ASHRAE Journal

5 8 A S H R A E J o u r n a l w w w. a s h r a e j o u r n a l . o r g J u n e 2 0 0 1

C

Avoiding CavitationIn Control Valves

About the Author

By Bengt Carlson

Bengt Carlson is a consultant, pri-marily for Belimo Aircontrols (USA).

ontrol valves usually are sized by picking a valve with a flow co-efficient (Cv) that produces the desired pressure drop. MarkHegbergs Practical Guide article from the November 2000

ASHRAE Journal addresses this aspect of valve sizing.

for dynamic pressure is an application ofBernoullis law:

02

2

=

++ gzvp

If the velocity is high enough, the pres-sure at the restriction can drop below thevapor pressure of the liquid and form va-por bubbles. As the liquid moves down-stream past the restriction, the flow areaopens up to the cross section of thepipe, and the velocity decreases. This re-duces the dynamic pressure and increasesthe static pressure. The downstream staticpressure is normally higher than the vaporpressure of the liquid. Therefore, thebubbles or cavities of vapor implode (seeFigure 1).

When a bubble implodes, all the energyis concentrated into a very small area. Thiscreates tremendous pressure (thousands ofpsi) in the small area, generating minuteshock waves. These shock waves pound onthe solid portions of the valve. Repeatedimplosions on a small surface eventuallycause fatigue of the metal and wear awaythis surface.

Moderate cavitation can be permissiblein a control valve. This causes little dam-age to the valve. However, increasing cavi-tation will become detrimental to the valvetrim and possibly to the valve body. Also,excessive cavitation can begin to chokethe flow through the valve. The flow ratewill be drastically reduced compared towhat the differential pressure and the Cv of

the valve would suggest. However, thiscondition is hardly a realistic concern inHVAC applications handling liquids be-cause the more serious problem of cavita-tion occurs first.

FormulaThe point when cavitation becomes

damaging can be expressed by the follow-ing formula:

( )PvP1dPKm

max=

Where:Km = valve recovery coefficient. Some

control valve manufacturers refer to aliquid pressure recovery factor, FL.Km = FL

2.

P1 = absolute inlet pressure (psia) Pv = absolute vapor pressure of liquid

(psia) dP max = maximum allowable pressure

drop through the valve (psi)The valve recovery coefficient, Km, de-

pends on the design of the valve. It is al-ways less than 1. Table 1 gives approximatevalues of Km for common types of valves.Table 2 shows the vapor pressure for wa-ter, which is used to calculate the onset ofcavitation.

For typical HVAC valves such as globeand control ball valves, a valve recoveryfactor of Km = 0.5 0.6 can be expected.If Km is not known, a conservative estimateof Km = 0.5 should be used.

The cavitation formula can be rear-ranged as follows:

dP allowed = 0.5 (P1 Pv)Example 1: dP max = 10 psi (69 kPa)

P1 = 20 psig = 34.7 psia(239 kPa)

Pv = 9.3 psia (64 kPa) for

While Cv and pressure drop are impor-tant, they are not the only considerations.Sometimes cavitation can occur, resulting innoise and rapid deterioration of the valvetrim. In extreme cases, cavitation can evenlimit the maximum flow through the valve.This happens in cases where the differentialpressure is too high compared to the outletpressure. It is not common in HVAC appli-cations, but it can happen. We need to knowhow to design to prevent cavitation and whatto do when it occurs.

GeneralCavitation is a perplexing phenomena

that sometimes occurs in hydronic systems.It can occur in pumps, heat exchangers andother parts. This text focuses on valves andhow cavitation can be eliminated.

Usually cavitation manifests itself withsharp noise. It sounds like gravel passingthrough the valve. Cavitation also shortensthe life of the valve. Usually the valve trimis destroyed prematurely, but the valvebody and the piping downstream of thevalve also can be affected. Pipe as far awayas 20 diameters downstream of the valvecan be affected.

