Trimming the impeller
Increasing the centrifugal pump performance by modifying the impeller 12-6
The following information will apply to closed, semi-open and open impellers unless noted otherwise:
- The affinity laws predict the result of this action, but they are not as accurate as we would like them to be, especially if we are making more than a 10% reduction in impeller diameter. There are several reasons why this is true :
- The affinity laws assume the impeller shrouds are parallel. This is true only in low specific speed pumps.
- There is increased turbulence at the vane tips as the impeller is trimmed because the shroud to casing clearance (Gap “A”) is increasing. This is sometimes referred to as “slip” .
- The liquid exit angle is changed as the impeller is cut back, so the head/capacity curve becomes steeper.
- Mixed flow (the popular version) are more affected than low specific speed, radial vane impellers (high head/low capacity).
- I would recommend using only 75% of the calculated cut to stay on the safe side.
- The greater the impeller reduction and the higher the specific speed of the impeller, the more the pump efficiency will decrease with impeller trimming.
- Impeller diameter reductions greater than 5% to 10% of the maximum will increase the NPSHR (net positive suction head required). If there is a close margin between NPSHA (net positive suction head available) and NPSHR (net positive suction head required) be sure to check with your pump manufacturer for information on how these two will be affected by an impeller reduction. Unfortunately many pump manufacturers do not publish this information along with their pump curve.
- Excessive shroud to casing clearance (Gap “A” ) and the resultant recirculation to the low pressure side of the pump will produce “eddy flows” around the impeller causing low frequency axial vibrations that can translate to mechanical seal problems. This can be a real concern in large pumps of over 250 horsepower (195 KW) or pumps pumping heads in excess of 650 feet (198 meters).
- For many years pump people have been machining the vane tips to reduce the vane passing frequency vibrations (Gap “B”) while carefully maintaining Gap “A”. The pulsating forces acting on the impeller can be reduced by 80% to 85% by increasing gap “B” from 1% to 6%.
- For impeller diameters up to 14 inches (355 mm) gap “B” should be at least 4% of the impeller diameter to prevent “Vane passing syndrome cavitation” problems. Above 14″ (355 mm) Gap “B” should be at least 6% of the impeller diameter to prevent this type of cavitation.
Although both the vanes and shrouds are often cut in end suction, volute type centrifugal pumps; it is not a good idea to do this in double suction designs. With these types of pumps you can reduce the vane diameters, but the shrouds should remain untouched.
- Structural strength is a consideration when deciding how much to reduce the vane diameter in double ended pumps because you could leave to much unsupported shroud. Some manufacturers recommend an oblique cut that will improve the vane exit flow and add some strength to the shrouds.
- Machining a radius where the trimmed vane meets the shroud is another good idea to add strength to the assembly. Square corners are never a good idea.
- Under filing will increase the pump capacity, especially for large circulating pumps. One look at the above diagram will make this obvious.
- The exit angle of the fluid will change resulting in a higher head at design flow, but no change in shut off head.
- Because of reductions in the wake of the fluid exiting the vanes. The efficiency of the pump should improve slightly. The smaller the size of the pump the larger the effect.
- The technique of under filing is critical. Sharp corners, where the vane joins the shroud, can initiate cracks and eventual impeller failure.
- At least 0.0125 inches (3 mm) of vane tip thickness must remain after the under filing.
- On February 18, 2018