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A pump impeller is a vaned rotating disk mounted on the shaft that converts the
torque transmitted from the driver into pump total dynamic head (TDH). There
are different types of impeller and it is a certainty an impeller will belong to more
than one type. The types can be grouped into:
There are different types of impeller and it is certain that an impeller belongs to
more than one type. These are according to:
1. Radial flow - In radial flow impeller the flow exits the impeller outlet area in a
radial direction, or perpendicular to the shaft. Torque is converted into TDH
through the centrifugal action of the impeller vanes.
2. Mixed flow - In mixed flow impeller the flow exits the impeller outlet area at a
certain angle from the radial direction. Torque is converted into TDH through the
combination of centrifugal force and the pushing action of the impeller vanes.
3. Axial flow - In axial flow impeller the flow exits the impeller outlet area in an
axial direction or parallel to the shaft. Torque is converted into TDH solely
through the pushing action of the impeller vanes.
[An impeller is also called a runner, and impeller vanes as impeller blades. Some
people refer to the pushing action of the axial impeller vanes as lifting action
which is a misnomer as it implies a vertical motion of flow; in fact the flow can be
either in horizontal or vertical direction depending on the orientation of the
The minimum diameter an impeller can be cut, or trimmed, usually depends on
whether the impeller is of radial, mixed flow, or axial flow (propeller) design.
Typically, the allowable minimum diameter, as a percentage of its maximum
diameter, is as follow:
Radial impellers 80%
Mixed flow impellers 90%
Axial flow impellers 95%
Because the performance of an impeller deviates more from the affinity laws as
the impeller cut increases it is a good practice to estimate the impeller cut
diameter from a reference performance curve obtained at a diameter closest to
the estimated diameter required for the service.
1. Single suction - The impeller has a suction (inlet) area on one side of the
impeller only; the pumped liquid enters on one side of the impeller only.
2. Double suction - The impeller has suction (inlet) area on both sides of the
impeller; the liquid enters on both sides of the impeller. Because each side
accounts for only half of the total discharge flow, this design has the advantage
of requiring less NPSHR than a comparable single suction design for the same
amount of discharge flow.
Mechanical design (shroud design):
1. Enclosed type - The impeller vanes have shrouds on both sides of the
impeller. This is the design of choice because the shrouds provides structural
strength; it yields the highest efficiency because it minimizes leakage flow back
to the suction of the impeller.
[An impeller shroud is the circular plate where the vanes are connected and
contains the impeller wear ring land.]
2. Semi-open type - The impeller vanes have shroud on the back side of the
impeller; the front of the impeller is open and the vanes are exposed and
unsupported. This design is ideal for pumping liquid with some amount of small
solids such as in slurry application; the downside of this design is that it has
lower efficiency than a comparable enclosed type impeller design because of
the leakage flow at the open face of the impeller.
3. Open type - The impeller vanes have no shroud on either side of the
impeller; the vanes are completely open and are connected only to the impeller
hub. Because the vanes lack strength due to the absence of structural support
from the shroud this type of impeller is used primarily in low head, low speed,
application such as in sewage pumping where its open type design is ideal for
[The absence of impeller shrouds makes dynamic balancing of an open-type
impeller difficult which is one reason it is limited to low speed application.]
Impeller erosion damage
Q - One of our 1850 KW slurry pumps is fitted with a mixed flow impeller. The
pump is handling an abrasive mixture of water and sand. At the point where the
blade is fixed to the outer shroud (opposite the point where the blade is fixed to
the hub of the impeller) the blades experienced severe (fast progressing) wear.
The blades are quite blunt at the tip so my guess is that because the flow
cannot follow the blade, a sort of spiral vortex forms at the tip of the blades that
causes the wear.
Has anyone ever had similar experience? Is there a way to modify the
existing impeller to reduce its wear?
A - This appears to be a classical example of erosion/corrosion damage. The
presence of sand particles is definitely the main factor and, in cases like this,
velocity is a major aggravating factor (that is why slurry pumps should be run at
low RPM.) This explains the severe erosion/corrosion damage at the outer tip
diameter of the impeller blades - this is the point of maximum peripheral velocity
at the inlet side of the impeller. If the pump is operating at off-peak capacity, it is
probable that suction recirculation may also be a contributing factor and the
area of the damage is typically where suction recirculation is more pronounced.
