CENTRIFUGAL PUMP

Centrifugal pump

• A centrifugal pump is one of the simplest pieces of equipment in any process plant.
• Centrifugal pumps are the most common type of dynamic pump or kinetic pump and are used most often in applications with the moderate-to-high flow and low head.
• These pumps all rely on the centrifugal force as the fundamental principle by which they operate.

Centrifugal pump increase the mechanical energy of the liquid by centrifugal action

Its purpose is to convert the energy of a prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. 

 

The energy changes occur by virtue of two main parts of the pump, the impeller, and the volute or diffuser. 

• The impeller is the rotating part that converts driver energy into the kinetic energy.
• The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Working of centrifugal pump

The process liquid enters the suction nozzle and then into the eye (center) of a revolving device known as an impeller. 

• When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration.
• As liquid leaves, the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. 
• Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. 
• The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.

Components of Centrifugal Pumps

• A centrifugal pump has two main components: 

A rotating component comprised of an impeller and a shaft.

A stationary component comprised of a casing, casing cover, and bearings. 

Stationary Components 

1. Casing

• Casings are generally of two types: volute and circular. 
• The impellers are fitted inside the casings. 
• Volute casings build a higher head.
• Circular casings are used for low head and high capacity. 
• A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. 
• One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacturers recommended capacity. 
• The circular casing has stationary diffusion vanes surrounding the impeller periphery that converts velocity energy to pressure energy. 
• Conventionally, the diffusers are applied to multi-stage pumps.
• The casings can be designed either as solid casings or split casings. 
• Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. 
• A split casing implies two or more parts are fastened together. 
• When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller.

2. Suction and Discharge Nozzle

• The suction and discharge nozzles are part of the casings itself. 

3. Seal Chamber and Stuffing Box

• Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. 
• When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. 
• When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. 
• Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. 
• When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. 
• When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. 
• The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control. 

3. Bearing housing

• The bearing housing encloses the bearings mounted on the shaft. 
• The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. 
• The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.

Rotating Components

1. Impeller

The impeller is the main rotating part that provides the centrifugal acceleration to the fluid. 

• The number of impellers determines the number of stages of the pump. 
• A single-stage pump has one impeller only and is best for low head service. 
• A two-stage pump has two impellers in series for medium head service. 
• A multi-stage pump has three or more impellers in series for high head service. 

2. Shaft

• The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. 
• It must do this job with a deflection less than the minimum clearance between the rotating and stationary parts.

• PERFORMANCE PARAMETER OF CENTRIFUGAL PUMP

1. CAPACITY
2. HEAD
3. BHP(Brake horsepower)
4. BEP (Best efficiency point)
5. SPECIFIC SPEED

• NET POSITIVE SUCTION HEAD