Best Method to Balance Torque Loadings on a Pumping Unit Gearbox

Abstract In general there are three methods available to the operator to determine the net torque loading on a pumping unit's gearbox. Two dynamic methods determine the instantaneous torque throughout the pumping cycle. Method 1 combines the measured surface dynamometer card and calculated torque factors with measured or calculated counterbalance moments from the crank and weights. Method 2 uses measured motor power with motor and drive efficiencies and the pumping unit speed to calculate gearbox torque. The third method combines the counterbalance effect (CBE) with the measured dynamometer loads and the torque factors to compute the net torque on the gearbox. The CBE test is a direct method of determining net gearbox torque at a specific crank position to estimate the maximum counterbalance moment. This static test is where the cranks and counterweights are held level until no upward or downward movement is noticed when the break is released. Field case studies applying all three methods to determining gearbox torque are presented in this paper. The pros and cons of using each method are discussed. Introduction The oldest and most common method of artificial lift used in producing oil wells is sucker rod pumping. In the United States and Canada, sucker rod lift is used in over 85% of artificial lift wells. The sucker rod lift system consists of four components:prime mover;pumping unit;counterbalance to the rod loading; and,sucker rods and associated downhole equipment. The function of the pumping unit is to convert the rotary motion of the prime mover to the vertical reciprocating motion of the polished rod. Therole of the prime mover is to furnish the necessary power to drive the system. To improve efficiency, reduce the size of the prime mover and gearbox, and to load the gearbox more uniformly, the sucker rod pumping mechanism is furnished with some type of counterbalance system, where the counterbalance effect at the polished rod is approximately equal to the buoyant weight of the rods plus half the fluid load on the plunger. Balanced or Unbalanced Gearbox Loading For each complete stroke, the net torque load on the gearbox is cyclic, usually having two maximum peaks and two minimum valleys. The peaks occur during the upstroke and the downstroke, and the valleys occur at the top and bottom of the stroke. For balanced operation, the magnitude of the peaks should be approximately equal. Pumping unit manufacturers use various types of counterbalancing and mechanical features to reduce the peak gearbox torque and to smooth out the cyclic effects of the load. The gearbox is said to be underbalanced or rod heavy if the upstroke peak is greater, whereas the gearbox is said to be overbalanced or weight heavy if the downstroke peak is greater. Since in a rotating system torque and power are directly related, the previous statements can be equally applied to the cyclic nature of the motor power developed during a complete stroke of the pumping unit.

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