MECHANICAL THEORY

Centrifugal Pump & Reciprocating Pump-Components, Working, Priming in Centrifugal Pump,Adv

Centrifugal Pump-Components, Working, Priming in Centrifugal Pump: In order to know about Centrifugal pump, we need to know about the centrifugal force.

Centrifugal force: If a force is acting away from the center of the circle called a Centrifugal force. Similarly, as the lifting of the liquid is due to the centrifugal action, these pumps are called centrifugal pumps.

centrifugal pump
centrifugal pump

Components of the Centrifugal pump:

The components of the Centrifugal Pump are as follows.

Impeller:

It is a rotor(runner) with backward curved blades or vanes and is made to rotate at high speeds inside a spiral casing or Volute casing. It is a rotor used to increase the kinetic energy of the flow.

The impeller is coupled with Primemover such as internal combustion engine or electric motor which makes the impeller to rotate inside the Volute casing.

Impellers are divided into 3 types,

  1. Open Impeller
  2. Semi-enclosed Impeller
  3. Enclosed Impeller

Casing:

The casing is airtight as well as watertight. It’s cross-sectional area gradually increases towards the outlet of the pump.

Commonly three types of casing are used in a centrifugal pump,

  1. Volute Casing
  2. Vortex Casing
  3. Casing with Guide Blade

Suction Pipe and Delivery Pipe:

Water enters into the impeller from the suction pipe and delivers out from the delivery pipe.

Eye of the Impeller:

The upper end of the Section pipe is connected to the inlet of the pump or center of the impeller called as the eye of the Impeller.

The lower end of the suction pipe dips into a liquid in a suction tank or a sump from which the liquid is to be pumped or lifted up.

Strainer or Foot Valve:

The liquid first enters the strainer or foot valve provided in order to keep the debris(such as dust, waste particles, etc.) away from the pump and opens only in an upward direction.

The delivery pipe is connected to the outlet of the pump to deliver liquid to the required height with the help of a delivery valve.

Driveshaft:

The driveshaft is fixed at the center of the pump such that it rotates in the same direction as the runner. Finally, from the driveshaft, electricity is generated.

Working of Centrifugal Pump:

centrifugal pump-impeller
centrifugal pump-impeller  Source

The liquid enters into the impeller from suction pipe axially, at the eye of a pump and flows radially along with the blades, into the casing and then flow towards the discharge pipe.

Water/liquid enters the impeller with a negligible tangential component of velocity.

During the outward flow of water, the cross-sectional area of the casing increases such that the velocity should decrease and there should be an increase in the pressure head.

This can be easily understood by the continuity equation and Bernoulli’s equation.

As per the continuity equation,

 A*V =Constant

Where 

A-Cross-sectional area

V-Velocity.

As the cross-sectional area of the casing increases, there should be a decrease in the velocity of the liquid.

As the velocity of the fluid decreases, there should be an increase in Pressure head and this statement is supported by the Bernoulli’s equation.

As per Bernoulli’s equation,

Potential Energy(P.E.) + Kinetic Energy(K.E.)+Pressure Energy = Constant.

The Potential energy taken is to be zero at the eye of the impeller. 

As Kinetic energy (K.E) =(1/2)mv2

As the cross-sectional area of the casing increases, there should be a decrease in the velocity of the liquid. The decrease in Velocity increases the Pressure head in the casing. In other words, the K.E. of the impeller is transformed into Pressure energy of water.

Priming in Centrifugal pump:

Priming is the operation in which the suction pipe, casing of the pump and the portion of the delivery pipe up to the delivery valve are completely filled with the liquid which is to be pumped out so that all the air(gas or vapor) from this portion of the pump is driven out and no air pocket is left.

The necessity of priming is due to the fact that pressure generated in the impeller is directly proportional to the density of the fluid. If the impeller runs in the presence of air, negligible pressure is generated because of the low density of air and hence no water will be lifted by the pump.

Hence, it is necessary to remove air from Section pipe and casing. This is called as Priming.

During priming, it must be noted that the delivery valve is kept closed.

Even after filling the casing with water and ensuring the completion of priming, the delivery valve is not immediately opened.

Application of Centrifugal Pumps:

  1. Industrial & Fire Protection Industry – boiler feed applications, Heating and ventilation, air conditioning, fire protection sprinkler systems, pressure boosting, etc.
  2. Oil & Energy – pumping crude oil, power generation plants
  3. Pharmaceutical, Food Industries, Chemical: sugar refining, food, and beverage production
  4. Agriculture, Waste Management & Manufacturing – Wastewater processing plants, drainage, gas processing, irrigation, municipal industry, and flood protection.

This is the detailed explanation of Centrifugal pump. Now, Let’s discuss Reciprocating Pump.


Reciprocating Pump:

The following are the main parts of the reciprocating pump.

  1. Cylinder
  2. Piston
  3. Water Sump(well)
  4. Suction Pipe
  5. Suction valve
  6. Delivery Pipe
  7. Delivery valve

Working of Reciprocating Pump:

The reciprocating pump essentially consists of a piston which moves to and fro in a cylinder. The cylinder is connected to the suction pipe and delivery pipe.

The suction pipe is connected between the cylinder and sump and the delivery pipe is connected between the cylinder and the required height to be delivered.

These pipes are provided with non-return valve or one-way valve called as suction and delivery valves respectively and that means, the liquid is admitted only in one direction.

Thus suction valve allows the liquid only to enter into the cylinder and the delivery valve permits only its discharge from the cylinder.

Reciprocating Pump
Reciprocating Pump

The piston is connected to the crank by means of a connecting rod. As the crank is rotated at a uniform speed by a driving engine or motor, the Piston moves to and fro (backward and forward) in a cylinder.

Before starting the operation, the piston is at the extreme left position, when the crank rotates from θ=0° to 180°, the Piston moves to its extreme right position.

Crank is rotated at a uniform speed by the motor to which the pump is coupled. The connecting rod converts the rotary motion of the crank into reciprocating motion of the Piston.

During the outward stroke of the Piston, a partial vacuum pressure below atmospheric is created inside the cylinder, which makes the liquid in the well or sump in which atmospheric pressure is acting to force the liquid through the suction pipe into the cylinder through a suction valve.

The outward stroke of the piston is called a suction stroke.

At the end of the stroke, the piston is at the extreme right position. The crank is at 180° and the cycle is full of liquid, suction valve is closed and the delivery valve is just at the point of opening.

When the crank rotates from 180° to 360°, the Piston moves from right to left.

The inward movement of the Piston causes the pressure of a liquid in a cylinder to rise above atmospheric, due to which suction valve closes and a delivery valve opens. The liquid is then forced up the delivery pipe and raised to the required height. This is called a delivery stroke.

At the end of this stroke, the piston is at the extreme left position; the crank is at 360° or 0° so that it has completed one full revolution and both suction and delivery valves are closed.

The same cycle is repeated as the crank rotates.

Type of Reciprocating Pump:

 According to the source of work and mechanism, the following are the types of reciprocating pumps

  1. According to the source of work
    1. Simple hand-operated
    2. Power-operated deep well
  2. According to the Air vessels
    1. Pump with air vessels
    2. Pump without air vessels
  3. According to the Mechanism
    1. Single-acting reciprocating pump
    2. Double-acting reciprocating pump
    3. Triple-acting reciprocating pump

This is the detailed explanation of the Reciprocating pump. If you like this article then share it to all and If you have any doubts, feel free to ask from the comments section.

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