At the time this article was published, Ed Nelson was a consultant to the turbomachinery and rotating equipment industries. He was a member of the Pumps & Systems editorial advisory board.
How this practice can affect pump efficiency, NPSHr, axial vibration and more.
What are the effects of trimming an impeller on radial vibration?
Careful machining of the volute or diffuser tips to increase Gap B while maintaining Gap A has been used for a number of years to greatly reduce the vane-passing frequency vibration. The pulsating hydraulic forces acting on the impeller can be reduced by 80 to 85 percent by increasing the radial Gap B from 1 to 6 percent.
Image 4. Impeller vane overfiling
There is no loss of overall pump efficiency when the diffuser or volute inlet tips are recessed, contrary to the expectations of many pump designers.
The slight efficiency improvement results from the reduction of various energy-consuming phenomena, such as the high noise level, shock and vibration caused by vane-passing frequency, and the stall generated at the diffuser inlet. Table 1, on page 72, gives recommended dimensions from Dr. Elemer Makay for radial gaps of the pump impeller to casing. If the number of impeller vanes and the number of diffuser/volute vanes are even, the radial gap must be larger by about 4 percent.
Table 1. Recommended radial gaps for pumps
When trimming an impeller from its maximum diameter to adjust the head and flow developed by a centrifugal pump, what is the best way to cut the impeller? Is it best to trim the impeller vanes and the shrouds or just the vanes?
No hard and fast guidelines for the mechanical aspects of impeller trimming exist, but there are several pump construction and hydraulic design factors to consider while making the decision of what to trim.
How the impeller is trimmed will greatly influence the hydraulic performance of the pump as well as the vibration levels experienced. Evaluate the hydraulic characteristics before you decide how to trim the impeller.
For volute type pumps, the entire impeller, vanes and shrouds may be cut as shown in Image 1. However, in some pumps, this method will alter Gap A (shroud-to-case clearance), leading to uneven flow distribution at the impeller exit area, which can cause axial vibration and other problems.
The double suction impeller type pump is especially sensitive to problems caused by increasing Gap A, so trimming the entire impeller is not a good choice. It is best to cut the impeller vanes obliquely (Image 2), which leaves the shrouds unchanged, or cut the vanes only (Image 3). Trimming the vanes only tends to even out the exit flow pattern and reduce recirculation tendencies at the exit area. Gap A should be about 0.050 inch (radial) for minimum vibration due to vane-passing frequency.
In most diffuser type pumps, it is best to trim only the vanes (Image 3) to control tip recirculation and the ill effects of an increased Gap A. This cut yields a more stable head curve.
The uniform flow reduces the tendency for tip recirculation and the possibility of suction recirculation is greatly reduced at the exit area.
Structural strength of the shrouds is a factor in the decision in how to trim the impeller.
There may be too much unsupported shroud left after a major reduction in diameter. The oblique cut leaves the shrouds unchanged and solves the structural strength problem and improves the exit flow pattern.
I frequently encounter “vane-passing” frequencies during vibration analysis of a pump. What are some of the methods that can be used to reduce this problem?
The most effective method of reducing vane-passing frequencies is carefully maintaining proper Gap A and Gap B clearances to reduce impeller-casing interaction.
Sometimes, impellers manufactured with blunt vane tips cause disturbances in the impeller exit area and in the volute area by generating hydraulic “hammer” even when the impeller outside diameter is the correct distance from the cut water (Gap B).
Image 5. Sharpening of impeller vanes
Corrections can be achieved by two methods:
Overfiling: This disturbance may be partly or entirely eliminated by tapering the vanes by “overfiling” or removal of metal on the leading face of the vanes (Image 4). This technique has the additional advantage of restoring the vane exit angle to near that of the maximum impeller design (i.e., before the diameter was reduced).
Underfiling: Sharpening the underside of the trailing edge of the vane (Image 5) can enlarge the outlet area of the liquid channel. This will generally result in about 5 percent more head near the best efficiency point (BEP), depending on the outlet vane angle. At least 1/8 inch of vane tip thickness must be left. Sharpening the vanes also improves the efficiency slightly. Where there are high stage pressures, you must sharpen the vanes carefully because the vanes are under high static and dynamic stresses.