October 2017

Maintenance and Reliability

Troubleshooting the repetitive failure and shaft seizure of a hot oil circulation pump

The Shaybah NGL recovery plant has a single-stage, double-suction between bearings, radial-split design for hot oil circulation.

The Shaybah NGL recovery plant has a single-stage, double-suction between bearings, radial-split design for hot oil circulation. The original design had four pumps in operation and two on standby. The pumps are rated for 3.16 Mgpm, with a total dynamic head (TDH) of 483 ft. The pumps are driven by 500-hp induction motors at an operating speed of 3,570 rpm. The pumps utilize API-610 Plan M for the cooling water piping inside the bearing housing water. These pumps were experiencing several thrust bearing failures, including pump seizure events, since commissioning in October 2015. Typically, pumps start up with high-thrust bearing temperatures and operate for 2 wk–3 wk, followed by a significant increase in bearing metal temperature and, in most cases, a thrust bearing failure.

Observations and findings

The primary factors that contributed to the reliability issues mentioned with these pumps are outlined here. These pumps have a single rolling-element radial bearing on the drive-end (DE), and a duplex set of rolling-element bearings mounted in a back-to-back configuration on the non-drive end (NDE).

Inadequate NDE bearing lubrication

FIG. 1. Unit E thrust bearing failure.
FIG. 1. Unit E thrust bearing failure.

All NDE thrust bearings exhibited relatively high bearing metal temperatures—some as high as 200°F—upon reaching stabilization temperature after startup. Three of the six pumps have experienced complete thrust bearing failure and, in some cases, resulted in pump seizure due to the severity of the failure. In all cases, the inner bearing of the NDE duplex bearing set, which is furthest from the oil ring, failed first. A visible inspection indicated that the cage experienced a severe failure, which was followed by skidding of the rolling elements. FIG. 1 shows the damaged NDE inner thrust bearing on Unit E.

A visible inspection of the rolling elements showed discoloration, which is consistent with a lubrication failure. All of the rolling elements showed two flat spots that were 180° apart (FIG. 2), which is consistent with the suspected skidding condition following the cage failure.

Based on the suspected lack of lubrication, Unit E’s breather caps were pulled to check for proper oil ring function. Upon inspection, the oil rings appeared to be relatively dry, without significant splashing. The DE’s oil ring was also inspected to compare the DE and NDE oil ring function, which showed a noticeable difference with visible splashing. Based on this observation and discussions with the pump vendor, it was decided to raise the oil level from the existing 50% sight glass level (recommended level) to 75% and recheck the oil ring function. Upon raising the level to 75%, no significant improvement in splashing occurred.

FIG. 2. Damaged balls with discoloration and flat spots 180° apart.
FIG. 2. Damaged balls with discoloration and flat spots 180° apart.


The oil level was then slowly raised until visible splashing was observed, which occurred at a sight glass level of approximately 90%. The observed splashing and oil ring function was similar to the DE oil ring. FIG. 3 shows the NDE sight glass level at approximately 90%. The oil level was left at 90%, and the NDE thrust bearing metal temperature was trended to identify any decrease in bearing metal temperature.

FIG. 3. Unit E pump NDE sight glass oil level.
FIG. 3. Unit E pump NDE sight glass oil level.


The Unit E pump NDE thrust bearing experienced a significant reduction in metal temperature from approximately 186°F to 156°F over a two-hour period. This observation was another indication that the bearings were not being properly lubricated.

Another indicator of improper lubrication was the significant difference—approximately 40°F—between the top and bottom bearing housing skin temperatures. After the oil level was raised, the temperature differential between the top and bottom of the NDE bearing housing was reduced to approximately 15°F–20°F. This drop in temperature indicated that the bearings were now receiving lubrication, resulting in heat transfer to the oil.

Inadequate warmup system and procedure

A review of the warmup and startup commissioning was performed prior to the startup of pump C. After reviewing the piping and instrumentation diagram (P&ID) and conducting field verification, it was determined that these pumps lacked provisions for a controlled warmup.

FIG. 4. View of the bypass line globe valve for warmup flow.
FIG. 4. View of the bypass line globe valve for warmup flow.

The warmup of the pumps was accomplished by opening the bypass valve around the discharge line check valve (FIG. 4). This flow was provided back through the pump discharge line, for an undetermined length of time, at the discretion of the operator. No restrictive orifice (RO) was provided in this line, so the bypass line globe valve was fully opened to provide an undetermined amount of warmup flow back through the pump. The warmup rate from the pump casing’s initial ambient temperature of approximately 90°F was monitored. Upon opening the bypass valve, the pump casing temperature went from approximately 90°F to 280°F within a 10 min–15 min period, which significantly exceeded the recommended warmup rate and resulted in the pump thermal shock. The recommended warmup flow and rate for this size pump and process temperature is about 6 gpm, with no more than a 50°F/hr increase, respectively.

