Thursday, December 8, 2011

Electrical Submersible Pumps

Electrical submersible pumps (ESPs), much like vertical turbine pumps in design, are typically used to pump liquid. Essentially, an electric motor drives the pump, and the fluid’s kinetic energy is increased. This energy is then partly converted into pressure, which lifts the fluid through the pump. ESPs are centrifugal pumps with vertical shafts, and as a result depend on basic rotating impellers to pressurize fluid.

Centrifugal Pump Basics

Centrifugal pumps feature rotating impellers, typically made from metal, which contain rotating vanes. These vanes transfer energy from the motor to the fluid they propel. As fluid enters the impeller, it accelerates as the impeller rotates. Eventually, the fluid exits the impeller’s vanes at an increased speed, and the kinetic energy is typically converted into pressure.

In an ESP, mechanical seals are used to prevent fluid from flowing into the motor—the motor is coupled to the pump itself, and the entire unit is submerged in the fluid it pumps. Without mechanical seals protecting the enclosed unit, the motor could short circuit and fail.

In cases where more than one impeller is used, the pump is said to be multistage. Multistage centrifugal pumps may feature multiple impellers located on one shaft, or impellers on separate shafts. The result of connecting impellers in a series is higher pressure; connecting impellers parallel to one another results in increased output. Regardless, the fluid will still garner its energy from the electric motor that drives the impellers.

ESP Applications

ESPs are used in many different applications. Single-stage pumps can be used for basic drainage and pumping, as in many industrial applications, and can also handle slurry pumping. Multistage pumps are more often found in water removal applications, and can be used in water and oil wells. Regardless of the application, double checking manufacturing specifications for a given ESP will help ensure its proper use.

ESPs and Oil Wells

Because ESPs can work with a variety of flow rates and depths, they are well-suited to work inside oil wells. When used accurately, an ESP pump can decrease well pressure at the bottom, enabling the withdrawal of a higher amount of oil than otherwise could be extracted under normal pressure conditions. Pump diameter size ranges from 90 millimeters (mm) to 254 mm, and pumps can be one to 8.7 meters long.

ESPs and Dewatering Gas Wells

Some gas reservoirs can produce a high amount of liquid, but because gas can damage ESPs, care must be taken when using an ESP to remove liquid from a gas well. However, ESP systems can be designed that enable the gas to flow freely up the pump’s casing, while the pump efficiently removes fluid. The gas flow depends largely on casing head pressure—there are typically four methods with which ESPs can be used to dewater gas wells, but depending on the exact well situation all ESP set-ups should be sufficiently researched before a method is employed.

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