Reducing pressure and controlling flowrates in pipes can be
accomplished via restriction plate orifices or throttle valves.
In all cases, it must be achieved without producing cavitation.
To avoid cavitation with any pressure reducing component, Eq. 1
sets the process operating conditions:
Where Ci is the incipient cavitation
coefficient obtained by tests for every type of component. The
ideal best component to run without cavitation would be with
Ci = 1 and P2 =
Control valves with an internal tortuous path almost
approach this value. However, restriction orifice plates, along
with globe and butterfly valves, have Ci
values from 1.8 to more than 6. Multi-hole restriction orifice
plates have Ci values from 1.2 to 4.
Newly developed multi-hole restriction plates do hold
promise.1 Tests with butterfly valves can achieve
Ci values as low as 1.05 for D = 4
in. and de/D = 0.2 or 1.14 for D = 24 in. and
de/D = 0.2.2,3
As shown in Fig. 1, the new restriction place/device may be
considered as a butterfly valve with the disc perforated by
several holes or an adjustable rotary multi-hole restriction
plate. Fig. 2 is a multi-hole restriction plate with D
= 30 in. For D = 8 in., Fig. 3 shows the flow
coefficient, Cv that relates the flow and
pressure drop as calculated by Eq. 2:
Cv = 1.17Q (P1
Fig. 1. Sketches of
the new flow/pressure
control device: a) as a control valve and
B) as an adjustable multi-orifice plate.
Fig. 2. New multi-hole
pressure/flow control plate.
Fig. 3. Flow
coefficient, Cv for D = 8-in.
and de/D = 0.2 or 0.5.
Fig. 4 illustrates the incipient cavitation coefficient,
Ci, for different values of de/D.
For P1 values greater than approximately 6
kg/cm2 abs, the pressure scale effects correction
and must be considered.4, 5 Fig. 5 shows the
corresponding values of Ci for restriction
orifice plates, butterfly valves and the new restriction plate.
The new Ci values are the minimum that
pertain to every de/D value with the appropriated
opening angle. They are close to 1 for de/D <
Fig. 4. Incipient
Ci for D = 8-in. and de/D = 0.2, 0.35
Fig. 5. Incipient
for ROs, butterfly valve and the new plate
for D = 8-in.
Characteristics and operation.
Two similar prototypes of the new restriction plate were
installed and are now running cavitation-free in CN Almaraz, a
Spanish nuclear power plant. The prototypes were constructed by
modifying two existing butterfly valves, drilling the discs
with a selected size and number of holes. But the holes were
done only in the external part of the surface of the disc
because the central part has a greater thickness, as shown in
Fig. 6. The prototypes were installed at the end of the
fire-water pump recirculation lines, and they discharge water
to a lake. The pipe-length downstream the prototypes is less
than 0.5 m with two 90° elbows. The prototype size is
D = 4 in. and de/D = 0.37.
Fig. 6. Design details
for new flow control systems.
When the opening angle is 0°, with the disc in the vertical
position, the prototype works as a multi-hole restriction
plate, and the measured values are:
P1 = 9.8 kg/cm2,
P2 = 0.039 kg/cm2,
P1 P2 = 9.761
kg/cm2, Q = 100 m3/h
The water temperature was approximately 60°F, so
Pv = 0.256 psia = 0.017 kg/cm2
abs. The prototype in this situation with the disc in the
vertical position has cavitation. This is in accordance with
the theory because a multi-hole restriction orifice plate for
D = 4 in. and de/D = 0.37 has
Ci = 1.25. From Eq. 1, the
Ci is calculated as:
(9.8 + 1.02 0.017)/9.761 = 1.107 < 1.25
Note: 9.8 + 1.02 = 10.82 kg/cm2 abs.
When the opening angle is only 5°, the cavitation
disappears, and the device runs without noise. The measured
P1 = 9.4 kg/cm2,
P2 = 0.12 kg/cm2,
P1 P2 = 9.28
kg/cm2, Q = 125 m3/h.
From Eq. 1, Ci is calculated as:
(9.4 + 1.02 0.017)/9.28 = 1.12
As there is no cavitation, then Ci must
be less than 1.12. According to these theoretical calculations,
for the restrictive plate with D = 4 in. and
de/D = 0.37, the minimum Ci value
is 1.08 for an opening angle of 20°. The prototype tests
show that in the opening range from 3° to 26°, there is
no cavitation or noise. The flowrate varies between 115
m3/h and 220 m3/h and the pressure drop
is between 9.5 kg/cm2 and 6 kg/cm2.
Upstream of the new device, a 4-in. globe valve was
installed. In the tests, when the new plate is in the
horizontal position (opening angle = 90°) and trying to
control the flow by throttling the globe valve, high cavitation
noise and valve vibration levels occurred. This condition
confirms that the globe valve should not be throttled in this
The new multi-hole device with the appropriated number and
size of holes and the corresponding opening angle approaches
the Ci = 1 value that can successfully
eliminate cavitation. HP
Ci Incipient cavitation
Cv Flow coefficient
D Pipe diameter, in.
de Equivalent diameter of the holes (in.), de =
do Hole diameter, in.
n Number of holes
P1 Upstream pressure, kg/cm2
P2 Downstream pressure, kg/cm2
Q Flow, m3/h
1 Casado, E., Look at orifice plates to cut
piping noise, cavitation, POWER, September
2 Casado, E., Avoid cavitation in butterfly
valves, Hydrocarbon Processing, August
3 Casado, E., Maximum throttling of manual
valves without damage, Hydrocarbon Processing, August 2008,
4 Tullis, J. P., Cavitation Guide for Control
Valves, NUREG/CR-6301, April 1983.
5 Considerations for Evaluating Control Valve
Cavitation, ISA-RP75.23-1995, June 1995.
Emilio Casado Flores is mechanical
head engineer in PWR nuclear power plants with EEAA in
Madrid, Spain. From 2007 to 2010, Mr. Casado was the
engineering manager at the Almaraz power upgrading project, PWR Spanish nuclear
power plant. His work experience includes mechanical,
thermal and hydraulic engineering design and operation
in petroleum refineries and nuclear power plants. Mr.
Casado Flores has published technical papers about
cavitation of restriction orifices, heat exchanger
operation, steam discharge through valves and pipes,
flow of saturated condensate through pipes and control
valves, etc. He has developed some plant tests about
cavitation and pressure drop in multi-hole perforated
restriction plates and throttled valves.