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ESP motor

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The electric submersible motor is simple in construction, rugged and reliable. In this article, the ESP motor compounds are detailed as well as their main functions.

Introduction to ESP motor:

  • ESP motor is installed below the motor seal and above the downhole sensor. In cases where a downhole sensor is not installed, the motor is installed at the very bottom of ESP string, generally attached to a motor guide.
  • ESP motor is an induction motor, two poles, three phases, squirrel cage type stator winding filled with specific motor oil, high dielectric strength (> 28 KV). The motor is rated for a specific horsepower, voltage, & current. Its role is to drive the downhole pump and seal section.
  • The ESP motor rotates at approximately 3500 RPM at 60 Hertz. The difference between actual and the synchronous speed (3,600 RPM) is called “motor slippage” and it is due to losses inside the motor. The actual RPM is usually noted on the motor nameplate (example:  3500 RPM / 60 Hz – 2917 RPM / 50 Hz).
  •  The ESP motor is constructed of rotors and bearings stacked on the shaft and loaded in a wound stator, the motor compounds will be detailed in the next section.
  • The motor contains synthetic dielectric mineral oil for lubrication, insulation, and for the homogeneous distribution of the heat generated inside the motor (cooling). Heat is then drawn off by the produced fluid past the housing OD on the way to the intake.

NB: ESP motor is close to the same design type as motors used on beam pumping units. Of course, it must be small in diameter in order to fit inside oil well casing sizes.

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Motor Seal

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The Motor seal is installed below the intake and above the electric motor. It is also named: Equalizer, balance chamber, or Protector. Seal section types, functions, components, and applications are detailed in this article.

In addition to the main function of transferring the motor torque to the pump shaft, the seal section performs four primary functions (Equalization, Expansion, Isolation, & “Aabsorbsion”):

  • Equalizes the pressure in the wellbore with the pressure inside the motor,
  • Provides area for motor oil expansion volume (induced by temperature changes in the motor),
  • Isolates the well fluid from the clean motor oil,
  • Absorbs the pump shaft thrust load (it houses the thrust bearing that carries the axial thrust developed by the pump, it can either be upthrust or downthrust, depending on the pumping conditions – obviously, for fixed impeller type only).

                        

PS: The motor, pump and seal are often submerged below several thousand feet of fluid. The seal section allows the pressure in the motor and the annulus to equalize, so that there is very little pressure across the shaft seals or the pothead connection.

PS: When selecting the protector, we need to be certain that the protector shaft is capable of delivering the full torque required without exceeding its yield strength which could result in a broken shaft.

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ESP: Gas handling device

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Gas  handling  devices  may  be  a  better  alternative  for  wells  prone  to  high  free  gas, slugging, foams and emulsions. These are essentially centrifugal pumps with large stages, mixed  (or  axial)  impellers,  large  vane  openings,  steep  vane  exit  angles  and  sometimes include multiple  vanes. 

Instead of separating, their purpose is to break large gas bubble into smaller ones thereby reducing the risk of gas locking and making it easier for gas to be re-absorbed into solution, and to homogenize the gas with liquid phases, prior to entering the pump intake.

An additional benefit of gas handlers is, because more gas is retained in the flow stream, this gas is then available to help lift fluids in the tubing above the ESP discharge head thereby reducing hydraulic horsepower requirements.

Applications: A Gas Handler is generally considered if the Free Gas Percentage at the intake of the pump is from 30% to 60% by volume.

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ESP: Pump Intake

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As the name suggests “ Pump Intake ” is where the well fluid enters the Submersible Pumping System. Care should be taken when designing a submersible pump intake because it is such a vital point in the system that when not designed properly may create all kinds of problems.

There are three types of intake Sections:

  • Standard Intakes or BOI (bolt on intake),
  • Integral (manufactured as part of the pump),
  • Gas Separators (static and rotary gas separators).

Standard and Integral Intakes:

Standard intakes (BOI and integral) do not separate gas.  Some gas separation might occur, but it will only be natural separation due to some of the gas not turning and going into the intake when the rest of the fluid does. Therefore, the standard intake is for wells that produce with a very low free gas to liquid ratio. The amount of free gas by volume at pump intake conditions should be no more than 10% to 15% by volume (depends if it is a radial or mixed flow stage)
                           

Usually, the pump intake is a separate component that bolts onto the bottom of the pump section. Occasionally, the pump is built in either a lower tandem or single configuration. In these cases, the pump intake as an integral part of the pump assembly.

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Pump impeller types

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The pump stages are of a fully enclosed curved vane design, whose maximum efficiency is a function of impeller types and designs. There are two basic types of stages used in oil well submersible pumps: stages with Mixed flow impeller and stages with Radial flow impeller.

 

The difference between these two types of designs is described by the pump impeller vane angles and the size and shape of the internal flow passages, as depicted in the graphs below.

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