30 November 1998
Damper or valve positive positioner
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c. Application of air consumption values to sizing of air compressor motors. An example of the
application of the values in table 2-2 is as follows:
5 IPs @ 8 ml/s (30 scim) each = 40 ml/s (150 scim)
15 valve and damper positioners @ 6 ml/s (20 scim) each = 90 ml/s (300 scim)
Total = 40 + 90 ml/s (150 + 300 scim) = 130 ml/s (450 scim)
Therefore, 130 ml/s (450 scim) x 3 (for 1/3 running time) = 390 ml/s (1,350 scim)
A manufacturer's catalog indicates that a typical hp compressor delivers about 390 ml/s (1,420 scim) and
of the time, while a hp compressor would run only 26 percent of the time. On the basis of this
calculation, a power circuit capable of serving a hp motor would provide adequate margin to compensate
for variances in the air consumption of control devices selected by the control system vendor.
12. DETERMINATION OF CONTROL VALVE FLOW COEFFICIENT (Kv / Cv).
a. The control valve flow coefficient (Kv (Cv)) is a number representing the quantity of water, at a given
temperature, that will flow through a valve with a given pressure drop. The designer will calculate and
select the flow coefficient (Kv (Cv)) of all modulating control valves required for the design, and show the
pressure against which the valve must close. The selection of the valve's Kv (Cv) provides the guidelines for
the vendor's selection of the valve's port size. The close-off pressure information gives the required
criterion for sizing the actuators for the valve selected by the vendor.
b. It should be noted that there is not yet an agreed upon international definition of control valve flow
coefficients expressed in SI units (Kv). Kv data seems to be commonly expressed in units of either 1) m /hr
@ ∆p = 100 kPa (1 bar), or 2) L/s @ ∆p = 1 kPa. Because of this, one needs to note the units associated
with published Kv data and explicitly cite units associated with Kv when producing specifications or drawings.
In this document Kv will be associated with the units of m /hr @ ∆p = 100 kPa. (For conversion purposes:
Kv (L/s @ 1 kPa) = 36 x Kv (m /hr @ 100 kPa).) A control valve flow coefficient expressed in inch-pound (I-
P) units is denoted as Cv and is generally expressed with units of gallons per minute (gpm), at 60 degrees
F, at a pressure drop of 1 psid.
c. A valve that is operated in a 2-position open and closed manner will be a line-size valve with the
largest available Kv (Cv), in order to reduce the pressure drop across the valve and the pumping
horsepower required. This applies to dual-temperature system changeover valves and dual-temperature
fan coil unit 3-way valves.
d. The designer will select the Kv (Cv) of the valve based on the maximum flow and the pressure drop
for the valve. The selected control valve will be checked against manufacturer's catalogs to insure that
such a valve Kv (Cv) is available in a product of the control valve manufacturers.