avoiding water hammer in steam systems.pdf

7/28/2019 AVoiding water hammer in steam systems.pdf

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The Health and Safety Executive employs a wide range of qualified and experienced

Specialist Inspectors who, in the course of their work, acquire a substantial

amount of information and expertise about workplace hazards. Much of this is

used in the preparation of official HSE Guidance Motes and formal advice.

However, other material which might be less developed could contain useful ideas

and be helpful to people involved in health and safety. Such material could

also stimulate discussions about problems and their solutions and encourage

others to come forward with ideas and practical improvements. Specialist

Inspector Reports are designed to publish this material.

Erquiries regarding this publication should be addressed to the Health and Safety

Executive at anyof the following piblic enquiry points.

Library & Information Services

Broad LaneSHEFFIELD S3 7HQTelephone (0742) 752539 Telex: 54556

Library & Information ServicesSt Hughs HouseStanley Precinct

Trinity RoadBootle

Merseyside L20 3QYTelephone 051-951 4381 Telex: 628235

Library & Information ServicesBaynards House

Chepstow PlaceWestbourne GroveLONDON W2 4TF

Telephone 01221 0870 Telex: 25683


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First printed March 1988

Reprinted April 1989


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kinetic energy may be absorbed by thewater itself but most is transmittedto the pipe and fittings and to theobstruction which stopped it, in theform of a shock load. This causeshigh stresses in the components andaccounts for the

hammering noise,which is usually associated withwater hammer occurrence.

9. The magnitude of the forcesinvolved depend upon the steampressure, velocity and mass of water,and pipe size. Unless the stressedcomponents can withstand the shockload which may in somecircumstances occur repeatedly

damage will result. Damage may not

always occur at the source of thehammer, for example, hammer at a bendor tee may cause damage to a valveseveral feet away.

10. Often damage caused by waterhammer is of a relatively minornature such as the buckling of afloat in a steam trap or leakage froma pipe joint and may thus gounrecognised as to cause. However,at the other end of the scale,

accidents have occurred in which amajor component has burst, see Figure2. Such failures have caused severescalding injuries and loss of life.The consequences of water hammerbecome most serious in pipes over50 mm diameter.

AVOIDING WATER HAMIIER

11. The best method of avoidingwater hammer is to remove the cause.

Draining Pjlines and Fittings

13. British Standard 806 (1986)"Specification for Design andConstruction of Ferrous PipingInstallations for and in Connectionwith Land Boilers", gives advice on

drainage. Where practicable, a

suitable fall should be provided inth pipwork in the direction of thesteam flow to ensure that condensateflows towards drainage points. Afall also enables pipes which areshut off to drain the condensatewhich arises as they cool.

14. Drainage points should be

provided to collect the condensate atintervals of about 30 to 60 metresdepending on pipe size and site

conditions. The drain point shouldbe provided with an adequately sizedpocket into which the condensate willfall. As a guide for small diametermains up to 4 or 5" diameter it willbe convenient to sake the pocketequal in diameter to the pipe asshown in Fiqure 3 but on larger mainssmaller pockets may be used.However, the pocket diameter shouldnot be less than 50% of the pipediameter for mains up to 450 mm

diameter. On mains larger than450 mm diameter the pocket diametermay be further reduced but should notbe lPss than 25% of the pipediameter. Adequate draining will notbe obtained by simply inserting asmall bore pipe into the bottom of amain.

15. If it is necessary to install ahorizontal main because of siteconditions or because the steam flowis reversible, then more frequentdrain points may be necessary.

12. A steam system usually consistsof a boiler, header, distributionmains and branch pipes but may alsoinclude items of plant such as dryingcylinders or heating coils which mayalso be prone to water hammer. Eachsection of any system should becarefully considered to ensure it isadequately drained under allconditions of operation, bearing inmind that closed valves are notalways steam tight.

16. Drainage should also be providedat all low points in the pipelinewhere condensate may collect.

17. Fittings used in the pipelinecan often be the means by whichcondensate may accumulate. Carefulconsideration should be given to theposition of fittings and, ifnecessary, drainage provided. Forexample, a globe valve, when fittedin a horizontal pipe with its spindlevertical, forms a weir owing to its


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shape for the valve seat. Water maycollect behind the weir to a depth ofmore than half the pipe diameter eventhough the valve may be open, Figure4. This may be avoided by fttingthe valve on its side with its

spindle horizontal. Similarly, astrainer in a horizontal pipe fittedwith its body vertically downwardwill very quickly become full ofwater. This not only becomes asource of water but also reduces theeffective area of the strainer screenas shown in Figure 5. The strainershould be fitted on its side to avoidthis problem. A concentric reducerused to change one pipe diameter toanother forms a dam in a horizontal

pipe as shown in Figure 6. Aneccentric reducer should be used.

