A reaction bonded silicon carbide material helped dramatically reduce the mean time between repair (MTBR) cycle due to premature bearing failures on an asphalt-lubricated bearing pump.

The application is difficult because it involves pumping abrasive asphalt at temperatures to 425ø F. The pump originally used antrification bearings, but these failed in one to three months because the solid particles in the asphalt loaded up the bearing raceways, prohibiting sufficient bearing lubrication.

A group of six pumps were used at an oil refinery in Canada to load paving asphalt into tractor trailers. The liquid being pumped has 5% solids and a viscosity of 260 centistrokes at the start-up temperature of 285ø F, and 30 centistrokes at the maximum temperature of 425ø F.

Pumps constructed with product-lubricated bearings - where the bearings are submerged in the pumpage - reduce the number of mechanical shaft seals from four to one, compared with an external bearing pump design, which places the bearings outside of the pumpage. Fewer mechanical seals provides substantial savings in both initial pump cost and overall maintenance. Moreover, by putting the bearings in the fluid, the shaft bearing span is significantly reduced, increasing the ability of the pump to perform under heavy loads.

The NIM twin-rotary screw pumps from Ingersoll-Dresser Pumps used in this application provide positive displacement performance. The pumps, which operate at 1,780 rpm, were originally fit with double-row ball thrust bearings and cylindrical roller radial bearings. Bearing lubrication is circulated from the pump discharge to the seal chamber and bearings, and then is reintroduced into the suction cavities.

Inlet pressure is minus-4.2 psig, and the discharge is 60 psig. The twin pumping screws generate a constant displacement and are hydraulically balanced due to the double-suction design of the casing. This design eliminates the axial load on the thrust bearing set. Although the hydraulic design of the pump was reliable, the unit experienced repeated bearing failures of the double-row thrust bearings in a short time due to raceway plugging.

Frederic Buse, senior engineering consultant for Ingersoll-Dresser Pumps, was assigned to solve the problem. The first material considered for the bearings was silicon carbide. Mechanical seal face components made from silicon carbide materials have surfaces that approach the hardness of diamonds and possess resistance to abrasion.

These materials are particularly well suited to corrosive applications and can handle a range of temperature extremes. Many grades are excellent thermal conductors and can be specified where high strength and stiffness are required.

Buse selected a reduction-bonded silicon carbide, Purebide-PR9242, from Pure Carbon Co, a Morgan Advanced Materials and Technology Business, St. Marys, Pa. Tribologically, reaction-bonded silicon carbides perform better than their sinter counterparts. Buse made minor design changes to the NIM pump to accept bearings made from the new material. Rework entailed a slight modification to the outboard ends of both drive and driven shafts and to the outboard end cover.

The bushings were pressed into 410 stainless-steel sleeves, and the journals were mounted onto 410 sleeves. The respective 410 sleeves were then installed in the bearing brackets and onto the two shafts. The outboard shaft sleeves and shaft end caps also were machined to accommodate axial thrust collars made of silicon carbide.

One pump with the new bearing design was installed in the refinery as a test case. The unit was in service for more than one year before it experienced an unexpected, prolonged dry run condition due to system control problems. The pump is being repaired by the customer using the PR-9242 bearing due to an increase in MTBR compared to the original bearing configuration. The material manufacturer estimates that the bearing life could be about three years in this application based on normal operating patterns once the system control is rectified.