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TITLE

Centrifugal Pump Performance Characteristic

INTRODUCTION

Centrifugal pump is one of the most common radial-flow turbo machines. A centrifugal

pump is a rotordynamic pump that uses a rotating impeller to increase the velocity of a

fluid. A centrifugal pump works by the conversion of the rotational kinetic energy,

typically from an electric motor or turbine, to an increased static fluid pressure. This

action is described by Bernoulli's principle. Centrifugal pumps are commonly used to

move liquids through a piping system. The fluid enters the pump impeller along or near

to the rotating axis and is accelerated by the impeller, flowing radially outward into adiffuser or volute chamber, from where it exits into the downstream piping system.

Centrifugal pumps are used for large discharge through smaller heads. Centrifugal pumps

basically consist of a stationary pump casing and an impeller mounted on a rotating shaft.

The pump casing provides a pressure boundary for the pump and contains channels to

properly direct the suction and discharge flow. The pump casing has suction and

discharge penetrations for the main flow path of the pump and normally has small drain

and vent fittings to remove gases trapped in the pump casing or to drain the pump casing

for maintenance.

The diagram shows a typical centrifugal pump that with the relative locations of the

pump suction, impeller, volute, and discharge. The centrifugal pump casing guides the

liquid from the suction connection to the center, or eye, of the impeller. The vanes of the

rotating impeller impart a radial and rotary motion to the liquid, forcing it to the outer

periphery of the pump casing where it is collected in the outer part of the pump casing

called the volute. The volute is a region that expands in cross-sectional area as it wraps

around the pump casing. The purpose of the volute is to collect the liquid discharged

from the periphery of the impeller at high velocity and gradually cause a reduction in

fluid velocity by increasing the flow area. This converts the velocity head to static

pressure. The fluid is then discharged from the centrifugal pump through the discharge

connection.

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Figure 1 : Basic elements of a Centrifugal Pumps

Centrifugal pumps can also be constructed in a manner that results in two distinct volutes,each receiving the liquid that is discharged from a 180 degrees region of the impeller at

any given time. Pumps of this type are called double volute pumps where they may also

be referred to as split volute pumps. In some applications the double volute minimizes

radial forces imparted to the shaft and bearings due to imbalances in the pressure around

the impeller. A comparison of single and double volute centrifugal pumps is shown

below.

Figure 2 : Types of centrifugal pump

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Figure 3 : Centrifugal pump

OBJECTIVE

To obtain performance characteristics for a variable speed centrifugal pump operating at

3 different impeller speeds. Performance characteristics of pump are pressure (head)

jump, power requirement, flow rate influence, and pump speed influence.

THEORY

Performance Characteristics

Consider the experimental arrangement for determining the head rise (or pressure rise)

gained by a fluid flowing through a pump, as shown schematically below:

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Figure 4 : Experimental arrangement for pump performance characteristic.

The pump head rise can be expressed using the energy equation as:

( )( )

( )

Typically the differences in elevations and velocities (between inlet and outlet) are so

small so that:

( )

The power P gained by the fluid is given by the equation:

The pump overall efficiency, η is the ratio of power actually gained by the fluid to the

shaft power supplied W shaft as given by the equation:

Where the shaft power, is the product of the Torque applied to the shaft and the angular

velocity, ω

W shaft = T shaft ω

Where

Typical performance characteristics for a centrifugal pump of a given size operating at a

constant speed are shown below.

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Figure 5 : Typical Performance Characteristic of a Centrifugal Pump.

APPARATUS

The experimental set-up consists of:

i. Water-flow bench and centrifugal pump.

ii. Instrumentation for data acquisition.

Instrument panel- Refer to the schematic diagram below.

i. Speed control to change pump speed. Pump speed can be varied over range of 0-

3000 rpm.

ii. Pump suction and delivery pressures

iii. Torque measurement.

Flow Measurement : using “V” notch wear.

i. Flow rate in the system can be measured relating the height of water seen in

the sight glass to graph T1 and reading off the flow rate in litter per minute.

See attachment.

Brake horsepower

Head

Shutoff head

Efficiency

H e a d ,

h

B r a k e

h o r s e p o w

e r , b h p

E f f i c i e n c y ,

η

Flow rate, Q0

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Speed Measurement.

i. Pump motor speed measurements are made using hand held digital

tachometer.

Pump Motor Speed Control

MOTOR DRIVE

PUMP SUCTION PUMP DELIVER

TORQUE (Nm)

TURBINE INLET

Figure 6 : Schematic of Instrument Panel

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EXPERIMENTAL PROCEDURE

Preliminaries

1. Students were adequately supervised by the lecturer and technician.

2. Pump discharge and suction valves were fully opened and the motor speed was

controlled to zero.

3. The electrical supply and motor drive were switched on. Motor control knob was

adjusted slowly to around half way position. No leaks were found in the system.

All gauges were checked and motor speed was reduced to zero.

Actual experiment

1. The suction valve was opened and the discharge valve was closed. Minimum

pump speed N 1 was selected by adjusting speed control to 50%. The speed of

pump in rpm was measured by using tachometer.

2. The discharge valve was fully opened and water allowed circulating. The

increments in flow were decided about eight points between zero and maximum

flow.

3. The discharge valve was closed (corresponding to no flow). All measurements

were read when the measured readings are steady.

4. The discharge valve was opened slightly for the first increment in volume flow.

Measurements were taken when readings are steady.

5. Step (4) was repeated. The valve was fully opened correspond to final

measurements. The entire experiment was repeated for the speed control position

of 75% and 90%. Furthermore, 10 readings were taken as opposed to 8 readings

in the first speed.