Monitor control valve position in all oil systems to determine component wear (rotary pumps, bearings and seals) to ensure corrective action is taken during a turnaround.
Marking the position of control valves (marking the stem and valve yoke with a straight edge) at the beginning of a run will give an instant indication of component wear for the following items:
- Rotary pumps (screw or gear) – if the bypass valve is closing over time.
- Bearing wear – if the lube oil supply valve is opening over time.
- Control component wear – if the control oil supply wear is opening over time.
- Seal wear – if the seal oil supply valve is opening over time.
Check the position of all marked control valves prior to the turnaround meeting to determine if the affected components need replacement during the turnaround.
Failure to mark and monitor control valve stem position in oil systems has led to many surprises and replacements soon after a turnaround.
Monitoring of valve stem position would have identified worn components and allowed replacement during a turnaround.
Remember that turnaround action does not affect product revenue, unplanned action does!
Replacement of an oil pump can take two days considering alignment.
Replacement of a bearing or seal can take 3-5 days!
This best practice has been used since the late 1980s since all field visits. This practice has saved many millions of USD by moving component replacement to the ‘turnaround revenue loss-free zone’!
Recommended checklist before shutdown
Check system transient functions (pump transfer) immediately before turnaround.
It is recommended that the following procedure be followed immediately before each scheduled shutdown of critical (unspared) machinery units:
Confirm that the auxiliary oil pump start switch actuates at the proper setting and starts the auxiliary oil pump.
Put the auxiliary oil pump into auto-start mode, and station an operator at auxiliary oil pump with instructions to immediately manually start auxiliary oil pump if compressor unit trips.
Trip the main oil pump and observe the following (with strip charts, if possible):
- Auxiliary oil pump starts without unit trip
- Lowest tube oil pressure during transient
- Lowest control oil pressure during transient
- That all control valves remain stable
The above procedure is the only way to ensure that the transient response of the oil system will not cause a trip during operation.
The amount of oil taken by the machinery components during operation is more than during the stationary unit case (when the unit is not operating).
Most functional oil system checks are performed at turnarounds with the unit off-line, only to find during operation that the system cannot respond to a transient event without a unit trip.
Performing the transient check just prior to the turnaround will allow ample time for any corrective action.
In many cases, oil system functional checks during a turnaround confirm that the transient function of the system is acceptable, only to find after the turnaround that a unit trip has occurred during a transient event.
As mentioned above, the amount of oil taken by the machinery components during operation is more than during the stationary unit case (when the unit is not operating).
The only way to accurately confirm oil console transient response is while the unit is in operation.
The only way to avoid exposure to a unit trip during plant operation is to perform the transient test immediately prior to a planned shutdown.
This best practice has been employed since 1990 to accurately check the transient response of critical machinery oil systems prior to a planned shutdown and to define an action plan for corrective action if required.