On bench testing of centrifugal pumps
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I would like to ask everyone: I have an OH2-type centrifugal pump that is used to transport gasoline. Recently, it has shown a problem in that both the pressure and flow rate at the pump’s outlet are lower than those of a spare pump of the same model and specifications (for simplicity, the abnormal pump will be referred to as Pump A, while the normal one will be called Pump B). The impeller of pump A, the inlet filter, the gap around the flange, etc. were inspected one after another; the impeller was even replaced with a new one (of the same size), but the flow rate of pump A was still slightly lower than that of pump B. Other parameters such as vibration, temperature, and the LQ value of the bearings for pump A were all within normal ranges. Later, an attempt was made to close both the outlet valves of pumps A and B, in order to observe the current during startup. It was found that the startup current of pump A, which does not operate at full capacity, was still 5A higher than that of the normally functioning pump B. Now, the specialized department plans to send Pump A to a certain pump factory for bench testing. I would like to ask, when Pump A has normal vibration and temperature levels, but its outlet pressure and flow rate are slightly low, what can be determined through bench testing? If the water used for testing there works fine when pumped, does that necessarily mean there’s no problem with the pump? Thank you!
Personal opinion: 1. First, move AB to a different position at the pump station to see if the problem persists, in order to rule out issues with the pipelines, valves, and other devices. 2. Examine the pump parameters in terms of the net positive suction head to rule out the possibility of critical cavitation; if the remaining head after calculation is very high, cavitation is impossible, and it is likely that there are differences within the A-B pump itself. 3. Since the impeller is the main component that does the work, a simple dimensional measurement of the impellers of the two pumps should be carried out. If the degree of blade twisting cannot be measured, it can be identified by visual inspection; A and B should be essentially identical for everything to be normal. 4. If nothing else works, testing on a test bench is the final option; full performance tests as well as cavitation head tests can be conducted. These tests can be compared with the initial pump factory test reports and with the parameters listed on the pump’s nameplate. The cavitation head test helps determine whether the pump meets the required standards and suits the operational conditions at the site. 5.1 On the test bench, the vibration of the pump may not be significant, but it is still possible to determine the value of the net positive suction head. Since vibration can be greatly influenced by the net positive suction head, the net positive suction head test can be used to determine whether pump A meets the required process parameters. 5.2 Information regarding flow rate and head (exit pressure) can be determined through performance tests.
Try swapping the rotors of the two pumps (except for the pump casings)
Since it is a production facility, it is not possible to shut down both pumps at the same time; therefore, swapping them is not feasible for now:dizzy: Thank you to the FPPZ moderator for their patient and thorough answers! ~
Has the pipeline been checked? There’s a spare impeller that can be tried
All the pipelines have been checked, and the replacement impellers that were purchased – which are identical to the original ones – have also been installed, but it still doesn’t work. The outlet check valve has also been inspected. Currently, I plan to replace the pump’s largest impeller; I’ll give it a try once the spare parts arrive. Since the relevant department suggested conducting bench tests, our workshop believes that the pump doesn’t generate much vibration to begin with; its flow rate and pressure are only slightly lower than those of the other pump. This means it does not meet the production requirements for handling high loads (reducing the load? Impossible – it affects the company’s profitability). Even if the bench tests show no problems, it still does not satisfy the requirements for actual production. Hehe
Are the results of the pump’s factory performance tests, cavitation tests, and vibration tests all satisfactory?
It has been in use for over 7 years, and the problems only appeared recently; the factory test reports showed everything was fine.
Have you checked the motor? Are the motor speeds of A and B exactly the same? If it’s only slightly smaller, it can be completely ignored; it’s not specified that the pumps must have 100% identical specifications. You can try swapping the rotors of pumps A and B and run the machine to see.
After more than seven years like this, could it be due to changes in the hydraulic components caused by corrosion and wear?