1. Field of Invention
The present disclosure relates to downhole pumping systems submersible in well bore fluids. More specifically, the present disclosure involves insulated bearings for a submersible pump.
2. Description of Prior Art
Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the well bore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used in this application employs an electrical submersible pump (ESP). ESPs are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via wireline. The pumping unit is usually disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect.
With reference now to FIG. 1, an example of a submersible ESP disposed in a well bore is provided in a partial cross sectional view. In this embodiment, a downhole pumping system 10 is shown suspended on production tubing 18 within a cased wellbore 8. The downhole pumping system 10 comprises a motor 12, a seal section 14, and a pump 16. Energizing the motor drives a shaft coupled between the motor 12 and the pump section 16. Rotors coaxially disposed on the shaft rotate with shaft rotation within the pump body. The centrifugal action of the rotors produces a localized reduction in pressure in the cavities thereby inducing fluid flow into the cavities.
The source of the fluid drawn into the pump comprises perforations 20 formed through the casing of the wellbore 10; the fluid is represented by arrows extending from the perforations 20 to the pump inlet. The perforations 20 extend into a surrounding hydrocarbon producing formation 22. Thus the fluid flows from the formation 22, past the motor 12 on its way to the inlets.
Because of the long length of the motor, the rotor is made up of a number of rotor sections. Each rotor section comprises a large number of flat disks called laminations secured by copper rods. The rotor sections are spaced apart from each other. A bearing assembly is located between each section for maintaining the shaft in axial alignment. The rotor sections are keyed to the shaft for rotation therewith, but are axially movable with respect to the shaft.
Traditionally, the bearing assemblies used in motors, seal sections and pumps of electrical submersible pumps (ESPs) are plain sleeve bearings, which provide radial support. These plain sleeve bearings are not heavily loaded since a large number of bearings are typically used and the ESP units are run in a near vertical orientation. The absence of a substantial load results in an unstable or marginally stable bearing operation that can result in metal-to-metal contact in the bearings, which accelerates bearing failure.
One example of a bearing assembly is provided in a cross sectional view in FIG. 2. Shown is a shaft 24, circumscribed by a sleeve 26 and bearing assembly 32 within the pump housing 28. The bearing assembly 32 radially encompasses a portion of the sleeve 26 and comprises an insert 34, an outer race 36 and a T-ring 38. The sleeve 26 is coupled to the shaft 24, such as by a key, and rotates along with the shaft 24. The sleeve 26, and therefore the shaft 24, is radially supported by the insert 34. A lubricant film (not shown) allows for sleeve rotation within the insert. The T-ring 38 which is disposed in the space between the outer race 36 and the stator 30, prevents bearing rotation.
Pump failure can be initiated by an electrical discharge from an electrical supply source into the bearing assembly. The discharge may produce sparks that in turn create pitting in the bearing components, such as between the sleeve and the insert. Although the pitting exceeds lubricant thickness enabling metal to metal contact, this condition often evades detection since the motor will continue to operate after the pitting episode and smear the evidence. Confirmation of this failure mode requires microscopic detection.