Pumpjack

It is used to mechanically lift liquid out of the well if there is not enough bottom hole pressure for the liquid to flow all the way to the surface. The arrangement is often used for onshore wells. Pumpjacks are common in oil-rich areas.

Depending on the size of the pump, it generally produces 5 to 40 litres (1 to 9 imp gal; 1.5 to 10.5 US gal) of liquid at each stroke. Often this is an emulsion of crude oil and water. Pump size is also determined by the depth and weight of the oil to remove, with deeper extraction requiring more power to move the increased weight of the discharge column (discharge head).

A beam-type pumpjack converts the rotary motion of the motor (usually electric) to the vertical reciprocating motion necessary to drive the polished-rod and accompanying sucker rod and column (fluid) load. The engineering term for this type of mechanism is a walking beam. It was often employed in stationary and marine steam engine designs in the 18th and 19th centuries.

Above ground

In the early 20th century, pumpjacks worked by rod lines running horizontally above the ground to a wheel on a rotating eccentric in a mechanism known as a central power. The central power, which might operate a dozen or more pumpjacks, would be powered by a steam or internal combustion engine or by an electric motor. Among the advantages of this scheme was only having one prime mover to power all the pumpjacks rather than individual motors for each. However, among the many difficulties was maintaining system balance as individual well loads changed.

Modern pumpjacks are powered by a prime mover. This is commonly an electric motor, but internal combustion engines are used in isolated locations without access to electricity, or, in the cases of water pumpjacks, where three-phase power is not available (while single phase motors exist at least up to 60 horsepower or 45 kilowatts, providing power to single-phase motors above 10 horsepower or 7.5 kilowatts can cause powerline problems, notably voltage sag on startup, and many pumps require more than 10 horsepower). Common off-grid pumpjack engines run on natural gas, often casing gas produced from the well, but pumpjacks have been run on many types of fuel, such as propane and diesel fuel. In harsh climates, such motors and engines may be housed in a shack for protection from the elements. Engines that power water pumpjacks often receive natural gas from the nearest available gas grid.

The prime mover runs a set of pulleys to the transmission, often a double-reduction gearbox, which drives a pair of cranks, generally with counterweights installed on them to offset the weight of the heavy rod assembly. The cranks raise and lower one end of an I-beam which is free to move on an A-frame. On the other end of the beam is a curved metal box called a horse head or donkey head, so named due to its appearance. A cable made of steel—occasionally, fibreglass—called a bridle, connects the horse head to the polished rod, a piston that passes through the stuffing box.

The cranks themselves also produce counterbalance due to their weight, so on pumpjacks that do not carry very heavy loads, the weight of the cranks themselves may be enough to balance the well load.

Sometimes, however, crank-balanced units can become prohibitively heavy due to the need for counterweights. Lufkin Industries offer "air-balanced" units, where counterbalance is provided by a pneumatic cylinder charged with air from a compressor, eliminating the need for counterweights.

The polished rod has a close fit to the stuffing box, letting it move in and out of the tubing without fluid escaping. (The tubing is a pipe that runs to the bottom of the well through which the liquid is produced.) The bridle follows the curve of the horse head as it lowers and raises to create a vertical or nearly-vertical stroke. The polished rod is connected to a long string of rods called sucker rods, which run through the tubing to the down-hole pump, usually positioned near the bottom of the well.