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G-4
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.
Initial Pressure Pressure Drop
15 psi 5 psi
50 psi 7.5 psi
100 psi 10 psi
Over 100 psi 10% of line pressure
(typically on/off applications)
Sizing a Valve
Pressure Drop for Steam
Recommended Pressure Drops
for Valve Sizing – Steam
1. With gravity ow condensate removal and inlet
pressure less than 15 psi, use a pressure drop equal
to the inlet gauge pressure.
2. With vacuum return system up to 7" Hg vacuum and
an inlet pressure less than 2 psi, a pressure drop
of 2 psi should be used. With an inlet pressure of
2 to 15 psi, use a pressure drop equal to the inlet
gauge pressure.
3. With an inlet pressure greater than 15 psi, use a
pressure drop equal to 80% of system differential
pressure. Example: Inlet pressure is 20 psig (35 psia)
and a gravity return at atmospheric pressure 0 psig
(14.7 psia), use a pressure drop of 16 psi.
4. When a coil size is selected on the basis that line
pressure and temperature is available in the coil of a
heating and ventilating application, a very minimum
pressure drop is desired. In this case, use the following:
pressure drop: