SUBJECT: All about impellers
- The open impeller is nothing more than a series of vanes
attached to a central hub for mounting on the shaft without any
form of side wall or shroud. This design is much more sensitive to
vane wear than the semi or closed impeller.
- The semi-open impeller incorporates a single shroud at the
back of the impeller. This is the most common design used in the
United States and the one you find on most ANSI standard pumps.
- The shroud often has "cast in" pump out vanes that will
help circulate lubricating liquid from the lantern ring
connection through the packing ahead of the lantern ring.
- Most modern pump designs allow you to adjust the semi- open
impeller without disassembling the pump. This is a tremendous
advantage if you want to maintain the pump efficiency by
adjusting the impeller to volute clearance for thermal
expansion and volute/impeller wear. Remember that if there is a
mechanical seal in the stuffing box any impeller adjustment can
interfere with the seal face loading. Those designs that adjust
to the volute (Goulds type) will unload the seal faces and
those that adjust to the back plate (Flowserve or Duriron type)
will increase the seal face loading.
- A typical volute or back plate clearance for a semi open
impeller would be 0.015 to 0.020 inches (0,4 to 0,5 mm). For
each 0.002 inches (0,05 mm) you increase this clearance, the
pump will lose about 1% of its capacity.
- The closed impeller has a shroud on either side of the vanes.
This is the most common design found with ISO standard pumps, oil
refinery applications and the design you see on double ended
- To maintain impeller efficiency you are required to replace
the wear rings after the original clearance has doubled. The
first problem is to determine when it has doubled, and then you
have to take the pump apart to replace them. The result is that
timely replacement is seldom done, and pump loss of efficiency
with resultant vibration becomes the rule.
- The general rule of thumb is that the pump will lose about 1%
of its capacity for each excessive 0.001 inches (0,025 mm) of
- Since the wear ring clearance is usually smaller than the area
of the balance holes drilled through the impeller, you will lose
the advantage of suction
recirculation as stuffing box pressure is very close to
- The impeller specific speed
number describes the shape of the impeller
- The shape of the head/ capacity curve is a function of
specific speed, but the designer has some control of the head and
capacity through the selection of the vane angle and the number of
- The pump with the highest specific speed impeller, that will
meet the requirements of the system, probably will be the smallest
and the least expensive. The bad news is that it will run at the
highest speed and be subject to maximum wear and damage from
flow impellers (low specific speed numbers)
- They should be specified for high head and low flow
- They seldom exceed 6 inches (150 mm) in diameter and run at
the higher motor speeds
- The casing is normally concentric with the impeller as opposed
to the volute type casings normally found in the industry..
- These impellers exhibit a flat head/capacity curve from shut
off to about 75% of their best efficiency and then the curve falls
- Radial flow impellers are normally started with a discharge
valve shut to save start up power.
flow impellers (high specific speed numbers)
- They run at the highest efficiency
- They have the lowest NPSH requirement.
- They require the highest power requirement at shut off, so
they are normally started with the discharge valve open.
can be manufactured from a variety of materials:
We would like a combination of a hard material to resist wear and
a corrosion resistant material to insure long life. This is often a
conflict in terms because when we heat treat a metal to get the
hardness we need, we lose corrosion resistance. The softer metals can
have corrosion resistance, but they lack the hardness we need for
long wear life. The best materials that combine these features are
called the "Duplex Metals". These duplex materials are now in their
second generation. They can be identified by letters and numbers such
If a new impeller is required because of cavitation,
the new design should incorporate those features we have learned that
will increase impeller performance:
- The use of large fillets where the vanes join the shrouds to
- Investment castings so that you can design in the compound
curves that produce less wear.
- The latest design iteration to help reduce radial thrust.
- Sharpened leading edges of the vanes to reduce losses.
- A reduction of shroud to cutwater clearance to lessen internal
- A conversion to the newer duplex metals.
can be designed for a variety of applications:
- The ideal impeller would have an infinite number of vanes of
an infinitesimal size.
- The conventional impeller design with sharp vane edges and
restricted areas is not suitable for handling liquids that contain
rags, stringy materials and solids like sewage because it will
clog. Special non-clogging impellers with blunt edges and large
water ways have been developed for these services.
