Eliminate Cavitation in Your Piping Systems _ Hydrocarbon Processing _ February 2012

2/4/2014 Eliminate cavitation in your piping systems | Hydrocarbon Processing | February 2012

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Eliminate cavitation in your piping systems

02.01.2012 | Casado Flores, E., EEAA, Madrid, Spain

New pressure control devices im prove fluid flow

Key words:

Reducing pressure and controlling flowrates in pipes can be accomplished v ia restriction plate orifices or throttle

valves. In all cases, it must be achieved without producing cav itation. To avoid cav itation with any pressure

reducing component, Eq. 1 sets the process operating conditions:

(P1 Pv)/(P1 P2) > Ci (1)

Where Ci is the incipient cav itation coefficient obtained by tests for every ty pe of component. The ideal best

component to run without cav itation 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

Design characteristics.

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 .17 Q (P1 P2)0.5 (2)



Fig. 1. Sketches of the new flow/pressure

control dev ice: 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 cav itation 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 cav itation coefficients,

Ci for D = 8-in. and de/D = 0.2, 0.35 or 0.5.



Fig. 5. Incipient cav itation coefficients

for ROs, butterfly valve and the new plate

for D = 8-in.

Characteristics and operation.

Two similar prototy pes of the new restriction plate were installed and are now running cav itation-free in CN

Almaraz, a Spanish nuclear power plant. The prototy pes were constructed by modify ing 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 prototy pes were

installed at the end of the fire-water pump recirculation lines, and they discharge water to a lake. The pipe-length

downstream the prototy pes is less than 0.5 m with two 90 elbows. The prototy pe size is D = 4 in. and de/D =

0.37 .



Fig. 6. Design details for new flow control sy stems.

When the opening angle is 0, with the disc in the vertical position, the prototy pe works as a multi-hole restriction

plate, and the measured values are:

P1 = 9.8 kg/cm2 , P2 = 0.039 kg/cm

2 , P1 P2 = 9.7 61 kg/cm2 , Q = 100 m3 /h

The water temperature was approximately 60F, so Pv = 0.256 psia = 0.017 kg/cm2 abs. The prototy pe in this

situation with the disc in the vertical position has cav itation. 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 )/9.7 61 = 1 .107 < 1 .25

Note: 9.8 + 1 .02 = 10.82 kg/cm2 abs.

When the opening angle is only 5, the cav itation disappears, and the dev ice runs without noise. The measured

values are:

P1 = 9.4 kg/cm2 , P2 = 0.12 kg/cm

2 , 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 cav itation, 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

prototy pe tests show that in the opening range from 3 to 26, there is no cav itation 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 dev ice, a 4-in. globe valve was installed. In the tests, when the new plate is in the horizontal

position (opening angle = 90) and try ing to control the flow by throttling the globe valve, high cav itation noise

and valve v ibration levels occurred. This condition confirms that the globe valve should not be throttled in this

situation.

The new multi-hole dev ice with the appropriated number and size of holes and the corresponding opening angle

approaches the Ci = 1 value that can successfully eliminate cav itation. HP

NOMENCLAT URE

Ci Incipient cav itation 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



sheena m undra

05.21.2012

P1 Upstream pressure, kg/cm2 abs

P2 Downstream pressure, kg/cm2 abs

Q Flow, m3 /h

LIT ERAT URE CIT ED

1 Casado, E., Look at orifice plates to cut piping noise, cav itation, POWER, September 1991.2 Casado, E., Avoid cav itation 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., Cav itation Guide for Control Valves, NUREG/CR-6301, April 1983.5 Considerations for Evaluating Control Valve Cav itation, ISA-RP75.23-1995, June 1995.

T he author

Em ilio 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 hy draulic engineering design

and operation in petroleum refineries and nuclear power plants. Mr. Casado Flores has published technical papers

about cav itation 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

cav itation and pressure drop in multi-hole perforated restriction plates and throttled valves.

pls look into cheng roatation vane it has been used in industrial piping for over 20

y ears. www.chengfluid.com

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Eliminate Cavitation in Your Piping Systems _ Hydrocarbon Processing _ February 2012

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