When a liquid passes through a pipe, thevelocity is comparatively low because ofthe relatively large cross section. As theliquid passes through a restriction such asan orifice or valve seat, its velocity in-creases. An increase in velocity increasesdynamic pressure, which reduces the staticpressure. This exchange of static pressure

J u n e 2 0 0 1 A S H R A E J o u r n a l 5 9

Valves

190F (88C)water.dP allowed = 0.5 ( 34.7 9.3) = 12.7 psi(88 kPa)

The recommended pressure drop for this valvewould probably be 4 to 10 psi (28 to 69 kPa). Cavita-tion would not be a problem in this application.Example 2: dP max = 10 psi (69 kPa)

P1 = 10 psig = 24.7 psia (170 kPa)Pv = 9.3 psia (64 kPa) for 190F (88C)water.dP allowed = 0.5 (24.7 9.3) = 7.7 psi(53 kPa)

The pressure drop across the valve must be less than7.7 psi (53 kPa) or cavitation will occur.

A high outlet pressure is advantageous for allvalves. Outlet pressure plus the differential pressureis the inlet pressure, P1. For a given pressure drop,the higher the outlet pressure, the higher the inletpressure, P1, and the greater the margin to avoid cavi-tation.

A low vapor pressure (Pv) is also advantageous.When calculating vapor pressure (Pv), take any gly-col concentration into account. Glycol lowers the va-por pressure of the mixture, which is advantageous.

A low-pressure drop through the valve (dP) is ad-vantageous for avoiding cavitation, but it should notbe changed. The pressure drop through the valve ischosen for valve controllability (usually 4 to 10 psi[28 to 69 kPa]).

Excessive flow causes high-pressure drop across thevalve. Sometimes valves are accused of cavitatingwhen the problem is that the flow is inadvertently higher than speci-fied. Calibrated balancing devices can provide a way to measurethe flow and limit it to the design value.

Figure 2 shows a cavitation diagram for a particular valve. Eu-ropean valve manufacturers often provide this information. The fig-ure shows the relationship between the incoming pressure and theallowable differential pressure. By entering the chart with the in-coming pressure (P1) and the temperature of the flowing fluid, theallowable differential pressure can be determined.

SolutionSuppose we calculate the maximum allowable differential pres-

sure and find that it is not high enough for the application. Whatcan we do?

The answer depends on whether we have a closed system oran open system. Closed systems present few problems. Open sys-tems, such as cooling tower bypass, are more difficult.

Closed SystemsDifferential pressure (dP) should be as low as possible without

sacrificing valve controllability. To accomplish that goal, the in-let pressure (P1) must be high enough.

The inlet pressure is the sum of the pressure drop and the out-let pressure. If the system has a bladder type expansion tank, theoutlet pressure can be increased by raising the fill pressure in the

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expansion tank. If the system has a conventional expansion tank,it must be located high enough in the system to provide the de-sired pressure at the valve outlet.

Valves located at the top of the building tend to cause a prob-lem. Outlet pressure decreases as elevation increases and the col-umn of water above the valve decreases.

The expansion tank should be about half full with water. Anempty tank with the bladder fully expanded exerts no pressure atall to the rest of the system, regardless of the fill pressure (seeFigure 3).

Figure 3 shows that the expansion tank is fully expanded so doesnot exert any pressure on the system. The overall system pressureis low, and the outlet pressure on the upper valve is low (0 psig).This system allows only 5.2 psi (36 kPa) pressure drop across thevalve, assuming 190F (88C) water.

Figure 4 shows a normal expansion tank. It is half full with ahalf compressed diaphragm that exerts pressure on the system.This system will allow a differential pressure of 16.7 psi (115 kPa),assuming 190F (88C) water.

If the problem persists even when the expansion tank is prop-erly filled, the system pressure can be raised by increasing the pre-charge pressure of the expansion tank. Of course, stay within therelief valve setting and safe limits for the system. A properly func-tioning expansion tank with a sufficiently high fill pressure solvesmany cavitation problems.

Distance

Distance

Distance

Pres

sure

Pres

sure

Pres

sure

Valve Inlet

Valve Inlet

Valve Inlet Valve Outlet

Valve Outlet

Valve Outlet

Figure 1a: Pressure drop accross a control valve

Figure 1b: Normal Conditions

Figure 1c: Cavitation Conditions

P1

P2

Pv

P

P1

Pv

P2

P1

P2

P

P

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6 0 A S H R A E J o u r n a l w w w. a s h r a e j o u r n a l . o r gJ u n e 2 0 0 1

ASHRAE JournalStatic pressure upstream valvePsi

240

210

180

150

120

90

60

30

00 10 20 30 40 50 60 70 80 90 100 110 Psi

356F 320F 284F248F

212F

68F

Pressure drop Cv

11 psi

22 psi

10 psi

15 psi

31 psi 7 psi

24 psi

4 psi

15 psi

3 psi

3 psi

1 psifriction + 10 psielevation

1 psi

5 psi

5 psi

5 psi

1 psi5 psi 10 psi

9 psi

0 psi

0 psi

5 psi

15 psi

10 psi

10 psi

Boiler

Coil

Coil

0 psi

4 psifriction + 5 psielevation

* pressure with pump off!"G46

Open SystemsOpen systems, such as cooling tower bypasses, present a chal-

lenge. High velocity is often the origin of problems in coolingtower bypass valves. Decreasing it may be possible.

A ball or butterfly valve need not be fully open. Limiting the valveopening reduces the flow and the resulting velocity in the bypass.The valve recovery factor (Km) also increases when a ball or but-terfly valve is slightly throttled. Ball and butterfly valves normallyhave such high capacity that limiting their stroke is not a problem.

It is also possible to raise the outlet pressure by locating thebypass valve at a level significantly lower than the cooing tower.Figure 5 shows the bypass valve at an elevation not much belowthe water level in the cooling tower sump. The outlet pressure onthe valve will be very low. Figure 6 shows the bypass valve at amuch lower elevation. There is a substantial water column be-tween the valve and the cooling tower sump to create a sufficientoutlet pressure.

A balancing valve (CBV-1) should be installed in series withthe bypass valve so the bypass flow is correct. The balancing valveis especially important. Without it, the pressure in the bypass lineis less than in the riser.

Pump pressure should not be higher than what is required tosupply the design flow to the cooling tower. Keeping the pumppressure low avoids a large differential pressure across the valves.A balancing valve (CBV-2) should be installed.

Location of Control ValvesHeating coils should have their control valve downstream of

the coil. From a cavitation point of view, valve location is not im-portant. On one hand, the water temperature and vapor pressureare lower at the coil outlet. On the other hand, the outlet pressureis higher if the valve is on the coil inlet. Avoiding overheating theactuator is a more important reason to locate the valve downstreamof the coil.

Cooling coils should also have their control valves downstream

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Coil

Coil

Boiler

3 psi

30 psi

30 psi54 psi

24 psi

4 psi

4 psifriction + 5 psielevation

33 psi

32 psi

5 psi

3 psi

1 psi

1 psi5 psi

28 psi

5 psi

34 psi

45 psi

23 psi

5 psi 1 psifriction + 10 psielevation

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Cooling Tower

CBV-1

P2

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J u n e 2 0 0 1 A S H R A E J o u r n a l 6 1

Valves

of the coil, at least if the actuator is electric. From a cavitation pointof view, it can be argued that the valve should be installed upstreamof the coil where the vapor pressure is lower and the outlet pres-sure is higher. That benefit is very minor. The reason for install-ing the actuator downstream is to avoid condensation on theactuator. Cavitation, if it occurs, can be solved by raising the fillpressure in the expansion tank.

Special ApplicationsThere are some special cases, such as hot water district heating

for example, where cavitation problems cannot be solved by rais-ing the pressure. In those cases special industrial-style valves areneeded. These valves have a higher valve recovery coefficient.They also have special trim that reduces the water pressure in twoor more stages. Sometimes the trim has a labyrinth design.

ConclusionCavitation problems often can be solved by raising the pres-

sure at the valve outlet. This normally can be done by adjustingthe fill pressure on the expansion tank.

The likelihood that valves will cavitate depends on how far theyare open to pass design flow. Globe and control ball valves are atlesser risk than butterfly and standard ball valves.

Valves handling hot water and valves located near the top of the

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Cooling Tower

CBV-1

P2

CBV-2

system are prone to cavitation. This is because there is no columnof water above the valve making the outlet pressure low.

Cavitation increases at low outlet pressure, high-pressure drop,high water temperature, and when using unsuitable valve designs.

Bibliography1. Clifford, G.E. 1984. Heating Ventilating and Air Conditioning.2. Neles-Jamesbury Bulletin T150-1.3. Johnson Control Engineering Data Book, Vb:4 General Valve

Data.

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