Counterweight Torque (and Counterbalance Equivalent)

Capital and power consumption costs for sucker-rod pumps depend, in part, on the counterweight torque. Counterweight torque is the primary variable because peak torque depends on how well the well is balanced. The secondary variables affect the ideal counterweight torque (motor slip, pumpjack efficiency, net lift, and torque equation). High slip motors allow more crank speed variation. Pumpjack efficiency (preventive maintenance) affects power consumption. Net lift changes the net crankshaft torque as the well pumps off. The general form of the net torque equation more accurately converts polished rod forces into equivalent moments at the crankshaft than the simplified counterbalance equivalent equation. Each combination of the primary and secondary variables may have a unique counterweight torque that minimizes the peak net crankshaft torque by equalizing the largest upstroke and downstroke torques. The peak torque affects capital costs for the assumed operating and preventive maintenance conditions by determining the smallest acceptable gearbox and motor sizes. Fully loaded motors use less electricity because power factor and electrical efficiency go up with the ratio of brake horsepower to motor horsepower. In short, it is possible to cut capital and power consumption expenses by managing the difference between the ideal and the actual counterweight torques throughout the life of the well. Pumpjacks are counterbalanced for an ideal operating condition that considers motor slip, mechanical efficiency, and a fluid level that falls to the pump. The latter element indicates that normally operating pumpjacks have multiple ideal counterweight torques. Careful torque predictions for the anticipated range of operating conditions can save capital by reducing gearbox and motor requirements. Specifying the smallest acceptable motor reduces power consumption because the power factor increases when the nameplate rating is close to the brake horsepower. Brake horsepower is affected by mechanical efficiency and counterweight torque. High mechanical efficiency comes with maintenance. A preventive maintenance goal is to reduce total operating cost by lowering power consumption. Thus power consumption and maintenance costs are an economic tradeoff for the life of the well. Counterweight torque also affects power consumption because it takes a bigger motor to handle the higher peak torques caused by improper counterweight torque. A well-balanced pumpjack uses less power because the ratio of brake to motor horsepower is higher when the crank is ideally counterweighted. Calculated counterweight torque accuracy depends on the selected equation. The net crankshaft torque equations are more SOUTHWESTERN PETROLEUM SHORT COURSE