experiment (9): centrifugal pump -...

Hydraulics Lab (ECIV 3122) Islamic University – Gaza (IUG)

Instructors : Dr. Khalil M. ALASTAL Eng. Mohammed Y. Mousa 1

Experiment (9): Centrifugal pump

Introduction:

Pumps fall into two main categories: positive displacement pumps and rotodynamic pumps. In a

positive displacement pump, a fixed volume of fluid is forced from one chamber into another. The

centrifugal pump is, by contrast, a rotodynamic machine. Rotodynamic (or simply dynamic)

pumps impart momentum to a fluid, which then causes the fluid to move into the delivery

chamber or outlet. Turbines and centrifugal pumps all fall into this category.

Centrifugal pumps are widely used in industrial and domestic situations. Due to the characteristics

of this type of pump, the most suitable applications are those where the process liquid is free of

debris, where a relatively small head change is required, and where a single operating capacity or a

narrow range of capacities is required. The general design is usually simple with few mechanical

parts to fail, however, and it is possible to operate a centrifugal pump outside ideal parameters

while maintaining good reliability.

The centrifugal pump converts energy supplied from a motor or turbine, first into kinetic energy

and then into potential energy.

The motor driving the impeller imparts angular velocity to the impeller. The impeller vanes then

transfer this kinetic energy to the fluid passing into the center of the impeller by spinning the fluid,

which travels outwards along the vanes to the impeller casing at increasing flow rate.

This kinetic energy is then converted into potential energy (in the form of an increase in head) by

the impeller casing (a volute or a circular casing fitted with diffuser vanes) which provides a

resistance to the flow created by the impeller, and hence decelerates the fluid. The fluid decelerates

again in the outlet pipe. As the mass flow rate remains constant, this decrease in velocity produces a

corresponding increase in pressure as described by Bernoulli ’s equation.

Exercise A

Purpose:

To create head, power and efficiency characteristic curves for a centrifugal pump.



Apparatus:

1. Centrifugal pump demonstration unit (Figure 1).

2. Interface device.

3. PC with a suitable software installed.

Figure 1: Centrifugal pump demonstration unit

Figure 2: Interface of one of the suitable softwares

Theory:

The operating characteristics of a centrifugal pump may be described or illustrated by using graphs

of pump performance. The three most commonly used graphical representations of pump

performance are:



Change in total head produced by the pump, Ht

Power input to the pump, Pm

Pump efficiency, E

The change in total head produced as a result of the work done by pump can be calculated as:

Ht = Change in pressure head + change in velocity head + change in elevation = Hs + Hv + He

Where:

Hs = Change in pressure head =

Where Pin is the fluid pressure at inlet in Pa and Pout is the fluid pressure at outlet in Pa.

Hv = Change in velocity head =

Where Vin is the fluid velocity at inlet in m/s and Vout is the fluid velocity at outlet in m/s.

He = Change in elevation.

The vertical distance between inlet and outlet, which is O.075m for the available pump.

The mechanical power input to the pump may be calculated as:

Pm = rotational force x angular distance = 2.π.n.t/60

where n is the rotational speed of pump in revolutions per minute and t is the shaft torque in N.m .

The efficiency of the pump may be calculated as :

Where Q is the volume flow rate in m3/s, and Pm is the mechanical power absorbed by pump:

Each of these parameters is measured at constant pump speed, and is plotted against the volume

flow rate Q through the pump. An example of this type of graphical representation of pump

performance is given in Figure 2.



Figure 3: Characteristic curves for a centrifugal pump

Examining Figure 3, the general performance of the pump can be determined.

The Ht-Q curve shows the relationship between head and flow rate. The head decreases as flow rate

increases. This type of curve is referred to as a rising characteristic curve. A stable head-capacity

characteristic curve is one in which there is only one possible flow rate for a given head, as in the

example here.

The Pm-Q curve shows the relationship between the power input to the pump and the change in

flow rate through the pump. Outside the optimum operating range of the pump this curve flattens,

so that a large change in pump power produces only a small change in flow velocity.

