VALVES AND ACTUATORS
38
control valves sizing
How to Calculate Kvs
Flow coefficient Kvs is the flow rate of water in m
3
/h passing through a fully open valve
at a 100 kPa pressure drop.
a) Liquids kvs= 10 x Q x
r
Dp
Q = flow rate m
3
/h
Dp = pressure drop (kPa)
r = relative density (kg/dm
3
)
The Dp pressure drop should be determined as follows:
- Equal or higher than the Dp of the circuit under control, in case of variable flow ap-
plications
- Equal or higher than the Dp of the supply circuit, in case of constant flow applications
b) Steam kvs =
100 x G x C
20.3 P
2
x Dpv
G = flow rate (kg/h)
C = 1 + 0.0013 (t-ts)
t = steam temperature in working conditions
ts = saturated steam temperature at P
2
pressure
P
2
= pressure downstream (kPa)
Dpv = pressure drop (kPa)
Choose the valve with the Kvs closest to the calculated one.
Water Systems
Two-way valve
For this application the pressure drop through the valve must be high, in order to have a
good control flow characteristic and a properly working system.
1) The valve pressure drop must be 30 to 50% of the pressure upstream the valve.
2)The valve pressure drop must be equal to, or higher than the pressure drop of the coil
or heat exchanger under control, in particular:
Temperature drop of heat exchanger Design of valve pressure drop
30 °C
Equal to pressure drop of heat exchanger
20 °C
Twice as pressure drop of heat exchanger
10 °C
Three times as pressure drop of heat exchanger
Three- way mixing valve
For mixing valve a high pressure drop is not normally required even when used in pri-
mary and secondary water circuits to control supply temperature to users.
As a general rule, the valve must have a pressure drop similar to the one of the heat
exchanger.
Three-way diverting valve
Three-way diverting valves are used to control flow to heat exchanger and, therefore,
the pressure drop through the valve. For proportional systems it must be high.
Note:When selecting pressure drop, you must not exceed the above-mentioned values
because an undersized valve could produce:
- Noisy operation and vibration of the plug
- Rapid wear of plug and seat due to high speed of the fluid through the valve.
