayne's throat wasbeginning to constrict. An ambulance arrived to rush he and Bobby to theHospital. As the injured workers were being cared for, Clyde turned his furyon his supervisor -- "You stupid son-of-a-bitch, we told you this wouldhappen."

What Happened

For four weeks asbestos workers had been removing asbestos insulation fromthe 2,200 foot section of steam main known as the G-Line and the 120 footH-Line. (See Figure 1) Like all steam mains at Fort Wainwright, Alaska, theG and H Lines ran underground in narrow utilidors filled with pipe.Originally, the contractor had tried to abate the steam main with the linesenergized. This proved to be near impossible for the workers. Utilidortemperatures reached 160 degree F as insulation was removed from the 325degree F pipe carrying 80 psig steam. Laborers who had to be suited-up andmasked to work in the asbestos laden environment were dropping like fliesfrom the heat and/or quitting. The contractor was forced to seek relief fromthe Owner. A compromise was negotiated after the first week-- steam wouldbe de-energized at midnight before each workday, asbestos abators would

start work at 4:00 a.m. and finish by noontime at which time steam would berestored. The asbestos removal contractor would be responsible for de-energizing and re-energizing the steam line daily. For the three weeks beforethe accident this was the procedure. By the beginning of the laborers'workday, temperatures in the utilidors were still around 120 degree F but,with frequent breaks to cool off and re-hydrate, conditions were tolerable.




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Fig 1 Isometric view of G- and H-Lines (no scale).

Unfortunately, the discomfort to the workers was not the only consequence ofremoving the insulation from active steam mains that had gone unforeseen.There was also the effect on the steam traps. 3/8" thermodynamic traps wereinstalled at each manhole except C-4 which contained a 1/2" trap. At thesystem's operating conditions, the 3/8" traps could remove 295 pounds ofcondensate per hour . With 3-1/2" of insulation, 300 feet of 12" pipegenerates 41 pounds of condensate per hour. Thus, for a typical pipe segment,the traps had better than a 7 to 1 safety factor for condensate removal with theline insulated. With the insulation removed, however, heat loss increasedalmost 18 fold so that condensate formation jumped to 729 #'s/hr over 300feet of pipe. At this rate of heat loss, the 3/8" traps had less than one-half the

capacity needed to keep up with the condensate production. This was notgood.

Abatement began at Manhole G-1 and headed south toward C-4 at the rate ofabout 125 feet a day. As abatement proceeded down the G-Line, local trapsserving the uninsulated portion of the line were overwhelmed withcondensate during the period the lines were energized each day. In the firsttwo weeks, however, this didn't cause a problem. Excess condensate merelyrolled down to C-4 on the south end and G-1 on the North end. Traps on thesouth end still serving insulated portions of the line had adequate capacity toremove the excess condensate. On the north end, the steam valve was left

closed so trouble was avoided. After two weeks of daily start-ups withoutserious incident, save some minor waterhammers, asbestos crew operatorsgrew confident that start-up of the steam line was no big deal. By thebeginning of the third week, insulation removal had reached Manhole G-9.Calculations show that at this point the rate of condensate being generated inthe southern section of the G-Line began to exceed the net capacity of thetraps to remove it.

Condensate accumulation during steam operation is potentially destructive,but even so, as long as condensate is religiously drained everyday beforestart-up, a catastrophic waterhammer accident might still be averted. Theproblem was--condensate wasn't being drained religiously. The asbestosworkers given responsibility for energizing the steam main daily didn't fullyappreciate the danger inherent in starting up a high pressure steam systemwith condensate in it. They did not routinely open drain valves to bleed the




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system of excess condensate either at night, when they shut the system down,or at noontime, when they re-admitted steam through the C-4 valve to re-energize the steam main. Their belief was that steam admitted through the C-4 valve would blow condensate to the far end of the main at G-1. Thus, intheir view, only the drain at G-1 "really" needed to be opened at start-up.Accordingly, there was a tacit understanding that the bleeder valve at G-1would be opened daily by the quality control supervisor for the primecontractor, and any condensate that wasn't drained at start-up, they apparentlythought, would be mopped up by traps after start-up.

