August 2017

Special Focus: Fluid Flow and Rotating Equipment

Analyzing axial cooling fans’ high noise, vibration and failures

Shipping pumps with 1,800 hp are equipped with five cooling fans that cool the oil going to the lube oil pumps to lubricate the bearings.

Allah, W. K., Saudi Aramco

Shipping pumps with 1,800 hp are equipped with five cooling fans (FIG. 1) that cool the oil going to the lube oil pumps to lubricate the bearings. Each shipping pump is equipped with a single lube oil cooler, with two fans per bay. Each fan is capable of 100% of the duty requirement. After commissioning, high noise and vibration were noticed on the cooler units.

FIG. 1. Cooling fan.
FIG. 1. Cooling fan.

Several reasons exist for the high vibration and noise of the cooler fans:

  • The misalignment of the shafts and the coupling have resulted in a significant out-of-balance force. This was not noticed during the original startup.
  • Long-term and vendor storage procedures onsite were not followed.
  • Poor installation of electrical motors and coupling alignments was also a factor. The vendor was not contacted during the installation.
  • Four fan coolers had 5 mm machined off the length of the fan shaft. This setup was only a precautionary action and was not a contributing factor to the failure of alignment of couplings and bearing failures.

The induced-draft lube oil cooling fans have an oil inlet temperature of 160°F, while the oil outlet temperature is 140°F. The inlet viscosity of the oil is 11.7, and the outlet viscosity is 16.5. The air inlet temperature is 126°F. The shipping pumps were commissioned in December 2015. During this time, noise and vibration in some of the air-cooled fan bearings were detected. The cooling fans were previously tested in mid-2015 and showed no signs of vibrations or noise. The vendor was asked to visit the site and investigate the issue, as well as check all 10 cooling fans to avoid future issues.

Observations and findings

All cooling fans were inspected with the vendor present. TABLE 1 shows the condition of each induced-draft fan, which were checked in late January 2016. The vendor’s observations included:

  • The motor was found to be poorly aligned, and the plastic insert in the flexible coupling was making noise.
  • The quantity of grease specified by the vendor in the installation and operation manual (IOM) was excessive and should be decreased.
  • The motors were shipped loose during transport; therefore, the driver alignment at the site was not completed by the vendor.
  • A significant amount of sand and dirt was present in the units, as well as in the finned tubes, which can adversely affect thermal performance.

Cooler fan B95 EM 0714B

The vibration readings showed that the bearings were starting to fail. The top bearing was removed due to the noise. The fan was removed, and an excessive amount of grease was found around the bearing, which was also removed and found to be in reasonable condition. The grease lines were checked for leaks, but none were found. The excessive amount of grease was being pumped to the unit (FIG. 2). It was inspected, and vibration measurements were captured. The type of noise, along with the results from the vibration tests, indicated that these bearings were starting to fail. Thus, it was determined that the bearings should be changed. FIG. 3 shows the bearing fault frequencies.

FIG. 2. Excessive amount of grease in cooler fan B95 EM 0744 A.
FIG. 2. Excessive amount of grease in cooler fan B95 EM 0744 A.

 

FIG. 3. Bearing fault frequencies above prior shaft modification.
FIG. 3. Bearing fault frequencies above prior shaft modification.

Cooler fan B95 EM 0744B

The fan was inspected and vibration measurements were captured. There was no issue with the bearings, but alignment was needed.

Cooler fan B95 EM 0754B

During the inspection of the fan, vibration readings showed that the alignment on the coupling was poor. After removing the coupling insert cover, it was found that the fan shaft coupling had moved and was poorly aligned. In addition, the key on the fan shaft had slipped down onto the motor shaft.

FAILURE ANALYSIS

Based on the findings and observations, four cooling fans required further action. The following actions were considered to return the cooling fans to their healthy condition:

  1. Replace the bearings on lube-oil cooling fan B on NGL shipping pump A.
  2. Replace the bearings on lube-oil cooling fan A on NGL shipping pump D.
  3. Check and realign the couplings on both the lube-oil cooling fans on NGL shipping pump D.
  4. All other units to be run tested. The parts required included two sets of bearings to change out on coolers B95 EM 0714B and B95 EM 0744A.

