Investigation of flow through centrifugal pump impellers 4

The speed, flow rate, suction pressure, discharge pressure, power and NPSHr net positive suction head required are measured during the test.. The suction and delivery pipe diameter are i

EXPERIMENTAL WORK

4.1 General Introduction

To verify numerical results, it is necessary to compare with experimental tests The pump performance test was carried out using pump test facility with cold water The speed, flow rate, suction pressure, discharge pressure, power and NPSHr (net positive suction head required) are measured during the test Some of these data will be used to compare with the numerical results

In the pump performance test, the most commonly used driver is an electric motor Due to fluctuations of power supply and load variation, the test speed usually fluctuates throughout the entire flow range It is necessary to correct the pump performance of the test reading to a reference speed based on affinity law below

2

1 2

1

Q

Q

2

1 2 2

1) (

H

H n

n

=

2

1 3 2

1) (

P

P n

n

where n is the rotational speed in rpm, Q is the flow rate in m3/h, H is the total head in meter, P is the horsepower in kw, and subscripts 1 and 2 refer to speed 1 and speed 2

respectively

It is practically impossible for centrifugal pump to maintain a very steady flow rate and head without fluctuation in the reading during test Therefore, it is imperative to take all the required readings simultaneously to ensure accuracy A test method is thus developed to meet this requirement and to improve the efficiency so that the accurate test results can be obtained immediately at the end of the test Figure 4.1 shows the schematic diagram of the test stand

In order to choose a good pump test standard that is easily understood and applied

by user, the most common pump test standard adopted by pump manufacturer today, that

is, ISO 2548 (Centrifugal, mixed flow and axial pumps-Code for acceptance tests-Class C) was chosen as the standard of our pump test stand This international standard is the first of a set dealing with acceptance tests of centrifugal, mixed flow and axial pumps; they correspond to three classes of tests A, B and C: class A is the most accurate and class C is the least accurate; the use of class A and B is restricted to special cases when there is a need to have the pump performance more precisely defined Class C is commonly used for the mass produced pumps

The pump test stand discussed below is designed to meet ISO 2548 Figure 4.2 shows the test stand instrument arranged according to ISO 2548 For pump performance test, two assumptions are made:

1) Test liquid is the cold water, and the compressibility of test liquid is negligible 2) Suction and delivery gauges are connected within 2 times pipe diameter from the suction and delivery flanges of the pump The frictional loss is negligible between pump flanges and gauge connection

4.2 Theoretical Background

4.2.1 Total Head Calculation

The most important parameter in the present study is the pump total head because

it will be used for comparison with the computational value Therefore, this section will introduce how to calculate the pump total head

Total head in meter at suction gauge position is given by

g

V g

P

2

2

and the total head in meter at discharge gauge position is given by

g

V g

P

2

2

The pump total head in meter is obtained from the difference between Eq (4.2) and Eq (4.1)

Pump total head H

g

V V g

P P h

2

2 2 1

2

− +

− +

−

=

where H is pump total head in meter, (h 2 -h 1) is also refered to as difference in gauge

height, P is pressure in Pa, V is velocity in m/s, h is the height in meter, and subscripts s and d refer to suction and discharge respectively

The velocity head is equal to the flow rate Q divided by the pipe cross-sectional

area, hence:

d d

A

Q

s

Q

where V and d V are the velocity head of the delivery and suction point in m/s; s

Ad and As are the cross-sectional area of the delivery and suction pipe in m2 The suction and delivery pipe diameter are input into the computer during the test

to calculate the pump total head based on the measured data The suction and delivery pressure (P and s P ) are measured by means of digital pressure transmitter with output d

pressure indication in MPa and 4-20 mA output signal to central processor of the computer to compute the pump total head with the information of difference in gauge height

4.2.2 Calculation of Pump Power Input and Efficiency

Assuming there is no loss in the power transmission with direct coupling drive between pump and motor, the pump power input can be written as

Pp (pump power input) = Motor power output

Motor power output = Pm (motor power input measured by the instruments) × ηm

364

H Q

p

u p

p

=

where Pu is pump power output, η is pump efficiency, and ηm is motor efficiency

4.2.3 NPSH Test

Net positive suction head (NPSH) refers to the energy of the test liquid at centerline of the pump above the vapour pressure of the test liquid A valve throttling method is used to measure the NPSHr for the pump being tested 3% head drop criterion, which is most commonly accepted internationally, is adopted

