Whether a pump is new, used and recently modified, or freshly refurbished with a critical service mission, end users routinely specify hydraulic performance acceptance tests to guarantee pump performance. Performance testing can be complex and should account for a multitude of variables.
Test loop designs can vary both in system design and instrumentation selection. Image 1 shows an example of an open pit, vertical test loop used to test large, high-capacity vertical pumps; however, test loop designs vary greatly across the industry.
Pump applications can represent a wide range of tolerance requirements for flow, head, efficiency, brake horsepower (BHP) and net positive suction head required (NPSHr). For these reasons, the pump industry adopted testing standards that are used to detail the methodology for testing centrifugal pumps and define whether a pump has met the performance specifications.
The end user must ensure that the testing standard, along with the acceptance grade, is properly specified from the beginning in the procurement contract, based on the specific pumping system requirements.
The following checklist is primarily based on testing to Hydraulic Institute (HI) 14.6-2016, although American Society of Mechanical Engineers (ASME) and American Petroleum Institute (API) specifications can be used with equivalent results.
End users must also specify in the purchase contract if they would like the acceptance test to be witnessed or non-witnessed. Witnessed testing will typically come with an increased cost; however, this practice allows the purchaser to be present during testing and data collection, ensuring the test’s integrity. Many modern test facilities offer remote witnessing via web cams and screen sharing. This eliminates travel expenses for the purchaser, while achieving the goal of a witnessed test.
For on-site witnessed tests, the following checklist will offer the end user a helpful guide to ensure an accurate and successful process.
Before the Test
❑ Verify that any necessary elevation corrections will be made to the head readings. Unless the pressure gauges are located precisely at the suction and discharge centerlines, a head correction must be made to account for the difference between the pump centerline elevation and the measurement elevation.
❑ Ensure that 10 pipe diameters of smooth bore, straight pipe are present before, and five pipe diameters are present after, the flow measurement instrumentation. This ensures a fully developed, uniform velocity profile at the flow measurement section.
❑ Verify that pressure tappings are no closer than two pipe diameters away from the suction/discharge flanges. This ensures a fully developed, uniform velocity profile at the pressure measurement section.
❑ Confirm that the methodology for calculating pump BHP meets expectations. If motor input power is used in conjunction with the motor nameplate efficiency, ensure that the test is being conducted at the motor’s rated or tested speed with a pure electrical signal (line power). If the motor is powered with a variable frequency drive (VFD), the nameplate efficiency is no longer valid, especially at different speeds. The most accurate method for determining BHP is a calibrated torque transducer and tachometer. Most commercially available torque transducers directly couple between the motor and pump (see Image 2). They are designed to give calibrated data for both torque and revolutions per minute (rpm).
❑ Verify that the rotational speed is being measured accurately with a calibrated tachometer. Using the motor’s rated speed for every flow point will result in inaccuracies. The motor slip changes as loading conditions change, moving along the BHP/torque curve from pump shutoff to run-out.
❑ Verify that the water being used meets cleanliness and chloride requirements specifically for pumps constructed from austenitic stainless steel to alleviate stress-corrosion cracking concerns.
It is prudent to do this well in advance of the test date, before the pump is filled with water.