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Laboratory for Heat and Power

Centrifugal pumps - characteristics Practical exercise

Authors:

Mitja Mori Boštjan Drobnič

Ljubljana, October 2010

Faculty of mechanical engineering

1.

Laboratory for Heat and Power

Target of the exercise

- Basics of centrifugal pumps installed in pipe systems and energy conversion in the pumps; - Basic characteristic curves obtained with the help of measurements of certain parameters; - Show the difference in characteristics at pump operation in parallel and serial mode;

2.

Basics

Armfield demonstration unit FM21 is an example

of radial

flow rotary

machine

(centrifugal pump), in which fluid flows into the rotor at a given radius and flows out at greater radius. In the pump kinetic, potential and pressure energy is changed. Fluid flows axially through the inflow part located in the center of the rotor (Figure 1), then the direction of flow is changed in the radial direction by the action of the rotor blade. The kinetic energy of the fluid is increased

Figure 1: Cross section of centrifugal pump

within the rotor and converted to pressure energy at the outlet. The interdependence between energy comes from the Bernoulli equation, which is a simplified equation of the conservation of energy, which takes into account only those forms of energy in the process of significantly changing stations. At each point of the system is:

c2 p g z const. 2 c

fluid speed

z p

height pressure

ρ

fluid density

g

gravitational acceleration

Centrifugal pumps - charactersistics

(1)

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Faculty of mechanical engineering

Laboratory for Heat and Power

The total energy of the fluid remains unchanged if there is no inlet or outlet power. When fluid is in motion, the part energy is used to overcome the flow resistance (power outlet), with the pump the energy is transported to the fluid. During the initial and end point in the system the energy balance equation can be written

c2 c2 p p m 1 g z1 1 WČ m 2 g z 2 2 Wtr ρ1 ρ2 2 2 m

mass of observed part of fluid

WČ

energy transported to the fluid by the pump

Wtr

energy loss due to friction

(2)

If we are dealing with uncompressible fluid it can be assumed that the density is constant. So the equation for pump work is

c22 c12 p p1 presist WP m g z 2 z1 2 2 g g g

(3)

In this energy loss was due to the flow resistance expressed by Δpresis pressure drop, which is a direct consequence of energy losses. The term in brackets is called the pump head and is directly related to the energy that must be supplied to fluid, so it could be moved from point 1 to point 2.

H

c22 c12 p p1 Δpup z 2 z1 2 2g ρg ρg

(4)

Pump head is at the same time a feature of the pipeline and pump. In the pipeline it represents the energy needed for the desired quantity (flow) of water, while at the pump it represents the energy that the pump is able to give the fluid at a given flow. Since the mass flow has significant impact on the pump and pipeline head, this dependence is expressed by the diagram for specific pipeline. This diagram is named pipeline or pump characteristic. At the same time pump head is criteria on the basis of which can be concluded if pump is required in the specific pipeline or not. First three parts of equation (4) can be positive, negative or equal to zero. H > 0, that means that pump is required in the system to provide desired mass flow; H = 0, the pump is not required, the flow will establish by its self; H < 0, the pump is not required, for desired fluid mass flow the reduction valve should be

installed to inhibit the fluid;

Centrifugal pumps - charactersistics

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Faculty of mechanical engineering

3.

Laboratory for Heat and Power

The UNIT

3.1.

Basic description

The unit FM21 that is shown on Figure 2 consists from two centrifugal pumps (1) and (2) driven by motors. Pumps are placed on the platform and connected to reservoir (3) via pipelines. The unit enables to study operation of: - Single pump - Two pumps serial mode - Two pumps in parallel mode That can be done by opening/closing specific valves: V1 (5), V2 (4), V3 (6) in V4 (7).

Figure 2:The unit for study centrifugal pumps operation and characteristics

The mass flow of the fluid through pumps is regulated by control valve V (8). The appropriate position of that valve enables changing total head and volume flow. To measure operational parameters required for further analysis appropriate sensors are mounted on the unit. The data are saved to computer via interface IFD6 (9).

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Faculty of mechanical engineering

3.2.

