May 2017

Special Focus: Maintenance and Reliability

Root cause analysis on bearing failure on an induced draft fan

This case study aims to provide the root cause analysis behind bearing failure of an induced (ID) fan at a new refinery startup in 2015.

Allah, W. K., Saudi Aramco

This case study aims to provide the root cause analysis behind bearing failure of an induced (ID) fan at a new refinery startup in 2015. After taking vibration readings during several runs, the reliability failure was determined to originate from the non-drive end (NDE) pillow block bearing, which was a double row ball bearing.

This examination will explain how bearing failure occurred during the regular run of the ID fan shown in FIG. 1. Due to the complicated nature of determining the causes of bearing failure, multiple parties were involved in the investigation. A total of four NDE bearings were damaged during the investigation process, which shed more light onto this equipment.

FIG. 1. The ID fan examined for bearing failure.
FIG. 1. The ID fan examined for bearing failure.

This ID fan has an NDE self-aligning double ball bearing (1220 K/C3). The mechanical run test (MRT) of this equipment was observed onsite. The test duration was 4 hr and was passed successfully. The equipment’s vibration and temperature readings were within normal range. The unit is driven directly by a 300-kW, 1,188-rpm electric motor. Fan vibration is protected by two seismic accelerometers, at horizontal and vertical directions, at each of the fan’s drive-end (DE) and NDE bearings.

Troubleshooting history

After the MRT, the ID fan tripped after 1 hr with high vibration readings on the NDE vertical. FIG. 2 shows a high vibration amplitude (7.36 mm/s) at ball pass frequency outer (BPFO) 4th harmonic frequency.

FIG. 2. Spectrum plot of the ID fan’s NDE vertical.
FIG. 2. Spectrum plot of the ID fan’s NDE vertical.

Observations

The unit was started at zero inlet guide vanes (IGVs) and openings, and gradually increased to 21%. At this point, the machine tripped. The overall velocity measurement at the fan’s NDE vertical direction was gradually increasing from 5 mm/s pk to 15 mm/s pk, which resulted in a machine trip after 1 hr from startup. The associated frequency spectrum at the fan’s NDE vertical shows a dominant frequency component of 42 kCPM, with side bands that gradually increase over time, as shown in FIG. 2.

Abnormal bearing noise was observed from the fan’s NDE bearing during operation, as well. However, the fan’s DE bearing vibration levels were at an acceptable level of below 5 mm/s pk during operation. The team started with the realignment process by removing the coupling (FIG. 3). After the alignment was checked, the ID fan was restarted only to trip again after a few minutes in operation. The reliability team then decided to check the NDE bearing. Upon examination, the NDE bearing had been damaged (FIG. 4).

FIG. 3. The coupling was removed to realign the laser.
FIG. 3. The coupling was removed to realign the laser.
FIG. 4. NDE bearing outer race damage.
FIG. 4. NDE bearing outer race damage.

Indicated velocity peaks and unusual noise from the NDE bearing suggest bearing deterioration/damage in the system. The NDE vertical vibration readings were high, as well. As a result, the team decided to install a new NDE bearing and realign the machine before running it again. A second bearing was installed and was damaged within one week, for the exact same root causes.

Consultant recommendation

A compressor consultant visited the site and inspected the failure mode of the NDE bearing. He determined that the bearing has been subjected to high axial load when it has low axial capability. The following recommendations were provided as action items:

  1. The existing NDE bearing outer race had zero clearance with the bearing housing, leading to high axial loading while in service. The required 0.004-in.–0.005-in. diametric clearance should be applied correctly. This measurement can be confirmed by using a feeler gauge.
  2. The fan’s DE thrust bearing must be fixed in place rigidly in axial direction, with no more than 0.003-in (0.075-mm) axial clearance allowed. Otherwise, the fan impeller forces will push the shaft hard against the NDE bearing and damage it. The thrust bearing must be locked not only on its inner race, but also axially in the bearing housing against its outer race. Only 0.003 in. is allowed as clearance in axial direction.
    This is indicated in FIG. 5.
  3. The fan may have been installed with an incorrect bearings pedestal height, meaning that the shaft is sloped downward toward the NDE side. This can be verified by using alcohol and a precision level placed on the shaft. If it is not level, then bearings must be raised or lowered, as needed. However, if a laser alignment contractor is available, then this is the most accurate method.
  4. The motor driver must be checked for incorrect coupling installation. The team should check the fan-motor installation manual for the coupling and ensure that the correct axial gap is included in the distance between hubs. Typically for this size motor, the axial pre-stretch gap is 0.04 in. (1 mm). Also, the motor must be set correctly. The team should compare the motor installation carefully to other manual and sister machines.
FIG. 5. Amount of clearance when bearing is axially locked.
FIG. 5. Amount of clearance when bearing is axially locked.

