ASHRAE Journal
Avoiding Cavitation
In Control Valves
By Bengt Carlson
the valve would suggest. However, this
condition is hardly a realistic concern in
HVAC applications handling liquids because the more serious problem of cavitation occurs first.
C
ontrol valves usually are sized by picking a valve with a flow coefficient (Cv) that produces the desired pressure drop. Mark
Hegberg’s Practical Guide article from the November 2000
Formula
ASHRAE Journal addresses this aspect of valve sizing.
The point when cavitation becomes
While Cv and pressure drop are important, they are not the only considerations.
Sometimes cavitation can occur, resulting in
noise and rapid deterioration of the valve
trim. In extreme cases, cavitation can even
limit the maximum flow through the valve.
This happens in cases where the differential
pressure is too high compared to the outlet
pressure. It is not common in HVAC applications, but it can happen. We need to know
how to design to prevent cavitation and what
to do when it occurs.
General
Cavitation is a perplexing phenomena
that sometimes occurs in hydronic systems.
It can occur in pumps, heat exchangers and
other parts. This text focuses on valves and
how cavitation can be eliminated.
Usually cavitation manifests itself with
sharp noise. It sounds like gravel passing
through the valve. Cavitation also shortens
the life of the valve. Usually the valve trim
is destroyed prematurely, but the valve
body and the piping downstream of the
valve also can be affected. Pipe as far away
as 20 diameters downstream of the valve
can be affected.
When a liquid passes through a pipe, the
velocity is comparatively low because of
the relatively large cross section. As the
liquid passes through a restriction such as
an orifice or valve seat, its velocity increases. An increase in velocity increases
dynamic pressure, which reduces the static
pressure. This exchange of static pressure
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ASHRAE Journal
for dynamic pressure is an application of
Bernoulli’s law:
p v

∆  + + gz  = 0
2
ρ


damaging can be expressed by the following formula:
Km =
2
If the velocity is high enough, the pressure at the restriction can drop below the
vapor pressure of the liquid and form vapor bubbles. As the liquid moves downstream past the restriction, the flow area
opens up to the cross section of the
pipe, and the velocity decreases. This reduces the dynamic pressure and increases
the static pressure. The downstream static
pressure is normally higher than the vapor
pressure of the liquid. Therefore, the
bubbles or cavities of vapor implode (see
Figure 1).
When a bubble implodes, all the energy
is concentrated into a very small area. This
creates tremendous pressure (thousands of
psi) in the small area, generating minute
shock waves. These shock waves pound on
the solid portions of the valve. Repeated
implosions on a small surface eventually
cause fatigue of the metal and wear away
this surface.
Moderate cavitation can be permissible
in a control valve. This causes little damage to the valve. However, increasing cavitation will become detrimental to the valve
trim and possibly to the valve body. Also,
excessive cavitation can begin to “choke”
the flow through the valve. The flow rate
will be drastically reduced compared to
what the differential pressure and the Cv of
dP max
(P1 – Pv )
Where:
Km = valve recovery coefficient. Some
control valve manufacturers refer to a
liquid pressure recovery factor, F L .
Km = FL2.
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, depends on the design of the valve. It is always less than 1. Table 1 gives approximate
values of Km for common types of valves.
Table 2 shows the vapor pressure for water, which is used to calculate the onset of
cavitation.
For typical HVAC valves such as globe
and control ball valves, a valve recovery
factor of Km = 0.5 – 0.6 can be expected.
If Km is not known, a conservative estimate
of Km = 0.5 should be used.
The cavitation formula can be rearranged 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
About the Author
Bengt Carlson is a consultant, primarily for Belimo Aircontrols (USA).
w w w. a s h r a e j o u r n a l . o r g
June 2001
Valves
190°F
Valve Inlet
(88°C)
Valve Outlet
Valve Type
Km
Pressure
Pressure
Pressure
water.
Globe Valve
0.50
P1
dP allowed = 0.5 ( 34.7 – 9.3) = 12.7 psi
V
P
o
r
t
B
a
l
l
,
P
0.40–0.45
P2
(88 kPa)
Characterized Disk
The recommended pressure drop for this valve
Standard Ball,
0.25–0.30
would probably be 4 to 10 psi (28 to 69 kPa). CavitaButterfly
tion would not be a problem in this application.
