ERT 205Fluid Mechanics Engineering

Experiment 5:Determination of Water Flow Rates and Head Losses for

Single, Series and Parallel Pump Configurations

Name:Ang Kien Hau

Members of Group A2:Huzaifah binti Ab RahmanIntan Liana binti Nordin

Ker Lee Shiang

Instructors’ Name:En. Razif Omar

Cik Khalilah Mohamad Yusuff

Date of Experiment:19th October 2010

Date of Submission:29th October 2010

Table of Contents

1 Objectives 1

2 Introduction 1

3 Theory 3

4 Equipment 6

5 Procedures 6

6 Results 8

7 Questions 9

8 Discussion 10

9 Conclusion 10

10 References 11

11 Appendix 12

ObjectivesThe main objective of this experiment is to determine flow and head of a single

centrifugal pump at single speed, flow and head of two similar pumps operating in a parallel configuration at the same speed and flow and head of two similar pumps operating in a series configuration at the same speed.

IntroductionA centrifugal pump is one of the simplest pieces of equipment in any process plant.

Its purpose is to convert 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 (Sahdev, n.d).

The process liquid enters the suction nozzle and then into 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. This imparts kinetic energy to the liquid causing it to exit the impeller’s vanes at a greater velocity than it possessed when it entered. The energy transferred to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, the higher will the velocity of the liquid energy transferred to the liquid be. This is described by the Affinity Laws (The Engineering ToolBox, n.d).

As the liquid leaves the eye of the impeller, a low-pressure area is created causing more liquid to flow toward the inlet. Since the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. The liquid that exits the impeller is collected in the casing or volute where its velocity is converted to pressure before it leaves the pump’s discharge. Figure 1 below depicts a side cross-section of a centrifugal pump indicating the movement of the liquid (Sahdev, n.d).

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Figure 1: Liquid flow path inside a centrifugal pump.If the discharge of a centrifugal pump is pointed straight up into the air the fluid

will pumped to a certain height, or so called the shut off head, or head. Imagine a pipe shooting a jet of water straight up into the air, the height the water goes up would be the head. This maximum head is mainly determined by the outside diameter of the pump's impeller and the speed of the rotating shaft. The head will change as the capacity of the pump is altered. The kinetic energy of a liquid coming out of an impeller is obstructed by creating a resistance in the flow. The first resistance is created by the pump casing which catches the liquid and slows it down. When the liquid slows down the kinetic energy is converted to pressure energy. This pressure energy is the resistance to the pump's flow that is read on a pressure gauge attached to the discharge line. Thus, it is to be clear that a pump does not create pressure, it only creates flow. Pressure is just only a measurement of the resistance to flow (The Engineering ToolBox, n.d).

The pressure at any point in a liquid can be thought of as being caused by a vertical column of the liquid due to its weight. The height of this column is called the static head and is expressed in terms of feet of liquid. The same head term is also being used to measure the kinetic energy created by the pump as mentioned earlier. In other words, head is a measurement of the height of a liquid column that the pump could create from the kinetic energy imparted to the liquid (Sahdev, n.d).

The head is not equivalent to pressure. Head is a term that has units of a length or feet and pressure has units of force per unit area or pound per square inch. The main reason for using head instead of pressure to measure a centrifugal pump's energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. Since any given centrifugal pump can move a lot of different fluids, with different specific gravities, it is simpler to discuss the pump's head and forget about the pressure. So a centrifugal pump’s performance on any Newtonian fluid, whether it's heavy (sulfuric acid) or light (gasoline) is described by using the term ‘head’ (Sahdev, n.d). It is important to understand that the pump will pump all fluids to the same height if the shaft is turning at the same speed. The only difference between the fluids is the amount of power it takes to get the shaft to the

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proper speed. The higher the specific gravity of the fluid the more power is required.

TheoryThe head of a pump in metric units can be expressed in metric units as:

h =P2 – P1

+V2

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ρg 2gWhereh = total head developed (m) P2 = pressure at outlet (N/m2)P1 = pressure at inlet (N/m2)ρ = density (kg/m3)g = acceleration of gravity (9.81 m/s2)V2 = velocity at the outlet (m/s)

Since pressure gauges often are calibrated in pressure – psi or bar, it may be necessary with a conversion to head - feet or meter, commonly used in pump curves. The static head corresponding to any specific pressure is dependent upon the weight of the liquid according to the following formula:

h =P × 2.31

SGWhereh = total head developed (ft) P = pressure at outlet/inlet (psi)SG = specific gravity

Newtonian liquids have specific gravities typically ranging from 0.5 (light, like light hydrocarbons) to 1.8 (heavy, like concentrated sulphuric acid). Water is a benchmark, having a specific gravity of 1.0.

The Affinity Laws are mathematical expressions that define changes in pump capacity, head, and BHP when a change is made to pump speed, impeller diameter, or both. These laws are only applicable under the conditions of constant efficiency of a

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pump. According to Affinity Laws, Head, h changes in direct proportion to the square of impeller diameter, D ratio, or the square of speed, N ratio:

h2 = h1 x [Doutlet/Dinlet]2

h2 = h1 x [Noutlet/Ninlet]2

WhereSubscript 1 refers to initial condition;Subscript 2 refer to new condition.

