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Here we have a pair of valves displaying similar ailment traits. Both displayed excessive oscillation (looping) during the unloading portion of the Valve Signature test as shown below.
They were both similar in construction. No other accessories were noted. Both left their respective manufacturing sites in tip-top working condition similar to what is shown here in valve "A’s" birth certificate (factory baseline signature).
Oscillatory behavior is the result of very aggressive tuning and/or a major restriction in airflow between the digital valve controller (positioner) and the actuator. Also, the suggested tuning set for valve "A" is K while the suggested tuning set for valve "B" is L. In viewing the graphs of the two valves, we found that the tuning set for valve "A" was indeed set at K, both during the original factory test as well as while it displayed the oscillatory behavior. This could be considered somewhat aggressive tuning, but keep in mind that it performed well prior to shipment. Valve "B" on the other hand was found to have a tuning set of G. This is considerably de-tuned from the recommended tuning set of L. This definitely is not aggressive tuning. We are now able to rule out the effects of aggressive tuning as the cause for the oscillatory behavior. We now concentrate our efforts on the possibility of a restriction downstream of the digital valve controller. In interviewing the customer for valve "A", it was disclosed that during its installation into the pipeline, the 3/8" OD tubing between the DVC and actuator had been damaged. The damaged tubing was replaced with ¼" OD tubing. The smaller tubing created the restriction that caused the oscillating performance. Note that for medium to large actuators, the positioner tubing should never be less then 3/8 inch. The oscillation problem was resolved by installing 3/8" tubing. In interviewing the customer for valve "B", it was learned that upon arrival at the plant site, a solenoid valve was installed between the positioner and actuator. This solenoid was safety related and installed to quickly dump the actuator pressure and thus close the valve under emergency situations. It was found that the solenoid in use had a much smaller capacity then the minimum recommended Cv of 0.49. The solenoid was the restricting factor between the DVC5000 and actuator, and thus created the oscillating environment. The solenoid was replaced with one containing a larger orifice allowing the valve to perform superbly again. Note – If you should encounter oscillatory/cyclic conditions with your DVC equipped valves, first check the configuration to verify that the tuning set falls within the recommended guidelines and then check for any restrictions (i.e. low capacity solenoid, undersized tubing, pinched tubing, needle valve, etc.) downstream of the DVC. |
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