TL;DR
Level control failure risks liquid carryover into the gas outlet or loss to the flare. Check the level transmitter, bridle, and control valve as the first three steps.
What you might see
- liquid level in separator not controllable
- level transmitter reading erratic or stuck
- level control valve not responding
- liquid carry-out to gas outlet or overflow to drain
Likely causes
Level transmitter bridle (standpipe) fouled with paraffin or scale causing sluggish response
Level control valve stuck or plugged preventing drainage
Transmitter calibration drift from temperature or process fluid density change
Float or displacer mechanism failed in differential pressure or displacer-type instruments
Required tools
- Calibrated level gauge or sight glass
- LOTO kit
- Transmitter calibration tool
- Valve positioner testing equipment
Safety first
- Level control valve operation at operating pressure sends liquid under pressure to downstream equipment. Coordinate with the control room before manually cycling the valve.
- Level bridle purge releases process fluid. Wear appropriate PPE for the fluid type including H2S protection if applicable.
Procedure
- 1
Compare the level transmitter reading to any sight glass or secondary level indication. A large discrepancy confirms instrument error.[1]
- 2
Purge or flush the level bridle per the general purge procedure. A plugged bridle gives a false steady reading.[1]
- 3
Command the level control valve to open and close from the control room. Verify valve position feedback matches the command.
Warning: Level control valve actuation with the separator at operating pressure will change liquid flow to the downstream system. Notify the control room operator before manually cycling the valve. - 4
Inspect the level control valve for plug fouling. Remove and clean the valve trim if fouling is suspected.
- 5
Calibrate the level transmitter against the sight glass at a known level point.
- 6
If the displacer or float mechanism is suspected, plan an inspection during the next planned maintenance outage. Replace the displacer or float if damaged.
- 7
After corrections, verify the level control loop response by introducing a small step change in the setpoint and observing the process variable response.
Sources
Sulzer Sulzer KnitMesh Process Separator / Knockout Drum general technical documentation, Sulzer
Sulzer KnitMesh process separator general level control and instrument maintenance procedures (general)
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