If changes are made to both impellers diameter and pump speed the equations can be combined to:

h2 = h1 x [(Doutlet x Noutlet)/(Dinlet x Ninlet)]2

This equation is used to hand-calculate the impeller trim diameter from a given pump performance curve at a bigger diameter.

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. Specific performance criteria must be met when considering these options.

Given a piping system which has a known design flow rate, Qdes and head requirements, hsys, the following pump selection criteria apply.

Pumps in Parallel:Assuming that the pumps are identical, each pump must provide the following:Qpump = 0.5 Qdes

hpump = hsys

Figure 2: Pumps in Parallel Configuration.

Pumps in Series:Assuming that the pumps are identical, each pump must provide the following:Qpump = Qdes

hpump = 0.5 hsys

Figure 3: Pumps in Series Configuration.

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For example, if the design point for a given piping system were Qdes = 600 gpm, and hsys

= 270 ft, the following pump selection criteria would apply:

1. Single pump system: Qpump= 600 gpm, hpump = 270 ft

2. Parallel pump system:Qpump= 300 gpm, hpump = 270 ft for each of the two pumps

3. Series pump system:Qpump= 600 gpm, hpump = 135 ft for each of the two pumps

When two or more pumps are arranged in parallel their resulting performance curve is obtained by adding their flow rates at the same head as indicated in the figure below:

Figure 4: Performance Curve for Pumps in Parallel Configuration.

Centrifugal pumps in parallel are used to overcome larger volume flows than one pump can handle alone. For two identical pumps in parallel the flow rate will double (moving from 1 to 2) compared to a single pump if head is kept constant. In practice the combined head and volume flow moves along the system curve as indicated from 1 to 3.If one of the pumps in parallel or series stops, the operation point moves along the system resistance curve from point 3 to point 1 - the head and flow rate are decreased (The Engineering ToolBox, n.d).

When two (or more) pumps are arranged in serial, their resulting pump performance curve is obtained by adding their heads at same flow rate as indicated in the figure below:

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Figure 5: Performance Curve for Pumps in Series Configuration.

Centrifugal pump in series are used to overcome larger system head loss than one pump can handle alone. For two identical pumps in series the head will be twice the head of a single pump at the same flow rate. With constant flow rate the combined head moves from 1 to 2. In practice the combined head and flow rated moved along the system curve to 3. If one of the pumps stops, the operation point moves along the system resistance curve from point 1 to point 2 - head and flow rate are decreased (The Engineering ToolBox, n.d).

Equipment

Figure 6: Series & Parallel Pump Apparatus LS-18001-23.

Flow Sensor Control Valve

Water Tank

Valve 1

Pump 2

Control Panel

Pump 1

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Valve 2

Valve 3

Valve 4

Valve 5

Gauge 1

Gauge 2

Gauge 3

Gauge 4

ProceduresExperiment 1: Flow/Head determination of a single centrifugal pump at a single

speed.1. The Series & Parallel Pump Apparatus was positioned near the water source.2. Water was been filled into the reservoir tank until ¾ of its volume before operating

the apparatus.3. The ELCB/MCB buttons was been pushed to on the apparatus.4. Power supply was been switched on.5. All control valves were made sure to be closed.6. The selection valves were been positioned according to Figure 7 in Appendix.7. Flow control valve was been fully opened.8. One pump was been selected for operation.9. The flow control valve was been adjusted to achieved required flow rate.10. The water flow rate from the flow meter and the corresponding pressure from

pressure meter 1 were recorded.11. Water was been refilled into the reservoir tank if the water is below the minimum

water level mark.12. Experiment was been continued by using different flow rates.

Experiment 2: Two similar pumps operating in a parallel configuration at the same speed.

1. The Series & Parallel Pump Apparatus was positioned near the water source.2. Water was been filled into the reservoir tank until ¾ of its volume before operating

the apparatus.3. The ELCB/MCB buttons was been pushed to on the apparatus.4. Power supply was been switched on.5. All control valves were made sure to be closed.6. The selection valves were been positioned according to Figure 8 in Appendix.7. Flow control valve was been fully opened.8. Two pumps were been selected for operation.9. The flow control valve was been adjusted to achieved required flow rate.10. The water flow rate from the flow meter and the corresponding pressure from

pressure meter 1 were recorded.11. Water was been refilled into the reservoir tank if the water is below the minimum

water level mark.12. Experiment was been continued by using different flow rates.

Experiment 3: Two similar pumps operating in a series configuration at the same speed.

