What Is a Sucker Rod Pump?

A sucker rod pump is a type of oil prospecting pump commonly seen in fields in Texas in the US and other areas where potential oil deposits exist deep underground. It is capable of pumping oil from a depth of 10,000 feet (3.05 kilometers) or greater, and somewhat resembles the movement of a large pendulum turned on its side. There are many other names for the sucker rod pump in the oil prospecting industry, including pumpjack, nodding donkey, and horsehead pump, but they are all designed based on the same principles.

Each component of a sucker rod pump has a unique name as well, which may seem difficult to understand unless someone is familiar with the machinery. The individual parts all serve as jointed links in a mechanical chain reaction that gradually transfers circular motion to a general up-and-down pumping motion. The whole mechanism operates similarly to a crankshaft assembly in an automobile or train locomotive, or the cushioning armature on doors that automatically close at a slow rate to let someone pass through.

First, an above-ground motor spins a nearby flywheel via a belt or chain, which is essentially a larger wheel that rotates more slowly but has greater torque or force per rotation. This flywheel has a nearly horizontal arm attached to it known as a crank arm, which moves up and down at roughly a 60° angle to the horizon. The crank arm is attached to a Pitman arm, which is the longest beam of the sucker rod pump machine. The Pitman arm is nearly horizontal, but angles down slightly where the end attaches to a weight known as the horsehead, an anvil-shaped counterweight that serves as a type of pendulum end. All of these components move together to pull and push on a Hanger cable that is suspended from the horsehead into a vertical shaft in the ground.

The Hangar cable is attached to a sequential series of parts known as the Polishing rod, Stuffing box, and Rod string, which together act as a sort of piston in the vertical chamber. In this vertical channel, they all move what's known as a Traveling Valve and Plunger to maintain a constant up-and-down pressure change in the shaft. A Standing valve farther down the shaft is fixed in place and channels all of the force of the piston action so that as the mechanism is forced down, oil pressure is increased in the ground, and, as it is withdrawn, this oil is sucked up from the underground reservoir and piped off.

The sucker rod pump design typically removes about 5.3 gallons (20 liters) of fluid for every cycle of the pump, which equates to one barrel of oil or 42 gallons (160 liters) for roughly every eight pumping cycles. A cycle on a pumpjack can take less than a minute, so the equipment can pump around 7 to 20 barrels of oil an hour depending on how fast it is set to run. This is very slow by standards such as those of offshore drilling platforms, but pumpjacks are relatively inexpensive and easy to set up and maintain in multiple locations.

This type of pumping unit is also known as a "thirsty bird," as it resembles a bird who is continually bobbing its head up and down, or a "grasshopper pump" due to the fact that its overall structure resembles that of a grasshopper, and its unique mechanical motion has many industrial applications. The sucker rod pump for oil prospecting is initially not very efficient at pumping oil out of the ground, as there tends to be a lot of trapped gas in new deposits. Optimally, the sucker rod pump has a volumetric pumping efficiency of 80% with oil, versus other compounds such as gas and water.

The most frequent use for sucker rod pump designs in oil prospecting is on land where there is known to likely be a rich deposit of oil. They do not pump a large volume of oil per stroke, but are built on a set of reliable mechanical principles that have been used in other applications as far back as the 18^th century. Small hand-operated versions of the sucker rod pump design still exist in many US park systems as well, where they can be used to mechanically pump water from an underground reservoir or aquifer.