Chemical Engineering October 2020 - 36

Temperature
transmitter
T1
Temperature
transmitter
Heat exchanger
T2
T1 299 F
Temperature
transmitter
Heat exchanger
T2
293 F
FIGURE 3. Measuring the inlet and discharge
temperatures can provide an estimated steam
pressure
testing program. These devices are
by no means the only piece of diagnostic
equipment that should be
used, but they can help provide
valuable information that would
otherwise not be available.
Estimating steam pressure
Plant engineers can estimate steam
and condensate pressures by using
temperature-testing devices to detect
the temperature of the steam
line inlet to the steam-trap station
and on the discharge line (Figure 3).
Knowing what steam and condensate
pressures are present in the
system will assist the people performing
the steam-trap station testing,
enabling them to quickly evaluate
system dynamics that can affect
the steam-trap station's operation.
Is the steam trap operational?
A temperature measurement will
allow the steam-trap station examiner
to determine whether
the steam trap is operational or
whether the steam trap station is
below the expected temperature. If
the latter is the case, then the plant
should initiate root-cause analysis
to determine the source of the
problem in the system.
For example, in Figure 4, the
temperature on the steam line entering
the process is 299°F; therefore,
the steam-trap body temperature
should be at or close to the
inlet temperature.
This is a true statement for 96%
of the steam process applications.
However, there are a few exceptions
when the heat-transfer units have
an extremely high condensing rate
or when there is a pressure drop in
the process. The following three examples
make this more clear.
Example 1: Equal temperatures.
The inlet and outlet temperatures
(process and steam trap) are mea36
FIGURE
4. In this setup, the steam inlet and
steam-trap temperatures are equal or nearly equal
sured to be the same or nearly equal
(Figure 4). This means the steam trap
is operational, and further testing
can be accomplished.
Example 2: Low outlet temperature.
In Figure 5, the steam
trap body temperature is very low
(210°F) compared to the inlet steam
temperature to the process. The
steam trap temperature is low, and
root-cause analysis needs to be
performed to find the reason, such
as the following:
* Undersized steam trap
* Fouled strainer
* High back-pressure in the condensate
line
* Other causes
Testing steam trap performance
Although surface-temperature
measurement can be very useful
in
evaluating various potential
conditions, using it alone for testing
steam-trap stations will have a
low accuracy for testing steam trap
performance. A steam-trap station
examiner should be extremely
knowledgeable of steam and condensate
system dynamics.
Different sources state that if there
is a high
temperature differential
across the steam-trap station, then
the steam trap is in good operational
condition. If there is no or a very low
temperature differential, then the
steam trap has failed and is blowing
or leaking steam into the condensate
system. Temperature measurements
must be part of the steam trap station
standard operating procedure
(SOP) to ensure the steam trap station
is operational.
Example 3: High temperature differential
across the steam-trap
station. Figure 6 indicates a high
temperature differential (inlet temperature
= 299°F; outlet temperature
= 214°F). However, the steam
trap is completely failed and is blowT1
299 F
Heat exchanger
T2
210 F
FIGURE 5. In this example, the steam trap has a
low body temperature
ing steam into the condensate line.
Then why is there a high temperature
differential?
If steam is blowing into a condensate
line that has zero pressure, the
steam temperature of the blowing
or leaking steam trap must be at
212°F, or the temperature of steam
at zero pressure. Now, when the
steam passes from a high pressure
to a lower pressure, superheat will
be generated, but the condensate
passing the steam keeps the steam
at saturated conditions.
Example 4: Steam-trap station
with a low temperature differential.
Figure 7 shows a very low
temperature differential (inlet temperature
= 299°F; outlet temperature
= 284°F), which should indicate
that the steam trap has failed
and is blowing steam in the condensate
line. In this example, there
is backpressure in the condensate
return line, which is normal in most
condensate lines due to design,
undersizing and elevations. The
condensate line pressure will vary
depending on the variables. With
pressure in the condensate line,
the condensate line temperature
should be at or close to the saturated
temperature at the condensate
line pressure.
Example 5: Low-pressure systems.
There will be a low temperature
differential across the steam
trap station based solely on the low
steam pressures in the steam system
and condensate line, as shown
in Figure 8. The steam trap could be
failed or working properly; the condition
of the steam trap is unknown
because both conditions will have
similar steam temperatures.
Measurement procedures
Temperature measurements (Figure
9) need to be taken upstream and
downstream of the steam trap station
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