Action considered

The team followed the consultant’s recommendation and determined that the pillow block inner diameter was modified after machining. The clearance between the bearing pillow block and the outer race was 0.07 mm–0.08 mm (FIG. 6). The team found that the stabilizing ring of the DE thrust bearing was not placed correctly; therefore, it was not locked correctly due to manufacturer error.

FIG. 6. The clearance between the bearing pillow block and the outer race was 0.07 mm–0.08 mm.
FIG. 6. The clearance between the bearing pillow block and the outer race was 0.07 mm–0.08 mm.

The height of the bearings was checked and verified. At the DE, the bearing level was 0.00175 in./ft. At the NDE, the bearing level was 0.001 in./ft. It was found that the NDE bearing was slightly high by 0.00075 in./ft. Therefore, shims were added to correct the leveling of the shaft (FIGS. 7 and 8).

FIG. 7. Measuring the NDE bearing slope using a level gauge.
FIG. 7. Measuring the NDE bearing slope using a level gauge.
FIG. 8. The height of the bearing on the DE and NDE sides.
FIG. 8. The height of the bearing on the DE and NDE sides.

Further investigation

The team decided to inspect the DE bearing. In the bearing manual, it was shown that the clearance between the rollers and the outer race was 0.08 mm. However, the measurement showed the clearance to be only 0.03 mm, which meant it was tightened too much.

The min. clearance = 0.0053 in.–0.002 in. = 0.0033 in. (0.08 mm).

To obtain the correct clearance between the rollers and the outer race, the team needed to torque the DE bearing until the right amount of clearance was achieved. To have a non-expansion pillow block, the thrust stabilizing ring needed to be installed correctly. Not using the stabilizing ring allows shaft and bearing axial expansion. The team has used it before installing the new DE bearing, as shown in FIG. 9.

FIG. 9. Stabilizing ring being installed on the DE bearing.
FIG. 9. Stabilizing ring being installed on the DE bearing.
FIG. 10. Checking axial clearance of the DE bearing.
FIG. 10. Checking axial clearance of the DE bearing.

The bearing is locked, and clearance in the axial direction for the DE bearing is ~0.03 mm (FIG. 10). The fan was started and all parameters were normal. However, the same symptoms showed up after a couple of months. Thus, the team has decided to install a new NDE pillow block due to the poor quality of work done on the previous one. The old NDE 1220 K/C3 double-row ball bearing was replaced with a 22220-Y double-row spherical roller bearing, since roller bearings have more load carrying capacity and more misalignment capability. After these implementations, the ID fan has been running smoothly for 1.5 yr without failure.

Key findings

This fan had a history of frequent NDE bearing failure since startup. In each failure, material chipped off and left an indentation pattern on the bottom load zone area. During the latest failure in July 2015, the NDE 1220 K/C3 double-row ball bearing was replaced with a 22220-Y double-row spherical roller bearing. Both the DE and NDE were fixed with 22220-Y double-row spherical roller bearings. During previous maintenance activity, pillow blocks of the DE and NDE were also interchanged. A new NDE pillow block was installed due to the poor quality of work on the previous one. The fan was started, and all parameters were found to be within the normal range of 5 mm/s pk to 6 mm/s pk, as measured on both DE and NDE bearing housings. HP

ACKNOWLEDGEMENTS

The author gratefully acknowledges the assistance of Abdulrahman Al-Khowaiter and Syed Uzair Shah for their work to develop this article.

The Author

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