June 2025
Valves, Pumps, Turbomachinery and Compression
Causes and countermeasures for a compressor lube oil system failure due to delayed oil pressure recovery by an auxiliary lube oil pump
This article shares insights from a lube oil system issue and recommends key points to consider when operating oil systems.
The lube oil system is one of the critical systems for rotating machinery that requires forced lubrication. Insufficient oil pressure and flow can cause significant damage to these machines. Despite its importance, the system operates under relatively low pressure and temperature conditions, meaning troubles are rare and it is challenging to assess system integrity in advance. This article shares insights from a lube oil system issue and recommends key points to consider when operating oil systems.
Chronology of events. The ethylene refrigeration compressor at a naphtha cracking center was installed in 1989 and had no history of significant issues related to the lube oil system before the incident. Regular turnaround maintenance was conducted every four years. During each maintenance period, with the compressor in a shutdown state, function tests were conducted to ensure that the main oil pump (MOP) would trip, causing the auxiliary oil pump (AOP) to auto start. No abnormalities were detected during these tests.
The sequence of events during the incident is illustrated in FIG. 1, and the alarm trip set points are listed in TABLE 1.
FIG. 1. Sequence of events.
On the day of the incident detailed here, a storm affected the steam conditions, leading to vibrations in the steam turbine. These vibrations caused the trip lever to disengage, resulting in the MOP trip. Approximately 1 sec after the MOP tripped, a pressure alarm low (PAL) was activated. The AOP started immediately after the PAL occurred, but it failed to recover the oil pressure, which continued to drop. About 3 sec later, a pressure alarm low-low (PALL) was activated. Although there was a 3-sec time delay set in the lube oil pressure trip interlock after the PALL activation, the PALL remained active, ultimately failing to recover the oil pressure and causing the compressor to trip.
ROOT CAUSE ANALYSIS (RCA)
Based on the site investigation, the RCA for each causal factor is identified as shown in FIG. 2.
FIG. 2. RCA of compressor trip.
RCA 1: Instability of the trip lever. The design of the steam turbine lever varies by vendor; typically, the lever is engaged with the notch of the latchet, ranging from 0.06 in.–0.08 in. The design ensures that, in the event of an overspeed condition, the mechanical trip assembly will protrude and activate the trip lever. However, unintended activation of the trip lever can occur due to equipment vibrations or external forces. This incident occurred on a day with a thunderstorm, and it appears that vibrations in the steam turbine, caused by poor steam conditions, activated the trip lever. The incident revealed that the trip lever's structure is unstable, emphasizing the need to implement the following countermeasures. These countermeasures are necessary to ensure that the trip lever activates only during overspeed conditions, as originally designed.
Countermeasures for RCA 1 include:
- Installation of a trip lever spring tensioner. The trip lever can be accidentally activated, and while it may seem beneficial to keep it fixed in place, it is important to note that the lever must trip during overspeed conditions. Therefore, by installing a spring tensioner, as shown in FIG. 3, the trip lever is stabilized to avoid activation from normal vibrations or external forces, while still allowing it to function as intended in overspeed situations. To avoid the risk of the trip lever activating unintentionally during normal operations, the installation of the spring tensioner should be performed during scheduled maintenance activities. Furthermore, after installing the spring tensioner, it is crucial to perform a turbine overspeed test to confirm that the trip lever operates as intended without any issues. If the trip lever does not activate during this test, the spring tensioner must be further adjusted to ensure the overspeed trip functions correctly.
FIG. 3. Trip lever with spring tensioner.
2. The addition of a logging sheet for inspecting the status of the trip lever. Previously, operators or diagnostic personnel checked the oil level only during patrols. Following the recent incident, a section has been added to the logging sheet for inspecting the status of the trip lever. As a result, during patrols, an additional check is now conducted to assess the status of the steam turbine's trip lever. The inspection is conducted visually by checking the position of the trip lever and the gap where the trip lever engages with the latchet.
3. Machining of the latchet notch. The latchet notch has been machined to the design maximum level. Prolonged use can lead to wear on the notch, increasing the likelihood of the trip lever disengaging unintentionally. However, if machining goes beyond the maximum, the overspeed trip assembly may not function properly. Therefore, it is important to be cautious and avoid machining beyond the maximum.
RCA 2: Low lube oil level. Even though the AOP was activated by the PAL signal, the compressor remained in the PALL condition until it tripped. Upon inspection, it was found that the check valve in the lube oil pump discharge line was installed horizontally, as shown in FIG. 4, and this horizontal piping was positioned higher than the operating level of the lube oil reservoir. As a result, this line configuration created an air pocket of approximately two gallons (gal) between the pump discharge line and the upstream side of the check valve.
FIG. 4. Level comparison between reservoir lube oil and AOP check valve.
Countermeasures for RCA 2 include:
- Adjustment of the reservoir oil level. The oil reservoir level gauge is marked with minimum, maximum and rundown levels (if there is a rundown tank). Fundamentally, even if the oil reaches the reservoir's minimum level, the discharge check valve of the AOP must be fully submerged in oil to prevent the formation of an air pocket. In the case of the compressor where the incident occurred, the lube oil system had the check valve positioned between the minimum and maximum levels of the oil tank. This means that when the oil level in the reservoir is at the maximum level, the horizontal piping is submerged, but at the minimum level, an air pocket forms in the horizontal section.
As a countermeasure, the minimum oil level in the reservoir has been raised to a point where air pockets do not form. It is important that even when the oil tank level reaches the minimum, the section from the pump discharge line to the upstream side of the check valve remains submerged in oil. For some compressors, where the check valve is positioned too high and it is difficult to raise the oil level, it is recommended to implement Additional Countermeasure 1, which will be discussed later.
RCA 3: Vent valve closed. Upon inspection, it was also found that the vent valve at the pump discharge was not properly opened. Due to the air pocket that formed around the pump's upstream and downstream, the AOP failed to recover the lube oil pressure even though it was activated. Although it is challenging to determine the exact volume of the air pocket at the time of the incident, it can be suggested that there could have been additional air pockets contributing to the issue.
Countermeasures for RCA 3 include:
- Maintain the vent line valve in the lock open position. In some lube oil systems, there is a vent line located between the pump discharge and the check valve. This line is usually made of tubing, and operators may mistakenly close the valve after startup, thinking it is not needed for continuous venting. Therefore, it is necessary to keep the valve for continuous venting in a "lock open" position and to regularly check it onsite. This is especially important for compressors using oil film seals, as vapor can be generated from oil that is not properly degassed, making continuous venting essential.
Additionally, valves that must be kept in a "lock open" position, such as vent valves and equalizing line valves, should be clearly marked on the piping and instrumentation diagram (P&ID) and tagged appropriately onsite to ensure these valves remain open at all times.
Note: During winter, the oil already in the oil cooler or oil filter can have a low temperature, which increases the likelihood of a sudden pressure drop when switching. To prevent this, the equalizing valve should always be kept open and managed together with the vent line valve.
Additional countermeasures to prevent an air pocket. There are some cases where the check valve is positioned higher than the reservoir, making it difficult to completely eliminate air pockets even when the oil level is raised. Increasing the size of the reservoir or modifying the piping to lower the check valve would involve a large scope of work and high costs, making these options impractical. A more feasible solution is to machine an additional hole in the AOP check valve for pump priming purposes, as shown in FIG. 5.
FIG. 5. Check valve for MOP (left); and a check valve for AOP with a hole (right).
Through the check valve hole, the AOP is maintained in a primed state while the MOP is in operation. For positive displacement pumps like screw pumps and gear pumps, you can observe them slowly back-rotating when they are primed through the check valve hole.
However, for this improvement to be effective, there must be a margin in the capacity of the pumps and lube oil system. The capacity margin can be checked by observing the opening of the control valve that bypasses oil from the MOP/AOP header to the reservoir. The flowrate of the oil returning to the system through the check valve hole when primed can be easily calculated using the orifice flow formula.
Since MOP and AOP check valves have the same appearance, there is a possibility of mistakenly installing an unmodified MOP check valve on the AOP line after maintenance. Therefore, when machining check valve holes, it is recommended to attach tags to distinguish between the MOP and AOP check valves, as shown in FIG. 6. Each check valve should have its own tag, and each connecting flange should also have its own tag to ensure correct identification and installation.
FIG. 6. MOP line and check valve tagging (top); and AOP line and check valve tagging (bottom).
Lube oil system function test. Most users conduct an AOP auto-start function test during regular maintenance to verify the integrity of the lube oil system. However, testing after the rotating machinery has been shut down makes it difficult to accurately assess the system's condition. In the year prior to the incident, a function test was conducted while the compressor was stopped, but no notable issues were detected. This is because—after the compressor is shut down and some time has passed—the lube oil reservoir level settles at the rundown level, which results in the removal of air pockets in the AOP discharge line.
To accurately verify the integrity of the oil system, the test should be conducted just before the rotating machinery is shut down at the start of regular maintenance. Internal procedures have been changed to perform the test at the compressor's minimum RPM right before the regular maintenance begins, according to the procedures outlined in TABLE 2.
Lessons learned. The lube oil system typically operates under non-severe conditions and rarely encounters issues during normal operation, making it challenging to identify vulnerabilities within the system. However, when issues occur, they can have a significant impact on equipment and plant operation. Therefore, it is important to identify vulnerabilities in advance and prepare accordingly to prevent potential issues.
The trip lever of the steam turbine that drives the MOP can trip due to vibrations, external forces or steam conditions. To prevent sudden trips, actions such as installing spring tensioner, checking lever notch conditions and setting the notch to the design maximum are necessary.
The AOP may have difficulty in recovering lube oil pressure if there are air pockets. Therefore, it is essential to identify potential points where air pockets could form and prevent their formation by raising the oil level, keeping vent valves open, and machining check valve holes.
Additionally, to accurately assess the integrity of the lube oil system, the function test should be conducted according to precise procedures under conditions similar to actual operations. It is recommended to perform the test while the compressor is running, just before regular maintenance begins.
The Authors
Hyoseok Jeong has 9 yr of experience as a rotating machinery engineer in petrochemical plants, especially in polymer plants. He specializes in maintenance, reliability and improvement. Jeong earned a BS degree in mechanical engineering and is certified as a Professional Engineer in Welding by the South Korean government.
Wooseok Eom has 6 yr of experience as a rotating machinery engineer in refineries and petrochemical plants. He specializes in gas turbine generators, maintenance, reliability improvement and technical support. Eom earned a BS degree in mechanical engineering in South Korea.
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