NPSH test is carried out by adjusting the pump capacity to the specified point Suction valve is throttled to create vacuum at the pump suction, meanwhile the discharge valve is adjusted to maintain the capacity to be constant The head will remain constant if the NPSHr is less than the NPSH available from the system The test is repeated by further suction valve throttling until the head cannot be maintained at constant capacity The NPSHr at 3% head drop is measured and calculated

The above test is repeated for different capacity so that a complete NPSHr curve for the pump being tested can be drawn

4.3 Experimental Facility

4.3.1 Description of Experimental Facility

A fully computerised pump test facility is designed and built to obtain more accurate pump performance test data This test facility is an open circuit and capable of testing pumps up to maximum flow rate of 2500 m3/h with maximum discharge pressure

of 20 kg/cm2; the maximum power available is 500 kw The flow is measured by a magnetic flow meter with accuracy within ± 0.25% of the readings The output signal is 0~10,000 Hz The suction and discharge pressure are measured by pressure transducers with output signal of 4~20 mA The power measurement is done by Ampere meter, Volt meter and Power Factor meter The test data are collected and processed by a personal computer to generate various test reports

Open and closed circuit test loops were considered during the test facility design The advantage of closed loop is that the pump suction is always flooded and it eliminates the requirement for priming before starting the pump It requires a large pressure vessel with proper cooling system to cool the water due to temperature rise during testing, a vacuum system is required to create a vacuum in the vessel in order to carry out the NPSHr test

The advantage of open test loop is simple construction and ease of operation A simple priming system is required to prime the pump before the start up To prevent turbulence that may affect the pump suction condition, two buffers were constructed in the underground water sump The total volume of water in the sump should be large enough to maintain the temperature rise of the water below 1 °C during the test The velocity of the flow in the sump is maintained below 0.3 m/s to ensure that the suction condition will not have an adverse effect on the pump performance

To run the test facility properly, special software has been developed to control its operation The program size is about 10 GB, it is edited by Visual Basic, Borland C++ and Assembler language The software can control the test facility in the following ways:

(1) Data input including testing method selection (using electric motor performance curve or using dynamometer), pump type selection (centrifugal pump or submersible pump), units to be used in measurement selection (metric or U.S.A.)

(2) Test run including taking readings for speed, power, suction and discharge pressure, voltage and current at each flow rate and converting these test data

to the specified speed

(3) Generating all the test report including pump original report, pump performance report, pump performance curve, curve for the verification of testing standard, pump NPSHr data report and pump NPSHr curve

4.3.2 Instrumentation

The instruments for the pump test stand shown in Figure 4.2 are:

• Danfoss MAG3100 digital output magnetic flow meter with flow calibrated

in m3/h A voltage is induced into a conductor which moves in a magnetic field With the electromagnetic measuring principle, the flowing fluid is the moving conductor The induced voltage is proportional to the flow velocity and is fed to the measuring amplifier by a pair of electrode Using the pipe cross-sectional area, the flow is calculated The transmitter converts the measured values coming from the sensors into standardised output signal

These output signals are transferred to the computer for processing and generating the test reports

• Endress and Hauser pressure transmitter calibrated in MPa The transmitter

is based on the principle that the pressure acts directly on the separating diaphragm with a filling liquid transmitting the pressure to a resistance bridge The bridge output voltage, which is proportional to the pressure, is measured, processed and converted to 4-20 mA output signal The output signal is then fed into the computer to be processed

These instruments are selected based on market availability, and their accuracies are required to meet the International Standard ISO 2548 for centrifugal pump testing The accuracies of various instruments are listed in Table 4.1 by referring to accuracy in percentage of reading These instrument accuracies are higher than those used for general industrial applications and able to meet the permissible systematic errors of measuring instruments set out in ISO 2548

4.4 Experimental Procedure

The pump test was first started with the discharge valve closed to measure the shut off head and power The test was continued by operating the discharge valve to vary the flow from shut- off to maximum flow

For each test, the rotational speed is set at 2900 rpm and 1450 rpm About 14 groups of readings from shut down to 122%-158% design flow rate are taken for performance test, and 5 groups of reading are taken for NPSHr test The readings for pump capacity, rotation speed, power input, pump suction pressure and discharge

pressure are also taken at each flow rate The performance test characteristics of design point are also recorded

