?Nodding donkey? redirects here. For the train, see British Rail "Pacer".

A diagram of a pumpjack.

Depending on the size of the pump, it generally produces 5 to 40 litres of a crude oil-water mixture (called emulsion) at each stroke. The size of the pump is also determined by the depth and weight of the oil to be removed, with deeper extraction requiring more power to move the heavier lengths of polished rod (see diagram at right).

A pumpjack converts the rotary mechanism of the motor to a vertical reciprocating motion to drive the pump shaft, and is exhibited in the characteristic nodding motion. 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 1700s and 1800s.

Above ground

This pumpjack located south of Midland, TX is a common sight in West Texas.

The prime mover of the pumpjack runs a set of pulleys to the transmission which in turn drives a pair of cranks, generally with counterweights on them to assist the motor in lifting the heavy string of rods. The cranks in turn 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, there is a curved metal box called a Horse Head or Donkeys Head, named so due to its appearance. A cable made of steel (or, occasionally, fiberglass) called a bridle, connects the horse head to the polished rod, a piston that passes through the stuffing box. The polished rod has a very 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 an almost completely vertical stroke. The polished rod is connected to a long string of rods called sucker rods, which run through the tubing all the way to the down-hole pump at the bottom of the well.

Down-hole

At the bottom of the borehole is the "down-hole pump". This pump consists of two ball valves: a stationary valve called the "standing valve" and a valve on the piston connected to the bottom of the sucker rods that travels up and down as the pump reciprocates, known as the "traveling valve". Reservoir fluid seeps from the formation into the bottom of the borehole through perforations that have been made through the casing and cement (casing is a larger metal pipe that runs the length of the well, which has cement placed between it and the earth. The tubing, pump and sucker rods are all inside the casing). When the rods are traveling up, the traveling valve is closed and the standing valve is open (due to the drop in pressure in the pump barrel). Consequently, the pump barrel fills with the fluid from the formation as the traveling piston lifts the previous contents of the barrel upwards. When the rods begin pushing down, the traveling valve opens and the standing valve closes (due to an increase in pressure in the pump barrel). The fluid in the barrel (which was sucked in during the upstroke) flows up through the traveling valve. The piston then reaches the end of its stroke and begins its path upwards again, repeating the process. Often, gas is produced through the same perforations as the oil. This can be problematic if gas finds its way into the pump because it can result in "gas locking", where not enough pressure builds up in the pump barrel to open the valves (due to compression of the gas) and little or nothing is pumped. To avoid this, the inlet for the pump is placed below the perforations. When the gas enters the well bore through the perforations, it bubbles up the annulus (the space between the casing and the tubing) and does not have a chance to make it into the pump. Once at the surface, the gas is collected through a tube connected to the annulus.

Gallery

<gallery> Image:Nodding donkey.jpg|A nodding donkey-type pumpjack (painted to resemble a toucan) in the United States Image:Pump_jack_animation.gif|A pumpjack in southern Alberta operating on natural gas. Location: 4-31-5-4 W4M </gallery>

See also

• Gas lift
• Progressive cavity pump
• Submersible pump

References

External links

• All Pumped Up : Evolution of Oilfield Technology, The American Oil & Gas Historical Society, Vol. 3, No. 3, September 2006

de:Tiefpumpe es:Bomba de varilla nl:Jaknikker pl:Kiwon uk:???????-???????

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