Basic Documentation
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Control Valve Sizing and Selection
Pressure Drop for Water Flow
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 specied by the engineer and the required ow is known, the
selection of a valve is simplied. 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 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 Steam
The same methodology should be applied for selecting a valve for
steam with the most important consideration is the pressure drop.
First, the correct maximum capacity of the coil must be determined.
Ideally, there should be no safety factor in this determination and it
should be based on the actual BTU heating requirements. The valve
size must be based on the actual supply pressure at the valve. When
the valve is fully open, the outlet pressure will assume a valve such
that the valve capacity and coil condensing rate are in balance. If
this outlet valve pressure is relatively large (small pressure drop),
then as the valve closes, there will be no appreciable reduction in
ow until the valve is nearly closed. To achieve better controllability,
the smallest valve (largest pressure drop) should be selected. With
the valve outlet pressure much less than the inlet pressure, a large
pressure drop results. There will now be an immediate reduction
in capacity as the valve throttles. For steam valves, generally the
largest possible pressure drop should be taken, without exceeding
the critical pressure ratio. Therefore, the steam pressure drop should
approach 80% of the system differential pressure.
Examining the pressure drops under “Recommended Pressure Drops
for Valve Sizing — Steam” , you might be concerned about the
steam entering the coil at 0 psi when a large drop is taken across
the control valve. Steam ow through the coil will still drop to
vacuum pressures due to condensation of the steam. Consequently,
a pressure differential will still exist. In this case, proper steam
trapping and condensation piping is essential.
Sizing Valves
Figure 1.