Based on these measurements, it was determined that the pump was experiencing a thermal shock condition during the initial warmup. A consequence of exceeding this warmup rate is differential thermal expansion between the pump casing and the rotor, which can result in damage to the pump’s internal components, including the thrust bearings. The existing configuration utilizes a bypass line around the discharge check valve, without an RO providing uncontrolled warmup back flow through the discharge line and back out of the suction nozzle. This configuration does not provide a controlled warmup flow (i.e., no RO), which resulted in more warmup to the top of the casing. It will not uniformly warm up the pump casing from the bottom to the top. Based on the recommended warmup flow of 6 gpm and the maximum warmup rate of 50°F/hr, the initial warmup should take a minimum of 4 hr. The difference between the top and bottom casings should not exceed 70°F.

After implementing the warmup orifice at the site, as a temporary solution, an acceptable increase in rate casing temperature was achieved. FIG. 5 is an illustration of increasing casing temperature over a 2-hr period. TABLE 1 shows the temperature ranges over the testing period.

FIG. 5. Illustration of the temperature increase in the casing during the warmup period.
FIG. 5. Illustration of the temperature increase in the casing during the warmup period.

 

Improper mounting of NDE thrust bearings

A visible inspection of the outer race of one of the failed bearings indicated evidence of improper mounting or installation of the bearings. FIG. 6 shows the outer race, which shows the visible evidence of these bearings being forced (hammered) during the mounting. The marks on the outer race at the rolling elements are an indication of this improper mounting issue.

FIG. 6. View of the outer race.
FIG. 6. View of the outer race.

Inadequate thrust setting or end float

After observation during bearing replacement activities, it was determined that the established thrust setting (axial float) was set at approximately 1 mm. The manufacturer recommended a thrust setting of 1 mm–5 mm. The tighter setting can also contribute to higher bearing operating temperatures. It was observed that pumps set at 3 mm have relatively lower operating temperatures compared to pumps that have a tighter clearance of 1 mm. The thrust setting is changed by adjusting the thickness of the bearing end cover shim.

Insufficient vibration and bearing temperature shutdown systems

FIG. 7. NDE vibration accelerometer and RTDs.
FIG. 7. NDE vibration accelerometer and RTDs.

The DE and NDE vibration accelerometers are not directly mounted to the bearing housing and do not meet API 670 requirements (FIG. 7). This was another contributing factor to the pumps experiencing complete failure of the NDE bearing without a shutdown of the machine. Without direct contact to the bearing housing, the accelerometers do not provide adequate warning of the increased vibration amplitude associated with the pending bearing failure. Another contributing factor to the bearing failures without a machine trip was the bearing temperature shutdown logic. Existing logic requires both the DE and NDE RTD faults for machine shutdown.

Changing the oil ring

The vendor has recommended changing the oil ring size, which was implemented on all six pumps. The bearing temperature indicated improvement, but the oil rings were inspected during the oil change and some minor rubbing marks were found.

Takeaways

After an anaylsis of the equipment, the following aspects were noted:

  • Inadequate bearing lubrication was identified as a significant contributing factor to the frequent thrust bearing failures with these pumps.
  • The inadequate lubrication appears to be related to an improper oil level in the bearing housing, as evident from the oil level adjustment and observed splashing, and confirmed with the observed significant reduction in bearing metal temperatures.
  • The inadequate warmup system and the observed thermal shocking of the pumps during warmup were also identified as significant contributing factors to the bearing failures.
  • The thrust setting or axial float was identified as an improvement area to reduce the operating temperatures of the thrust bearings.
  • An improper bearing-mounting technique—the basis of the preliminary bearing failure analysis—was identified, and was a contributing factor to the poor reliability of the thrust bearings.
  • Inadequate bearing temperature shutdown logic required both DE and NDE bearing temperature faults to shut down the machine.
  • Inadequate vibration shutdown protection, due to improper mounting of the seismic accelerometers at both the DE and NDE locations. This configuration also contributed to the bearing failure events without a machine trip.

The following are some recommendations:

  • Revise the warmup system to incorporate an RO to maintain a proper pump warmup rate. It is recommended to revise the warmup system per the manufacturer’s recommendation. The startup procedure for these pumps should include warmup requirements to ensure that the pumps do not experience thermal shocking and maintain a continuous warmup in the standby condition.
  • Ensure that the warmup is conducted in a controlled manner, as per the recommended warmup rate of less than 50°F/hr. The difference between the topand bottom casings should not exceed 70°F.
  • The thrust setting, or axial float, should be set at 3 mm–4 mm on future NDE thrust bearing replacements to ensure clearance for these bearings and to minimize the bearing operating temperature.
  • Reconfigure the bearing RTD shutdown logic, which requires both DE and NDE faults to trigger a pump shutdown. This configuration resulted in complete failure of the NDE bearing without a shutdown of the machine, due to requiring faults on both DE and NDE bearings.
  • Revise the DE and NDE bearing housing accelerometers, which were incorrectly mounted.
  • Future thrust bearing replacements should be performed by following the procedure outlined in the maintenance and repair manual to ensure proper installation and mounting. HP

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