TM 5-810-15
sized for the maximum expected condensate load
float to open the trap until it is drained and the float
and a safety factor. Oversizing a trap can increase
loses buoyancy. A float trap does not have to be
losses, both for good traps as well as traps that fail
primed as an inverted bucket trap does. However,
open. It is recommended that a safety factor of 2:1
buildup of solids and sludge in the trap body can
be used to size a trap in a constant pressure usage
prevent the float from sinking and closing the
and safety factor of 3:1 be used if the pressure
valve. The discharge from the float trap is generally
varies. The major steam loss from a trap is at a
continuous. This type is used for draining
failed-open condition and can cost thousands of
condensate from steam headers, steam heating
dollars a year if not detected. However, a failed
coils, and other similar equipment. When a float
close trap may cause extensive damage because of
trap is used for draining a low pressure steam
corrosion or water hammer from condensate that is
system, it will be equipped with a thermostatic air
not discharged. Selection of the failure mode
vent.
(3) Thermostatic. The thermostatic trap opens
depends on the design conditions and maintenance
practices. All traps are subject to freezing,
and closes by means of a force developed by a
particularly due to condensate flow blockage. Of all
temperature sensitive actuator. A basic problem for
types of traps, float traps are most subject to
all thermostatic traps is keeping the actuating
freezing. Steel bodied traps resist freezing better
temperature close to the saturation temperature so
than iron. Trap cost is an important consideration
the condensate will be hot, but not allow live steam
as initial expense may not justify selection because
to blow out the trap. If the actuating temperature is
of maintenance characteristics and life span. The
not close to the saturation temperature, there is a
inverted bucket type of trap has the longest life,
possibility that O2 and CO2 may dissolve in the
followed by the float trap, thermostatic trap and the
water, and also, that the condensate may back up
thermodynamic trap consecutively for long life.
to an unacceptable level. The discharge from this
d. Types. There are four basic types of steam
type of trap is intermittent. Thermostatic traps are
traps: inverted bucket, float, thermostatic and
used to drain condensate from steam heating coils,
thermodynamic.
unit heaters and other similar equipment. Strainers
(1) Inverted bucket. This type of steam trap is
are normally installed on the inlet side of the steam
the most widely used. When properly sized, steam
trap to prevent dirt and pipe scale from entering the
loss is kept to a minimum. The inverted bucket trap
trap. The thermostatic trap is the most common of
contains an inverted bucket inside the trap body.
all trap types used for two pipe steam heating sys-
The inverted bucket is fastened to a linkage in such
tems. When this type of trap is used for a heating
a manner that it will close the trap outlet when
system, at least 2 feet of pipe will be provided
steam enters from beneath the bucket. As the steam
ahead of the trap to cool the condensate. This
cools and condenses (assisted by a bucket vent),
permits condensate to cool in the pipe rather than
the bucket loses its buoyancy and the trap opens
in the coil, and thus maintains maximum coil
releasing the condensate. Gases mixed with the
efficiency. Thermostatic traps are recommended for
steam pass through the inverted bucket trap partly
low pressure systems up to a maximum of 15 psi.
by way of the bucket vent and partly in any steam
When used in medium or high pressure systems,
discharged by the trap. The discharge from the
they must be selected for the specific design
inverted bucket trap is intermittent and requires a
temperature. In addition, the system must be
differential pressure between the inlet and discharge
operated continuously at that design temperature.
(4) Thermodynamic disc traps. Thermody-
of the trap to lift the condensate from the bottom of
the trap to the discharge connection. They are
namic disc traps are used for intermittent service,
resistant to water hammer, operate well at low
where they operate well under variable pressure
loads, and fail open. Bucket traps may be subject to
conditions, are resistant to damage from freezing
damage from freezing and have only fair ability to
and water hammer, and fail open. A disadvantage
handle start-up air loads. Inverted bucket traps are
of the thermodynamic trap or disc trap is poor gas
well suited for draining condensate from steam
handling. The pressure drop when air or 002 are
lines or equipment where an abnormal amount of
flowing in the trap resembles that of steam, so that
air is to be discharged and where dirt may drain
large amounts of air will close the trap. Therefore,
into the trap.
another air removal method is necessary for startup
(2) Float. A float trap has a small chamber
of a steam system. Thermodynamic disc traps will
containing a float and linkage that multiplies the
not be used where high back pressures, or low load
float*s buoyancy. The condensate will cause the
conditions might occur. They are best suited for
8-12
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