CENTRIFUGAL PUMP- MEASURING PUMP PERFORMANCE

 CENTRIFUGAL PUMP- MEASURING PUMP PERFORMANCE

• Pump performance is usually described by curves of measured head and power consumption versus measured flow,  From these measured curves, an efficiency curve can be calculated. The measured pump performance is used in development projects for verification of computer models and to show that the pump meets the specification. 

• During production, the performance curves are measured to be sure they correspond to the catalogue curves within standard tolerances.

• Flow head and power consumption are measured during operation in a test bench that can imitate the system characteristics the pump can be exposed to. By varying the flow resistance in the test bench, a number of corresponding values of flow, differential pressure, power consumption and rotational speed can be measured to create the performance curves. Power consumption can be measured indirectly if a motor characteristic that contains corresponding values for rotational speed, electrical power, and shaft power is available. Pump performance depends on rotational speed and therefore it must be measured.

• During development, the test is done in a number of operating points from shut-off, i.e. no flow to maximum flow and in reversal from maximum flow to shut-off. To resolve the performance curves adequately, 10 - 15 operating points are usually enough. 

• Maintenance tests and final inspection tests are made as in house inspection tests or as certificate tests to provide the customer with documentation of the pump performance. Here 2 - 5 predefined operating points are usally sufficient. The flow is set and the corresponding head, electrical power consumption and possibly rotational speed are measured. The electrical power consumption is measured because the complete product performance is wanted.

• Grundfos builds test benches according to in-house standards where GS241A0540 is the most significant. The test itself is in accordance with the international standard ISO 9906.

1).FLOW:

• To Measure the flow, Grundfos uses magnetic inductive flowmeters. These are integrated in the test bench according to the in-house standard. Other flow measuring techniques based on orifice, vortex meters, and turbine wheels exist.

2). PRESSURE:

• Grundfos states pump performance in head and not pressure since head is in-dependent of the pumped fluid, see section 2.4. Head is calculated from total pressure measured up and down stream of the pump and density of the fluid.

• The total pressure is the sum of the static and dynamic pressure. The static pressure is measured with a pressure transducer, and the dynamic pressure is calculated from pipe diameters at the pressure outlets and flow. If the pressure transducers up and down stream of the pump are not located at the same height above ground, the geodetic pressure enters the expression for total pressure. 

• To achieve a good pressure measurement, the velocity profile must be uniform and non-rotating. The pump, pipe bends and valves affect the flow causing a nonuniform and rotating velocity profile in the pipe. The pressure taps must therefore be placed at a minimum distance to pump, pipe bends and other components in the pipe system.

• The pressure taps before the pump must be placed two pipe diameters upstream the pump, and at least four pipe diameters downstream pipe bends and valves, see figure 6.3. The pressure tap after the pump must be placed two pipe diameters after the pump, and at least two pipe diameters before any flow disturbances such as bends and valves.

• The pressure taps are designed so that the velocity in the pipe affects the static pressure measurement the least possible. To balance a possible bias in the velocity profile, each pressure tab has four measuring holes so that the measured pressure will be an average, see figure 6.4. 

• The measuring holes are drilled perpendicular in the pipe wall making them perpendicular to the flow. The measuring holes are small and have sharp edges to minimise the creation of vorticies in and around the holes, see figure 6.5.

• It is important that the pressure taps and the connection to the pressure transducer are completely vented before the pressure measurement is made. Air in the tube between the pressure tap and transducer causes errors in the pressure measurement.

• The pressure transducer measures the pressure at the end of the pressure tube. The measurements are corrected for difference in height Δz between the center of the pressure tap and the transducer to know the pressure at the pressure tap itself, see figure 6.4. Corrections for difference in height are also made between the pressure taps on the pump’s inlet and outlet side. If the pump is mounted in a well with free surface, the difference in height between fluid surface and the pressure tap on the pump’s outlet side must be corrected, see section 6.2.4.

 3). TEMPERATURE:

• The temperature of the fluid must be known to determine its density. The density is used for conversion between pressure and head and is found by table look up, see the chart ”Physical properties of water” at the below image.

4). Calculation of head:

• The head can be calculated when flow, pressure, fluid type, temperature and geometric sizes such as pipe diameter, distances and heights are known. The total head from flange to flange is defined by the following equation:

• Figure 6.6  shows where the measurements are made. The pressure outlets and the matching heads are marked with a ( ’ ). The pressure outlets are thus found in the positions S’1 and S’2and the expression for the total head is therefore

5). GENERAL CALCULATION OF HEAD:

• In practise a pump test is not always made on a horizontal pipe, see figure 6.7. This results in a difference in height between the centers of the pump in and outlet, z’1and z’2 , and the centers of the inlet and outlet flanges, z1and z2 respectively. 

• The manometer can, furthermore, be placed with a difference in height compared to the pipe centre. These differences in height must be taken into consideration in the calculation of head. Because the manometer only measures the static pressure, the dynamic pressure must also be taken into account. 

• The dynamic pressure depends on the pipe diameter and can be different on each side of the pump. Figure 6.8 illustrates the basic version of a pump test in a pipe. The total head which is defined by the pressures p1and p2and the velocities U1and U2in the inlet and outlet flanges S1 and S2can be calculated by means of the following equation:

Using the measured sizes in S’1and S’2, the general expression for the total head is

6).POWER CONSUMPTION:

• Distinction is made between measurement of the shaft power P2and added electric power P1. The shaft power can best be determined as the product of measured angular velocity w and the torque on the shaft which is measured by means of a torque measuring device. The shaft power can alternatively be measured on the basis of P1.

• However, this implies that the motor characteristic is known. In this case, it is important to be aware that the motor characteristic changes over time because of bearing wear and due to changes in temperature and voltage.

• The power consumption depends on the fluid density. The measured power consumption is therefore usually corrected so that it applies to a standard fluid with a density of 1000 kg/m3 which corresponds to water at 4°C. Head and flow are independent of the density of the pumped fluid.

7). ROTATIONAL SPEED:

• The rotational speed is typically measured by using an optic counter or magnetically with a coil around the motor. The rotational speed can alternatively be measured by means of the motor characteristic and measured P1.

• This method is, however, more uncertain because it is indirect and because the motor characteristic, as mentioned above, changes over time.

• The pump performance is often given for a constant rotational speed. By means of affinity equations, described in section 4.5, the performance can be converted to another speed. The flow, head and power consumption are hereby changed but the efficiency is not changed considerably if the scaling of the speed is smaller than ± 20 %.