Advancements in Extreme Pressure Rotary Sealing

Abstract

Polymeric rotary seals used in various oilfield equipment face challenging demands, including rotation for extended periods of time while sealing high differential pressure (?P). Such seals are typically mounted in a housing and compressed radially against a rotatable shaft, and prevent fluid from escaping through the clearance between the shaft and housing. One of the damage mechanisms that limits seal life is extrusion. High ?P forces seal material to extrude into the shaft-to-housing clearance. Factors such as shaft defection and runout overstress the extruded material, causing pieces to break away. Another damage mechanism is the accelerated adhesive wear that occurs when the PV (pressure times velocity) capacity of the seal material is exceeded for conventional rotary seals, or as hydrodynamic rotary seals transition toward boundary lubrication. In static sealing, extrusion is minimized by reducing shaft-to-housing clearance. In rotary sealing, the clearance has to be large enough to accommodate shaft deflection, runout, etc. Failure to provide adequate clearance results in heavily loaded metal-to-metal contact that damages the shaft, the seal, and the housing. This paper describes an innovative sealing arrangement that dramatically increases the PV capability of rotary seals, and summarizes key results from an extensive laboratory test program. Test conditions that were varied include shaft diameter, velocity, ?P, temperature, seal material, and lubricant. In the most extreme tests, each seal was exposed to a ?P of 7,500 psi and a velocity of 240 ft/minute for 1,000 hours, and survived in excellent condition. Potential applications for the new technology include rotating control devices (RCDs), washpipe assemblies, cementing heads, and hydraulic swivels. The new high ?P sealing arrangement is based on three technical advances: The seal is lined with a plastic having excellent high pressure extrusion resistance. The seal incorporates an advanced hydrodynamic inlet geometry that is sufficiently aggressive to produce hydrodynamic interfacial lubrication when plastic seal materials are used. Hydrodynamic lubrication with plastic seals significantly increases the PV capability of the seals beyond what is achievable with elastomer seals. An axially force balanced, radially pressure balanced backup ring having a very small clearance with the shaft is interposed between the rotary seal and the shaft-to-seal housing clearance.   The extrusion resistance of the hydrodynamic plastic seal, combined with the axially and radially balanced backup ring, allows this rotary sealing arrangement to reliably operate at ~5 times the PV value of conventional high pressure polymeric seals for durations in excess of 1,000 hours.