In deciding the size of the control valve that (Port Size) we need to calculate the Cv value of the process (Calculated Cv) by calculating Cv has educated more than 50 years, which is involved in. such as the Renault number (Reynolds number), flow rate (Flow), the Shock (Choking), the involved joints in pipes (Fitting) and the other by the values ​​of these.
This reference standard IEC 60534-2-1: 1998 (International Electro technical Commission), which is standard valve manufacturer mainly quoted. The coefficient Cv IEC standards are highly complex.
I recommend using the control valve manufacturer has provided. However, to be considered under the verification e Cv easier we have a simple formula Cv value can be calculated following them.
Cv value calculation
A) Simple Cv Value Formula: Case of the Liquid
Q LÂ :Â Flow rate (m3 / H).
â–³ P :Â Differential pressure (kg / cm 2 )
G LÂ :Â Specific Gravity (Water = 1)
B) imple Cv Value Formula: Case of the Gas
I) Non-choked flow  ΔP <P1 / 2
Q GÂ :Â Flow rate (M 3 / H @ 15 o C , 1atm.)
â–³ P :Â Â Â Differential pressure (kg / cm 2 )
G GÂ :Â Â Â Specific Gravity (Air = 1)
T :Â Â Temperature ( o C)
P MÂ :Â (P1 P2) / 2
II) choked flow ΔP ≥ P1 / 2
Q GÂ Â :Â Flow rate (M 3 / H @ 15 o C, 1atm.)
P 1Â Â :Â Â Inlet pressure (kgf / cm 2 , ABS)
G GÂ Â :Â Â Â Specific Gravity (Air = 1)
T Â :Â Â Temperature ( o C)
C) Simple Cv Value Formula: Case of the Steam (Vapor)
I) Saturation Vapor: Non-choked flow  ΔP <P1 / 2
Q s  :  Flow rate (kg / H)
Y Â Â :Â Â Â Â Vapor Correction factor (Water Vapor = 19.4)
â–³ P :Â Â Â Differential pressure (kg / cm 2 )
P M: Â (P1 P2) / 2
II) Vapor Saturation: choked flow ΔP ≥ P1 / 2
Q s : Flow rate (kg / H)
Y Â :Â Â Â Vapor Correction factor (Water Vapor = 19.4)
P 1Â : Â Â Inlet pressure (kgf / cm 2 ABS.)
III) Wet Vapor: Non-choked flow  ΔP <P1 / 2
Q s  : Flow rate (kg / H)
X Â Â :Â Â Quality of Vapor (X = 1 is Dry Steam)
Y Â :Â Â Â Â Vapor Correction factor (Water Vapor = 19.4)
â–³ P :Â Â Differential pressure (kg / cm 2 )
P M: Â (P1 P2) / 2
IV) Wet Vapor: choked flow ΔP ≥ P1 / 2
Q s : Flow rate (kg / H)
X Â :Â Â Â Quality of Vapor (X = 1 is Dry Steam)
Y :Â Â Â Â Â Vapor Correction factor (Water Vapor = 19.4)
P 1Â : Â Inlet pressure (kgf / cm 2 ABS.)
v) Superheated Vapor: Non-choked flow  ΔP <P1 / 2
Q s : Flow rate (kg / H)
Y Â : Â Â Vapor Correction factor (Water Vapor = 19.4)
â–³ P :Â Differential pressure (kg / cm 2 )
S :Â Â Â Superheat ( o C)
P MÂ :Â Â (P1 P2) / 2
VI) Superheated Vapor: choked flow ΔP ≥ P1 / 2
Q s : Flow rate (kg / H)
Y :Â Â Â Â Â Vapor Correction factor (Water Vapor = 19.4)
S :Â Â Â Â Â Superheat ( o C)
P 1Â :Â Â Inlet pressure (kgf / cm 2 ABS)
In the example for calculating it. Time calculation aware of it. The manufacturing unit will have a total abs.
Example # 1
Application : Fluid Water
Flow rate 50 M 3 / HR.
Inlet Pressure 10 kgf / cm 2 G.
Outlet Pressure 5 kgf / cm 2 G.
Pressure different 5 kgf / cm 2 G.
Temperature 120 degree C
Density 943 kg / M 3
Because the fluid is a liquid So we will use the formula
Q L = 50 M 3 / HR.
G L = 943 kg / M 3 Â Convert to (UNIT Water = 1) = 0.943.
ΔP = 5 kg / cm 2.
So,
Cv = 1.17 (50) x sqrt (0.943 / 5).
Cv = 25.4
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Example # 2
Application : Fluid Gas
Flow rate 48,000 Sm3 / hr
Inlet Pressure 32 kgf / cm2 Convert to absolute pressure = 33.03 kgf / cm2 (A).
Outlet Pressure 15 kgf / cm2 convert to absolute pressure = 16.03 kgf / cm2 (A).
Pressure different 17 kgf / cm 2.
Temperature 240 degree C
Density 0.86 Air = 1
Because the fluid is a gas and ΔP ≥ P1 / 2 so we will use the formula.
Cv = (48000 / (248 × 33.03)) x sqrt (0.86 x (273 240)).
Cv = 123.08
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How to calculate PM in the formula for calculating CV related to Case of the Gas? Is P1 added to P2 or subtracted or multiplied?