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1. The Series & Parallel Pump Apparatus was positioned near the water source.2. Water was been filled into the reservoir tank until ¾ of its volume before operating

the apparatus.3. The ELCB/MCB buttons was been pushed to on the apparatus.4. Power supply was been switched on.5. All control valves were made sure to be closed.6. The selection valves were been positioned according to Figure 9 in Appendix.7. Flow control valve was been fully opened.8. Two pumps were been selected for operation.9. The flow control valve was been adjusted to achieved required flow rate.10. The water flow rate from the flow meter and the corresponding total pressure from

pressure meter 2 were recorded.11. Pressure for pump 1 can be obtained from pressure meter 1.12. Water was been refilled into the reservoir tank if the water is below the minimum

water level mark.13. Experiment was been continued by using different flow rates. Caution was been

taken to stop the experiment when the total pressure reaches 5 bars.

ResultsExperiment 1Flow Rate, Q (L/min) Pressure, P (psi) Head, h (ft) Head, h (m)

2.0 46 106.26 32.393.1 44 101.64 30.983.8 42 97.02 29.574.9 40 92.40 28.166.0 24 55.44 16.906.5 10 23.10 7.04

Experiment 2Flow Rate, Q (L/min) Pressure, P (psi) Head, h (ft) Head, h (m)

2.8 51 117.81 35.913.4 50 115.50 35.203.5 49 113.19 34.505.0 43 99.33 30.285.3 38 87.78 26.765.8 36 83.16 25.35

Experiment 3

Flow Rate, Q (L/min)

Pressure for Pump 1, P1

(psi)

Total Pressure, PT (psi)

Head, h (ft) Head, h (m)

3.9 34 68 157.08 47.88

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4.6 31 62 143.22 43.655.1 29 58 133.98 40.845.6 26 53 122.43 37.326.2 24 48 110.88 33.806.5 22 44 101.64 30.98

Example of Calculation:

For Experiment 3, at Q = 6.5 L/min, PT = 44 psi,

h =P × 2.31

SG

h =44 psi × 2.31

1.0h = 101.64 ft

1 ft = 0.3048 m,101.64 ft = (101.64 × 0.3048) m

= 30.98 m

Questions1. Explain the conditions when the centrifugal pump should be used in series.

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Centrifugal pumps should be used in series when it is necessary to overcome larger system head loss than one pump can handle alone. For two identical pumps in series the head will be twice the head of a single pump at the same flow rate.

2. Explain the conditions when the centrifugal pump should be used in parallel.Centrifugal pumps should be used in parallel when it is necessary to overcome larger volume flows than one pump can handle alone. For two identical pumps in parallel the flow rate will double compared to a single pump if head is kept constant.

DiscussionFrom the results and graphs, we can see that for all the data measured, pressure is

decreasing with increasing flow rate. This relationship between pressure and flow rate can be proven through Bernoulli Equation, which stated that as velocity increases the air pressure will be decreasing.

When two pumps are arranged in parallel their resulting performance curve is obtained by adding their flow rates at the same head. In other words, for two identical pumps in parallel the flow rate will double compared to a single pump if head is kept constant. This can be seen from the graph line of parallel configuration compared to the single pump one. However, it can be noticed that the flow rate of two pumps in parallel is lower that twice the value of flow rate for a single pump one. This is most probably due to the friction losses along the pipes and fittings which being ignored in this experiment.

When two pumps are arranged in series, their resulting pump performance curve is obtained by adding their heads at same flow rate. In other words, for two identical pumps in series the head will be twice the head of a single pump at the same flow rate. This can be seen from the graph line of series configuration compared to the single pump one. However, it also can be noticed that the head of two pumps in series is lower that twice the value of head for a single pump one. Just like experiment 2, this is due to the friction losses along the pipes and fittings which being neglected.

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ConclusionFrom the graphs, the parallel configuration has the highest flow rate at the same

pressure compared the other two; while the series configuration has the highest pressure as well as head at a same flow rate compared to the other two.

References

J.Evans, n.d. A brief introduction to centrifugal pumps, [pdf]Available at: http://www.pacificliquid.com/pumpintro.pdf[Accessed 20 October 2010].

M. Sahdev, n.d. Centrifugal pumps: basics concepts of operation, maintenance, and troubleshooting, part I. [pdf]Available at: http://www.maintenanceworld.com/Articles/engresource/centrifugalpumps.pdf[Accessed 20 October 2010].

The Engineering ToolBox, n.d. Centrifugal pump. [Online]Available at: http://www.engineeringtoolbox.com/centrifugal-pumps-d_54.html[Accessed 20 October 2010].

The Engineering ToolBox, n.d. Converting pump head to pressure. [Online]Available at: http://www.engineeringtoolbox.com/pump-head-pressure-d_663.html[Accessed 20 October 2010].

The Engineering ToolBox, n.d. Pumps in parallel or serial connection. [Online]Available at: http://www.engineeringtoolbox.com/pumps-parallel-serial-d_636.html[Accessed 20 October 2010].

Y.A. Cengel & J. M. Cimbala, 2006. Fluid mechanics: fundamental and applications. 1st Ed. Singapore: McGraw-Hill.

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Appendix

Figure 7: Single Pump Configuration.

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Figure 8: Parallel Pump Configuration.

Figure 9: Series Pump Configuration.

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