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Valve Flow Calculations
 

How to use Cv

 
Valve coefficient (Cv) is a number which represents a valve's ability to pass flow. The bigger the Cv, the more flow a valve can pass with a given pressure drop. A Cv of 1 means a valve will pass 1 gallon per minute (gpm) of 60oF water with a pressure drop (dp) of 1 PSI across the valve. A Cv of 350 means a valve will pass 350 gpm of 60oF water with a dp of 1 PSI.

FORMULA 1

 
FLOW RATE LBS/HR (Steam or Water)

  

Where:
     
dp = pressure drop in PSI
     
F = flow rate in lbs./hr.
     
= square root of a specific volume in ft3/lb.
(downstream of valve)


FORMULA 2

  
FLOW RATE GPM (Water or other liquids)

  

Where:
     
dp = pressure drop in PSI
     
Sg = specific gravity
     
Q = flow rate in GPM


LIMITATIONS

 
Above formulas are not valid under the following conditions:

  1. For compressible fluids, where pressure drop exceeds half the inlet pressure.
  2. For non-compressible fluids, where pressure drop causes cavitation or flashing.
  3. For dual-phase flow such as steam-water mixtures.


EXAMPLE

  1. What is the pressure drop through a 14" Class 900 pressure seal gate valve with 400oF water, 1500 PSIG, and 5,000,000 lbs/hr flow rate?

    Solution: Use formula 1.
F = 5,000,000
     
= 0.137 (from chart for water at 400oF)
     
Cv = 9150

 
  1. What is the pressure drop through a 14"x12"x14" Class 900 Venturi pressure seal gate valve with the same conditions as example 1.

    Solution: Use formula 1.

    Cv = 6285
  2. What is the smallest Class 2500 gate valve that will have less than a 5 PSI pressure drop in 900oF, 1200 PSI steam service at a flow rate of 500,000 lbs/hr?

    Solution: Use formula 1.
F = 500,000
     
= 0.785 (from chart for steam at 900oF, 1200 PSIG)
= = 2.24
 

  
The closest higher Cv is a 10" Class 2500 gate with a Cv of 3130. This is the smallest valve with a pressure drop less than 5 PSIG.


Selected Properties of Water and Steam

For Water (Downstream)

All
Pressures
Temperature (oF)
100 200 300 400 500 600
0.127 0.129 0.132 0.137 0.143 0.154
 
For Steam (Downstream)

Pressure
PSIG
Temperature (oF)
Sat 500 600 700 800 900 1000 1100
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1600
1800
2000
2500
3000
4000
5000
6000
1.97
1.46
1.21
1.06
0.950
0.865
0.801
0.747
0.701
0.662
0.628
0.598
0.570
0.545
2.20
1.61
1.30
1.11
0.979
0.895
2.32
1.68
1.38
1.19
1.06
0.960
0.880
0.815
0.759
0.710
0.670
0.629
0.590
0.559
2.44
1.77
1.46
1.26
1.12
1.02
0.941
0.875
0.821
0.773
0.732
0.695
0.662
0.633
0.58
0.54
0.50
0.41
0.31
0.17
0.16
0.16
2.54
1.85
1.52
1.32
1.18
1.08
0.994
0.926
0.870
0.816
0.781
0.745
0.710
0.683
0.63
0.59
0.55
0.48
0.42
0.32
0.24
0.19
2.64
1.95
1.60
1.38
1.23
1.13
1.04
0.972
0.914
0.865
0.823
0.785
0.752
0.723
0.67
0.63
0.59
0.52
0.46
0.38
0.32
0.28
2.74
2.00
1.65
1.43
1.28
1.17
1.08
1.01
0.955
0.917
0.872
0.813
0.789
0.752
0.70
0.66
0.62
0.55
0.49
0.42
0.36
0.32
2.84
2.07
1.71
1.47
1.33
1.22
1.12
1.05
0.99
0.94
0.90
0.86
0.83
0.79
0.74
0.69
0.65
0.58
0.52
0.44
0.39
0.35

From: NVF valve knowledge

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