User Guide

4 © Copyright 2006 TAC All Rights Reserved. F-24396-1
VALVE SIZING AND SELECTION
Water Proportional
Proportional valves are usually selected to take a pressure drop equal to at least 50% of the
“available pressure” (that is, the pump pressure differential available between supply and
return mains with design flow at the valve location). As “available pressure” is often difficult
to calculate, the normal procedure is to select the valve using a pressure drop at least equal
to the drop in the coil or other load being controlled (except where small booster pumps are
used) with a minimum recommended pressure drop of 5 psi (34 kPa). When the design
temperature drop is less than 60°F (33°C) for conventional heating systems, higher pressure
drops across the valve are needed for good results (Table-3).
Table-3 Conventional Heating System.
Secondary Circuits with Small Booster Pumps: 50% of available pressure difference
(equal to the drop through load, or 50% of booster pump head).
Water Capacity
See Table-4 for water capacity of VB-7332 series valves.
Note: For four-pipe application, refer to Table-6 for valves in series.
Table-4 Water Capacity in Gallons Per Minute for VB-7332 Series.
C
v
Equation
Where:
Cv = Coefficient of flow
GPM = U.S. gallons per minute (60°F, 15.6°C)
ΔP = Differential pressure in psi (pressure drop)
Cavitation Limitations
on Valve Pressure
Drop
A valve selected with too high a pressure drop can cause erosion of discs and/or wire
drawing of the seat. In addition, cavitation can cause noise, damage to the valve trim (and
possibly the body), and choke the flow through the valve.
Do not exceed the maximum differential pressure (pressure drop) for the valve selected.
The following formula can be used on higher temperature water systems, where cavitation
could be a problem, to estimate the maximum allowable pressure drop across the valve:
Pm = 0.5 (P
1
– Pv)
Where:
Pm = Maximum allowable pressure drop (psi)
P1 = Absolute inlet pressure (psia)
Pv = Absolute vapor pressure (psia) (refer to Table-5)
Note: Add 14.7 psi to gauge supply pressure to obtain absolute pressure value.
Design Temperature
Load Drop °F (°C)
Recommended Pressure Drop
a
(% of Available Pressure)
Multiplier on
Load Drop
60 (33) or More 50% 1 x Load Drop
40 (22) 66% 2 x Load Drop
20 (11) 75% 3 x Load Drop
a
Recommended minimum pressure drop = 5 psi (34 kPa).
Valve Body
Part Number
C
v
Rating
Differential Pressure (
ΔP in psi)
12345678910152035
VB-7332-0-4-3 2.2 2.2 3.1 3.8 4.4 4.9 5.4 5.8 6.2 6.6 7.0 8.5 9.8 13
VB-7332-0-4-4 4.44689101112121314172026
C
v
=
GPM
ΔP=
GPM
C
v
()
GPM = C
v
ΔP
2
ΔP