Vibration in rotating equipment, like fever in the human body, indicates that an underlying issue exists but does not identify the exact cause. One source of vibration in centrifugal pumps is cavitation due to insufficient net positive suction head (NPSH) margin.
Cavitation stemming from inadequate NPSH margin progresses through three stages:
- Head loss occurs first. The NPSH test uses this head loss to determine the actual NPSH required (NPSHR) value (or NPSH3, depending on the situation). Although cavitation initiates at this stage, impeller erosion is typically negligible, allowing this stage to be considered nondestructive testing.
- As the NPSH margin is further reduced, cavitation enters its second phase. Pitting and erosion damage the impeller.
- With additional NPSH margin reduction, structural vibration and audible noise generation mark cavitation's third stage. Experiments show cavitation produces a distinct 147 Hz tone and associated harmonics. This tonal signature can facilitate NPSHR determination during pump testing. (Chudina,2003)
While pump operation yields noise influenced by factors like size, geometry, and coupling alignment, this low-frequency tone remains discernible. Trends manifest in higher-frequency sound spectra as well.
Based on author's point of view, potential solutions to address vibration issues during pump commissioning may be considered in the following order based on likely cost and feasibility:
- ?Recalculate the net positive suction head available (NPSHA). Options to increase NPSHA could include raising inlet drum height if located on a multi-level platform or reducing upstream pressure drops by modifying orifice plates.
- Inspect and clean inlet strainers to remove any debris. If strainers are clean, consider a higher mesh rating or temporarily removing strainer screens. Note this may impact the pump warranty.
- Check for inadequate priming and air entrapment. Consider venting options to release trapped air or modify suction piping layout to facilitate better filling and venting.
- Validate pump selection and working conditions. Replace the pump or add minimum continuous recirculation flow if oversized.
- Inspect bearings and replace them if wear, damage, or inadequate lubrication is evident.
- Verify published pump net positive suction head required (NPSHr). Arrange pump manufacturer shop testing if uncertain.
If vibration persists after systematically addressing the above items, further investigate root causes like misalignment, soft foot, or other mechanical installation issues.
In addition to the article by Chudina, which is highly recommended, here is additional sources if readers are interested :
- Chudina, M. (2003) “Noise As an Indicator of Cavitation in a Centrifugal Pump,” Acoustical Physics, 49(4), pp. 463–474. doi: 10.1134/1.1591303.
- Bachus, L. and Custodio, A. (2003) Know and understand centrifugal pumps. Burlington: Elsevier. Available at: INSERT-MISSING-URL (Accessed: December 11, 2023).
Maintenance Engineer at Yanbu Aramco Sinopec Refining Company (YASREF) Ltd.
1 年Thanks Amir. It is really informative subject. I would like to add one contribution cause that could increase the required net positive suction head (NPSHr) that eventually will lead to cavitation, which is Wear Ring Clearance. Wear rings are crucial component in centrifugal pump that minimize the internal recirculation of fluid from higher pressure zone to lower pressure zone (front and back impeller shrouds). In case that the clearance of front wear ring is too high, higher the recirculation flow from higher pressure zone to the suction side at impeller eye inlet that can increase the turbulence & NPSHr, and lead to cavitation. In case that the back wear ring clearance is too high, higher pressure flow will be recirculated into the mechanical seal chamber, so seal in this case will deal with discharge pressure not suction one, leading to seal failure. Furthermore, higher axial forces will be generated on the back shroud toward suction inlet causing higher load on the thrust bearing. Thanks,