Table 1: Approximate valve
Distance
Example 2: dP max = 10 psi (69 kPa)
recovery coefficients (Km).
Figure 1a: Pressure drop accross a control valve
P1 = 10 psig = 24.7 psia (170 kPa)
Vapor
Pv = 9.3 psia (64 kPa) for 190°F (88°C)
Temperature
Pressure
Valve Inlet
Valve Outlet
water.
(°F/°C)
(psia/kPa)
dP allowed = 0.5 (24.7 – 9.3) = 7.7 psi
P1
32/0
0.09/0.62
(53 kPa)
P
P2
The pressure drop across the valve must be less than
4/4.4
0.12/0.83
7.7 psi (53 kPa) or cavitation will occur.
50/10.0
0.18/1.2
Pv
A high outlet pressure is advantageous for all
75/23.9
0.43/3.0
valves. Outlet pressure plus the differential pressure
Distance
is the inlet pressure, P1. For a given pressure drop,
100/37.8
0.95/6.6
the higher the outlet pressure, the higher the inlet
Figure 1b: Normal Conditions
125/51.7
1.9/13
pressure, P1, and the greater the margin to avoid caviValve
Inlet
Valve
Outlet
tation.
150/66
3.7/26
A low vapor pressure (Pv) is also advantageous.
P1
175/79
6.7/46
When calculating vapor pressure (Pv), take any glyP
col concentration into account. Glycol lowers the vaP2
190/88
9.3/64
por pressure of the mixture, which is advantageous.
200/93
11.5/79
Pv
A low-pressure drop through the valve (dP) is advantageous for avoiding cavitation, but it should not
210/99
14.1/97
be changed. The pressure drop through the valve is
Distance
212/100
14.7/100
chosen for valve controllability (usually 4 to 10 psi
Figure 1c: Cavitation Conditions
[28 to 69 kPa]).
Figure 1: Pressure profile across a Table 2: Vapor pressure of
Excessive flow causes high-pressure drop across the valve.
water.
valve. Sometimes valves are accused of cavitating
when the problem is that the flow is inadvertently higher than speci- expansion tank. If the system has a conventional expansion tank,
fied. Calibrated balancing devices can provide a way to measure it must be located high enough in the system to provide the dethe flow and limit it to the design value.
sired pressure at the valve outlet.
Figure 2 shows a cavitation diagram for a particular valve. EuValves located at the top of the building tend to cause a probropean valve manufacturers often provide this information. The fig- lem. Outlet pressure decreases as elevation increases and the colure shows the relationship between the incoming pressure and the umn of water above the valve decreases.
allowable differential pressure. By entering the chart with the inThe expansion tank should be about half full with water. An
coming pressure (P1) and the temperature of the flowing fluid, the empty tank with the bladder fully expanded exerts no pressure at
allowable differential pressure can be determined.
all to the rest of the system, regardless of the fill pressure (see
Figure 3).
Solution
Figure 3 shows that the expansion tank is fully expanded so does
Suppose we calculate the maximum allowable differential pres- not exert any pressure on the system. The overall system pressure
sure and find that it is not high enough for the application. What is low, and the outlet pressure on the upper valve is low (0 psig).
can we do?
This system allows only 5.2 psi (36 kPa) pressure drop across the
The answer depends on whether we have a closed system or valve, assuming 190°F (88°C) water.
an open system. Closed systems present few problems. Open sysFigure 4 shows a normal expansion tank. It is half full with a
tems, such as cooling tower bypass, are more difficult.
half compressed diaphragm that exerts pressure on the system.
This system will allow a differential pressure of 16.7 psi (115 kPa),
Closed Systems
assuming 190°F (88°C) water.
Differential pressure (dP) should be as low as possible without
If the problem persists even when the expansion tank is propsacrificing valve controllability. To accomplish that goal, the in- erly filled, the system pressure can be raised by increasing the prelet pressure (P1) must be high enough.
charge pressure of the expansion tank. Of course, stay within the
The inlet pressure is the sum of the pressure drop and the out- relief valve setting and safe limits for the system. A properly funclet pressure. If the system has a bladder type expansion tank, the tioning expansion tank with a sufficiently high fill pressure solves
outlet pressure can be increased by raising the fill pressure in the many cavitation problems.
