Jet Pumps (Ejecto Pumps)
One of the major drawbacks of the centrifugal pumps and reciprocating pumps is the limitation of suction lift. These pumps can lift water from a maximum depth of only 7.5 m. To overcome this drawback deep well pumps such as turbine pumps, submersible pumps and jet pumps are developed. In these pumps, the rotating part is immersed in water and so there is no problem of suction lift.
But in case of turbine pumps, the power for rotating part is transmitted through a long shaft, and in submersible pumps both elevator motor and pump are submerged deep into water and the electrical energy is carried by cables to the motor immersed in water. Therefore installation, servicing and maintenance of these pumps are time consuming.
To overcome the above difficulties, the jet pumps are developed in these types of self priming deep well pumps. There is no transmission of either mechanical energy or electrical energy from the top of the well to the standing water level. The water ejector, also known as a jet pump, is used for a longer period and is now widely found in domestic wells in conjunction with a centrifugal pump. It can lift up to 40 metres and has a capacity of 20-40 litres per minute.
Operation Principle of Jet Pumps
Jet pumps or Ejecto pumps is working based on the principle of conversion of hydraulic energy from one form to another form. Jet pumps has a ejector assembly which is immersed in water. It consists of a nozzle having converging portion and venturi having a divergent portion.
When high pressure water is supplied by radial flow pump or from other sources to the nozzle in the ejector assembly. When high pressure water is passed through the nozzle pressure head is converted into velocity head. This creates a drop in pressure head i.e. below atmosphere pressure head at the throat of venturi. As a result, foot valve situated at the bottom, open and water gets sucked through the side passage with high velocity and then passes through the venturi (Diverging passage). When high velocity head is again converted into pressure head, enabling the water to rise up.
The principle of Jet pumps can be summarized up as:
- To create partial vacuum and lifting from the well
- To impart sufficient pressure energy to water so that it is raised from depth to the desired level
Working Operation of Jet Pumps
The jet pump is a centrifugal pump with a jet ejector and one or two impellers and diffusers. A jet ejector is made of a compatible venturi and nozzle. At high pressure, the nozzle receives water. Water speed (velocity) is significantly improved, but the pressure falls as water flows through the jet. The considerably higher water speed plus low pressure around the tip of the nozzle allows suction to form around the jet nozzle. Water is drawn and transported in the water stream around a jet nozzle.
The discharge of a high-velocity jet into a suction chamber creates a vacuum, which allows a solvent to be lifted into the chamber and mixed with the fluid from the jet. A part of the discharge flow from the motor-driven centrifugal pump at the top of the well is recirculated back down into the well to the jet pump converging nozzle. This is the driving force.
In the converging nozzle, the ejector transforms the static energy in the incoming fluid to kinetic energy, and then it entrails the well water from the inlet pipe. As a consequence, an intermediate-velocity mixed flow flows into a diverging nozzle, where a large portion of the kinetic energy is transferred to static pressure. If there is a large head loss in the discharge line between the ejector and the centrifugal pump, it must be taken into account when choosing an ejector.
Application of Jet Pumps
Jet pumps are often used in mines and for pumping oil. These are also used to feed steam boilers. In this case, steam from the boilers, instead of water passes through the nozzle of jet pumps.
Advantages of Jet Pumps
Jet pumps have a number of practical benefits over other types of lift in many field applications, including flexibility. A jet pump will deliver wells with depths varying from 300 m to 5,500 m and production rates ranging from less than 50 bbl/d to more than 20,000 bbl/d by changing the size of the nozzle and throat.
Since there are no moving parts to cause mechanical wear, jet pumps can run for many years with no chance of failure and little maintenance. They’re much more durable and resistant to corrosive and abrasive well fluids than most downhole lift systems. Jet pumps are capable of handling large amounts of free gas in the output port.Disadvantages of Jet Pumps
Disadvantages of Jet Pumps
The possibility of cavitation (the forming of vapour cavities) at the entry of the throat segment caused by the accelerated movement of the production fluids as they approach the pump body is one of the design flaws of jet pumps. Fluid pressure will drop to vapour pressure as the fluid’s velocity increases rapidly. At this low pressure, vapour cavities form, restricting flow through the throat. If the pressure in the pump increases, these vapour cavities can collapse, causing cavitation damage and corrosion of internal pump components. In most oil wells, cavitation-induced erosion rates are modest, according to field experience.
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