Saturday, March 30, 2019

BEARING CASE STUDY 1 – BEARING DAMAGE IN HORIZONTAL PUMP


Bearing defects can be of several types. It can be caused by several factors too. For example, due to excessive cyclic loads (fatigue failure), cracking and flaking away of the races (spalling), removal of metal due to the presence of foreign particles (abrasion), compromised metal due to oxidation (corrosion), cracked mating surfaces under load (fretting), impact damage (true brinelling), electrical discharge through bearings (fluting/EDM(Electric Discharge Machining)), removal of bearing metal due to vibration/movement of bearing at rest (false brinelling), metallic removal from one surface and deposition on other due to excessive load and sliding under dry condition/improper lubrication (smearing), deep scratching of bearing surfaces due to presence of large foreign particles (scoring/wire drawing), denting due to inclusion of foreign particles between rolling surfaces (debris denting), damage due to heat and load (hot working), cracking of bearing races due to excessive loads (fracture), failure due to misalignment or other factors that damages the retainer/cage (retainer failures).

In this case study, we will discuss the bearing failure in one of the horizontal pump, it’s detection, causes and rectification. For more details on different modes of bearing failures, CLICK HERE.

The monitored pump was kept as a standby for a long period of time. During the startup, observed heavy rubbing sound from the pump side. Vibration readings were collected from the pump and motor to identify the defect. The spectrum revealed very high amplitude at high frequency from pump bearings, indicating bearing damage. Stroboscope was used to check the coupling condition while in operation and revealed coupling damage. Excessive radial and axial play observed in the pump shaft. Coupling and bearings were replaced.
Random high peaks in the waveform also confirmed severe bearing damage.

Equipment details:
·         Damaged Bearing: FAG 6306 C3
·         Lubrication type: Oil
·         Pump type: Horizontal overhung, OH1*
·         Rated speed: 1780
·         Highest peak at 44.68x rated speed.
* As per API 610, OH1 = Horizontal overhung, foot mounted pump.
Upon replacement, the damaged bearing was studied for the cause of failure.
It was found that the reason for failure is corrosion, surface fatigue and spalling. The bearing got damaged due to ageing and water ingress in the bearing housing. The pump was located in an open space, exposing it to atmospheric changes. The bearing housing vent plug became a path of water entry into the housing, accelerating the damage of bearing by corrosion.
The coupling got damaged after the bearing deterioration.

Random high peaks in the waveform also confirmed severe bearing damage.


Equipment details:
·         Damaged Bearing: FAG 6306 C3
·         Lubrication type: Oil
·         Pump type: Horizontal overhung, OH1*
·         Rated speed: 1780
·         Highest peak at 44.68x rated speed.
* As per API 610, OH1 = Horizontal overhung, foot mounted pump
.
Upon replacement, the damaged bearing was studied for the cause of failure.
It was found that the reason for failure is corrosion, surface fatigue and spalling. The bearing got damaged due to ageing and water ingress in the bearing housing. The pump was located in an open space, exposing it to atmospheric changes. The bearing housing vent plug became a path of water entry into the housing, accelerating the damage of bearing by corrosion.
The coupling got damaged after the bearing deterioration.


Recommendations were provided to lubricate the pump regularly, flush clean the bearing housing with oil every 3 months, attach Teflon tape on vent plug thread (temporary solution) and to provide a rain shield over the pump. Proper alignment was ensured to improve the reliability of the equipment. Standby pump switchover frequency was revised to 1 month.

Root cause:
1.    Ageing of bearings.
2.    Water ingress in bearing housing of the pump.

In addition, test the lube oil quality in Emerson CSI 5200 oil analyzer or similar oil analyzers and cross-check the viscosity of oil with that recommended by the manufacturer to improve reliability.

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