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 60^oF water with a pressure drop (dp) of 1 PSI across the valve. A Cv of 350 means a valve will pass 350 gpm of 60^oF water with a dp of 1 PSI.

FORMULA 1

　
FLOW RATE LBS/HR (Steam or Water)

　　

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)

　　

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 400^oF 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

2. 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
   
3. What is the smallest Class 2500 gate valve that will have less than a 5 PSI pressure drop in 900^oF, 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.

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