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In viewing the Valve Signature curve, no major anomalies are seen. But when one compares the analyzed data to the specified data we see that the tested seat load is approximately 2/3 of the required seat load (363 lbs. vs. 549 lbs. specified). We also see that the packing friction is approximately twice what one would expect from the installed packing (492 lbs. vs. 235 lbs. specified). With the reduced seating force, this valve could face premature seat failure.
The maintenance history on this valve showed that it was just re-packed with a new set of graphite packing. Evidently, the packing was errantly over tightened resulting in the excessive friction. During the sizing procedure, an actuator is selected that will provide enough force to overcome the valve’s frictional forces (which always opposes motion) and the unbalance forces (caused by the process pressure acting on the unbalance area of the valve trim.) The actuator must also contain enough additional force to provide seat loading. Because the total amount of available actuator force is limited via the installed spring, the diaphragm area, and the air supply, any additional force required to travel the valve through any excess friction must come from some area within this limitation. The only force available is that which was reserved for seat loading. Any increase in friction will diminish seat load. Thus one is robbing Peter (seat load) to pay Paul (excess in packing friction). It is always a good practice to validate a valve’s performance/operability following a rebuild. It not only verifies that the assembly was done properly but also provides a benchmark for future diagnostic testing. An excellent means for providing this validation would be by conducting a dynamic scan test with either the FlowScanner or ValveLink software. |
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