Lube Oil Pressure Decay After a Filter Swap
During a routine shift, maintenance changes the duplex lube oil filter on a 120 MW steam turbine while the unit runs at 95 MW. Within two minutes the operator sees bearing oil header pressure fall from 1.6 bar to 1.25 bar, bearing metal temperatures on the No. 2 journal rise from 78 °C to 86 °C, and the oil cooler outlet temperature drift up 3 °C. The standby lube oil pump has not auto-started. The DC emergency oil pump is available and its auto-start is armed at 0.8 bar. The alarm setpoint for low lube oil pressure is 1.1 bar. The operator suspects air was introduced into the filter housing during the swap.
1. Assess the risk and decide whether to trip, transfer, or continue, explaining the reasoning.
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The header pressure has fallen 0.35 bar and bearing metal temperature has risen 8 °C, which indicates reduced oil film and a real risk of bearing damage. The standby pump has not started, so there is no automatic redundancy. However, pressure is still above the 1.1 bar alarm and well above the 0.8 bar emergency pump setpoint, so this is a degrading condition, not yet a loss-of-oil event.
I would not trip immediately on pressure alone, because a trip at 95 MW is itself a transient that changes bearing loading. Instead I would treat this as an emergency: start the standby lube oil pump manually to restore header pressure, verify the filter swap is complete and the housing is properly vented, and watch bearing temperatures. If bearing metal temperature continues to climb or pressure falls below 1.1 bar, I would trip the unit and rely on the emergency oil pump. The decision hinges on trend, not a single reading.
2. Explain why air ingress into the filter housing would cause this pattern and what you would do to confirm and correct it.
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Air in the oil system reduces the effective flow through the bearings and can cause the pump to lose prime or cavitate, lowering discharge pressure. Air also insulates and reduces heat transfer in the coolers, which explains the cooler outlet temperature rise. Bearing temperature rises because the oil film is thinner and possibly aerated, reducing load capacity.
To confirm, I would check for pump noise and vibration, look at the sight glass for foaming or entrained air, and compare pump discharge pressure with the header pressure to see if the drop is across the filter. I would vent the filter housing at its high point, confirm the standby filter is fully in service and the swap valve is correctly positioned, and check the oil reservoir level and seal oil system for air ingress. After venting, I would trend pressure recovery before concluding the problem is solved.
3. What protective and procedural gaps does this event reveal, and what would you recommend?
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The standby pump failing to auto-start is a serious gap: redundancy existed on paper but not in function. I would recommend testing the standby pump auto-start logic and its pressure switch, verifying the DC emergency pump auto-start, and reviewing the filter swap procedure to require venting and a pressure check before returning the filter to service. The procedure should also require the standby pump to be proven running, or at least proven ready, before any filter work on a running machine.
I would also review alarm response: the low-pressure alarm at 1.1 bar is close to the emergency pump setpoint of 0.8 bar, leaving little margin. Consider raising the alarm or adding a pre-alarm. Finally, document the event, bearing temperature excursion, and inspection results, and schedule a bearing inspection if temperatures do not return to baseline.