Laboratory for Heat and Power

Instrumentation and sensors

On the unit specific sensors are mounted: 1. Differential sensor SPW1 (10) Differential sensor consists from piezzoresistance sensor and converter that converts pressure difference on the orifice (11) to voltage. With pressure difference on known orifice we are able to calculate volume flow in different operational points. 2. Differential pressure sensors SPW3 (12 in 13) Differential sensor consists from piezzo-resistance sensor and converter that converts pressure difference in pump to voltage. From data total head and theoretical power is calculated. 3. Optical proximity sensors SSO1 (14 in 15) Optical proximity sensors measure rotational speeds of pump’s rotors. 4. Temperature sensor STS1 (16) Temperature sensor is classical NiCr-Ni thermocouple. 5. Power mesters SWA1 (17 in 18) Power meters are inverters that enable speed regulation and measure electrical power of motors.

3.3.

Program interface

Graphical interface in the program on the computer enables observing operational parameters and pump characteristics. With the program we can store measured and calculated data in all operational points of the system. The main menu is shown on the Figure 3. Sensors and program interface unit FM21 enables to observe and save operation parameters: (on figure 3 shown by blue rectangle): Differential pressure on orifice to measure volume flow, Δp0 (Orifice Pressure), Differential pressure between inlet and outlet pressure on pump 1, Δp1 (Pump 1 Pressure), Differential pressure between inlet and outlet pressure on pump 2, Δp2 (Pump 2 Pressure), Rotational speed of pump 1, n1 (Motor 1 Speed), Rotational speed of pump 2, n2 (Motor 2 Speed), Electrical power of motor 1, Pem1 (Motor 1 Power), Electrical power of motor 2, Pem2 (Motor 2 Power), Water temperature, T (Temperature).

Centrifugal pumps - charactersistics

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Faculty of mechanical engineering

Laboratory for Heat and Power

Figure 3: The main menu of software for observing operational parameters of FM21 unit

In the software calculated values are also shown: (green rectangle on figure 3): Water volume flow V (Volume Flow), Pump’s total head (Total Head), Power output – theoretical power (Power Output), Pump’s efficiency (Efficiency),

To obtain relevant calculated data, some parameters need to be properly set before measurements (purple rectangle on figure 3): Orifice constant (Orifice Cd) C d = 0,74 Operational mode of pumps (Mode)

→ Single = one pump in operation → Series = two pumps operating in serial mode → Parallel = two pumps operating in parallel mode In addition to present above, the software enabled graphical and tabular presentation of measured and calculated data. That can be done with buttons in the main menu (orange rectangle in figure 3). To save one measured data set button GO has to be clicked once (red rectangle in figure 3), new set of measurements are start with button ‘’Next Results’. By clicking it empty table is opened and results are presented with new additional curves on diagrams.

Centrifugal pumps - charactersistics

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Faculty of mechanical engineering

4.

Laboratory for Heat and Power

Theoretical background

4.1.

Characteristic curves of centrifugal pumps

Characteristics of centrifugal pumps are total head, power, efficiency

usually presented in forms of total head H vs. volume flow, Power P vs. volume flow and efficiency η vs. volume flow for different rotational speeds of centrifugal pumps (figure 4). In diagram it is shown that efficiency peaks at specific volume flow and drops after, total head is falling

total head power efficiency

c b

a

a b c

a b

from low to higher volume flows, where c

the drop is very significant. Optimal flowrate

conditions correspond with point where efficiency is at peak value.

Figure 4: Characteristics of centrifugal pump for different rotational speeds (na > nb > nc)

With specific pump manufacturers gives diagram that represent pump working area. The diagram represents one parameter (efficiency or power) as a function of other two parameters (total head and

total head, power, efficiency

volume flow) in the form of isoclines (figure 5).

