Valve Sizing
In selecting the suitable valve to use for an application, the following items must be
considered:
1. What is the fluid medium of usage.
In chillers, the fluid is water. There are valves specially designed other fluids e.g. steam and
air, which are not suitable for water.
2. What is maximum operating pressure and temperature.
3. What is the valve duty required - is it for throttling, shut-off, balancing, mixing, etc.
4. What is the pipe size to be connected
Do not oversize or undersize a valve to suit the pipe size. Size the valve according to the
flow requirements. Use reducers where applicable.
5. What is the flow rate required through the valve
6. What is the flow characteristics required - linear, equal percentage (See following pages)
7. What is the piping connection method to the valve - threaded, flanged
Most of the above mentioned information may be obtained from the valve catalogs provided
by the valve manufacturers.
In sizing the valve, the general accepted method is by means of the C
v
(flow coefficient).
Different valves will have different C
v
values.
Formula:
Q = C
v
* (∆p)
Definition: The C
v
rating of any valve is the amount of water,Q (GPM) at standard conditions
(60°F, specific graviti = 1) which will pass through the valve with a pressure drop, ∆p of 1 psi
with the valve in a full open position.
By using conversion factors, we can have the flow coefficient K
v
in metric units:
1 C
v
= 0.857 K
v
Definition: The K
v
rating of any valve is the amount of water (m
3
/hr) at standard conditions
(20°C, specific graviti = 1) which will pass through the valve with a pressure drop of 1 kg/cm
2
with the valve in a full open position.
If the reference pressure is 1 bar, then:
1 C
v
= 0.867 K
v
Section 3 Page 58
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