TL;DR
If the valve cannot deliver the required flow at 100% travel, check calibration first, then inspect the trim for seat or plug wear. An undersized Cv trim requires a body change.
What you might see
- process flow lower than setpoint with valve fully open
- positioner commanding 100% but process still short
- valve body flow coefficient lower than design Cv
- calibration check shows travel below 100%
Likely causes
Positioner calibration drift causing the valve to stop before reaching physical travel limits
Seat or plug wear reducing the effective Cv at the rated travel
Trim undersized at original selection, unable to meet the process Cv at any travel
Actuator spring force insufficient to fully stroke against the process differential pressure
Required tools
- Fisher positioner calibration tool or FIELDVUE DVC6200 communicator
- Stem travel gauge or ruler
- Combination wrench set for bonnet studs
- Process flow and pressure measurements from plant instrumentation
- Replacement trim per Fisher easy-e part numbers
- LOTO kit
Safety first
- Process pressure inside the valve body is a serious hazard. Confirm zero pressure before removing the bonnet.
- Fluid temperature can remain high inside the body even after isolation. Allow cooling before opening.
Procedure
- 1
Put the loop in manual and command 100% open. Use a position indicator or measure stem travel mechanically to confirm the valve is reaching its physical stop.[1]
- 2
If the stem does not reach the physical stop, the positioner calibration has drifted. Re-calibrate the positioner per the Fisher FIELDVUE DVC6200 or model-specific calibration procedure.[1]
- 3
If the stem reaches 100% travel but flow is still below the setpoint, the problem is the trim Cv. Calculate the actual Cv from the measured flow and differential pressure.
- 4
Compare the calculated Cv to the trim Cv from the Fisher valve datasheet. A calculated Cv within 10% of the rated Cv suggests trim wear. Greater than 15% shortfall suggests an undersized trim.[1]
- 5
To inspect the trim: isolate the valve, bleed the process pressure, and remove the bonnet.
Warning: Process fluid is present in the valve body. Follow the plant valve-isolation and bleed-down procedure. Verify zero pressure before removing the bonnet. - 6
Inspect the plug tip and seat ring for erosion, wire-draw erosion, or pitting. A plug tip showing visible grooves or seat ring showing a worn seating line requires trim replacement.
- 7
Replace the trim with the Fisher-specified seat ring and plug for the body size and service class. Verify the Cv matches the process requirement before ordering.[1]
Sources
Fisher easy-e ED, ET, EZ Sliding-Stem Control Valve Instruction Manual, Fisher Controls (Emerson)
Fisher easy-e ED, ET, EZ Sliding-Stem Control Valve Instruction Manual, trim inspection and positioner calibration (general)
View source
More guides for Fisher (Emerson) easy-e ED/ET/EZ
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Stem packing leakage is either backed-off packing gland nuts or packing rings at end of life. Re-torque gland nuts first; if leakage continues, replace the packing set.
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