- Paper pulp impellers are fully open and non-clogging. The
screw conveyer end projects far into the suction nozzle permitting
the pump to handle high consistency paper pulp stock.
- Vortex pump designs have recessed impellers that pump the
solids by creating a vortex (whirl pool effect) in the volute and
the solids move without ever coming into contact with the
impeller. You pay for this feature with a greater loss of pump
- An axial flow impeller called an Inducer (it works like a
booster pump) can be placed ahead of the regular pump impeller, on
the same shaft, to increase the suction pressure and lessen the
chance of cavitation. In some instances this can allow the pump to
operate at a higher speed with a given NPSH. The inducer will
contribute less than 5% of the total pump head, and although low
in efficiency the total efficiency of the pump is not reduced
significantly. The total reduction in NPSH required can be as much
People often inquire about forward curved
vanes. Tests have shown:
- Both the capacity and efficiency were reduced.
- There was a slight increase in head.
- The impeller exhibited unstable characteristics at the low end
of capacity range.
- The impeller exhibited steep characteristics at high end of
- Increasing the number of vanes tends to flatten out the curve
and steady the flow.
Impellers can be single or double suction
- Because an over hung, single suction impeller does not require
an extension of the shaft into the impeller eye it is preferred
for applications handling solids like sewage. The suction eye is
defined as the inlet of the impeller just before the section where
the vanes start. In a closed impeller pump the suction eye is
taken as the smallest inside diameter of the shroud. Be sure to
deduct the impeller shaft hub to determine the area.
- Double suction pumps lower the
NPSH required by about forty percent.
- Most double suction impellers are constructed so that the
stuffing box is at suction pressure. This causes you to lose
the advantage of suction recirculation to prevent seal failure
when handling solids. You are going to have to flush many of
these seals with a clean, compatible liquid that will dilute
your product to some degree.
- Looking at the axial thrust in single stage pumps.
- The axial thrust generated is higher than in closed
impellers because of the hub. Pump out vanes and balance holes
are a common solution to this problem.
- A mechanical seal can add to this axial thrust. The amount
is dependent upon the design of the seal. Balanced designs
create less thrust.
- Balancing holes are not desirable with closed impellers
because leakage back to the impeller inlet opposes the main
flow creating disturbances. A piped connection to the pump
suction can replace the balance holes
- Theoretically there shouldn't be any thrust in a double
suction closed impeller, but:
- An elbow with the inlet piping running parallel to the
shaft will cause an uneven flow into the impeller eyes. This
uneven flow will cause thrusting of the impeller in one
direction depending upon the flow difference. The eye is
taken as the smallest inside diameter of the shroud.
Remember to deduct the area occupied by the impeller
- The two sides of the discharge casing may not be
symmetrical causing an axial thrust.
- Unequal leakage through both sets of packing can upset
the axial balance. Leaking seals can do the same thing.
- Impellers can be cut down to keep the application close to the
pumps best efficiency point :
- Theoretically up to twenty five percent of an impeller
diameter can be removed, but any time you remove more than ten
percent of the maximum impeller diameter the affinity laws are no
longer accurate because of slippage between the impeller outside
diameter and the pump volute.
- Changing the impeller diameter changes the head, capacity and
- The capacity can be increased by under filing the vane tips,
but the discharge head and the power requirement will
automatically adjust to the values where the pump curve intersects
the system curve.
- If you intend to cut down the impeller diameter, the impeller
should be cut down in at least two steps and tested after each
- After cutting down the impeller diameter the discharge vanes
should be reshaped to a long gradual taper to increase the pumps
performance. Chamfering or rounding the discharge tips will
frequently increase the losses and should never be done.
- Over filing is removing metal from the leading edge of the
blade. This seldom produces any increase in the vane spacing and
produces a negligible change in pump performance.
- Under filing is removing metal from the trailing edge of the
blade. If properly done it will increase the vane spacing and can
increase the capacity by as much as ten percent.
- If the inlet vane tips are blunt, over filing will increase
the inlet area and the cavitation characteristics can be
- Cutting back the tongue increases the throat area and
increases the maximum capacity. The head/capacity is then said to
"carry out further".
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