2000 Hook-up Book - Spirax Sarco

Increasing attention is being paid
in modern plants to means of
assessing steam trap performance.
While it is important to
know if a trap is working normally
or is leaking steam into the condensate
return system, most of
the available methods of assessing
trap operation are of much
more restricted usefulness than is
appreciated. To explain this, it is
necessary to consider the mode
of operation of each type of trap
when operating and when failed,
and then to see if the proposed
test method can distinguish
between the two conditions.
Temperature Test Methods
One well established “method” of
checking traps is to measure temperature,
either upstream or
downstream. People use pyrometers,
remote scanners and
temperature sensitive crayons or
tapes, while generations of maintenance
men have thought they
could assess trap performance by
spitting onto the trap and watching
how the spittle reacted! Certainly, if
a trap has failed closed, the temperature
at the trap will be lower
than normal, but equally the equipment
being drained will also cool
down. The trap is not leaking
steam since it is closed, and this
failure is only a cause of problems
in applications like steam main
drips where the condensate not
discharged at the faulty trap is carried
along the steam line. More
usually, the temperature on the
inlet side of the trap will be at or
close to the saturation temperature
of steam at whatever pressure is
reaching the trap. Even if the trap
were blowing steam, the temperature
remains much the same.
The one exception is in the
case of a temperature sensitive
trap, especially one of the bimetal
pattern. If this fails open, then the
temperature at the inlet side will
rise from the normal subcooled
level to saturation values, and this
rise may be detectable if the
steam pressure is a known, constant
value.
Measuring temperatures on
the downstream side of a trap, by
whatever method, is even less
likely to be useful. Let’s look first
at a trap discharging through an
open-ended pipe to atmosphere.
The pressure at the trap outlet
must be only just above atmospheric,
and the temperature just
above 212°F.
With any condensate present
with the steam at temperatures
above 212°F on the inlet side, the
condensate, after passing through
the trap will flash down to 212°F
and this temperature is the one
that will be found. Any leaking
steam will help evaporate a little
more of the condensate without
increasing the temperature.
Again, the only exception which
may be encountered is the low
pressure steam heating system
where thermostatic traps normally
discharge at temperatures
below 212°F into atmospheric
return. A temperature of 212°F
here may indicate a leaking trap.
Discharge of condensate into
a common return line is more
usual than discharge to an open
end, of course. The temperature
in the return line should be the
saturation temperature corresponding
to the return pressure.
Any increase in this temperature
which may be detected will show
that the return line pressure has
increased. However, if trap “A”
discharging into a line blows
steam and the pressure in the
line increases, then the pressure
and temperature at traps “B” and
“C” and all others on the line will
also increase. Location of the
faulty trap is still not achieved.
Visual Determinations
The release of flashing steam
from condensate nullifies the
effectiveness of test cocks, or
three-way valves diverting a trap
discharge to an open end for test
purposes. It also restricts the information
which can be gained from
sight glasses. Consider a trap discharging
to an open end some
500 lbs. per hour of condensate
Testing Steam Traps
from a pressure of 125 psi. The
steam tables show that each
pound of water carries 324.7 BTU
which is a144.5 BTU more than it
can carry as liquid at atmospheric
pressure. As the latent heat at 0
psig is 970.6 BTU/hr., then
144.5/970.6 lbs. of flash steam are
released per pound of condensate,
or 14.29%, which is some
74.45 pounds per hour. The volume
of steam at 0 psig is 26.8 cu.
ft. per pound, so some 1,995 cu. ft.
per hour of flash steam is
released. The remaining water,
500 - 74.45 = 425.55 lbs. has a
volume of about 7.11 cu. ft. per
hour. Thus, the discharge from the
trap becomes 1995/1995 + 7.11 =
99.65% steam and 0.35% water,
by volume.
It is sometimes claimed that
an observer can distinguish
between this “flash” steam and
leakage steam by the color of the
steam at the discharge point.
While this may be possible when a
trap is leaking steam but has no
condensate load at all, so that only
steam is seen at the discharge, it
is obvious that the presence of any
condensate will make such differentiation
virtually impossible. It
would be like trying to distinguish
between 99.65% steam with
0.35% water, and perhaps 99.8%
steam with 0.20% water!
Trap Discharge Sounds
In a closed piping system, trap
discharge sounds may be a good
indicator of its operation. A simple
stethoscope will be of little value,
but the sound produced at ultrahigh
frequencies measured by an
ultrasonic instrument eliminates
background noise interference.
Live steam flow produces a
greater and steady level of ultrasound,
while flashing condensate
tends to have a crackling sound
and the level changes with the
trap load. The problem is that the
instrument requires the operator
to make a judgement as to trap
condition which will only be as
reliable as his training and experience
provide for.
SYSTEM DESIGN
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