"The Case of the Haunted Valve"

In this feature, the valve was a small 1-½" Fisher Design DBQ valve with a direct acting Type 657 size 50 actuator.  This guy was employed as an on-off valve activated by a solenoid valve and thus no other instrumentation was needed (i.e. I/P or positioner).   We made a house call to examine the valve while the plant was up and running.  To facilitate the diagnostic examination and isolate the valve from the process pressures, a block valve upstream of the valve was closed.  Upon completion of the initial examination, the following test results were observed:

• The friction was within expected limits.

• The travel was low for the 1 1/2" rated but probably OK providing capacity is not an issue.

• The benchset was as specified and thus, the spring rate would be correct.

• The seat load was more then adequate for the required shutoff classification.

But, as we look at the results, we see that the valve is fully closed at approximately 15 psig actuator pressure but then opened at approximately 19 psig.  It actually opened with a higher force applied then what was required to initially close it.  We see that by the spacing of the data points that the valve moved very rapidly towards the open position and then just as abruptly, went back closed.  What could be causing this abnormal behavior?   Is the valve haunted?  Is it possessed?  Is it developing a mind of its own?  Are there supper natural powers present?  Let’s take a look at this with a level head.

We know that it is physically impossible for the valve to open with more actuator force than that which was required to close it, without additional forces being present.  But where could these forces be coming from?  As stated earlier, there was a block valve upstream of the valve being tested.  Suppose the block valve was leaking.  As the test valve was sitting in the closed position during the pause portion of the Dynamic Scan test, the leaking block valve was slowly building the pressure within the piping between it and the test valve.  Once this process force (piping pressure multiplied by the valve’s unbalance area) became greater then that of the actuator force holding the valve closed, the valve would open.  Once the valve opened, the pressure in the pipe would decay allowing the actuator force to again close the valve.  The pressure again began to build causing the valve to open quickly at approximately the closing pressure of 15 psig.   By now the ramping exhaust signal from the FlowScanner is such that the actuator pressure is below the 15 psig required to close it and thus the pressure within the pipe can no longer build.  The test now concludes in the normal fashion.

Here we see that the diagnostic test not only showed that the valve was in perfect health with very tight shutoff capability but also that the block valve was ailing and thus required some maintenance attention.

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