Application of Polycrystalline Diamond (PCD) Bearing for Reactor Circulation Pump

Abstract

The axial flow loop reactor pump installed in a polymer unit facilitates slurry circulation in the reactor. The pump is installed with triple mechanical seal having API seal flushing plans 32, 53C and 52. The seal cartridge contains a radial bearing for shaft support to ensure stability on the cantilevered pump design. There have been multiple failures in the past with low seal MTBF. The failure analyses conducted indicate similar failure mechanism- seal support bearing failure leading to primary seal leak and subsequent shutdown. The failure mechanism has been attributed to skidding and sliding wear on one of the rows of rollers in the bearing. The skidding/sliding is caused by the insufficient load on one of the rows of rollers caused by axial misalignment between the inner and outer race. This misalignment can happen during transients when thermal growth differences require outer race to slide inside the seal housing fit. The lubrication of the bearing and sliding joint can be further impacted by seal oil contamination stemming from seal face geometric instability allowing reverse flow of process across seal face. This process contamination and viscosity loss can cause rapid deterioration of the bearing due to poor lubrication. To address these vulnerabilities, a Polycrystalline Diamond (PCD) bearing is installed to replace existing spherical roller bearing for the rotor support system inside the mechanical seal cartridge. Also due to PCD bearing design, the bearing is less sensitive to lubrication issue caused by C3 ingress into seal oil. This article presents a summary of investigation around the root cause of multiple seal failures in a specialty pump application and ‘fit for purpose’ mitigation taken as a result of investigation along with the capabilities of PCD bearing technology to tackle a design with inherent vulnerability.