55

60

efficie ncy 65 68 %

power 50 kW 40 30 20

10

flowrate

Figure 5: Specific shell chart for centrifugal pump, rotor diameter 375 mm

On the basis of parameters that measured, parameters are calculated on the basis of which characteristics of centrifugal pumps can be specified for all operational points: Water density, Volume flow, Centrifugal pumps - charactersistics

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Faculty of mechanical engineering

Laboratory for Heat and Power

Water speed on the inlet and outlet of the pump, Total head of single pump or total head of pumps in parallel or series mode, Power of single pump or power of pumps in parallel or series mode, Efficiency of single pump or power of pumps in parallel or series mode.

4.1.1.

Water density

Water density in kg/m3 can be obtained with the help of thermodynamic tables or calculated with polynomial equation prescribed by ISO standard. Temperature is input data in following equation in °C. ρ = 0,000015324364·T 3 – 0,00584994855·T 2 + 0,016286058705·T + 1000,04105055224

4.1.2.

(5)

Water volume flow in the pipe

Unit FM21 has orifice installed in the pipeline that enables to calculate water volume flow on the basis on pressure drop:

CD π d V

2 ρ Δp0 4 ρ 2

(6)

In eq. 7 Cd = 0,74 is orifice coefficient, d = 0,024 m pipeline diameter, ρ fluid density calculated with eq. (5) in Δp0 measured pressure drop through orifice.

4.1.3.

Water velocity on the inlet and outlet of the pump

c1* c2*

V A

(8)

Where A is cross sectional area of the pipe of the value 0,00029865 m2.

4.1.4.

Total head

Total head of single pump is pressure difference in different form that should be made by centrifugal pump, to transport water of specific volume flow in given system from start to end point. General equation of total head for centrifugal pump is:

H

p p 2 p1 1 2 z 2 z1 c2 c12 c12* c22* loss 2g g g II

I

Centrifugal pumps - charactersistics

III

(7)

IV

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Faculty of mechanical engineering

Laboratory for Heat and Power

Parts of equation represent energy differences between points 1 and 2. These parts of equation can

2

be also negative that means that energy difference 2*

(pressure, geodetically or speed difference) between starting and end point helps to push fluid through pipeline. Parts of equation represent: I

1*

Pressure difference

II

1

Height difference

III

Kinetic energy difference

IV

Pressure drop because local and linear losses

Slika 6: Points in pipeline system

At observed system point 1 is at pump inlet and point 2 at pump outlet. That means that eq. part I pressure difference between these two points is used, height difference is (eq. part II) z2 – z1 = 0,048 m. Because water velocities in points 1 and 2 are equal, the eq. part III is 0, eq. part IV can be neglected due to very small energy losses between points 1 and 2 due pressure losses.

4.1.5.

Power and efficiency

At given total head theoretical power is:

gH Pt m

(9)

Because compression is not ideal (isentropic), the pump uses internal power for compression that is bigger than theoretical power due to internal efficiency.

Pi

Pt

n

(10)

In the pump the part of power is used for mechanical losses. Therefore effective power has to be introduced that is larger than internal power due to mechanical efficiency.

Pe

Pn ηm

(11)

Electromotor also has electrical and mechanical losses and uses electrical power to overcome these loses. Therefore electrical power is larger than effective power.

Pem

Pe ηem

(12)

The overall efficiency is

η ηn ηe ηem

Centrifugal pumps - charactersistics

Pt Pn Pe P t Pn Pe Pem Pem

(13)

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Faculty of mechanical engineering

Laboratory for Heat and Power

In this case power of single pump is calculated with eq. (9) with the help of total head. Electrical power is measured with inverters. Overall efficiency is calculated with eq. (13).

4.2.

Serial and parallel mode of two centrifugal pumps

If single pump is not able to give adequate total head or volume flow, two or more pumps can be connected in serial or parallel mode. In that case total head is calculated as in single mode: a) Pumps in serial mode:

H KOMB H1 H 2

(14)

b) Pumps in parallel mode:

H KOMB

H1 H 2 2

(15)

In the case of two pumps in series mode total head is larger at constant volume flow and in the case of two pumps in parallel mode volume flow is larger at constant total head (figure 7). setu p

total head

serial

sing le p

parall el set up

um p

flowrate

Figure 7: Single pump total head vs. total head of two pumps in parallel and series mode

4.3.

