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"Seating Profile -
The Good, The Bad, and The Ugly."
Much has been said about
seating profile. But before we get to far into the conversation, I must warn you
that although a bad seating profile (degradation from benchmark comparison) typically
indicates a worn or bad seat, a good seating profile does not necessarily mean that the
seat is good and will meet the design shutoff classification.
What is "Seat
Profile"? Seating profile is the expanded view of the valve signature (net
pressure vs. travel curve) at the closed position and is used to provide a comparative
indication as to the integrity of the seat. It is characterized as an abrupt change
in slope from that shown throughout the normal travel range. The change in slope is
due to the higher spring rate or stiffness of the actuator parts reacting to the forces
being applied to the seat (i.e. stretching of the actuator yoke legs, expansion of the
diaphragm housing, compression of the valve/actuator stems, etc.). When using the
FlowScanner, the profile can appear as a "stair stepping" action or as a very
fine "saw tooth" action. This appearance is due to the resolution of the
travel transducer used during the test. Travel transducers such as the 50"
Celesco and 4" DRC have a resolution of 0.001". Each 0.001" movement
provides one count, or travel measurement point, and thus produces the stair step image
when plotted against change in net actuator pressure. On the other hand, the
25" Celesco and 2" DRC have a resolution of 0.0001 and hence the smaller steps
or saw tooth profile. Although the appearance may differ between transducers, the
slope would remain the same.
Seating Profile
is best utilized as a comparative tool. By obtaining a "benchmark" test of
the valve when the seating surfaces are known to be good, it is just a simple matter to
overlay any succeeding tests and view for any developing changes in the profile. To
illustrate this, let’s view the following examples.
Here we see an example of a
benchmark test of a good seat. Notice the sharp transition in slope as the valve
makes its initial seat contact. This is the area in which you pay close attention.
You should see the sharp transition plus you should note the amount of travel that is
displayed from this point of seat contact until the actuator runs out of force (zero
pressure for reverse acting actuators, full supply pressure for direct acting actuators).
The travel to force relationship shows the seating material softness and/or the
stiffness of the actuator parts. This travel will be greater for a PTFE seat then
that of a metal seat. It will also increase when there is damage or wear present on
the seat. |
By zooming in on the seating area, we can obtain a well-defined
seating profile. Typically a 0.050" or 0.060" travel range is ample zoom for
defining the seating profile. Any further zooming just muddies the waters and can
make even a good profile look ugly.
Here we see the results of
further zooming in on the above profile. Increased zoom does have its place though.
For instance, if you are looking for subtle changes between today’s trace with
one taken previously. This is typically done by overlaying the two graphs.
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Speaking of overlays, it is
now time to compare today’s test results with those from the original
"benchmark" test. Notice that the spring rate transition on today’s
test is not nearly as crisp as it was on the original test of this valve. Also
notice that the travel from point of seat contact is much more then that of the original
test. This is a very reliable indication that there is a problem in the seating area
of this valve. In this case, the seating areas of both the valve plug and seat ring
were found to damaged from debris flowing through the pipeline. |
Now that we have viewed a
"Good" and a "Bad" seating profile, let’s close with an
"Ugly" one.
In viewing this
seating profile, we see the valve contacting the seat and then what appears to be the
start of a typical seating profile. But then we see an additional 0.020" travel
with no increase in force. This is very atypical. The valve was opened for
inspection. The seat ring had nearly eroded away. |
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