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W

ater hammer a high-pres-sure surge or wave createdby the kinetic energy of the

moving liquid is not onlya system issue, but primarily a safetyconcern. Understanding the nature andseverity of water hammer in a steam-and-condensate system will allowprevention of its destructive forces.

A better understanding will also helpwith the introduction of preventativemeasures into system designs, steamsystem startups, maintenance andinstallations, which can contribute topersonnel safety, reduce maintenancecosts, and reduce system downtime.

In its most severe form, water ham-mer can cause injuries or even fatali-ties. Unfortunately, 82% of the steamsystems in North America are expe-riencing some type of water hammer.Many mistakenly believe that waterhammer is an unavoidable and natu-ral part of steam-and-condensate sys-tems; this is entirely false. Water ham-mer is never normal, it is abnormal.If the system is properly designed andcorrectly operated, water hammer willnot occur. It is possible for high-pres-sure steam systems to function with-out water hammer over a long opera-tional life.

Water hammer can occur in anysteam or condensate line. Its effectscan be more pronounced in heteroge-

neous or condensate bi-phase systems.Condensate bi-phase systems containtwo states, the liquid (condensate) anda vapor (flash or generated steam).This bi-phase condition is found insteam systems where condensate co-exists with generated steam or flashsteam. Typical examples include heatexchangers, tracer lines, steam mains,condensate-return lines and some-times pump-discharge lines.

A common example of water ham-mer occurs during the startup or ener-

gizing of a steam system. If the steamline is energized too quickly withoutproper warm-up time and the conden-sate created during the startup is notproperly removed, water hammer willbe the result.

Effects of water hammer

The effect of water hammer cannot beunderestimated. Its forces have beendocumented to result in the collapse ofelements within all designs of steamtraps including the cracking of steamtrap bodies. Water hammer can over-stress pressure gauges, bend internalsystem mechanisms and otherwise

impair inline analytical equipment.Ruptured piping systems and pipefittings, broken pipe welds, as well

as valve, pipe support, and heat-ex-changer-equipment tube failures canall occur with prolonged exposure towater hammers effects. When severe,it can result in not only damage toequipment, but also significant injuryto plant personnel.

Water hammer may be occurringand yet remain silent to personnel.This means that water hammer is notalways accompanied by audible noise.For example, a steam bubble may besmall in size and yet the collapsing

bubble creates a thermal shock thatis not heard by the human ear. How-ever, damage to steam and condensatecomponents is still occurring.

The continuing banging and otheraudible sound that may accompanywater hammer should be interpretedas the way the steam system is tryingto communicate with plant person-nel. This audible noise should be analarm meaning fix the water ham-mer problem or damage will occur.This water-hammer sound meanssomething in the system is wrong andneeds to be corrected.

Evidence gathered while conduct-

Feature Report

40 ChemiCal engineering www.Che.Com april 2008

Feature Report

The Number OneProblem in a Steam System:

Water

HammerThere is only one time

to correct water hammer

immediately

Kelly Paffel, Swagelok Company

Figure 1. This drawingof a standard heater-unit

installation shows a steamline off the top and conden-

sate line that incorrectlyreturns to the bottom of the

condensate header

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ing root-cause analysis on steam-com-ponent failure suggested that waterhammer causes 67% of premature

component failures.

Causes of water hammer

The following four conditions havebeen identified as causes of the violentreactions known as water hammer: Hydraulic shock Thermal shock Flow shock Differential shockThe following is a description of eachof these causes.

Hydraulic shock. A small percent-

age of the water-hammer problemsfound in steam systems are caused byhydraulic shock. This condition can beeasily described by using the exampleof a household faucet. When the fau-cet in a home opens, a uniform massof water moves through the pipes fromthe point where it enters the house tothe outlet of the faucet. This could be a200-lb quantity of water moving at 10ft/s or about 7 mph.

When the faucet is suddenly shut, itcould be compared to a 200-lb hammercoming to a stop. There is a notice-able bang heard in the system whenthe faucet is closed. This shockwave

sound is similar to a hammer hitting apiece of steel. The shock pressure waveof about 300 psi is reflected back and

forth from end to end until the energyis dissipated in the piping system.

This is the same action that cantake place in the suction or dischargepiping in a steam and condensate sys-tem. Pumps are often installed withcheck valves. As the pump starts andstops, hydraulic shock can occur asthe flow rapidly stops and the check

valves restrict the flow in one direc-tion. Slow closure of the valve, justlike slow closure of a faucet, is the so-lution to this problem. When a column

of water is slowed before it is stopped,its momentum is reduced graduallyand, therefore, damaging water ham-mer will not be produced.Thermal shock. One pound of steamat 0 psig occupies 1,600 times the

volume of a pound of water at atmo-spheric conditions. This ratio dropsproportionately as the condensate linepressure increases. When the steamcollapses, water is accelerated into theresulting vacuum from all directionswith great speeds.

In bi-phase condensate systems,steam bubbles may be introducedbelow the level of condensate in a con-

densate line. For example, a branchline from a steam trap may be piped tothe bottom or side of a condensate main

header (Figures 1 and 2). The pressurein the condensate line is lower thanthe flash steam temperature (lowerpressure yields lower temperature).The condensate cools the flash steambubble and the steam bubble collapsesimmediately. While collapsing, a void iscreated in the volume of the pipe andcondensate rushes to fill this void, thuscausing an audible pinging sound.

Flow shock. Flow shock is mostcommonly caused by lack of properdrainage ahead of a steam-line-isola-

tion valve or steam control valve. Forexample, consider a steam-line-isola-tion valve (typically used with pipe of3-in. dia. or larger) opened withoutthe use of a warm-up. When the large

valve is opened, steam rushes down acold pipe producing a large quantityof condensate at high velocity. Thiscondensate will continue to build inmass as it travels along the pipe anda large wave of condensate is created(Figure 3). The wave will travel at ahigh velocity until there is a suddenchange in direction, possibly an elbowor valve in the line. When the conden-sate changes direction, the sudden

ChemiCal engineering www.Che.Com april 2008 41

Figure 2. Red circles show improper connectionto the condensate header. Instead of connectinginto the side of the condensate header,the returns should enter in the top ofthe manifold (condensate header)

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Figure 4. This AutoCAD print shows the standardinstallation of an isolation valve in a steam system.

Two main points are the warm-up valve and the drip legpocket with a steam trap ahead of the isolation valve.

This installation will prevent water hammer duringstartup, but it will also promote long valve life

Figure 3. As steamrushes accros this cold

pipe, a large wave of highvelocity condensate is

formed, creating a waterhammer effect on the elbow fitting

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stop will generate water hammer.When a steam control valve opens,

a slug of condensate enters the equip-ment at a high velocity. Water hammer

is produced when the condensate im-pinges on the heat exchanger tubes orwalls. Additionally, water hammer fromthermal shock will result from the mix-ing of steam and condensate that fol-

lows the relatively cooler condensate.Differential shock. Differentialshock, like flow shock, occurs in bi-phase systems. It occurs whenever

steam and condensate flow in thesame line, but at different velocities.This is commonly seen in condensate-return lines.

In bi-phase systems, the velocity of

the steam is often 10 times that of theliquid. If condensate waves rise and filla pipe, a seal is formed temporarily be-tween the upstream and downstream

side of the condensate wave. Since thesteam cannot flow through the con-densate seal, pressure drops on thedownstream side. The pressure differ-ential then drives the condensate sealat a high velocity downstream accel-erating it like a piston. As it is drivendownstream, the wave of condensatepicks up more liquid, which adds tothe existing mass of the slug and the

velocity increases.Just as in the example above, the

slug of condensate gains high momen-

tum and will be forced to change direc-tion due to a tee, elbow or valve in theline. The result is usually great dam-age when the condensate slug poundsinto the wall of a valve or fitting whilechanging direction.

Since having a bi-phase mixture ispossible in most condensate returnlines, correctly sizing condensate re-turn lines becomes essential.

Condensate normally flows at thebottom of a return line with the as-sistance of gravity. Condensate flows

naturally because of the pitch in thepipe and also because the higher ve-locity flash steam above it, pulls italong. The flash steam moves at ahigher-velocity because it moves bydifferential pressure.

