CEMP-E
TI 810-11
30 November 1998
3
Q = flow, m /hr
∆p = pressure drop through valve, in bars (100 kPa = 1 bar)
G = specific gravity of the liquid.
Q
Cv =
(eq. 2-4 IP)
∆p
G
Where:
Cv = flow coefficient, gpm @ ∆p = 1 psi
Q = flow, gpm
∆p = pressure drop through valve, psi
G = specific gravity of the liquid.
The specific gravity of water is 1. As the result of using equation 2-4, Kv = 17.64 (Cv = 20.41). If the
designer specifies a Kv of 17 (Cv of 20), the vendor can provide a valve with a Kv in the range of 17 to 21.25
( Cv in the range of 20 to 25). If the vendor provides a valve with a Kv of 21.25 (Cv of 25), the pressure drop
would be 28.5 kPa (4 psid) as calculated by using equation 2-4. This valve selection would make the
pressure drop through the valve 50 percent of the drop through the total piping circuit. Knowing that a plus
25 percent deviation in valve Kv (Cv) selection is allowable, the designer could select a Kv (Cv) which would
result in a larger pressure drop through the control valve. With a Kv of 14 (Cv of 16), the valve pressure
drop will be 65.7 kPa (9.76 psid) as calculated by the use of equation 2-4. This is less than the maximum
allowable valve pressure drop of 71.9 kPa (10.43 psid), and the total circuit pressure drop would be 27.6
kPa (4 psid) (coil and piping) plus 65.7 kPa (9.76 psid) (valve) = 93.3 kPa (13.76 psid). A Kv of 14 (Cv of
16) would result in the pressure through the valve being 71 percent of the drop through the piping circuit. If
the designer specifies a Kv of 14 (Cv of 16), the vendor can provide a valve with a Kv in the range of 14 to
17.5 (Cv in the range of 16 to 20). Cavitation will not be a problem in this example if the water temperature
used is 111 degrees C (200 degrees F), or less. In most HVAC control valve applications, water
temperatures are less than 111 degrees C (200 degrees F). When the hot water temperature is scheduled
from outside air, 111 degree C (200 degree F) water will occur only on design heating days. Also,
cavitation should not be a problem in hydronic systems when expansion tank pressures are adjusted so that
cavitation is not present at the pump inlet. Selection by the designer of a control valve Kv of 17 (Cv of 20)
rather than a Kv of 14 (Cv of 16) would be technically acceptable according to the criteria of this EI, but
would not be a good engineering practice. If the larger Kv (Cv) is used as the basis of design, the contractor
may provide a valve with a Kv of 21.25 (Cv of 25). A Kv of 21.25 (Cv of 25) may result in the valve pressure
drop being less than 50 percent of the drop through the circuit. The basis of design valve Kv (Cv) is selected
because of the drop expected through the basis of design coil and the piping system configuration shown by
the designer. The coil provided by the contractor, field piping conditions and aging of the piping system are
factors which could result in the installed valve being less than 50 percent of the drop through the coil when
the contractor provides a valve with the largest Kv (Cv) permissible.
c. Review of manufacturers' catalogs shows the ready availability of valves with Kv in the range of 14 to
22 (Cv in the range of 16 to 25), as listed in table 2-3.
2-23
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