Laser scanning of pumps30 November 2014
Laser scanning technologies can be used during scheduled outages to identify impeller hydraulic imbalance in centrifugal pumps. By Jim Aubrey and Greg Piotrowski
More than 30 years of high-energy centrifugal pump repairs have identified a significant number of open-vane impellers that exhibit undesirable hydraulic imbalance during operation. It has been determined that this imbalance is caused by non-uniformly spaced vanes resulting from casting errors at foundries.
Modern laser scanning and modelling technology can be utilized during regularly-scheduled maintenance to efficiently identify these asymmetries before the pumps are put back into service.
In early November 2011, a large single-stage centrifugal pump was removed from service at a nuclear power plant after only 18 months of service due to excessive vibration. The pump was sent to Rotating Equipment Repair, Inc. (RER), a company that specializes in the reverse engineering, repair, and manufacture of new parts for high-energy centrifugal pumps. The pump was fully disassembled, cleaned, and inspected.
The inspection revealed heavy one-sided wear on the shaft sleeves (five total), exceeding 0.375in (9mm) at the lowermost sleeve, and lessening at each sleeve location travelling up the pump. All sleeves were worn at the same angular position, from contact with the pump's cutless rubber bearings. Heavy cavitation damage was observed on only one of the impeller's six vanes. The cutless rubber bearings were heavily worn and the clearances were excessive, resulting in high vibration readings observed by plant personnel.
Finding the failure mode
Typical failure modes were ruled out. Shaft misalignment occurs when the motor-to-pump alignment is not properly performed. If the shaft is not centred within the pump bearings before being coupled to the motor, the shaft will rub against one side of the bearings. This causes high vibration, shaft wear and one-sided bearing wear. However, the one-sided wear was seen on the shaft, not the bearings.
Register fits are used within a pump to create one common centre line between pump components. Typically the fit-to-fit clearances within a pump are specified to be 0.001"-0.003" (0.02-0.07mm) loose. If these clearances are too loose, components containing bearings will be misaligned. The consequences of excessive register fit clearances are the same as shaft misalignment: high vibration, shaft wear and one-sided bearing wear. This was again ruled out as a cause because the one-sided wear was seen on the shaft, not the bearings.
Dynamic unbalance occurs when the centre of mass is not aligned with the central axis of the rotor. When the rotor starts to spin, the unbalance causes high vibration, bearing and shaft wear. This was ruled out as a cause by verifying the dynamic balance of the impeller using a Schenck balance machine.
RER's engineers next turned to their 3D laser scanning technology to check the symmetry of the impeller's vane geometry. A laser scanner was used to accurately acquire millions of XYZ coordinates in a matter of minutes. The resulting so-called point cloud was imported into a software package, which turned the point data into a surface mesh made up of millions of polygons. This surface mesh was then analysed to determine the vane symmetry and spacing about the impeller's central axis. The analysis was summarized in a report showing how the impeller's surface geometry deviated with respect to a perfectly-symmetric impeller.
The deviation report identified two vanes, adjacent to one another, which were unequally spaced by 0.25in or 6.4mm (1.03°) and 0.5 in (2.05°) when compared to the symmetric model. The impeller keyway was used to establish the correct angular position of the impeller vanes with respect to the shaft sleeve wear. The wear lined up perfectly with the mid-point of the two improperly-spaced vanes.
RER's engineering team, in consultation with plant engineering, concluded that the improperly-spaced vanes were generating a large hydraulic imbalance during pump operation. This imbalance was causing the impeller to orbit eccentrically within the pump bearings, resulting in the heavy one-sided sleeve wear.
Making a new impeller
To fix this condition, RER manufactured a new impeller from the symmetric solid model. The manufacturing process utilized five-axis CNC machining to create a wood pattern. The wood pattern then generated a sand mold, which was used by a steel foundry during the casting process. After the new impeller was finish-machined, the same laser-scanning process was used to confirm that the new impeller was correct to the solid model and that the vane geometry was symmetric about the central axis.
The sleeves were also upgraded to incorporate a nickel-tungsten carbide laser deposition-welded surface approximately 0.030-inch deep for surface wear resistance.
Hydraulic performance of the new impeller was verified virtually with computational fluid dynamic analysis, and physically in a high-capacity performance test loop.
The pump was installed and put into service in March 2012, and has been running since with very low vibration readings. Based upon these vibration readings, RER and plant engineering estimate that the bearings (which were replaced, but not upgraded) are still in good condition. They have recommended running the pump through its previously scheduled maintenance until the next refuelling outage, and possibly longer.
About the authors
Jim Aubrey and Greg Piotrowski, Rotating Equipment Repair, Inc