As the third week began, the severity and frequency of waterhammer began toaccelerate. Residual condensate accumulated in the steam pipe at C-4 due notonly to operation of the uninsulated steam main, but also due to condensateformed at start-up that went undrained. Early in the third week, heavybanging forced workers to evacuate the utilidor. Clyde, one of the more vocalevacuees, warned the abatement supervisor "this thing sounds like its ready toexplode... What are you going to do about it?"

By Wednesday of the third week, all the insulation had been stripped from theG and H Lines. The lines were completely bare. By the next morning, the dayof the Accident, I calculate that enough condensate would have accumulatedat C-4 to completely fill the line adjacent the valve and extend over 300 feetup the steam line toward G-9 . In addition, condensate accumulated in the 120foot long H Line. Due to a design oversight, there was no drain or trapupstream of the gate valve at H-1. The contractor, not comprehending thepitch of the H Line, did not realize that condensate would accumu-late againstthe H-1 valve during the three weeks of on-off steam operation. Hence theline filled with condensate as depicted in Figure 2.

Figure 2 H-Line full of condensate to overflowing.

Condensate also accumulated each night in the double-elbow riser to thesouth of the C-4 valve. (See Figure 3 below). During the period aftermidnight when the C-4 valve was closed, steam would condense in theuninsulated double-elbow riser and come to rest against the south side of theclosed valve. From midnight until noon the following day, enough condensateaccumulated to almost fill the riser.

Figure 3. Conditions Before C-4 Valve Opened




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One other event shaped the conditions just prior to the accident. On the day ofthe accident, there was a change in the start-up procedure. In order to isolateanother steam main for repair work, the contractor needed to energize the GLine early. The asbestos crew was instructed to start up the G Line an hourand fifteen minutes before their quitting time. In addition, but unbeknownst tothe asbestos crew, the contractor's quality control supervisor decided toexpedite warm up the G-Line by admitting steam through the G-1 valve at thefar end of the G Line as well as through C-4. The G-1 valve was opened asmuch as 30 minutes before Bobby first cracked open the C-4 steam valve.This had the likely effect of sweeping undrained condensate residing againstthe G-1 valve on the north end of the line south to C-4, and completely fillingthe H-Line as explained in the sequence offigures a thru c in sidebar atend of article. The situation, then, 15 minutes before the Accident as Bobbyreadied to crack open the C-4 valve, is as shown in Figure 3.

Subcooled condensate filled the steam line on both sides of the C-4 valve aswell as completely filling the H Line. High pressure steam admitted throughG-1 had pressurized the steam main and was sitting atop the condensate on

the north side of C-4. The south side of the valve was also under steampressure which, based on testimony, was likely slightly less that that on thenorth side.Now, put yourself in Bobby's place, except, assume you know all theinformation described above, i.e., in your mind's eye, you can 'see' the buildup of condensate shown in Figure 3 and figure out, based on the ease withwhich the valve's handwheel spun, that there is full steam pressure atop thecondensate. Ask yourself two questions:

#1. Is this Situation Dangerous? Some steam people would say "no, aslong as there is no fast moving steam, there's no danger of

waterhammer. Opening C-4 slowly and incrementally should preventsteam or condensate from moving quickly and thus prevent awaterhammer." This is wrong, dead wrong. High pressure steam incontact with subcooled condensate is dangerous. It's a recipe forCondensation Induced Waterhammer. The sidebar (See sidebar nearbottom page) explains why this type event is 10 to 100 times morepowerful that conventional "steam flow" driven waterhammer.

#2. What Would You have Done in Bobby's place? If your answer is,"I'd first open the C-4 bleeder valve to drain the condensate," you'retoast. Although this is the answer most steam operators would give, it

will trigger the accident. Neither the bleeder valve nor the steam valvecan be opened without provoking this accident. To understand why, it'scrucial for steam fitters and operators to understand the mechanism ofCondensation Induced Waterhammer.

