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Control Valve Cv calculation

Cv value calculation

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.

A)     Simple Cv Value Formula: Case of the Liquid

CV Value Formula for Liquid

Q Flow rate (m3 / H).

P : Differential pressure (kg / cm 2 )

G L : Specific Gravity (Water = 1)

B)      Simple Cv Value Formula: Case of the Gas

I) Non-choked flow   ΔP <P1 / 2

CV Value Formula for Gas & Non Choked Flow

Q Flow rate (M 3 / H @ 15 o C , 1atm.)

P :   Differential pressure (kg / cm 2 )

G G :   Specific Gravity (Air = 1)

T :  Temperature ( o C)

P M :  (P1 P2) / 2

II) choked flow ΔP ≥ P1 / 2

CV Value Formula for Gas & Choked Flow

Q G   Flow rate (M 3 / H @ 15 o C, 1atm.)

P 1    :   Inlet pressure (kgf / cm 2 , ABS)

G G   :   Specific Gravity (Air = 1)

T  :  Temperature ( o C)

C)      Simple Cv Value Formula: Case of the Steam (Vapor)

I) Saturation Vapor: Non-choked flow   ΔP <P1 / 2

CV Value Formula for Steam & Saturation Vapour Non choked flow

Q s  :   Flow rate (kg / H)

Y    :    Vapor Correction factor (Water Vapor = 19.4)

P :   Differential pressure (kg / cm 2 )

P M:   (P1 P2) / 2

II) Vapor Saturation: choked flow ΔP ≥ P1 / 2

CV Value Formula for Steam & Saturation Vapour choked flow

Q Flow rate (kg / H)

Y   :   Vapor Correction factor (Water Vapor = 19.4)

P 1 :   Inlet pressure (kgf / cm 2 ABS.)

III) Wet Vapor: Non-choked flow   ΔP <P1 / 2

CV Value Formula for Steam & Wet Vapour Non choked flow

Q s  Flow rate (kg / H)

X    :  Quality of Vapor (X = 1 is Dry Steam)

Y  :    Vapor Correction factor (Water Vapor = 19.4)

P :  Differential pressure (kg / cm 2 )

P M:  (P1 P2) / 2

IV) Wet Vapor: choked flow ΔP ≥ P1 / 2

CV Value Formula for Steam & Wet Vapour choked flow

Q Flow rate (kg / H)

X  :    Quality of Vapor (X = 1 is Dry Steam)

Y :     Vapor Correction factor (Water Vapor = 19.4)

P 1 :  Inlet pressure (kgf / cm 2 ABS.)

v) Superheated Vapor: Non-choked flow   ΔP <P1 / 2

CV Value Formula for Steam & Superheated Vapour Non choked flow

Q Flow rate (kg / H)

Y  :    Vapor Correction factor (Water Vapor = 19.4)

P : Differential pressure (kg / cm 2 )

S :   Superheat ( o C)

P M :  (P1 P2) / 2

VI) Superheated Vapor: choked flow ΔP ≥ P1 / 2

CV Value Formula for Steam & Superheated Vapour choked flow

Q Flow rate (kg / H)

Y :     Vapor Correction factor (Water Vapor = 19.4)

S :     Superheat ( o C)

P 1 :  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

CV Value Formula for Liquid

Q L = 50 M 3 / HR.

G L = 943 kg / M 3   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

================================================== =====

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 Value Formula for Gas & Choked Flow

Cv = (48000 / (248 × 33.03)) x sqrt (0.86 x (273 240)).

Cv = 123.08

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7 comments

RAMESHB January 26, 2016 at 1:33 pm

Sir,
l am working in power plant plz send electrical questions and answers
Sir urged arrivals very useful
thank u sir

Reply
S Bharadwaj Reddy January 26, 2016 at 3:20 pm

Hello Ramesh,
For Electrical Questions & Answers Please Click Here

Reply
aslam January 27, 2016 at 3:11 pm

Sir, I am aslam khan please explain hydraulic control valve working principles and also explain i/p working hydraulic oil possible

Reply
Mohit Bansal February 13, 2016 at 1:13 pm

Please share the calibration procedure (steps) for calibration of flow Instrumentation like – orifice flowmeter ,venturi, Coriolis, vortex, Magnetic flowmeter.

Reply
Ramu Sunke March 15, 2016 at 2:50 am

Hi Bharadwaj

This is Ramu from Anrak. I have seen your website. Very much useful for all. You are doing very good job. Well done Bharadwaj. Be continue.

Reply
Hardik Makvana July 11, 2016 at 4:21 am

Dear sir,
Please sir share the DP type transmitter basic principal and how its work..??

Reply
DEvanathan December 29, 2018 at 9:34 pm

Devanathan,

Dear sir,

please share Convertion table for all units like pressure,temp,mm water column, mercury column, psi to bar,and bar to kg–etc.

Reply

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