A beam pumping unit is a simple machine with an unforgiving selection process. The motor turns, the gearbox reduces, the crank and pitman convert rotation into a walking beam oscillation, and the horsehead lifts the rod string. Pick a unit with too little torque and the gearbox fails on the first cold morning; pick one with too much stroke and the pump does not fill. The selection is a match between the well's inflow, the pump displacement and the structural rating of the unit. On the lease the crew reads the result off the polished rod load and the current draw, not off the sales brochure.
How a Pumping Unit Converts Rotation to Lift
Power from an electric motor, diesel engine or gas engine goes through a belt drive into a three-shaft gearbox, where two stages of reduction bring the speed down and the torque up. The crank rocker mechanism then turns that rotation into the up and down motion of the walking beam, and the horsehead carries the polished rod clamp that lifts the rod string. Every part of that chain is rated, and the rating that matters most at the lease is the gearbox torque. A unit that is correctly rated but badly balanced will still fail, which is why the balance procedure is part of the installation rather than an afterthought.
Stroke Length and Pump Fillage
Stroke length sets the displacement per cycle, and the unit is built with two or three stroke options on the same crank. A C-912D-470-240 unit offers 240, 211 and 181 inches of stroke, which is 240 inches (20 feet) down to 181 inches (15.1 feet), while the smallest units in the range run strokes near 30 inches (2.5 feet). The choice is made against pump fillage: a long stroke at a slow speed usually fills better in a gassy well than a short stroke at high speed, because the pump has more time to take fluid. The pump shop sets the pump bore against the same inflow figure, so stroke and pump size are chosen together rather than in sequence.
Rated Torque and Gearbox Selection
Rated torque runs from 25,000 inch-pounds on the smallest unit to 912,000 inch-pounds on the largest, and the model code carries the number so the lease crew can read it off the nameplate. Torque capacity has to cover the peak load at the polished rod, which occurs at the top of the upstroke when the rod load and the counterweight imbalance are both at maximum. A unit selected on average load rather than peak load will run hot and fail early. The peak is what the gearbox teeth see, and the nameplate number is a peak rating rather than a working average.
Balancing and Prime Mover Choice
Crank counterweights are adjusted by gear-rack transmission, which makes the balance change a deliberate operation rather than a guess. A well-balanced unit draws a nearly constant current across the stroke; an unbalanced one shows a peak on the upstroke and a light load on the downstroke. The prime mover follows the duty: an induction motor for grid power, a gas engine where fuel is available at the lease, and a variable speed drive where the inflow changes through the year. The field decision is usually made on power availability, but the operating cost follows the balance and the fillage more than the motor type.
Check the nameplate torque against the calculated peak polished rod load.
Balance the unit with a current trace, not by feel.
Match stroke and speed to the pump fillage seen on the dynamometer card.
Pumping Unit Checks on the Lease
The routine checks are structural and mechanical. Look at the Samson post bearings, the crank pin and pitman bearings for play, check the brake for a positive stop, and confirm the horsehead is aligned with the wellhead so the polished rod runs true through the stuffing box. A unit that is out of alignment wears the packing, loads the rod string sideways and shortens the life of both. A workover crew that checks alignment before running the rods saves the next crew a packing change.
A pumping unit is selected once and lived with for years. Match the torque to the peak load, the stroke to the inflow, and the balance to the counterweight, and the lease runs at a steady current instead of a daily adjustment.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