Condensation Induced Waterhammer

A condensation induced water hammer is a rapid condensation event. It couldalso be aptly termed a "rapid steam bubble collapse". It occurs when a steampocket becomes totally entrapped in subcooled condensate. As the steamgives up its heat to the surrounding condensate and pipe walls, steam changes

from a vapor to a liquid state. As a liquid, the volume formerly occupied bythe steam shrinks by a factor of from several hundred to over a thousand,depending on the saturated steam pressure. Likewise, the pressure in the void




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drops to the saturated vapor pressure of the surrounding condensate. (Forexample, the saturated vapor pressure of condensate at ambient temperature isless than 1 psia). This leaves a low pressure void in the space formerlyoccupied by the steam that the surrounding condensate, under steam pressureitself, will rush in to fill. The resulting collision of condensate generates anoverpressurization which reverberates throughout the condensate filledportion of the pipe. How severe is the over-pressurization? Remember thatwater is virtually incompressible. In a collision, it does not give. Think of thelast time you did a belly flop off the low dive. The water felt pretty "stiff"didn't it?

The specific factors which influence the severity of a condensation inducedwaterhammer are: (1) the steam pressure, (2) the degree of condensatesubcooling, (3) the presence of non-condensables left over in the void, and (4)the size of the void. If the steam pressure is high, the condensate is subcooled,non-condensables are absent, and the void is large enough for a slug to pickup some velocity, the overpressure resulting from an event can easily exceed1000 psi. This is enough pressure to fracture a cast iron valve, blow out a

steam gasket, or burst an accordion type expansion joint. And, in fact, failureof each of these components in separate condensation induced waterhammeraccidents has resulted in operator fatalities.

One might ask at this point, "But wait, isn't it common for steam andcondensate to come into contact in a steam system?" Good design andoperating practice aim to avoid mixing high pressure steam and excesscondensate by making sure steam mains are properly trapped and live steamis kept out of condensate return systems. Nevertheless, it does happen.Condensate lines, for instance, are often heard to pop and bang when steamsquirts into them through traps. Why don't the collapsing steam bubbles

destroy condensate pipes? They can over time. But the shock wavesgenerated are not catastrophic because the pressure in a condensate system isgenerally low-- on the order of just a few psi, subcooling is not great, and thesteam bubbles are small. Of course, high pressure steam can contactsubcooled condensate in steam lines when something goes wrong-- forexample when a trap assembly becomes plugged with scale causing a drip legto fill with condensate. Why don't situations like this result in destructivecondensation induced waterhammer? One reason is pipe geometry. A steambubble must become entrapped for there to be a collapse. In a vertical pipesuch as a drip leg where steam is above the condensate, it's difficult to entrapthe steam because natural buoyancy tends to keep the two fluids separate . In

fact, research experiments show that it's difficult to entrap a steam void in anypipe sloped downward in the direction of steam flow more than 1/2" in 1.0foot . At slopes less than this, however, and in upwardly sloped pipes, it's adifferent story.




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Fig 4 Steam entrapment and slug formation in a horizontal pipe.

At Fort Wainwright, the pipe slope to C-4 is 1/4" in 10.0 feet-- normal for asteam line. Thus the line is nearly horizontal. How does steam resting atopsubcooled condensate in a nearly horizontal line become entrapped? Thesequence below explains. (Fig 4)

1. Steam residing over subcooled condensate losses heat to the

condensate and the surrounding pipe which causes the steam tocondense. The continual loss of steam, induces fresh steam to flow in toreplace it. Steam flow over condensate will tend to draw up a wave inthe condensate via the Bernoulli effect .

2. If the rate of heat transfer is rapid enough for a given condensatelevel, the induced steam velocity will draw up a wave high enough toseal the pipe.

3. The creation of a seal immediately isolates the downstream steampocket from the upstream supply creating a steam pocket. Ongoing

condensation in the isolated steam pocket drops the pressure causing aslug to accelerate into the void.

The formation of a condensate seal is a necessary condition for a rapidcondensation event in a horizontal line. Often, however, heat transfer is notrapid enough to induce sufficient steam flow to seal the pipe and cause arapid steam bubble collapse. In fact, generally, to initiate a condensationinduced waterhammer in a horizontal line where neither steam or condensateis flowing through the pipe, a "trigger" is needed. That's because in a non-flowing situation, heat transfer between the steam and condensate is retardedby a stagnant layer of hot condensate and non-condensables laid down by

steam as it condenses atop cooler condensate. This interphase boundary layerinsulates the steam void. On the one hand, the layer prevents rapidcondensation, but on the other, it allows a potential steam void to grow inmagnitude and potential energy like an over expanded balloon. Often times




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there will be no rapid condensation event if the layer goes undisturbed. Steamwill fill a pipe atop subcooled condensate without incident. Minor collapsesmay occur, but due to the lack of rapid heat transfer, they will be mild and gounnoticed. If, however, the insulating layer is disturbed in such a way that thelayer is breached at some point, then the local intrusion of subcooledcondensate can result in a chain reaction which shatters the entire insulatinglayer. In a millisecond, the rate of heat transfer can increase a thousand foldinducing a rapid steam influx which seals the pipe and sets off a rapidcondensation event resulting in condensation induced waterhammer. The key,then, to whether or not an event is initiated depends on the occurrence of atrigger to cause interface shattering.

