STEAM TRAPS
DOE-HDBK-1018/2-93
Miscellaneous Mechanical Components
A control orifice runs through the disk from top to bottom, which is considerably smaller at the
top than at the bottom. The bottom part of the disk extends through and beyond the orifice in
the seat. The upper part of the disk (including the flange) is inside a cylinder. The cylinder
tapers inward, so the amount of clearance between the flange and the cylinder varies according
to the position of the valve. When the valve is open, the clearance is greater than when the
valve is closed.
When the trap is first placed in service, pressure from the inlet (chamber A) acts against the
underside of the flange and lifts the disk off the valve seat. Condensate is thus allowed to pass
out through the orifice in the seat; and, at the same time, a small amount of condensate (called
control flow) flows up past the flange and into chamber B. The control flow discharges through
the control orifice, into the outlet side of the trap, and the pressure in chamber B remains lower
than the pressure in chamber A.
As the line warms up, the temperature of the condensate flowing through the trap increases. The
reverse taper of the cylinder varies the amount of flow around the flange until a balanced
position is reached in which the total force exerted above the flange is equal to the total force
exerted below the flange. It is important to note that there is still a pressure difference between
chamber A and chamber B. The force is equalized because the effective area above the flange
is larger than the effective area below the flange. The difference in working area is such that the
valve maintains at an open, balanced, position when the pressure in chamber B is approximately
86% of the pressure in chamber A.
As the temperature of the condensate approaches its boiling point, some of the control flow
going to chamber B flashes into steam as it enters the low pressure area. Because the steam has
a much greater volume than the water from which it is generated, pressure builds up in the space
above the flange (chamber B). When the pressure in this space is 86% of the inlet pressure
(chamber A), the force exerted on the top of the flange pushes the entire disk downward and
closes the valve. With the valve closed, the only flow through the trap is past the flange and
through the control orifice. When the temperature of the condensate entering the trap drops
slightly, condensate enters chamber B without flashing into steam. Pressure in chamber B is
thus reduced to the point where the valve opens and allows condensate to flow through the
orifice in the valve seat. The cycle is repeated continuously.
With a normal condensate load, the valve opens and closes at frequent intervals, discharging a
small amount of condensate at each opening. With a heavy condensate load, the valve remains
open and allows a continuous discharge of condensate.
Orifice-Type Steam Trap
DOE facilities may use continuous-flow steam traps of the orifice type in some constant service
steam systems, oil-heating steam systems, ventilation preheaters, and other systems or services
in which condensate forms at a fairly constant rate. Orifice-type steam traps are not suitable for
services in which the condensate formation is not continuous.
ME-05
Rev. 0
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