Flash steam occurs in return lineswhen condensate discharges intothese lines that are operating at alower pressure. The lower pressurecauses a percentage of the condensateto flash back to steam at the givensaturation pressure. If the lines are

also undersized, additional pressureis created in the line. This pressurepushes the flash steam at relativelyhigher velocities toward the conden-sate receiver, where it is vented to at-mosphere. Heat loss of the flash steamwhile moving in the line causes someof the flash steam to condense, whichcontributes to this pressure differenceand amplifies the velocity. Becausethe flash steam moves faster than thecondensate, it makes waves. As longas these waves are not high enoughto touch the top of the pipe and do notclose off the flash steams passageway,there is not a problem. This is why

Feature Report

42 ChemiCal engineering www.Che.Com april 2008

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larger-sized condensate return linesare preferred.

To control differential shock, the con-densate seal must be prevented from

forming in a bi-phase system. Steammains must be properly trapped andcondensate lines must be properlysized. The length of horizontal lines tothe traps inlet should be minimized.

Steam-main drainage is one of themost common applications for steamtraps. It is important that water isremoved from steam mains as quicklyas possible, for reasons of safety andto permit greater plant efficiency. Abuild-up of water can lead to waterhammer, and as we have already dis-

cussed, the water hammer can haveany number of adverse effects on thesteam and condensate components ofa system.

Prevention or resolution

There are a variety of design or systemchanges that can be implemented toprevent or eliminate water hammer.

Proper training and well-docu-mented, standard operation proce-dures (SOPs) should be provided toplant personnel for steam system

startups, shut downs, maintenanceand general operation. Maintenanceprograms, in particular, should be de-signed to take a pro-active approachon water hammer. Pipe insulation, forinstance, should be regularly checkedand repaired as needed. Doing this willsave energy and reduce accumulationof condensate in the piping system.

Installation standards for steamcomponents should be implementedand rigorously enforced to ensure cor-rect steam and condensate design. For

steam traps, these standards shouldinclude their proper sizing and gen-eral suitability for each application.Steam-line-drip steam traps must beproperly specified and placed on thesteam system (Figure 4). Warm-up

valves should be included on steam-line-isolation valves that are 2-in. dia.or larger. Do not crack open largesteam-line-isolation valves with thehope of avoiding condensation-inducedwater hammer. This will not guaran-tee safe operation.

Condensate line-sizing is crucial toinsure proper operation of the steamsystem as under-sizing condensate

lines is one of the largest contributorsto water hammer. To be correct, con-densate connections of branch lines tothe main condensate line should enteronly through the top (Figure 5).

Systems that have a modulatingcontrol valve should have a drip-leg-trap (Figure 6) upstream of the

valve to remove condensate dur-ing a closed condition for the valve.

Always gravity drain away fromprocess applications with a mod-ulating control valve. The con-densate can be drained intoa pressurized-condensate-return lineonly if a proper differential is main-tained.

Finally, be sure to properly label thesteam and condensate lines and to re-move abandoned steam and conden-sate lines from the system. Adherence

to these basic heuristics will provide asuitable foundation for the reductionof water hammer and water-ham-mer-related losses in most industrialsteam systems.

Edited by Matthew Phelan

Author

Kelly Paffel is a technicalmanager at Swagelok Com-pany (Email: kelly.paffel@plantsupport.com). A rec-ognized authority in the in-dustry, Paffel has publishedmany technical papers onthe topics of steam and com-pressed air system designand operation. He has morethan 36 years of experiencein steam, compressed air sys-

tems and power operations and a Steam SystemLevel V certification in Steam Systems. He is a

member of the Department of Energys (DOE)Steam Best Practices Committee and SteamTechnical Committee and is chairman of Quan-tum Steams Roundtable, a non-profit organiza-tion for steam professionals.

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Figure 6.This drawing de-picts a standard

dripleg-trap instal-lation

ChemiCal engineering www.Che.Com april 2008 43

Figure 5. The green circle indicates the properconnections to the main condensate header