maintenance is required for this trap because of its
simple construction. Too, the trap is usually constructed
of stainless steel.
THROTTLING TRAP.The operation of the
throttling trap (fig. 3-17) is based on the principle that
the flow of water through an orifice decreases as its
temperature approaches that of the steam used. The rate
of flow of the condensate may be adjusted by raising or
lowering a stem (needle valve) that fits into a tapered
seat. This throttling trap has no moving parts.
Condensate that is slightly cooler than steam enters
the trap, travels up through a baffle arrangement, and is
discharged through an orifice. If the condensate
discharge rate is higher than the inlet rate, the water
(condensate) level in the chamber drops. This allows
steam to enter the baffle passage and heat the
condensate. The amount of water flashing into steam
increases; so the volume of steam-water mixture
Pointers on Operating Procedures
handled by the orifice increases and thereby reduces the
capacity of the orifice. The reduced flow through the
orifice permits the level of condensate in the chamber to
rise until the heater water in the baffle passage has been
completely discharged and replaced with water that is
slightly cooler. Then the cycle is repeated. Air is vented
from this trap through the same passage as the
The throttling trap can be replaced without
disturbing any of the piping.
Figure 3-17.A throttling trap.
bimetallic-element trap, as shown in figure 3-18,
contains bimetallic elements that bend when heated.
The metals in the bimetallic strip generally are Emvar
and copper. The copper expands rapidly when heated,
but Emvar expands very little. Therefore, the bimetallic
strip bends when it is heated. This trap may be used for
higher or lower steam pressure by increasing or
decreasing the number of bimetallic leaves in the trap.
This trap works basically the same as the
thermostatic trap. When steam enters the trap, the
element is heated and bends, thus closing the valve. As
steam condenses, the elements cool and straighten out
to allow the valve to open and let the condensate escape.
The bimetallic trap can be repaired without disturbing
any of the piping.
To help ensure trouble-free service of steam traps,
follow the proper operating procedures carefully. Some
important factors involving operating procedures are
Steam traps should be operated within the
capacity rating and pressure differentials recommended
by the manufacturer. Use traps for the correct pressure
and temperature. If operating pressures change, it may
be necessary to change trap sizes, or internal parts, to fit
the new pressure conditions.
Traps should be insulated where heat must be
conserved. Some types of traps, which depend on the
cooling effect of the condensate for operation, should be
left bare. Check the manufacturers instructions
Where continuity of service is a requirement, a
three-valve bypass is usually provided to permit
drainage while the trap is being overhauled. Bypasses
are also used to speed up the discharge of condensate
and air when you are starting a system. In normal
operation, however, the bypass valve should be kept
closed to prevent steam from being wasted.
Check valves, located in the discharge line, are
important in parallel installations to prevent the
discharge of one trap from backing up into that of
another. Also, when condensate from the trap must
discharge to a higher elevation, a check valve prevents
backflow of condensate.
Inverted bucket traps must be primed for
operation by providing a condensate seal in the bottom