June 2001
ASHRAE Journal
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ASHRAE Journal
Static pressure upstream valve
Psi
240
356°F
320°F
284°F
5 psi
248°F
5 psi
5 psi
1 psi
212°F
210
0 psi
11 psi
Coil
68°F
0 psi
0 psi
180
1 psi
friction
+ 10 psi
elevation
150
1 psi
5 psi
5 psi
10 psi
3 psi
22 psi
9 psi
Coil
120
90
10 psi
15 psi
4 psi
friction
+ 5 psi
elevation
4 psi
60
3 psi
24 psi
15 psi
30
Boiler
15 psi
10 psi
10 psi
0
0
10
20
30
40
50
60
70
80
90
100
110
Pressure drop
Psi
31 psi
Cv
* pressure with pump off
Figure 2: Control valve cavitation diagram.
Figure 3: Effect of inadequate expansion tank fill.
Open Systems
Open systems, such as cooling tower bypasses, present a challenge. High velocity is often the origin of problems in cooling
tower bypass valves. Decreasing it may be possible.
A ball or butterfly valve need not be fully open. Limiting the valve
opening reduces the flow and the resulting velocity in the bypass.
The valve recovery factor (Km) also increases when a ball or butterfly valve is slightly throttled. Ball and butterfly valves normally
have such high capacity that limiting their stroke is not a problem.
It is also possible to raise the outlet pressure by locating the
bypass valve at a level significantly lower than the cooing tower.
Figure 5 shows the bypass valve at an elevation not much below
the water level in the cooling tower sump. The outlet pressure on
the valve will be very low. Figure 6 shows the bypass valve at a
much lower elevation. There is a substantial water column between the valve and the cooling tower sump to create a sufficient
outlet pressure.
A balancing valve (CBV-1) should be installed in series with
the bypass valve so the bypass flow is correct. The balancing valve
is especially important. Without it, the pressure in the bypass line
is less than in the riser.
Pump pressure should not be higher than what is required to
supply the design flow to the cooling tower. Keeping the pump
pressure low avoids a large differential pressure across the valves.
A balancing valve (CBV-2) should be installed.
Location of Control Valves
Heating coils should have their control valve downstream of
the coil. From a cavitation point of view, valve location is not important. On one hand, the water temperature and vapor pressure
are lower at the coil outlet. On the other hand, the outlet pressure
is higher if the valve is on the coil inlet. Avoiding overheating the
actuator is a more important reason to locate the valve downstream
of the coil.
Cooling coils should also have their control valves downstream
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ASHRAE Journal
June 2001
7 psi
28 psi
5 psi
1 psi
5 psi
23 psi
34 psi
Coil
5 psi
1 psi
5 psi 1 psi
friction
+ 10 psi
elevation
3 psi
32 psi
Coil
45 psi
4 psi
friction
+ 5 psi
elevation
4 psi
30 psi
3 psi
24 psi
Boiler
54 psi
33 psi
30 psi
Figure 4: Properly filled expansion tank provides sufficient
pressure to avoid cavitation.
Cooling Tower
P2
CBV-1
Figure 5: Inadequate column of liquid above valve makes
P1 too low.
w w w. a s h r a e j o u r n a l . o r g
Valves
of the coil, at least if the actuator is electric. From a cavitation point
of view, it can be argued that the valve should be installed upstream
of the coil where the vapor pressure is lower and the outlet pressure is higher. That benefit is very minor. The reason for installing the actuator downstream is to avoid condensation on the
actuator. Cavitation, if it occurs, can be solved by raising the fill
pressure in the expansion tank.
Cooling Tower
Figure 6: Cooling
tower bypass and
balancing valve arrangement to avoid
cavitation.
P2
Special Applications
There are some special cases, such as hot water district heating
for example, where cavitation problems cannot be solved by raising the pressure. In those cases special industrial-style valves are
needed. These valves have a higher valve recovery coefficient.
They also have special trim that reduces the water pressure in two
or more stages. Sometimes the trim has a labyrinth design.
Conclusion
Cavitation problems often can be solved by raising the pressure at the valve outlet. This normally can be done by adjusting
the fill pressure on the expansion tank.
The likelihood that valves will cavitate depends on how far they
are open to pass design flow. Globe and control ball valves are at
lesser risk than butterfly and standard ball valves.
Valves handling hot water and valves located near the top of the
CBV-1
CBV-2
system are prone to cavitation. This is because there is no column
of 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.
Bibliography
1. 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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June 2001
ASHRAE Journal
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