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:
(P1 Pv)/(P1 P2) > Ci (1)
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 = Pv.
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 P2)0.5 (2)
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 restrictive
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 < 0.3.
Fig. 4. Incipient cavitation coefficients,
Ci for D = 8-in. and de/D = 0.2, 0.35 or 0.5.
Fig. 5. Incipient cavitation coefficients
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 values are:
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 situation.
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 coefficient
Cv Flow coefficient
D Pipe diameter, in.
de Equivalent diameter of the holes (in.), de = do(n)0.5
do Hole diameter, in.
n Number of holes
P1 Upstream pressure, kg/cm2 abs
P2 Downstream pressure, kg/cm2 abs
Q Flow, m3/h
1 Casado, E., Look at orifice plates to cut piping noise, cavitation, POWER, September 1991.
2 Casado, E., Avoid cavitation in butterfly valves, Hydrocarbon Processing, August 2006.
3 Casado, E., Maximum throttling of manual valves without damage, Hydrocarbon Processing, August 2008, pp. 5557.
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.
|The author |
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.