Fluid Mechanics Applications/A26: Submersible Pumps

You might have listened about submersible pumps especially in our homes if water gets finished in reservoir tank, we are asked to switch on the submersible pump. It is also used in industries especially refineries to pump up raw material either liquid or gas. You may also have seen gases or fluid getting transferred over long distance especially in case of LPG gas. Submersible pump is basically a centrifugal pump. In this chapter we will come across that how this transfer of fluid or gas takes place and what are the parameters involved in this process.

A pump is a machine or device that is used for raising, compressing or transferring fluid that may be liquid or gas. A centrifugal pump is a device which converts the mechanical energy into the hydraulic energy by centrifugal force on the liquid and is used to move liquids through a piping system.

Components
A Centrifugal pump has two main components:
 * 1) Stationary Components: CASING, CASING COVER & BEARINGS
 * 2) Rotating Components: IMPELLER, SHAFT

Parts
The pump’s casing protects the whole assembly and protects it from being damaged as well as from the forces that fluid discharge from the pump and convert its velocity into pressure. The casings design does not influence Total Dynamic Head (TDH) but essential to reduce friction losses. It supports the shaft bearings and takes the centrifugal forces of the rotating impeller and axial loads that is caused by pressure thrust imbalance.
 * Casing :

The performance of the pump depends on the impeller dia and design. The pump’s TDH is basically defined by the impeller’s inner and outer dia and the pump’s capacity is defined by the width of the impeller vanes.
 * Impeller :

The shaft connects impeller and drive unit. Drive unit in most cases is an electric motor but can also be a gas turbine. It is generally activated by the radial force caused by unbalanced pressure forces in the casing and an axial force generated due to the pressure difference between front and backside of the impeller.
 * Shaft :

The bearings keep the shaft at the place to ensure radial and axial clearance.
 * Bearings :

There are several seals fitted into the casing in order to protect the bearings against fluid and prevent leakage.
 * Sealing :

Working Principle
A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. The fluid enters the pump near the rotating axis and streams into the rotating impeller. The impeller consists of a rotating disc with several attached vanes. Generally vanes are sloped backwards, away from the direction of rotation. When the fluid enters the impeller at a particular velocity then due to the suction system, it is captured by rotating impeller vanes. The fluid is accelerated by pulse transmission following the curvature of the impeller vanes from the impeller centre (eye) outwards. It reaches its maximum velocity at impeller’s outer dia and leaves the impeller into a diffuser or volute chamber.

So the centrifugal force assists accelerating the fluid particles because the radius at which the particles enters is smaller than the radius at which the individual particles leaves the impeller. Now the fluid’s energy is converted into static pressure, assisted by the shape of the diffuser or volute chamber. The process of energy conversation in fluids mechanics follows the Bernoulli principle (Equation 1) which states that the sum of all forms of energy along a streamline is the same on two points of the path. The total head energy in a pump system is the sum of potential head energy, static pressure head energy and velocity head energy.

As a velocity of the fluid in centrifugal pump increases, then it is essentially a velocity machine. After the fluid left the impeller, it then flows at a higher velocity from a small area into a region of increasing area. So the velocity is decreasing and so the pressure increases as explained by Bernoulli’s principle. The result is an increased pressure at the discharge side of the pump. As fluid is displaced at the discharge side of the pump thats why more fluid is sucked in in order to replace it at the suction side, causes the flow.

Classification of Pumps

 * On the basis of curved vane Impeller
 * 1) Open Impeller - It consists of blades attached to its eye.
 * 2) Semi-Open Impeller - They are constructed with a disc attached to one side of the vanes.
 * 3) Enclosed Impeller - They have discs attached to both sides of the vanes.


 * On the basis of number of points in pump where fluid can enter
 * 1) Single Suction Impeller - They allow the fluid to enter its centre from only one side.
 * 2) Double Suction Impeller - They allow the fluid to enter from both sides simultaneously.


 * On the basis of number of stages fluid passes
 * 1) Single Stage Pump - It is equipped with only a single impeller.
 * 2) Double Stage Pump - It consists of two or more impellers fitted to the same shaft in a single casing.


