A nodding donkey, or pump jack, is used to extract oil from wells that lack internal pressure. The device has a horse or donkey head that moves up and down, activating a pump to bring oil to the surface. Pump jacks are used for extraction wells that produce small amounts of oil. The design includes a beam with a counterweight and a pitman’s arm, powered by a central power or electric motor.
A nodding donkey is a type of pump commonly used to extract oil if the oil is not brought to the surface naturally by internal pressure. The device has a component resembling a donkey or horse’s head, which bobs up and down during operation. This vertical motion moves a long suction rod through the borehole, which activates a pump at the lower end. The oil, or an emulsion of water and oil, is then pushed to the surface where it can be collected. Nodding donkey pumps are commonly known as pump jacks, although other terms such as thirsty bird, horse head pump, and grasshopper pump are also used.
Many oil fields do not require pumping to extract the hydrocarbons. Because oil deposits tend to be pressurized, it is often enough to drill a pipe into a supply to suck the oil to the surface. This isn’t always the case, and even a well that starts out under pressure can lose that pressure as the deposit depletes. If the pressure at the bottom end of the tube isn’t great enough to push the oil up, you usually need some type of pump. Pump jacks were first designed around 1925 to solve this problem, and similar designs are still in use.
Pump jacks are generally associated with wells that do not produce a large amount of oil, which are often referred to as extraction wells. Some of these wells simply lack large deposits of oil, while others have depleted over time. Many of them produce 10 barrels or less of oil each day. Five to 40 liters (1.3 to 10.5 gallons) of liquid can be pumped for each nodding donkey ride, depending on the configuration.
The basic design of a nodding donkey consists of a beam mounted on scaffolding so it can lean forward and backward. One end of the beam has a component resembling a donkey’s head and the other typically has a pitman’s arm connected to a counterweight. The end with the counterweight is also connected to a component capable of powering the device.
Early versions of the nodding donkey were powered by rods that connected to a device called a central power. This power source often drove many pump jacks simultaneously, although modern versions often use individual electric motors. In both cases the power source rotates the counterweight, which pushes the pitman’s arm up and down. This in turn causes the nodding donkey to perform the action after which it is named. A pump at the lower end of the well is then driven by a suction rod connected to the pump jack.
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