For example, to test the fourth model pump M4 at rotational speeds of 2900 rpm and 1450 rpm, 14 groups of readings from shut down to 140% and 145% design flow rate were taken for performance test respectively, and 5 groups of readings were taken for NPSHr test The performance test characteristics of design point at rotational speed 2900 rpm are: Q = 270.84 m3/h, H = 154.21 m, BHP = 147.91 kW, η = 76.90% The performance test characteristics of design point at rotational speed 1450 rpm are: Q = 181.20 m3/h, H = 36.56 m, BHP = 22.23 kW, η = 81.15%

All the above readings are recorded automatically in the computer At each flow rate, 3 readings of rotational speed, input power, suction pressure and discharge pressure were taken within 0.5 second, and the mean readings were obtained by averaging them

The pump performance curves are plotted based on the above test data These curves are η -Q curve (efficiency verses capacity curve); H-Q curve (head verses capacity curve); P-Q curve (power verses capacity curve) and NPSHr-Q curve (NPSHr verses capacity curve)

4.5 Results and Discussion

4.5.1 Model Impellers Design Characteristics

As discussed in Chapter 1, three of model impellers named M3, M4 and M5 will perform experimental test on the test stand described above and the experimental results will be used to compare with the numerical ones for validation The design characteristics

of these model impellers are summarised as follows:

For model impeller M3 at rotational speed of 2900 rpm, the design specifications are: Q = 27 m3/h, H = 96 m, and ns = 30 At 1450 rpm, the design specifications for M3 are: Q = 14 m3/h, H = 24 m, and ns = 30 The impeller inner diameter is 46 mm; outer diameter is 269 mm; and the outer width is 6 mm with 4 straight vanes

For model impeller M4 at rotational speed of 2900 rpm, the design specifications are: Q = 270 m3/h, H = 148 m, and ns = 68; at 1450 rpm, the design specifications for M4 are: Q = 180 m3/h, H = 36 m, and ns = 81 The impeller inner diameter is 106mm; outer diameter is 337mm; and the outer width is 21mm with 6 twisted vanes

For model impeller M3 at 2900rpm, the design specifications are: Q=70m3/h, H=66m, and ns=64; at 1450rpm, the design specifications for M3 are: Q=45m3/h, H=14m, and ns=82 The impeller inner diameter is 70 mm; outer diameter is 221 mm; and the outer width is 14 mm with 6 twisted vanes

4.5.2 Experimental Results Discussions

Figures 4.3-4.5 show the experimental H-Q curves for model impellers M3, M4 and M5 at two rotational speeds of 2900 rpm and 1450 rpm The converted curves using similarity law are also drawn on each graph to make comparison with the experimental data It is found that the pump head H in column of water decreases as the flow rate Q through the pump impeller increases and there is no linear relation between H and Q For the actual pump head H, we have the following relation:

loss

H

where H is theoretical pump head; T h is head losses; a, b and c are constant loss

It can be seen that although theoretical pump head H decreases linearly as flow T

rate Q increases, head losses h doesn’t follow linear relation with Q If only two main loss

types of energy losses that occur in the pump are considered, friction loss at the blades and passage surfaces and shock loss due to the mismatch between the blade angle and the inlet flow direction, and neglect other types of losses, head losses h loss will be propotional to Q 2

From the above figures, a close relation between experimental curve and predicted curve using similarity law (4.1) is also shown on each graph; this shows the similarity law is valid for all the test curve, with this, all the test data given can be formulated using similarity law given in Eq (4.1) This also proves the pump test facility

is capable of performing to the required standard

Figures 4.6-4.8 show the experimental η-Q curves for model impellers M3, M4 and M5 at two rotational speeds Several observations can be made from these figures Firstly, it is found that the efficiency of the pump η increases as the flow rate Q increases However, the efficiency starts to decrease as the flow rate increases further

This can be verified from relationship (4.6) The water power of the pump (Pu) will initially increase more than the motor power output (Pp) when the flow rate (Q) is

increased Thus, the pump efficiency (η) will also be increased, however, after it reaches its maximum efficiency, the shock loss, frictional losses, circulation loss and other losses that occur in the pump will become more prominent and increase more rapidly than the

head supplied Hence, the power output of the pump (Pu) will decrease gradually and the motor power of the pump (Pp) will increase continuously As a result, the pump

efficiency begins to drop with increasing flow rate The experimental results verify this