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Techniques
for Testing Steam
Trap Operation |
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The observer must know
if real steam or flash steam is being discharged. Differentiating
between flashing condensate and steam requires considerable experience.
Visual distinction between 99.6 and 100-percent steam is extremely
difficult. If the effluent from the trap contains a substantial
number of water droplets, the trap is probably not blowing steam.
A more positive indication that the trap is blowing steam is
a short, nearly transparent zone -- usually with a blue tint
-- at the end of the discharge line. A good trap may easily be
judged to be defective. Only through repeated observations will
the inspector become proficient.
If the discharge from the trap is intermittent and the trap closes between
discharges with no appreciable discharge of steam or vapor, the trap is
in good operating condition. Listening to the trap discharge through a
sounding device, such as a stethoscope or a metal rod placed against the
trap, is another method of determining trap performance. This method is
satisfactory only if noises from adjacent traps or mechanical devices are
not transmitted to the listening device. When several traps are close together
in the piping system, ultrasonic testers, responding only to frequencies
above 35 kilohertz, are useful. High-frequency sound is quickly attenuated
in piping, and, consequently, does not travel from one trap to another.
Testers may be supplied with earphones, loudspeakers, or meters so the
discharge can be seen or heard. Each type of trap has a definite response,
so experience is necessary in using ultrasonic testers to evaluate performance.
Thermostatic traps, when properly sized for the load, will discharge intermittently.
Therefore, if the trap is operating properly, a loud hissing sound will
be heard during discharge; no sound will be heard when the trap is closed.
If hissing continues after closing, the trap is leaking. Hissing does not
necessarily mean that the trap is losing steam. If the condensate load
is greater than the amount of leakage, steam will not be lost. However,
if the condensate load is less than the leakage, the hissing will continue
for a prolonged period and the trap will cycle only when the condensate
load exceeds the leakage, allowing subcooling of the condensate. If the
trap continues to cycle regularly, it is probably functioning properly.
Some thermostatic traps tend to throttle under certain load conditions.
When these conditions exist, the test instrument will emit a continuous
hissing. When this occurs, the trap can be tested only by reducing the
load. Under light-load conditions, even these traps cycle rhythmically.
Bucket traps operate intermittently. The tester will easily learn to distinguish
between normal and malfunctioning operation. When the trap is working properly,
a hissing noise will be heard during discharge, and, when the trap closes,
the sound stops. Continuous hissing indicates that the trap is blowing,
which may result from a loss of prime under light loads, or a malfunction
of the trap mechanism.
Thermo-dynamic trap opening and closing frequency depends on the trap load
and the mechanical condition of the trap. If the trap cycles fewer than
10 times per minute, it is operating normally. But, if the trap is worn,
its cycling rate will increase significantly; the trap will sound somewhat
like a machine gun. If the thermo-dynamic trap discharges continuously,
the loading may exceed capacity, or return line pressure may be too high.
The return line pressure and the pressure upstream of the trap should be
checked.
Impulse traps have a piston in a conical cylinder and are intermittent
in operation. A bleed hole is drilled through the piston, allowing flow
from inlet to outlet even when the trap is closed. Therefore, with the
trap closed, a hissing sound will be heard. But the trap is not necessarily
wasting steam. If it is properly sized, it will still operate intermittently.
A loud roar when the trap is in the full discharge position will be followed
by a much lower noise level. If a loud noise is heard continuously, the
trap is either overloaded or is stuck in the open position and should be
repaired.
Float-thermostatic traps have a tendency to discharge continuously, particularly
at low or moderate pressures, and modulate according to the load ahead
of the trap. Under these conditions, ultrasonic testers are of no value.
However, when float-thermostatic traps are used at high pressures, they
tend to discharge intermittently, and, if the tester indicates a rhythmic
intermittent discharge, the trap is working properly.
Table II shows amounts of steam lost through various orifice sizes. A 1/16
in. dia. hole, or a trap with 1/16 in. of wear, will leak steam worth nearly
$500 a year (based on steam at $3.50/1000 lb and 125 psig). |
| Steam losses at pressures greater than 300
psig may be calculated from: |
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| If the condensate
load is greater than the capacity of the leakage path,
no steam will be lost. When the condensate load decreases,
steam will be lost. Regular trap testing can help save
steam and, thus, energy. |
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