e. To insure good control characteristics, the pressure drop through the control valve at full flow must

be greater than the pressure drop through the coil and piping circuit (without the control valve) between the

point where the piping circuit connects to the supply and return mains. The pressure drop through the

control valve will then be at least 50 percent of the total pressure drop through the circuit. The valve Kv (Cv)

must be specified accordingly. For liquid service, the vendor may supply a control valve with a plus 25

percent deviation from that specified. The designer must check to insure that the pressure drop through the

valve will be acceptable at both the specified Kv (Cv) and at a Kv (Cv) 25 percent greater than specified. As

the Kv (Cv) increases the pressure drop through the valve decreases. For steam service, the vendor must

supply a control valve with a Kv (Cv) not less than the value specified and not larger than the manufacturer's

next larger value.

13. CALCULATION OF LIQUID CONTROL VALVE FLOW COEFFICIENT (Kv / Cv).

a. A physical phenomenon known as cavitation is a cause of valve failure. Cavitation is caused when

the velocity through the valve creates an absolute pressure lower than the vaporization pressure of the

liquid. To avoid cavitation, use equation 2-3 to determine the maximum allowable pressure drop through

open valves.

∆

(eq. 2-3)

Where:

∆pm = maximum allowable pressure drop through the valve, kPa (psid)

km = valve pressure recovery coefficient (use 0.45)

pe = absolute pressure entering the valve, kPa (psia) (by design calculations).

pv = absolute vapor pressure of the liquid, kPa (psia) (from steam tables).

For example: If 111 degree C (200 degree F) water with an equivalent vapor pressure of 79.5 kPa (11.53

psia) is to flow through a valve with an inlet pressure of 138 kPa (239 kPa (abs.)) (20 psig (34.7 psia)), the

maximum allowable pressure drop through the valve will be, as a result of using equation 2-3, 71.9 kPa

(10.43 psid).

b. The pressure drop through the control valve must be at least 50 percent of the total pressure drop,

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at full flow, through the circuit. Continuing with the above example, assume a coil with a flow of 11.35 m /hr

(50 gpm) and a 27.6 kPa (4 psig) drop through the coil, piping and fittings between the mains. The

maximum allowable pressure drop to avoid cavitation was calculated to be 71.9 kPa (10.43 psid). An initial

valve pressure drop selection of 41.4 kPa (6 psid) will prevent cavitation and provide sufficient valve

pressure drop for good control. The Kv (Cv) can be calculated from equation 2-4:

(eq. 2-4 SI)

∆p

Where:

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Kv = flow coefficient, m /hr @ ∆p = 100 kPa (1 bar)

2-22

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