Back at the Accident

Now, return to Manhole C-4 fifteen minutes before the Accident. Bobby hadopened the bleeder valve at C-4 for the first time per a special instructionfrom the QC supervisor. He then proceeded to crack open the C-4 steamvalve. Both of these actions presumably resulted in condensate draining fromthe system on the north side of the C-4 valve. The pipe volume vacated by thedraining condensate at C-4 drew in steam along the top of the pipe from thenorth to replace it (Figure 5). Fifteen minutes after the first crack, Bobbyopened the C-4 valve again, this time lifting the disk 1/2" off its seat. Thisaction further speeded the removal of condensate and the advance of steamalong the top of the pipe towards C-4.

Figure 5. Steam Encroachment As Condensate is Drained




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Photo 1:The model pictured above was constructed to simulate the accident.

From this point on, I'll describe what we understand happened based on testswith this model and others used to understand the accident. (Fig 6)

Fig 6 Frame-by-frame "animation" of a condensation-induced waterhammerevent.




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As the tongue of steam reached down the nearly horizontal line toward C-4, itprobably collapsed several times as seals developed, but the collapses werenot violent enough to be termed waterhammer events. When the steam finallyreached the vertical opening to the H Line, the steam licked up around thecorner seeking to buoyantly flow up into the H Line riser. This was the triggernecessary to set off the event. The tip of the tongue distended, then detached,releasing a bubble containing steam and non-condensables which rose up intothe vertical H Line while an equal volume of subcooled water spilled downinto the G Line. The remainder of the steam tongue snapped back into the GLine after releasing the bubble. The release of the bubble and the exposure tothe cool condensate assaulted the stability of the boundary layer. It caused aripple to reverberate down the length of the steam-condensate interfaceperturbing it and quickening heat transfer. This could have been sufficient totrigger the event. It depends on how much air has seeped into the systemduring cooling to suppress a collapse. As condensate continued to drain,steam advanced toward the H Line opening a second time again peakingaround the corner, and again releasing a bubble of steam and non-condensables. This time the interface shattered. The entrapped steam pocket

collapsed hard drawing a slug of water from the north toward C-4 crashinginto the collapsing void faster than the eye could follow. The collision of theslug with the condensate at C-4 created an overpressurization that reboundedthrough-out the water filled portion of the system including up the H Linewhere Clyde and Don would have been working.

At Fort Wainwright, we believe the overpressure caused the double-elbowriser at C-4 to compress as shown below.(Fig 7).

Figure 7 Illustration of a valves deflectionduring the waterhammer event.

The pipe and valve flanges twisted in response to the deflection of the double-elbows riser. The twisting flange caused the cast iron valve body to crack atthe flange neck causing first condensate, then steam to spray from the valve.




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The actual damage to the valve is shown in Photo 2.

Photo 2 This shows the actual damage to the valve.

Could this accident have been prevented?

Of course. Numerous procedural blunders should be obvious to experiencedsteam operators and their supervisors as they read this article, not the least ofwhich is assigning responsibility for start up of a high pressure steam system

to asbestos workers. But I'm most interested in putting this question to theguy who's in the last line of defense-- the steam operator standing in Bobby'sshoes with his hands on the valve's handwheel just moments before theaccident. Suppose from the feel of the valve's handwheel, he surmises thatthere must already be full steam pressure on the steam line, and he believesnot only that the line contains subcooled condensate, but that it is FULL andresting against the valve he is about to open. The question is: Is it possible,given the circumstances with which he's confronted, to avoid this accident?

