Page 45 - Valve Selection Handbook

32 Valve Selection Handbook

If the straight length of pipe after the valve throat > 12 D, the pressure
loss cannot exceed the Borda-Carnot loss, which is:

where
v d = flow velocity in valve throat
V D = flow velocity in pipeline
This maximum pressure loss occurs if the taper angle (cc/2) of the dif-
fuser > 30°. With a decreasing taper angle, the pressure loss decreases
and reaches its lowest value at a taper angle of 4°.
If the straight pipe after the valve throat < 12 D, the pressure loss
increases and reaches its maximum value when the static energy convert-
ed into kinetic energy gets completely lost, in which case:

The friction loss approaches this maximum value if the valve with a
converging/diverging flow passage is mounted directly against a header.

Flow Coefficient C v

The flow coefficient C v states the flow capacity of a valve in gal
(U.S.)/rnin of water at a temperature of 60°F that will flow through a
valve with a pressure loss of one pound per square inch at a specific
opening position, as defined by the equation:

where
Q = U.S. gaVmin
2
Ap 0 = reference differential pressure = 1 lb/in.
2
Ap = operating differential pressure in lb/in.
Po = density of reference fluid, water = 62.4 lb/ft 3
p = density of operating fluid in lb/ft 3