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Cavitation

Prevention 

Written by Cla-Val Technical Products Department

 Pumps & Systems, October 2008

The Telltale Signs of Cavitation

In ater distribution systems here hi!h pressure di""erentials and hi!h "lo rates are

 present# valves# pipin! and e$uipment really ta%e a beatin!& When automatic control valves

are e'posed to such conditions# they ill o"ten e'hibit vibration and e'cessive noise# lettin!the operator %no that the pressure di""erentials and hi!h "lo rates are ta%in! their toll&

When these (symptoms( appear# they are clear indications that cavitation is occurrin! and

ill be "olloed by diminished per"ormance and "ailure&

Cavitation in valves occurs hen the velocity o" the "luid at the seatin! area becomese'cessive# creatin! a sudden# severe reduction in pressure belo the vapor pressure level

and causin! the "ormation o" thousands o" tiny bubbles& )s "luid velocity subse$uentlydecreases# the pressure level rises# causin! the vapor bubbles to collapse and release asubstantial amount o" ener!y that literally eats aay the metal sur"aces o" the valve interior&

This can eventually result in multiple per"ormance issues# includin! loss o" "lo capacity

and erosion dama!e& The rule o" thumb is* ith relatively lo donstream pressureconditions# the hi!her the pressure drop across the valve# the !reater potential "or dama!e&

+ee the inset "or applications here cavitation commonly occurs&

What Does Cavitation Damage Look Like?

It is not pretty& In "act# it is startlin! hat the "orce o" the vapor bubbles impin!in! on a

valve,s metal sur"aces can do& The collapse o" vapor bubbles can cause local pressure aveso" up to #000#000-psi# causin! deterioration o" any sur"ace ith hich they come in

contact& The noise and vibration that accompany the valve dama!e are also a concern#

 posin! a potential sa"ety ha.ard "or personnel or%in! in close pro'imity to the cavitatin!

valve&

Determining the Potential for Cavitation

The "irst step in avoidin! this commonly occurrin! problem is to analy.e the system to

measure the potential "or valve cavitation& )nalysis should ta%e into account parameters

such as pipe and valve si.e# ma'imum and minimum "lo rates# static/dynamic inlet and

outlet pressure# ater temperatures and elevation above sea level& This analysis can be per"ormed usin! commercially available so"tare or another approach such as the analytical

method described in ydraulics o" Pipelines* Pumps# Valves# Cavitation# Transients by 1&

Paul Tullis o" tah +tate niversity 3copyri!ht 484 by 1ohn Wiley 5 +ons# Inc&6&

7olloin! Tullis, methodolo!y# "lo tests are conducted to determine a +i!ma curve "or the

valve& +i!ma is de"ined as a cavitation inde' that identi"ies the level o" cavitation that ill

occur ith various "lo and pressure conditions throu!h a restriction device 3such as a

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valve6&

+i!ma is de"ined as 3P2 - Pv6 / 3P - P26

Where*

P valve inlet pressure 3P+I6P2 valve outlet pressure 3P+I6

Pv !a!e vapor pressure 3P+I6

To "urther clari"y# the conditions causin! cavitation are directly proportional to the %inetic

ener!y o" "luid# hich is related to delta P 3see 7i!ure 6&

Preventing Cavitation in Valves

Once the potential "or cavitation in a system is understood# the ne't step is determinin! the

 best method to prevent it "rom occurrin!& +ome o" the most common and e""ective measures

are*

& Installin! to valves in series here e'tremely hi!h pressure di""erentialse'ist

2& sin! bac%pressure-producin! devices such as one or more ori"ice plates

donstream o" a valve that is e'posed to cavitation-causin! conditions9& sin! valves e$uipped ith anti-cavitation trim in systems here e'treme pressure di""erentials and hi!h velocity "lo rates are present

Pros and Cons of the Most Common Cavitation Prevention Measures

Two Valves in Series

Installin! to valves in series ill e""ectively miti!ate the incidence o" cavitation& )n added

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 bene"it is that the second valve acts as a bac%up in the event the "irst valve "ails# ensurin! at

least some level o" pressure-reducin! "unctionality in the application& The ne!ative in this

scenario is that space at valve installations is o"ten limited and there simply is not enou!hroom to install to valves& In addition to these concerns# there is also the cost o" the second

valve to consider&

Orifice Plates

)n obvious bene"it o" usin! ori"ice plates as bac%pressure devices to reduce cavitation,se""ect is the relatively lo cost to purchase and install them in the pipeline& n"ortunately#

hen ori"ice plates are used in this ay# they are only e""ective ithin a narro "lo ran!e

and ill cause a reduction o" "lo capacity ithin the system&

)nother ne!ative is that ori"ice plates can actually cause cavitation to occur# creatin! the potential "or dama!e to donstream "ittin!s and valves&

nti!Cavitation Valves and Trim

+everal automatic control valve manu"acturers o""er products that are desi!ned to reduce oreliminate cavitation& :y nature# any anti-cavitation device ill result in reduced "lo

capacity& 7ortunately# the ne!ative e""ects can be minimi.ed hen the valve is properly

si.ed "or the application& This is achieved by per"ormin! comprehensive and thorou!h "lotestin! ith a si.in! pro!ram to ensure that the ri!ht si.e valve is chosen to provide

cavitation protection&

Despite this caveat# usin! an anti-cavitation valve is !enerally considered the most e""ective

approach# especially i" an e'istin!# installed valve can be retro"itted ith anti-cavitation

trim&

Whether an e'istin! valve is enhanced ith the cavitation-"i!htin! components or a ne

valve e$uipped ith such trim is used# the cavitation solution is sel"-contained and should

 be able to provide a ider ran!e o" "lo rates and smooth operation ith lo levels o"vibration and noise&

It is important to note that there are occasionally applications here a system,s pressure and

"lo rates are so e'treme that the only e""ective measure a!ainst cavitation is a combination

o" a valve out"itted ith anti-cavitation trim# installed in con;unction ith ori"ice plates&7ortunately# this scenario is not the norm&

Designing a Cavitation!"ree S#stem

In the best-case scenario# distribution systems ould be desi!ned ith cavitation prevention

measures as an inte!ral part o" the system& It !oes bac% to ta%in! that all-important "irst stepo" per"ormin! a complete cavitation study be"ore valves are selected# purchased and

installed in a pipeline&

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customers by considerin! cavitation hen desi!nin! their systems&

They can provide "urther value by consultin! ith valve manu"acturers and speci"yin!

valves that are properly si.ed and e$uipped ith anti-cavitation trim or other options& The

lon!-term bene"its are si!ni"icant* loer maintenance costs= "eer e$uipment "ailures= lessdontime associated ith valve replacements= and a system that per"orms ith optimum

e""iciency&

In the case o" an e'istin! pipeline# the most direct approach to minimi.in! or even

eliminatin! cavitation is either replacin! e'istin! valves ith cavitation-"i!htin! valves orretro"ittin! them ith anti-cavitation trim&

Whichever avenue the ater company chooses# operatin! a ater distribution system ith

little or no cavitation is possible& It is ;ust a matter o" hich approach ill or% best ith

the company,s operational and "inancial parameters&

 For more information, contact the Cla-Val Technical Products Department.