The answer is YES. But, there's only ONE WAY. Cut the steam off. Don'topen the C-4 steam valve. Don't open the bleeder valve. You've got to exit the

manhole and close the G-1 steam valve, then drain the lines to empty thecondensate. This is what must be done to avoid a condensation inducedwaterhammer in the situation described. Trying to drain the condensate withhigh pressure steam atop the subcooled condensate will trigger a rapidcondensation event.

In Conclusion...

Here's what I want steam fitters and operators to know:

1. High pressure steam in contact with subcooled condensate is an

unstable and potentially explosive mixture.

2. Don't admit steam into a line filled with subcooled condensate. In




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fact, always be wary of admitting steam to any cold steam line if youcannot be absolutely certain that the line's been completely drained.

3. Allowing subcooled condensate to flow into a steam filled line ismore dangerous than admitting steam into a line with subcooledcondensate.

4. If you suspect a pressurized steam line is filled with subcooledcondensate, don't attempt to drain the condensate. Shut the steam offfirst, then drain the condensate. If you do open a drain, and the linehammers, close it and get the steam off. The line may continue tohammer until you get the steam off.

5. A mixture of steam above subcooled condensate can sit dormant inan isolated steam line like a loaded gun awaiting a triggering event.Opening a valve to admit steam or opening a bleeder to draincondensate can trigger an event. Don't let yourself or those yousupervise inadvertently pull that trigger without first making sure thegun is unloaded.

THE END

S I D E B A R

Condensation Induced Waterhammervs.

Conventional "Steam Flow" DrivenWaterhammer

"Waterhammer" according to a major steam trap manufacturer's engineeringguide is "the impact caused by a sudden stopping of a rapidly moving slug ofwater". The guide goes on to explain that" [unless] condensate is removed from low points...ripples form on thecondensate surface ...until condensate so restricts steam flow that a slug ofcondensate is carried down the main by the steam. The slug of water travels atthe speed of steam (which may be in excess of 100 mph) until some

obstruction is reached...[and]...the slug of water is suddenly stopped oftenwith disastrous results..."

The waterhammer described above is but one type of waterhammer. I term it"steam flow driven waterhammer". It describes an impact event where a slugof fast moving water strikes a stationary object and gives up its momentummuch like an ocean wave striking a sea wall. The formula for the maximumimpact pressure over the target area is:




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where "rho" is fluid density and "v" is slug velocity. For water at 60 #/c.f. andv = 100 mph, Pmax = 279 psi. Lab experiments indicate that peak pressuresfor actual events are typically less than the maximum theoretical value .

Condensation Induced Waterhammer is a different animal. The pressure pulsegenerated by a Condensation Induced Waterhammer is due to thecompression of water by a piston formed by a moving plug of water. This isthe same phenomena which generates waterhammer in a hydronic singlephase system (i.e. plumbing). The formula to calculate the magnitude of themaximum pressure pulse is:

where "c'" represents the speed of sound in water--about 4,300 fps. Note thatthis formula is similar to the former except that "c" replaces one "v". What'sthe relevance of the speed of sound? The sonic speed squared, "c2", is inessence a shorthand notation for the ratio of the stiffness of the material,represented by Young's Modulous "E", divided by the density of the material;i.e.

Clearly, the magnitude of a pressure pulse reverberating through, say, a pieceof steel compressed upon collision would in some measure be a function of

the stiffness of the steel. Same for water. Hence, the dependence on "c" isreally a dependence on "E".

At 4,300 fps, "c" is roughly two orders of magnitude larger than "v". Thus,the overpressurization generated by Condensation Induced Waterhammer canbe 10 to 100 times greater than that caused by steam flow drivenwaterhammer.

S I D E B A R

Condensate Collection in Vertical Lines




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Condensate will fill a vertical take-off like the H-Line against gravity if thehorizontal line beneath it becomes filled or nearly filled with condensate. Toillustrate this point, I have exaggerated the rise of the H-Line in the figuresbelow. Fig. A shows steam flowing into all open portions of the steam lineand condensing. The condensing steam causes a reduction in local pressurethat induces steam movement to flow in to replace it.

If the horizontal section of steam pipe fills or be-comes nearly full, acondensate seal forms that isolates the steam downstream of the seal.Condensing steam in the pocket causes the pressure to fall. The fallingpressure in the isolated steam pocket will then suck up condensate into thepocket to fill the void. The result is shown in Fig. C.


Condensation Induced Water Hammer

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