 * On the basis of Functioning
 * 1) Regenerative Turbine Pumps - The rim of the pump’s impeller has vanes on either sides that rotate in the casing of the pump.
 * 2) End Suction Pumps - Single stage pumps having the casing that takes in suction on one end and gives out discharge from the top.
 * 3) Multistage Stackable Pumps - Multiple chambers are linked together, allowing fluid to enter at first chamber and leave where the pressure is increased.
 * 4) Split Case Pumps - Pump casing is split into two chambers unlike that of an inline or end suction pump.

Pump Parameter and Selection
Head in general is used to define the energy supplied to a liquid by a pump and is expressed in units of length. In absence of any velocity it is equal to the height of a static column of fluid that is supported by a pressure in the point of datum. Total dynamic head (TDH) is the difference between total dynamic discharge head and total dynamic suction head (Equation 2.1).
 * Total Dynamic Head (TDH)

Total dynamic discharge (suction) head is practically the pressure read from a gauge at the discharge (suction) flange converted to length units and corrected to the pump centre line plus the velocity head at the point of the gauge (Equation 2.2). These two values represent the total amount of energy of the fluid at the discharge and suction flange of the pump. Mathematically it is the sum of static discharge (suction) head and velocity at the discharge (suction) flange minus total friction head in the discharge (suction) line. The difference of these values gives you the TDH which represents the energy added to the fluid. TDH does not depend on the delivered fluids density. A higher density only increases the pressure and therefore the required power at a constant flow rate.

(Volumetric) Flow rate is the volume of fluid passing through the pump per unit of time. It is calculated as area times fluid velocity (Equation 3). It depends on the impeller's geometry and RPM.
 * Flow rate (Q)

The work performed by a pump is a function of TDH, flow rate and the specific gravity of the fluid. Pump input (P) or brake horse power (bhp) is the actual power delivered to the pump shaft. Pump output $$(P_{hydr})$$ or hydraulic horse power (whp) is the energy delivered to the fluid per time unit (Equation 4). Due to mechanical and hydraulic losses in the pump, $$(P_{hydr})$$ is always smaller than P. Therefore efficiency is defined as $$(P_{hydr})$$ divided by P (Equation 5).
 * Power and Efficiency (P, η)

The impeller geometry is optimized to provide highest flow rate at a certain speed at a given diameter at its point of best efficiency (BEP). If operating a pump off its (BEP), losses due to increasing turbulences and recirculation will increase and reduce efficiency. These effects are caused by a mismatch of the pump’s design flow rate and the actual flow rate. The difference between inlet vane angle and approaching flow angle is increasing as moving away from the BEP as well as losses between impeller vane exit and the diffuser. Result of this is an increased flow between the impellers shrouds and the casing.

Problems in Pump

 * Cavitation - Cavitation occurs when the static pressure in a fluid lowers than the fluids vapour pressure, mostly caused by high velocities. Due to Bernoulli’s law, static pressure decreases when velocity is increases. If this happens, the fluid locally starts boiling and forms gas bubbles which need more space than the fluid would take. In a centrifugal pumps’s impeller, the bubbles are moving to an area of decreasing pressure. If the pressure now exceeds the vapour pressure, the gas condensates at the bubble’s inner surface and so collapse rapidly. This implosion of gas bubbles causes high, temporarily pressure fluctuations of up to a few 1000bar. As the fluid flows from higher to lower pressure, this flow causes a jet of the surrounding fluid, which may hit the surface. These high energy micro‐jets cause high compressive stress weakening the material. Finally, crater‐shaped deformations and holes known as cavitation pitting occur.Other reasons for cavitation can be a rise of fluid temperature, a low pressure at the suction side or an increase of delivery height. Cavitations in centrifugal pumps mainly occur at the impeller leading edges but also at the impeller vane, wear rings and thrust balance holes. To avoid cavitation, it is important to deliver sufficient NPSH and to keep fluid temperature low. High fluid temperatures can occur if the pump is on to keep the pressure up but no fluid is taken out.
 * Corrosion - Corrosion is breaking down of essential properties in a material due to chemical or electrochemical reactions with its surroundings. There are several types of corrosion and many factors it depends on, like fluid temperature, contained elements and pH‐value. Most common and dangerous corrosion in pumps is the so called uniform corrosion. This is the overall attack of a corrosive liquid on a metal. The chemical reactions between fluid and metal surface lead to uniform metal loss on the moistened surface, known as corrosive wear. To minimize corrosive wear it is important to select a resistant pump material.