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"Actuator Leak
Inspection"
On reverse acting
spring and diaphragm actuators (i.e. Fisher Type 667), a sealing mechanism must be in
place to prevent actuator pressure from escaping between the actuator stem and casing
joint. This actuator stem seal is typically an o-ring seal or some type of packing
arrangement depending on the actuator’s manufacture. Any pressure leakage in
this area is very difficult to detect. It is very seldom heard because of other
ambient noises within the area. It is also next to impossible to feel because of
its location (well within the spring barrel of the actuator yoke). In the past, the
best means for detecting actuator leakage was to lock pressure within the actuator and
then monitor for any pressure decay. If this test proved leakage to be present, one
would then use a soap solution to search for leakage between the top and bottom casing
joint in an attempt to locate the source of the leak. Experience has shown that in
more cases then not, the source of leakage has been the actuator stem seal. With
FlowScanner diagnostics, one can detect actuator leakage during a routine "Dynamic
Scan" test. This test can detect a leakage well before it becomes detrimental
to the valve’s performance. In other words, while the leak is still very
small. To illustrate this, lets view the following signatures obtained from a
Dynamic Scan test on a sliding stem valve.
Total Valve
Notice that the total valve performance is very good. The zero
calibration may be a little low, but in all, this valve displays excellent performance.
The D.E.B. is low indicating very good response plus the controllability is very
linear. No major anomalies are seen. |
"I/P
Performance"
| Here we are viewing the I/P performance. Again
excellent performance. The zero calibration is a little low but can be easily
adjusted. The D.E.B. is very good, as well as the linearity of the device. |
"Positioner
Performance"
| The trend of excellent performance continues with the
positioner. Here we see that the calibration, D.E.B., and linearity are all well
within the expected operating perimeters of this instrument. |
| The apparent excellent performance continues with the valve.
Here we see that the stops have been contacted. The friction is uniform
throughout the stroke and within specified limits. The benchset and travel have been
adjusted correctly. And there is ample seat load. All is well. |
| Here we see that the actuator stroke pressure did not reach
the full available supply pressure. We also see that as the actuator pressure is
being exhausted and the valve is traveling closed, that there is some demand on the supply
regulator. |
When
checking for an actuator leak, one would view the maximum actuator pressure achieved as
well as the behavior of the supply pressure. The maximum actuator stroke pressure should
equal that of supply. If the actuator pressure does not peak out at the supply pressure
reading, one should suspect a leak. To verify that there is a leak present, look to see
what the supply pressure is doing while the actuator pressure is being exhausted. As the
pressure is being exhausted, the supply regulator should be sitting at set point with no
demand on it, since at this point, the positioner should only be unloading pressure that
had been locked in the actuator. Notice that in this case, the actuator pressure never
reached full supply and that there is demand on the regulator during the exhaust portion
of the test. This can be more easily seen when we expand on the supply pressure as shown
in the curve below.
As I’m sure you’re
all aware, leaks typically will get worse instead of better. If this leak were left
unattended, the performance of the valve would soon deteriorate.
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