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Engineering
G-3
Sizing a Valve
A pressure drop must exist across a control valve if ow
is to occur. The greater the drop, the greater the ow at
any xed opening. The pressure drop across a valve also
varies with plug position – from minimum when fully open,
to 100% of the system drop when fully closed.
To size a valve properly, it is necessary to know the full
ow pressure drop across it. The pressure drop across a
valve is the difference in pressure between the inlet and
outlet under ow conditions. When it is specied by the
engineer and the required ow is known, the selection of a
valve is simplied. When this pressure drop is not known,
it must be computed or assumed.
If the pressure drop across the valve when fully open is
not a large enough percentage of the total system drop,
there will be little change in uid ow until the valve
actually closes, forcing the valve’s characteristic toward
a quick opening form.
Figure 1.
Control Valves
Selection and Sizing
Figure 1 shows ow-lift curves for a linear valve with
various percentages of design pressure drop. Note the
improved characteristic as pressure drop approaches
100% of system pressure drop at full ow.
It is important to realize that the ow characteristic for any
particular valve, such as the linear characteristic shown
in Figure 1 is applicable only if the pressure drop remains
nearly constant across the valve for full stem travel. In
most systems, however, it is impractical to take 100% of
the system drop across the valve.
A good working rule is, “at maximum ow, 25 to 50%
of the total system pressure drop should be absorbed
by the control valve.” Although this generally results in
larger pump sizes, it should be pointed out that the initial
equipment cost is offset by a reduction in control valve
size, and results in improved controllability of the system.
Reasonably good control can be accomplished with
pressure drops of 15 to 30% of total system pressures.
A drop of 15% can be used if the variation in ow is small.
Recommended Pressure Drops
for Valve Sizing – Water
1. With a differential pressure less than 20 psi, use a
pressure drop equal to 5 psi.
2. With a differential pressure greater than 20 psi, use a
pressure drop equal to 25% of total system pressure
drop (maximum pump head), but not exceeding the
maximum rating of the valve.
Pressure Drop for Water Flow