18. Care must be taken to ensurethat when a section of a system isshut down it is not isolated from itsdrainage or its drainage becomesineffective. Steam leakage past avalve may result in an accumulationof water. Extra drain points may berequired to cater for this situation.

19. Condensate is best removed fromthe drain pocket automatically bymeans of a suitable steam trap.There may however be situations wherea manual drain can be usefullyemployed in addition to a steam trap.

20. The job of a steam trap is todischarge condensate while notpermitting the discharge of livesteam. Steam traps may be requiredto operate at pressures from afraction of a bar to the maximum ofthe steam system. They may have tooperate with superheated or saturatedsteam and may discharge condensate atthe steam temperature as soon as itarises, or they may be required tohold it back until it has given upsome of its sensible heat. The

quantity of condensate will alsovary. To cope with the variety ofconditions, many different sizes and

designs of trap have been produced,each having advantages and

disadvantages. In choosing a steamtrap, its limitations must be borne

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in mind and the manufacturer shouldalways be consulted.

Inspection and Maintenance

21. If a steam trap is continuously

to function correctly it must beregularly inspected and properlymaintained. There are a number ofmethods and devices available tocheck if a trap is passing livesteam. It is more difficult to checkif the trap is passing condensate andmore important, the correct amount ofcondensate.

22. A trap with an intermittentaction and on which the outlet can be

seen, can be checked by observing thedischarge. If the trap dischargesinto a condensate return main, a

sight glass may be used to observethe discharge.

23. Traps with continuous dischargecannot be easily checked. Clearly itis possible to see if the trap is

discharging condensate but it is notpracticable to check if it is thecorrect quantity to keep the mainclear

ofcondensate.

24. The temperature of a steam trapwill also give some indication as towhether it is functioning. A cold orcool trap should be immediatelyinvestigated.

25. Isolating valves in a condensatedrain line should be avoided if

practicable. However they may beneeded for maintenance of traps wherea system or part of a system cannotbe shut down.

26. It is recommended that aschedule of all traps in a system bemade for inspection and maintenanceat regular intervals.

Other Factors

27. Although adequate drainage isthe main way of avoiding of waterhammer, there are other factors whichshould be considered.


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a. In the design of a system, piperuns should be as short and asstraight as practicable.

b. Adequate lagging will reduce therate at which condensate isformed. Lagging on outdoorlines should be

waterproofed.c. Where a system is modified or

operational requirements change,redundant branches or linesshould be removed or effectivelyisolated as near to the supplyas practicable.

d. Operation of valves in a steamsystem should be carried out

slowly and gradually, not onlyto allow warmup of the pipe toavoid thermal shock, but also toallow condensate more time todrain. This will also allowcondensate which may haveaccumulated on the upstream sideof a valve to disperse at acontrolled rate. Particularcare will be required to achievethis when a valve is chainoperated. Gradual operationwill not generally be

practicable where a valve Is

power operated and specialconsideration should be given tothe positioning of these valvesand drainage of the pipe.

UCIWG T ECT F WAT JOR28. Grey cast iron is widely usedfor the manufacture of valves andother components for pressures up toabout 25 bar. Indeed the majority ofvalves are iron. It is cheaper than

steel and produces Sound castingsmore easily,, particularly castings ofintricate shapes. However grey castiron is weak in tension and has poorresistance to shock loading. In themajority of cases where a valvefailure has been attributed to waterhammer, the valve was made of castiron. A more ductile material suchas steel or SG iron would be moresuitable where water hammer islikely, and would reduce the risk of

failures but would not eliminate it.It is not unknown for steel and

bronze fittings to fail either

diretly or indirectly as a result ofwater hammer.

CONCLUSION

2. Water hammer can cause severe

damageand

personalinjury, even

death.

JO. The ways to avoid it are

a. good design

b. adequate draining of pipeworkand fittings under allconditions of operation

c. propnr inspection ar)maintenance of the

d. choice of appropriate materials

e. careful operation of valves.

31. Potentially the most hazardoussituation is the steam main or majorbranch which has been shut down, andlarge quantities of condensate mayhave accumulated. Care must be takenbefore start up to make certain thata system is effectively drained and

the start up procedure rninimises therate at which condensate forms.


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avoiding water hammer in steam systems.pdf

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