Subject : 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:
- We will be using the term Gap "A" to describe the
clearance between the impeller shrouds to the volute or
- Gap "B" describes the clearance between the impeller
vanes and the casing or volute.
- "D" describes the diameters of the vanes and
happens when you reduce the impeller vane diameter? Several
- 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
- 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 Unfortunately many pump manufacturers do not publish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
- 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
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.
filing the exit vanes is usually a good idea. Please look at
the following diagram :
- (d) is the distance before filling
- (df) is the distance after filing
- Under filing will increase the pump capacity, especially for
large circulating pumps. One look at the above diagram will make
- 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
- At least 0.0125 inches (3 mm) of vane tip thickness must
remain after the under filing.
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