Compared to radial flow, mixed-flow impeller is more susceptible to suction
recirculation because of its larger eye diameter/maximum diameter ratio. The
blunt tip of the inlet blades does not help either because it causes a greater
amount of flow disturbance or entrance shock. The sharp corner in a
square-shaped blade tip is also a stress-concentration point. If possible slow
down the pump but this is probably not an acceptable option depending on the
service. You can consider using a more abrasion-resistant impeller material
such as CD4MCu, or other duplex stainless steel. There are a number of
available technologies - diffusion type coating and plasma spray coating, among
them - to coat the blade inlet tips with hard overlay material to make the blades
more erosion resistant. It also helps to grind a smooth radius at the blade tips
and remove any sharp edge (profile the blade tip similar to that of airplane wing.)
Impellers in multistage pumps must be mounted staggered on the shaft so that
their discharge vane tips do not line up together. If the vane tips line up, they will
pass the volute lips at the same time and the simultaneous impeller-volute
interaction will result in higher pump vibration level at impeller vane-passing
To prevent this increase in vibration, it is important that the impeller discharge
vane tips are intentionally staggered so that each impeller vane interacts with
the volute lips one at a time.
How this is done, and what precautions to make in staggering the impellers, are
discussed in a full version of this article.
Impeller with staggered vanes
Q - I noticed that in some large double-suction impellers the center rib (or
center shroud) extends all the way to the impeller outside diameter (OD), and
the vanes on both sides of the outlet do not line up.
In most double suction impellers the center rib extends only to about halfway of
the OD, and the impeller vanes line up at the outlet.
What are the reasons for this difference in impeller design?
A - We can give two reasons why some impellers are designed with full center
rib and staggered vanes:
One, a full center rib improves the structural integrity of the impeller vanes in
high speed or high pressure service, or in high specific speed design where the
width of the impeller vanes is wide. Without a full center rib, the vanes are at risk
of developing cracks.
Two, it allows the left hand (LH) vanes to be staggered from the right hand (RH)
vanes so that only half the vane width passes the volute lips at a time. It diffuses
the energy being imparted by the vanes to the volute lips and minimizes the
vibration induced by the impeller vane/volute lip interaction.
The downside to the full center rib design is that the pump efficiency is slightly
reduced to its higher disk friction loss, specially in low specific speed pumps
where the impeller passageway is narrower. For this reason, a full center rib
design is to be used selectively.
Q - In impellers with staggered vanes, how are the vanes spaced apart? Are
they spaced equally, or at some angular distance based on other factors such
as vane number, or discharge angle?
A - The answer to this question is ...
Impeller diameter trimming
Q - What is the best way to cut, or trim, an impeller diameter?
a. Cut both the impeller vanes and shrouds to the same diameter.
b. Cut the vanes only, and leave the shrouds uncut.
A - In most cases it is best to cut both the impeller vanes and the shrouds to
the same diameter for these reasons:
- The uncut portion of the shrouds causes additional and unwanted friction
loss resulting in additional brake horsepower and reduced pump
- With uncut shrouds it is more difficult to reach inside the impeller
discharge passageway to clean the casting and to profile the discharge
tip of the vanes.
- The added weight of the uncut shrouds may increase the shaft deflection
specially in overhang horizontal pumps.
In rare instances it may be preferable to cut the vanes only and leave the
shrouds uncut, such as:
- When the impeller requires a large cut in diameter and is operating at low
flow condition. The combination of large volute clearance (B-gap) and low
flow operation can induce discharge flow recirculation. In this case,
leaving the shrouds at a bigger diameter than the vane would help
mitigate this problem.
- When a pump is anticipated to operate at a higher head at some future
time where it may be more economical and faster to simply weld up and
extend the vane diameter (to obtain the increase in head) rather than to
buy a replacement impeller at a bigger diameter. However this approach
is advisable only in big, high capacity and low head (high specific speed),
low speed pumps such as those in large de-watering, sewage control,
flood-control, irrigation, and similar light to medium service, where the
impeller is not subjected to high pressure and high peripheral speed, and
when the liquid is not highly corrosive or abrasive.
rings still mounted on its shrouds. Can you identify at least one
way to improve the design of this impeller?