On this occasion, our ongoing series from the PIA’s Australian Pump Technical Handbook investigates why and how pump testing is used.
Factory performance tests
Pumps are tested for a variety of reasons, not all of which are solely to meet the customer’s test requirements.
The final user’s interest in the machine is generally confined to the following:
- The total head at the duty flow.
- The power consumption at the duty flow and head directly indicates the pump efficiency.
- Suction performance, defined by NPSH.
- The mechanical integrity of the machine.
The customer may also be interested in the following:
- The level of noise emitted by the machine.
- The vibration level of the machine.
- A functional check on auxiliary equipment.
- The starting capability under simulated site conditions.
- To verify the curve shape and rise to zero flow.
AS/ISO9906:2018 covers the Australian Standard for pump performance testing and is a direct adoption of ISO9906:2012, Rotodynamic pumps – Hydraulic performance acceptance tests – Grades 1, 2 and 3.
It should be noted that the ANSI/HI 14.6-2016 standard is harmonised with the ISO standard, so testing under either standard will enable a similar test result.
Where required and specified, ISO and HI/ANSI standards are available to provide guidelines for acceptable test levels for pump noise and vibration testing. BS EN ISO 20361 provides guidelines on noise tests, and Europump has also published Pump vibration standard guidelines.
The centrifugal pump test standard applies to water or liquids with similar viscosity. Since a centrifugal pump may eventually be pumping viscous liquid, ISO 17766 provides conversion factors to predict its actual site performance.
Pump test tolerances
The allowable tolerances and error limits are clearly defined in the separate testing grades allowed in the standard. For information about selecting suitable acceptance grades, refer to section 10.3 of the Australian Pump Technical Handbook.
Every measurement is inevitably subject to inaccuracies, even if the measuring procedure, the instruments used and the analysis directives fully comply with prevailing acceptance rules. The test standards consider these inaccuracies (errors of measurement) when determining an allowable tolerance.
The test code expresses the rules for deciding whether a pump meets the guaranteed conditions of a contract. These rules vary greatly for Grade 1, 2, and 3 tests, and the allowable errors in the measuring equipment used for Grade 1 tests are more stringent than those required for Grades 2 and 3.
The verification of each guarantee shall be accomplished by comparing the results obtained from the tests with the values guaranteed in the contract, including their associated tolerances. See the following table:
|
Test parameter |
Grade | 1 | 2 | 3 |
Guarantee requirement |
|||
| ΔTQ | 10% | 16% | 18% | |||||
| ΔTH | 6% | 10% | 14% | |||||
| Acceptance Grade | 1U | 1E | 1B | 2B | 2U | 3B | ||
| Rate of flow | tQ | +10% | ±5% | ±8% | +16% | ±9% | Mandatory
|
|
| Total head | tH | +6% | ±3% | ±5% | +10% | ±7% | ||
| Power | tP | +10% | -4% | +8% | +16% | +9% | Optional | |
| Efficiency | tɳ | ≥0% | -3% | -5% | -7% | |||
All tolerances are percentages of the values guaranteed.
While the test standard nominates default acceptance grades for various industries and pump kilowatt ratings, the most common test is Acceptance Grade 2B.
For a pump performance test, the guarantee point (duty point) must specify a guaranteed flow and a guaranteed head. Unless specified in a contract, the power or efficiency is an optional guarantee value. Power is directly measured, while efficiency is a derived value.
In verifying if a guaranteed Grade 1B, 1E, 2B or 3B condition is met a tolerance cross with the horizontal line and the vertical line is drawn through the guarantee point. Similarly, Grades 1U and 2U guaranteed conditions are met by an L with the horizontal line and the vertical line drawn from the guarantee point.
The guarantee on the head and flow rate has been met if the H–Q (measured head and flow) curve cuts or at least touches the vertical and/or horizontal line (see Fig.2 for Grade 2B acceptance).
The efficiency shall be derived from the measured H–Q curve, which is intersected by the straight line passing through the specified flow rate guarantee, head guarantee, and the zero of the Q-H axes, and where a vertical line intersects the efficiency curve.
The guarantee condition on efficiency () is within tolerance if the efficiency value at this point of intersection is higher than or at least equal to x (1 – ), as per Fig 2.
When it is contracted to be tested, a measured NPSHR value shall not exceed the guaranteed NPSHR value.
To determine the complete hydraulic performance of a pump, it is necessary to measure the following:
- Rate of Flow: This can be done using classical venturi flow meters, orifice plates, volumetric measuring tanks, weirs, electromagnetic meters, or velocity measuring devices used in a known cross-sectional area of the pipe. A throttle valve on the pump discharge controls the flow rate.
