Lift System Reliability and Upthrust

Shallow well CBM development in the Powder River Basin has proved to be a great reliability challenge. Most of the 12,000 operating wells have experienced multiple failures. The most common failures involve the artificial lift system. Each failure has provided a learning opportunity. The screened liner completion has improved reliability. Also, for open-hole completions, the flushing screen/degassing pump shroud has improved reliability and is a low cost alternative to using a liner. Toward the end of increasing the reliability of artificial lift systems, the WellTamer, was developed to address the primary failure mechanisms experienced by multi-stage electric submersible pumps used in CBM dewatering applications. Following, in a question and answer format, is a discussion of the lessons learned to date. - What considerations led system designers to select multi-stage electric submersible pumps as the optimum method of artificial lift for CBM de-watering in the Powder River Basin? Multi-stage submersible pumps were selected because they have the appropriate characteristics, including; lift, flow, cost, availability and a long history of reliability. - Why have multi-stage submersible pumps, when used in CBM de-watering applications, demonstrated such poor reliability? The most prevalent cause of premature pump/motor failures is upthrust. This failure mode has been observed in at least 2/3 of the pump and motor failures examined. Other causes of failure included; poor motor cooling in an oversized hole, jamming and wear due to ingested solids and gas locking due to entrained gas - What is unique about CBM de-watering applications that cause upthrust to occur? Historically, in water wells, check valves have been installed at the pump discharge and about every 100 feet up the string. This practice is recommended specifically to maintain a discharge head on the pump for the next restart. In CBM production, however, check valves are typically eliminated to solve two serious problems; surface-freeze and gas locking at the check valves. Each time the pump cycles off, there is a gravity driven back flush through the pump. The flush must continue until equilibrium is reached and the pump stops spinning backwards. Specifically, it is this run and flush cycle that sets the stage for upthrust to occur on every restart. Premature failure is guaranteed. - What is upthrust? Upthrust is axial thrust on the pump impellers generated by starting the pump at the no head, max-flow operating point. It occurs on every start-up if there is inadequate head at the pump discharge. Every multi-stage submersible pump, regardless of manufacturer, is subject to upthrust. Firm statements in every pump and motor manufacturer’s technical literature states that upthrust will cause premature failure.   - What is meant by the phrase, “inadequate head at the pump discharge”? Each pump requires a specific discharge head to prevent upthrust. Differential head is the height difference between the higher water level in the drop pipe and the lower water level in the well casing. If this differential head is not equal to or greater than the manufacturer’s requirement, it is inadequate to prevent upthrust. - What causes upthrust? When a pump is started with no differential head at the discharge, the pump is initially operating at the extreme right hand end of the pump curve. This operating point, called “run-out,” is where you have no head and maximum flow. With no differential discharge head available to create a counteracting down force, the extreme flow rate lifts each impeller stage with considerable force.   How does upthrust cause pump failure? When multi-stage submersible pumps operate at run-out, the lifting force on each stage accumulates along the stack of impellers. At the top stages of a 25 - 30 stage pump the axial thrust may be several hundred pounds. Friction, excessive heat, and mechanical wear eventually cause the upper stages of the pump to fail, typically beyond repair. - How does upthrust cause motor failure? When the wet-end is in upthrust, there may be enough torsional load on the splined motor/pump coupling that when the pump shaft lifts with the impellers it may pull the motor rotor up into its upper thrust bearing. Usually, the thrust capabilities of the motor’s upper thrust bearing are exceeded in both magnitude and duration. As a result, the motor’s upper thrust bearing fails. Also, both bending moments and radial loads may be applied to the rotor shaft and bearings during wet-end failure. Does every brand of multi-stage submersible pump have the same upthrust characteristics? The amount of up-thrust generated is a function of each manufacturer’s impeller/diffuser/bowl design. Some manufacturers vent the impellers back to the intake side or add flutes on the bottom side to reduce the pressure difference across the impeller. Any reduction in up-thrust by design comes at the cost of efficiency. Other designs fix the individual impeller stages to the drive shaft. The intent is to reduce the stage-to-stage transfer of axial thrust forces. In this case, when the pump is operating at “run-out”, the combined lifting force of all stages is transferred directly to the drive shaft. As a result, both the pump and the motor upper thrust bearings must bear the excessive upthrust load and will fail prematurely. - How can upthrust failures be eliminated? The most economical method of preventing upthrust is to install a calibrated choke at the pump discharge. This choke should provide only enough artificial discharge head to prevent upthrust at start-up. At the operating point the parasitic loss will be minimal and insure that at start-up the pump never experiences upthrust, ever. - What reliability might one expect from an upthrust protected submersible pump that is used in CBM de-watering applications? Manufacturers report that pumps and motors, when operating in clean, cool water and protected from upthrust, typically live over 10 years with over 100,000 restarts. They also agree that, when operated in a controlled environment and protected from upthrust, multi-stage electric submersible pumps and motors used in CBM applications should enjoy this historical reliability. Pump and Axial-Thrust Curves  With no discharge head upthrust occurs at start-up. This pump operating point is called run-out. The “zero axial thrust” operating point occurs to the right of BEP. The head at this operating point is the minimum discharge head required to keep the pump from upthrust on startup.