The E-Q curve shows the pump capacity at which the pump operates most efficiently. In the

example here, the optimum operating capacity is 0.7 dm3/s, which would give a head of 1.2m. When

selecting a pump for an application where the typical operating capacity is known, a pump should

be selected so that its optimum efficiency is at or very near that capacity.

Equipment set up:

If the equipment is not yet ready for use, proceed as follows:

1. Ensure the drain valve is fully closed.

2. If necessary, fill the reservoir to within 20cm of the top rim.

3. Ensure the inlet valve and gate valve are both fully open.

4. Ensure the equipment is connected to the interface device and the interface device is connected

to a suitable PC. The red and green indicator lights on the interface device should both be

illuminated.



5. Ensure the interface device is connected to an appropriate mains supply, and switch on the

supply.

6. Run the software. Check that 'IFD: OK' is displayed in the bottom right corner of the screen and

that there are values displayed in all the sensor display boxes on the mimic diagram.

Procedures:

1. Switch on the interface device, then switch on the pump within the software using the pump

on/standby button.

2. Using the software, set the speed to 80%. The interface will increase the pump speed until it

reaches the required setting. Allow water to circulate until all air has been flushed from the

system. Slightly closing and opening the inlet valve and gate valve a few times will help in

priming the system and eliminating any bubbles caught within the valve mechanism. Leave the

inlet valve fully open.

3. In the results table, rename the spreadsheet (Select Format > Rename Sheet) to 80%.

4. Close the gate valve to give a flow rate Q of 0. (Note that the pump will not run well with the

gate valve closed or nearly closed, as the back pressure produced is outside normal operating

parameters. The pump should begin to run more smoothly as the experiment progresses).

5. Select the (Go) icon to record the sensor readings and pump settings on the results table of the

software.

6. Open the gate valve to allow a low flow rate. Allow sufficient time for the sensor readings to

stabilise then select the (Go) icon to record the next set of data.

7. Increase the flow in small increments, allowing the sensor readings to stabilise then recording

the sensor and pump data each time.

8. Using the arrow buttons on the software display, reduce the pump speed to 0%. Select "Save" or

"Save as…" from the "file" menu and save the results with a suitable file name.

9. Switch off the pump within the software using the power on/standby button, then switch off

the interface device and close the software.

Results:

Using the graph facility, plot a graph of head against flow rate. On the secondary axis plot a graph of



mechanical power and efficiency against flow rate.

Alternatively, the results sheet may be exported to an alternative spreadsheet program (or results

may be manually plotted on graph paper) to produce a chart.

Conclusion:

Examine and describe the shapes of the graphs obtained, relating this to the changing performance

of the pump as the flow rate changes. Locate the point of maximum efficiency and the flow rate at

which it occurs.

Exercise B

Purpose:

To obtain a head - flow curve for the piping system through which the fluid is to be pumped. To

determine the operating point of the pump .

Apparatus:

1. Centrifugal pump demonstration unit (Figure 1).

2. Interface device.

3. PC with a suitable software installed.

4. Tape measure.

Theory:

System analysis for a pumping installation is used to select the most suitable pumping units and to

define their operating points. System analysis involves calculating a head - flow curve for the

pumping system (valves, pipes, fittings, etc.) and using this curve in conjunction with the

performance curves of the available pumps to select the most appropriate pump(s) for use within

the system.

The system curve is a graphic representation of the flow rate in the system with respect to system

head. It represents the relationship between flow rate and hydraulic losses in a system. Such losses



are due to the system design (e.g. bends and fittings, surface roughness) and operating conditions

(e.g. temperature).

Assuming that:

Flow velocity is proportional to volume flow rate.

Losses in the system are proportional to the square of the flow velocity.

It follows that system head loss must be proportional to the square of the volume flow rate, and the

system head-flow graph will therefore be parabolic in shape.

