Main oil line (MOL) pumps are responsible to deliver the produced oil from an oilfield to a processing and storage facility where oil is exported through a loading station. MOL pumps failure is considered disastrous due to the fact that it introduces a safety hazard and leads to business and operational interruption. This paper studies a failure prevention approach of MOL pump bearing isolator/seal using computational mechanics simulation to predict the failure and maximize pump reliability.
This research conducted is a case study of failed MOL pump bearing isolator, made of Teflon, which occurred during the pump's Factory Acceptance Test (FAT). The pump capacity is in the magnitude of 250,000 barrel/day. A Finite Element (FE) axisymmetric coupled steady state temperature-displacement model of the bearing isolator was developed. The FE model mimics the actual pump operating conditions to simulate the seal performance. FE results of two different materials, Teflon and Bearing Bronze, were assessed and compared against seal behavior during pump testing. Recommendations to prevent future on-site failures were made and executed based on FE outcomes.
During MOL pump FAT, a Teflon bearing isolator repeatedly failed while running under its normal operating conditions. It is found that the isolator's rotating part is deflecting during operation due to the effect of centrifugal force and thermal growth. This deflection causes the rotating part outer diameter to contact, radially, with the stationary part leading to frictional heat generation and subsequent component rupture. The built FE model was utilized to analyze the failure pattern. The FE model predictions were able to identify the exact location of the failure, under the actual operating speed and temperature. Excellent agreement between FE analysis results and actual failure was observed. In order to prevent this failure from occuring at site, change of material was proposed to improve the seal performance. Another FE analysis was built utilizing Bearing Bronze material, which is a commonly used material for this application, to examine the bearing seal integrity under the same operating conditions. FE results showed that Bronze is able to perform without failure due to relatively negligible deformations. Bronze material seal was then adopted and tested with no failure. The risk of Teflon seal premature failure was hence eliminated.
Bearing isolator selection criteria are not sufficiently covered in any international standard. Operating companies depend solely on manufacturer recommendations. Bearing seal failure due to pump rotation can be analyzed only with advanced engineering simulation due to its dynamic behavior and access limitations. The outcomes of this assessment were able to predict/prevent MOL pump bearing seal failure. This will allow taking action before failure takes place, and provide a material selection guideline for such machine element.