experiment (10): series and parallel pumps...

Fluid Mechanics Lab Experiment (10): Series and parallel pumps

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

Experiment (10): Series and parallel pumps

Introduction:

Centrifugal pumps are often used together to enhance either the flow rate or the delivery pressure

beyond that available from the single pump. For some piping system designs, it may be desirable to

consider a multiple pump system to meet the design requirements. Two typical options include

parallel and series configurations of pumps which require a specific performance criteria. In serial

operation the heads of the pumps are added and in parallel operation the flow rates (capacities) of

the pumps are added.

The experimental unit provides the determination of the characteristic behavior for single

operation and interaction of two pumps. The apparatus consists of a tank and pipework which

delivers water to and from two identical centrifugal pumps. The unit is fitted with electronic

sensors which measure the process variables. Signals from these sensors are sent to a computer via

an interface device, and the unit is supplied with data logging software as standard.

Purpose:

To investigate the result on discharge and total head of operating pumps in series and in parallel.

Apparatus:

1. Series and parallel pumps demonstration unit (Figure 1).

2. Interface device.

3. PC with suitable software installed.

Figure 1: Series and parallel pump demonstration unit



Figure 2: Interface of one of the suitable softwares

Setting the flow path

The system may be configured to drive flow using single,

series or parallel pumps. The system valves are as shown:

Valves should be set to configure the system as follows.

The software should also be set to the corresponding flow path to ensure that the correct

calculations are performed.

Single Pump:



Series Pumps:

Parallel Pumps:

The two pumps are motor-driven centrifugal pumps. On pump 1, the speed of the motor is

adjustable to give a range of 0 to 100%, allowing operation as a single pump for pump performance

analysis. Pump 2 is an identical model but is run at its design speed, which is equivalent to a setting

of 80% on the variable-speed pump for a 50Hz electrical supply, or 100% for a 60 Hz supply.

Exercise A (Series pumps)

Theory:

A single pump may be insufficient to produce the performance required. Combining two pumps

increases the pumping capacity of the system. Two pumps may be connected in series, so that water

passes first through one pump and then through the second. When two pumps operate in series, the

flow rate is the same as for a single pump but the total head is increased. The combined pump head-

capacity curve is found by adding the heads of the single pump curves at the same capacity.



Figure 3: Pump curve for two pumps in series

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 10cm of the top rim.

3. Check that both pumps are fitted with similar impellers (the impellers may be viewed through

the clear cover plate of each pump).

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

5. Set the 3-way valve for flow in series.

6. 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.

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

supply. Switch on the interface device.

8. 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. Both pumps must be used at the same setting in this experiment to ensure identical

performance. As the speed of pump 2 is fixed at its design operational point, pump 1 should be

set to match - select 80% for a 50Hz electrical supply, or 100% for 60 Hz.

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

3. If results are already available for a single pump across its full flow range, load those results into

the software now and jump to the section of this exercise using two pumps. If results are not

available then proceed as follows:



Single pump performance:

a. Close pump 2 outlet valve and open pump 1 outlet valve.

b. In the software, on the mimic diagram, set the 'Mode' to 'Single' by selecting the appropriate

radio button.

c. Rename the results sheet to 'Single'.

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

software.

e. Close the gate valve to reduce the flow by a small amount. Select the (Go) icon again.

f. Continue to close the gate valve to give incremental changes in flow rate, recording the sensor

data each time.

g. After taking the final set of data, fully open the gate valve.

Series pump performance:

4. Create a new results sheet using the (New) icon. Rename this new results sheet to 'Series'. In the

software, on the mimic diagram, set the 'Mode' to 'Series' by selecting the appropriate radio

button.

5. Open pump 2 outlet valve, close pump 1 outlet valve and wait for any air to circulate out of the

system.

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

software.

7. Close the gate valve to reduce the flow by a small increment. Select the (Go) icon again.

8. Continue to close the gate valve to give incremental changes in flow rate, recording the sensor

data each time.

9. After taking the final set of data, fully open the gate valve again.

Pumps in parallel exercise may be performed immediately after this experiment without closing the

software; otherwise, save the results and ensure they are available for exercise B when required. (It

may also be advisable to save the results from this exercise before starting exercise B even if

continuing straight on, to ensure that the data is not lost in the event of a computer failure. The

results sheet may be overwritten with the combined results once exercise B has been completed).

Results:

On a base of flow rate, plot a graph of total head gain for the single pump and for two pumps

connected in series. Calculate the difference between the total head gain for single and series

pumps.

Conclusion:

Does the total head gain for the two pumps in series match the theoretical prediction of twice the

head gain for a single pump (assuming the two pumps used gave identical performance)?



Exercise B (Parallel pumps)

Theory:

A single pump may be insufficient to produce the performance required. Combining two pumps

increases the pumping capacity of the system. Two pumps may be connected in parallel, so that half

the flow passes through one of the pumps and the other half through the second pump. When two

pumps operate in parallel, the total head increase remains unchanged but the flow rate is increased.

The head-capacity curve is found by adding the capacities of the single pump curves at the same

head.

Figure 4: Pump curve for two pumps in parallel

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 10cm of the top rim.

3. Check that both pumps are fitted with similar impellers (the impellers may be viewed through

the clear cover plate of each pump).

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

5. Set the 3-way valve for flow in parallel.

6. Fully open the pump 1 outlet valve and pump 2 outlet valve. Opening both valves fully ensures

that the outlet pressure on both pumps is equal.

7. 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.

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

supply. Switch on the interface device.

9. 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.



10. In the software, on the mimic diagram, set the "Mode" to "parallel" by selecting the appropriate

radio button.

Procedures:

1. Both pumps must be used at the same setting in this experiment, to ensure identical

performance. As the speed of pump 2 is fixed at its design operational point, pump 1 should be

set to match - select 80% for a 50Hz electrical supply, or 100% for 60 Hz.

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

3. Exercise A should be performed before this experiment, and the results loaded into the software

if the software is not still open from that exercise. If the software is still open from exercise A,

then create a new results sheet by selecting the (New) icon. Rename the current (blank) results

sheet to 'Parallel'.

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

software.

5. Close the gate valve to reduce the flow by a small increment. Select the (Go) icon again.

6. Continue to close the gate valve to give incremental changes in flow rate, recording the sensor

data each time.

7. After taking the final set of data, fully open the gate valve. Set Pump 1 to 0% and switch off both

pumps.

Results:

On a base of flow rate, plot a graph of total head gain for the single pump and for two pumps

connected in parallel. Calculate the difference between the capacity for single and parallel pumps.

Conclusion:

Does the total head gain for the two pumps in parallel match the theoretical prediction of twice the

capacity of a single pump (assuming the two pumps used gave identical performance)?

Compare the graphs for pumps in series and pumps in parallel, and describe the similarities and

differences.

experiment (10): series and parallel pumps...

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