Cooler fan B95 EM 0714B, shipping pump A

The following observations and actions were taken:

  • The top bearing was found to be in an operable condition.
  • The bottom bearing was removed and replaced.
  • Prior to the removal of the fan shaft, the clearance between the fan shaft and the motor shaft was measured and was found to be less than 1 mm (FIG. 4).
FIG. 4. The surface clearance between the fan shaft and the motor shaft was less than 1 mm.
FIG. 4. The surface clearance between the fan shaft and the motor shaft was less than 1 mm.


Due to the clearance being less 1 mm, there was no further room for adjustment to the slotted bolt holes on the motor frame (FIG. 5). Therefore, the vendor decided to reduce the shaft length by 5 mm to allow for an ideal clearance of 3 mm–5 mm between shafts.

FIG. 5. No further room for adjustment on slotted bolt holes.
FIG. 5. No further room for adjustment on slotted bolt holes.


The machined shaft and new bearings were installed and greased, and the motor coupling was aligned. A test run was then performed, and the fan experienced no further vibration or noise.

Cooler fan B95 EM 0754B, shipping pump E

After disassembling the fan drive assembly, it was discovered that the top bearing was starting to fail, and there was evidence that water had entered the bearing (FIG. 6). The clearance between the motor shaft and the fan shaft was measured, and was found to be less than 1 mm. Just like in the previous example, there was no further room for adjustment on the slotted bolt holes on the motor frame; therefore, 5 mm was machined off the end of the fan shaft. The machined shaft and new bearings were installed and greased, and the motor coupling was aligned. The fan was tested, and no further vibration or noise were detected.

FIG. 6. Evidence of water entering the bearing.
FIG. 6. Evidence of water entering the bearing.

Cooler Fan B95 EM 0744A, shipping pump D

The coupling covers on both fans were removed and the couplings were checked for alignment. The radial misalignment tolerance was greater than the acceptable tolerance of 0.991 mm. The misalignment measured around 1.5 mm on both fans. The clearance between the motor shaft and the fan shaft was measured and determined to be less than 1 mm. Both fans were machined to allow for the ideal clearance of 3 mm–5 mm. The machined shaft and bearings were installed and greased, and the motor coupling was also aligned. The fan was tested, and no further vibration or noise were detected (FIG. 7).

FIG. 7. Trends showing no sign of bearing fault frequencies after the modifications.
FIG. 7. Trends showing no sign of bearing fault frequencies after the modifications.

Takeaway

There were several reasons behind the failure of the cooling fans:

  • The misalignment of the shafts and coupling resulted in a significant out-of-balance force. This problem was not noticed during the original startup.
  • Long-term storage procedures and vendor storage procedures were not followed onsite.
  • There was poor installation of the electrical motors and coupling alignments. The vendor was not contacted during the installation.
  • Four cooling fans had 5 mm machined off the length of fan shaft. This was only a precautionary action, and not a contributing factor to the failure of the alignment of the couplings and bearing failures.
  • Design problems in the fan bearings resulted in a lack of proper sealing from environmental dust. Improved releasable bearings with protective seals are needed to replace the original equipment manufacturer (OEM) bearings; otherwise, dust and water contamination will cause a high frequency of failures.

Recommendations

The following actions are recommended with regard to the reliable operation of cooling fans:

  • The site should have two spare sets of bearing assemblies, along with coupling spares.
  • If grease has been leaking excessively from the bearings, it is recommended to check the grease lines during bearing replacement to ensure that the grease is not contaminated and flowing. The lubrication should be maintained by ensuring regular intervals and that the correct quantities are applied.
  • The accurate alignment of the motor is critical for the long-term reliability of the unit. Some vendors recommend that a dial indicator is placed on both shafts when they are aligned, which will ensure good alignment. While the coupling tolerates some misalignment, the better aligned the shafts are, the less vibration there will be. This step will increase the life of the coupling and bearings.
  • To align the shafts, the motor mount should be shimmed to the appropriate height to ensure that the motor shaft position and angle matches the fan shaft.
  • It is recommended to upgrade the bearings with new and improved sealing designs that protect from dust and water. HP

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