- Suction and Discharge Pressure: Pressure gauges, transmitters and transducers are the most common devices for direct pressure measurement. Often, gauges in factory test facilities read directly in metres of water, deleting the need to convert a kPa pressure reading into metres of water head. Pressure tappings should be placed in a parallel section of pipe located at a distance of two diameters from the pump flanges. Grade 1 requires four tapping points around the pipe, whilst grades 2 and 3 only need one tapping on the horizontal centreline. The height of the gauges above or below the pump datum must be included in the head calculation. Most factory pump tests eliminate this by positioning the suction and discharge gauges at the same height (via flexible tubing), whereby the correction becomes zero. The pump datum is the shaft centreline for horizontal pumps. As the total pressure in the suction and discharge pipes includes a velocity head component, these must be calculated and included in the total head.
- Power Measurement: The majority of pumps are driven by electric motors. The normal method for determining the pump input power is to measure the motor input kW using a suitable power analyser. The pump input power is then calculated using motor efficiency figures corresponding to the actual load. Where the pump is not directly coupled, e.g., belt or gearbox drive, consideration needs to be made for additional drive losses.
- Speed of Rotation: Optical frequency response tachometers are the most common method for measuring the pump input speed (r/min). Reflective tape is attached to the pump shaft to trigger the optical response device.
If a speed-changing device, such as a belt drive or gearbox, is used between the driver and pump, the loss of this device must be known to arrive at the pump’s input power. Modern variable-speed devices, such as frequency inverters, have electrical losses that need to be considered to arrive at the total input power.
It should be noted that the rotation speed is not usually a guaranteed performance value. However, it is a significant value to know, especially for performance diagnostic evaluation either while testing or later on site.
Whenever possible, a manufacturer’s test on an engineered pump should be at the specified rated speed and power. Under these conditions, NPSHR tests can clearly determine the zone of suction instability or onset of cavitation. This is particularly important where low NPSHA conditions exist at the site.
Where pumps cannot be tested at the rated speed, e.g., high-head pumps such as boiler feed units or units with variable speed drives, problems may be encountered where testing facilities require performance testing. In many such cases, reduced speed tests are conducted, and the affinity laws, as described in Section 6.4 of the Australian Pump Technical Handbook, are used to translate the test data to reflect the performance at the actual site operational speed. When it is agreed between the purchaser and supplier, the test standard allows test speeds to be within 50 to 120 per cent of the specified rotation speed (actual site operational speed).
In most instances, the pump is installed on the test bed coupled to its own driver, and both are mounted on a combined baseplate. Thus, the behaviour of the entire unit can be monitored during factory tests. Manufacturers outlay considerable capital to provide and maintain an adequate test facility. While testing costs are relatively expensive, the potential improvement in plant/station reliability outweighs and justifies these additional costs. If the reliability of operations is improved and there is a reduction in standby equipment results, then the cost will be justified. Specifying a factory test provides baseline data that will be available for all future analyses of a pumping system.
Site Tests
Many clients specify testing of pumps after initial installation at the site to prove the guaranteed performance. If a pump has been factory tested and further tests are required after installation, reproducing the factory test results can often be difficult. Often, instrumentation and the layout are such that accuracy of measurement is not possible, and the results can differ greatly from those obtained in the factory test facility.
Actual site testing should be regarded more as part of the unit’s commissioning. This means ensuring that the unit operates satisfactorily both mechanically and hydraulically. The commissioning personnel should also ensure that the driver is not overloaded. Most often, site tests are performed using pressure gauge readings in kPa or bar, so it is important to convert these readings into metres of head and include the velocity head in the suction and discharge pipes and the height of the gauges relative to the pump datum.
Pump test layout, report and calculations
Typical Pump Performance Test Layout
Fig. 1
Calculations have been included for point 1 of the test sheet.
Fig. 2
Fig 15.3
Refer to Chapter 15 of the Australian Pump Technical Handbook for a sample calculation.
Summary
This chapter details how the Australian Pump Test Standard operates, how to select the correct test grade for your requirements, and how to translate the test results recorded to determine if these results meet the requirements of your test grade chosen, ensuring that a successful pump test is undertaken. It also provides comprehensive guidance on pump test tolerances. It outlines the acceptable deviations in pump performance from stated specifications. By correctly understanding and applying these tolerances, industry professionals can ensure accurate pump evaluation, reliable equipment selection, and effective dispute resolution.
Image: Cozine/stock.adobe.com
