MECHANICAL SEALS • PUMP SEALS • GLAND PACKINGS

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Mechanical Seals in Submersible Pumps

Mechanical seals have been part of great improvements in the efficiency of submersible pumps. A submersible pump (also referred to as an electric submersible pump or sub pump) is a kind of device that has a motor that is hermetically sealed and close-coupled onto the pump body.   The entire pump assembly gets submerged into whatever fluid needs to be pumped.

This kind of pump has the advantage of being able to prevent pump cavitation.  This is a problem that frequently occurs when there is a big difference in elevation between the fluid surface and pump.  A submersible pump, on the other hand, is able to plush fluids to the surface.  A jet pump, on the other hand, pulls fluids.  A submersible pump is much more efficient than a jet pump.

There is a submersible pump style that is used for industrial purposes.  The electrical cable and outlet pipe are not connected.

How Submersible Pumps Work

Electrical submersible pump installations have multistage centrifugal submersible pumps containing mechanical seals that operate from a vertical position.  Over the years, their operational and constructional feature have undergone continuous evolutions, including the introduction of mechanical seals and cartridge mechanical seals.  However, the basic operation principles haven’t changed.  After produced liquids have been subjected to extreme centrifugal forces coming from the impeller’s high rotational speed, they start to lose kinetic energy inside the diffuser.  This is where kinetic energy is converted to pressure energy.  This is the major operational mechanism for mixed flow and radial pumps.

A mechanical coupling underneath the pump connects the protector or gas separator to the pump shaft.  Well fluids enter via an intake screen into the pump.  The pump stages lift the fluids.  The bushings, or radial bearings, are distributed along the shaft length and provide radial support for the pump shaft as it turns in high rotational speeds.  A portion of the axial forces are taken up by the optional thrust bearing.  However, the protector’s thrust bearing absorbs most of the force.

Applications

There are many different applications that use submersible pumps.  The uses for single stage pumps include slurry pumping, general industrial pumping, sewage pumping and drainage.  Pond filters also use them.  Typically, a multiple stage submersible pump is lowered into a bore hole.  They are used in oil wells, water wells and for water abstraction.

Other applications that submersible pumps are used for include water supply systems, irrigation systems, mine dewatering, artificial lifts, offshore drilling rigs, deep well and water well drilling, pumping, borehole drilling, fire fighting, groundwater pumping, seawater handling and sewage treatment plants.

When using certain kinds of liquid, special attention needs to be paid to the kind of ESP that is needed.  Pumps used with either combustible liquids or water that potentially contains combustible liquid contamination, needs to be designed so that it will not ignite the vapors or liquids.

Oil Well Use

Oil production uses submersible pumps to provide artificial lift that is relatively efficient.  They have the ability to operate over a wide range of depths and flow rates.  When the pressure at the well’s bottom is decreased (through either increasing drawdown or decreasing the bottom-hole flowing pressure), a significantly greater amount of oil from the well is produced than from natural production.  Typically, the pumps are powered by electricity and called Electrical Submersible Pumps (or ESP).

An ESP system has both sub-surface components (located inside the well hole) and surface components (inside the production facility such as an oil platform).  Surface components include transformers, surface cables and motor controller (frequently it is a variable speed controller).  Typically, sub-surface components include cables, seal, motor and pump.  Sometimes, there is a gas separator that is installed.

Cartridge Mechanical Seals

In terms of the pump, the unit is multi-stage.  The specific number of stages is determined by what the operating requirements are.  Each stage includes a diffuser and driven impeller.  The diffuser direct flow for the pump’s next stage.  Pumps range in diameter from 254mm to 90mm, with lengths ranging from 8.7 meter to 1 meter.  The motor for driving the pump typically is an induction three phase, squirrel cage motor that has nameplate power rate ranging from 560 kW down to 7.5 kW.

New kinds of ESP may come with a oil/water separator.  This allows for the re-injection of water into the reservoir with no need for lifting to up to the surface.  Throughout the world, are are a minimum of 15 oilfield ESPs brands used.  ESPs, until recently have been very expensive to install because an electric cable downhole was required.  Jointed tubing was needed for the cable to be wrapped around and the cable had to be connected at every joint.  Now there is coiled tubing umbilicals available.  They allow both electric cable and piping to be deployed using one unit of conventional coiled tubing.

There are several different components in an ESP system.  They turn the centrifugal pump staged series, which increases the well fluid pressure, pushing it up to the surface.  Energy for turning the pump originates with an alternating-current, high voltage (3-5 kV) source that drives a special motor capable of working at high temperatures (maximum 300 °F or 149 °C) as well as high pressures to 5,000 psi that work with deep wells to 3.7 km (12,000 feet) with high energy requirements (to 750 kW or 1000 horsepower).  There are dramatically lower efficiencies for ESPs.

At pump intake, there are significant gas fractions, more than 10%.  Due to tight clearances and high rotational speeds to 67 Hz or 4000 rpm, they aren’t too tolerant of certain solids like sand.

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