The objective of this experiment is to investigate the operating characteristics of a centrifugal pump used to transport water. Explore characteristic curves of varying head, power and efficiency vs. the volumetric flow rate. Obtain a head-flow curve for a centrifugal pump operating at inherent speed.
In a centrifugal pump, the mechanical energy of the liquid is increased by centrifugal action. The liquid enters through a suction connection concentric with the axis of an impeller. The impeller is a high speed rotary element with radial vanes integrally cast in it. Liquid flows outward in the spaces between the vanes and leaves the impeller at a considerably greater velocity with respect to the ground than at the entrance to the impeller. The liquid leaving the outer periphery of the pump is collected in a spiral casing called a volute. It then leaves the pump through a tangential discharge connection. In the volute the velocity head of the liquid from the impeller is converted into pressure head. The power is applied to the fluid by the impeller. The impeller is directly connected through a drive shaft to an electric motor.
The use of characteristic curves to present the operating characteristics of a pump is commonly done in industry. Characteristic curves can be used to aid engineers in the selection of pumps needed for their process and determine the maximum efficiency of a pump over a range of operating conditions.
The volumetric flow rate, Qv, for this pump is calculated from pressure drop readings obtained from an orifice meter
Where Cd is the orifice discharge coefficient, r is the density of the fluid, and DPo is the pressure drop across the orifice meter.
The pump head, H, is found from a derivation of Bernoulli's Equation.
In this equation, v is the velocity of the fluid (v1=Qv/A1; v2=Qv/A2), g is the gravitational constant, DPp is the pressure drop on the pump, and z is the height of the fluid from a reference point. The pump head is calculated from data obtained from the orifice meter and measurements of pipe lengths.
The efficiency of the pump Egr, can be calculated from the following equation
The power supplied by the motor, Pgr, is controlled by the dial located on the wattmeter. The power absorbed by the fluid, P, is found using the following equation where
Values of Constants
Description of Constant
Orifice Plate Diameter d
Orifice discharge coefficient (Cd)
Change in elevation between pressure points
Radius of pipe
A diagram of the apparatus is shown in Figure 1.
The equipment is comprised of a centrifugal pump (6) driven by an electric motor (19) mounted on a base. Also attached to the base is a clear acrylic reservoir (11) and associated pipework for continuous circulation. Water is used as the fluid and a drain valve (10) at the base of the reservoir allows the water to be drained after use.
Sensors are incorporated on the unit to facilitate analysis of the pump performance when connected to the parallel port of a suitable microcomputer via an Armfield "POD" interface. Additional pressure taps are included in the pipework to permit connection to appropriate calibration instruments.
The fluid flow through the centrifugal pump is regulated by a flow control valve (16) installed in the pump discharge pipework. The head/flow produced by the pump can be varied by adjustment of this valve. A valve (9) is installed in the inlet pipework to permit the effect of suction losses to be investigated.
A spare impeller is attached to the base in order to allow visual inspection of the impeller which is installed inside the volute of the water pump.
Figure 1. FM 20 Centrifugal Pump Unit
A brief description of the sensors that monitor the performance of the pump is given below.
A differential pressure sensor, SPW1, is connected to channel 1 on the IFD4.
A differential pressure sensor, SPW3, is connected to channel 2 on the IFD4.
A rotational speed sensor, SSO1, is connected to channel 3 on the IFD4.
A temperature sensor, STS1, is connected to channel 4 on the IFD4.
An integrating wattmeter, SWA1, is connected to channel 5 on the IFD4.
Before starting the experiment, familiarize yourself with the centrifugal pump demonstration unit. Determine where the orifice meter is, where the pump is, and which direction the pump will transport the water.
Check to ensure that the water in the reservoir is within three (3) to five (5) inches from the top of the column. If the water is lower than five inches, air can become entrained in the pump when pumping at high speeds.
Supply power to the surge protector outlet strip below the computer monitor and to the IFD4 Interface Console located below the centrifugal pump unit. The IFD4 interface is a control panel that supplies power to the pump controller and to the sensors. It also serves as the interface to relay the data to the computer.
The pump controller (Integrating Wattmeter SWA1) is located to the left of the IFD4. The dial on the controller adjusts the speed of the pump.
Turn on the computer. Once Windows 95 is loaded start the FM20 Centrifugal Pump Demonstration Unit program by double clicking on the FM20 Capture icon on the Desktop. See Attachment 1 for additional information on the FM20 software.
Turn on the three power switches located on the IFD4 Interface Console. Make sure the two valves at locations 9 and 16 on Figure 1. are fully open.
To familiarize yourself with how this apparatus works, click on the Diagram button located in the upper left corner of your monitor. Use the dial on the pump controller (SWA1) to vary the power to the pump. This changes the pump rpm and flow rate. Click on the Tables button to view the data supplied by this program. Data is written to this table when you click on the Take Sample button.
When you are familiar with the apparatus, click on Labsheets on the menu bar and select the first experiment you were asked to perform. Information about the experiment and the steps needed to perform it will appear in a window. This information can be printed on the printer. Follow the instructions to complete each experiment you were asked to perform.
The best way to describe the operating characteristics of a centrifugal pump is through the use of characteristic curves:
This figure shows the interrelation of discharge pressure or head H, capacity Qv, and efficiency Egr, and power input Pgr, for a given pump at inherent speed (motor speed changes with load). The H – Qv curve shows the relation between the total head and capacity. The pressure increase created by a centrifugal pump is commonly expressed in terms of the head of the fluid following. This discharge head H is independent of the density of the fluid.
In the above figure, the head increases continuously as the capacity is decreased; this type of curve is referred to as a rising characteristic curve. A stable head – capacity characteristic curve is one in which only one capacity can be obtained at any one head. Pump selection should be made such that stable operating characteristics are available.
The Pgr – Qv curve shows the relation between power input and pump capacity. The Egr – Qv curve relates pump efficiency to capacity. For a pump having the characteristics of the above figure, maximum efficiency would occur at a volume flow rate of 0.7 dm^3/s, and the total head of about 8 [m].
The pump running at the speed given by the instructor.
Valve V1 should be fully open, and remain so during the experiment
A technical version report must be submitted in both printed copy and electronic copy on the due date (see the announcement board).
The FM20 software program running in Windows 95 on a PC will display the measurements from the measurement sensors and calculate appropriate variables. A maximum of 30 data samples can be collected in a table. The following variables are also calculated and placed into the table.
The software can display the following performance curves and copy them to a printer.
The data can be saved to a text file which can be imported into a spreadsheet. The file that is created is a comma delimited file. At this point in time, the files will have to be saved to a 3.5" floppy disk. They can also be saved to the student directory on the hard drive and then copied to a floppy disk.
NOTE: A 3.5" floppy disk will be needed for this experiment.
To start the software double click on the FM20 Capture icon on the Desktop. The software will prompt you for USER DATA. Fill out the information asked for on the form and click on OK. The software will finish loading. Information about the unit, software and instructions for 6 exercises are included in the help files. A short summary of the items on the menu bar is given below. A more detailed explanation can be found in the software help file.
A short summary of each button on the button bar is given below.
The software allows the display of two of the variables on the left vs volume flow rate with two different y-axis units. Each graph has a set of three buttons associated with it.