Pipeline characteristics

While the pump characteristic shows the pump rates, the pipeline characteristic means the amount of energy (total head), which is required to transport certain amount of fluid through pipeline. Characteristic of the pipeline shows the dependence

total head

ability to supply energy to fluid at different flow

IV

of the total head from the volume flow for specific

III

pipeline at specific operating conditions (figure 8).

II I

The required total head, according to equation (4) consists of four parts/articles. Parts I and II are independent of volume flow, while part III and IV Centrifugal pumps - charactersistics

flowrate

Figure 8:Pipeline characteristics with marked proportions of articles in eq. (4) Page: 10 from 12

Faculty of mechanical engineering

Laboratory for Heat and Power

are increasing by the square of fluid velocity or volume flow. When determining the

pump

total head

characteristics of the pipeline the properties of A

operational point

line pipe

B

the pipeline should not be changed, therefore we should not change the position of the red valve V, which regulate water volume flow. Different volume flows of water that is necessary to determine the characteristics of a pipeline in this case are achieved by varying

flowrate

Slika 9: Operating point and water volume flow regulation by variing pipeline characteristic(A) or pump characteristic (B)

the operating parameters and characteristics of the pump. This is achieved by varying the rotational speed of the pump.

Pump with known characteristics in the pipeline system with specific characteristic will also be able to establish an operating point that is exactly specified by the intersection of the two characteristic curves (Fig. 9). If you want to change the water flow is necessary to change one of the characteristics. In this case the characteristic of the pipeline is usually changed by a valve that increases/decreases pressure loses in the pipeline, part IV of the equation (4). Characteristic of the pump is usually changed with the rotation speed of the pump.

5.

Exercise instructions

5.1.

Introduction

Before measurements interface IFD 6 should be turned ON with red button, run the computer and appropriate software. Check if orifice coefficient is set to 0,74. Before putting pumps in operation, regulation valve V should be fully opened and valves V1, V2, V3 in V4 closed. That set up enables pumps to be started with minimum load. After start blue valves should be properly set according to the case (single pump, pumps in serial or parallel mode). At the same time mode should be properly set also in the software (Mode). Before measurements check it out if new table is opened (button Next results), so data are not mixed with previous measurements. After measurements data should be saved in Excel file for further analysis.

5.2.

Experiments

The exercise is divided in three parts and in every part pump and pipeline characteristic should be calculated in different operating conditions.

Centrifugal pumps - charactersistics

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Faculty of mechanical engineering

5.2.1.

Laboratory for Heat and Power

Characteristic of single pump

With changing pressure losses with red regulating valve, determine pump characteristic in operation region of the pump from maximal to minimal water volume flow. According to maximal volume flow pick the gradient of volume flow change in the way that whole volume flow region is covered (approximately 10 points). Repeat measurements for rotational speeds of: a) 45 Hz b) 35 Hz c) 25 Hz Be careful that before every measurement rotational speed of the pump is constant. At low rotational speed rotational speed of the pump is increased during regulation valve closing. That happens because the load of the pump is decreased.

5.2.2.

The characteristic of two pumps

On the same way determine the characteristic of the two pumps in series or parallel mode at 45 Hz. With the help of blue valves set the system to appropriate mode, so pumps are working: d) parallel e) in series Pick the step that you will obtain 10 measuring points in whole operating region. Take into account that in parallel mode water volume flow is almost double as in serial or single.

5.2.3.

Pipeline characteristic

With blue valves the operation of one pump has to be set. The pipeline characteristic is obtained with rotational speed varying (with potentiometer on inverter). At maximal rotational speed (approx. 45 Hz) set the volume flow with red regulation valve to: f) 100 % g) 66 % h) 33 % and lower the rotational speed by 5 Hz to approx. 15 Hz.

5.3.

Tasks

1. Set up the unit FM21 and make measurement according to instructions. 2. On the basis of results calculate total head, theoretical power and overall efficiency of the pump in all measured points. 3. Show results in graphical form.

Centrifugal pumps - charactersistics

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