A predicted system head-flow curve may be calculated using standard coefficients for the system

design and a measurement of the system head at zero flow. The simplest method of calculation is

Hazen-Williams equation for major pipe losses. This uses a coefficient based on the pipe material,

along with values for the pipe length and diameter and the flow rate within the system. This is not

the most accurate method and is only valid for water flowing at ordinary temperatures (approx. 5

to 40°C), but it is sufficient for many practical purposes. Accuracy may be improved by adding a

second equation for calculating the minor losses due to pipe fittings. The resulting calculation is as

follows:

h = total head loss in system = hf + hm

where :

hf = major losses in pipe =

hm = minor losses in pipe =

Where L is the total pipe length, V is the flow velocity, d is the pipe diameter, C is a coefficient

obtained from standard values (acrylic pipe = 140) and k is a coefficient obtained from standard

values, as follows:

Pipe entrance 0.5 (reservoir to pipe)

Pipe exit 1 (pipe to reservoir)

90° Bend 0.3

45° Bend 0.4

Ball Valve Negligible when fully open

Gate Valve 2.1 (half open)

As noted previously, pump characteristic curves illustrate the relationship between head,



discharge, efficiency and power over a wide range of possible operating conditions, but they do not

indicate at which point on the curves the pump will operate. The operating point (or duty point) is

found by plotting the pump head-discharge curve with the system head-flow curve. The

intersection of the two curves gives the duty point for the pump in that system, as illustrated in

figure 4 below.

It will be seen that the optimum operating condition is achieved if this operating point coincides

with the maximum point in the efficiency-discharge curve of the pump.

Figure 4: Definition sketch for determination of pump operating point

Equipment set up:

If the equipment is not yet ready for use, proceed as follows:

1. Ensure the drain valve is fully closed.

2. If necessary, fill the reservoir to within 20cm of the top rim.

3. Ensure the inlet valve and gate valve are both fully open.

4. Ensure the equipment is connected to the interface device and the interface device is connected

to a suitable PC. The red and green indicator lights on the interface device should both be

illuminated.

5. Ensure the interface device is connected to an appropriate mains supply, and switch on the

supply.

6. Run the software. Check that 'IFD: OK' is displayed in the bottom right corner of the screen and

that there are values displayed in all the sensor display boxes on the mimic diagram.



Procedures:

1. Measure the pipe length of the system, not including the path through the pump. Keep the

measurement as close to the centerline of the pipework as possible. Enter the result in meters

on the mimic diagram screen in the box for Pipe Length.

2. Add up the coefficient values for all the pipe fittings in the system. Do not include the entry and

exit into the pump but do include the pipes entering and exiting the reservoir, all bends. valves

and flow meter. Assume the pressure sensors have no effect on the coefficient. Enter the total

on the mimic diagram screen of the software in the box for coefficient k.

3. Switch on the interface device, then switch on the pump within the software. In the software,

set the pump to 100%.

4. Allow water to circulate until all air has been flushed from the system.

5. Select the (Go) icon to record the sensor readings and pump settings on the results table of the

software.

6. Set the pump to 90%, and select the (Go) icon again.

7. Repeat while reducing the pump speed in 10% steps, recording a data sample at each step, with

a final set of data taken at 0%.

8. Select the (New) icon to create a new results sheet.

9. Set the pump to 70% (the design speed of the pump).

10. Select the (Go) icon to record the sensor readings and pump settings on the new results table in

the software.

11. Close the gate valve to give a small but noticeable reduction in flow rate. Allow a few moments

for the system to settle then select the (Go) icon again.

12. Repeat while closing the gate valve in small increments, recording the data at each step, until

the valve is fully closed.

13. Set the pump to 0%, then select 'Save' or 'Save As .. .' from the 'File' menu and save the results

with a suitable file name (e.g. the date and the exercise).

14. Switch off the pump within the software, then switch off the interface device.

Results:

On a base of flow rate, on one y-axis plot the system head from the first set of data and the total



head from the second set of data. On the second y-axis plot the pump efficiency from the second set

of data.

Mark the point on the graph at which the system head curve and pump curve (total head curve)

intersect to obtain the duty point of the pump.

Conclusion:

Compare the graph obtained with the example given.

Compare the point of intersection of the system head curve and pump head curve with the curve

for pump efficiency.

The k value for the gate valve was greatly simplified for this experiment. A more accurate value varies

depending on whether the valve is fully open or partially open (0.26 for 1/4 closed, 2.1 for 1/2 open, 17

for 3/4 closed). Discuss the effect on the results obtained on having used a single average value for

the gate valve.

experiment (9): centrifugal pump -...

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