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Axial and Mixed
Flow Pumps: A
Simplified
Engineering
Design Manual
CIMMYT-Bangladesh
Eric Lam
2
Introduction
What is an axial flow and mixed flow pump? Axial flow pumps (AFP) and mixed flow pumps (MFP) are pumps designed for
pumping water at high flow rates against low lifts. These are ideal for crop
irrigation, aquaculture, flood control, and wastewater handling. The amount of
flow and lift capacity are dependent on how the pump is built. There are a few
commonalities between all AFP and MFP pumping systems.
1. Engine—provides power to the pump. Rated by horsepower and RPM.
2. Power coupling—connects the pump to the engine.
3. Bearing housing—holds the bearings that keeps the shaft in alignment and
able to freely rotate. Contains a head shaft.
4. Discharge casing—redirects flowrate out, while sealing the head shaft.
5. Delivery pipe—allows for flow of water. Contains the line shaft and line shaft
bushings.
6. Impeller—imparts energy on the water, causing it to flow. Housed
within the intake casing, in front of the stator,
and behind the intake screen.
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Total Dynamic Head (meters)= Lift + Friction
Lift (meters)
Bottom Clearance (meters)
Submergence (meters)
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Water and Diesel Power
How much power should I use to run the pump? The power needed by the farmer, in terms of water delivery and engine capacity,
can be approached based on 1. the farmer’s need, 2. the engine available, or 3. the
pump available. In all three cases, the water-horsepower and break-horsepower
requirements should be well defined
Water-horsepower is the power needed to operate a pump if the pumping
system was 100% efficient.
w.hp = water-horsepower , kW ρ = density of water, 1000 kg/m3
Q = water flow, m3/s g = acceleration of gravity, 9.81 m/s2 tdh = total dynamic head, lift plus friction
Break-horsepower is the power needed to operate a pump with inefficiencies
included.
b.hp = break-horsepower, kW w.hp = water-horsepower, kW
pump = pump efficiency drive = drive efficiency
Example Known Values Calculated Values
A farmer needs to irrigate a field, he has a 12 horsepower diesel engine, connected to 2 V-belts. He will pump for 12 hours a day, at an average 32ºC.
b.hp = 12 horsepower or 9 kW drive = 40% pump = 70%
He needs a pump with a w.hp rating of 2.5 kW or 3.4 hp. If he uses direct coupling, he can use a smaller engine with the same pump or a smaller pump with the same engine.
A farmer needs a pump that can do 25 liters per second, at a pressure of 6 meters.
Q = 25 l/s tdh = 6 m
He needs an engine with a b.hp flywheel output rating of 1.5 kW or 2.0 hp.
A farmer has a pumping system capable of delivering 2.0 kW with a total efficiency of 0.30.
w.hp = 2.0 kW or 2.7 hp He can buy a pump rated to deliver 60 l/s at 1 m tdh, 15 l/s at 4 m, or 9.0 l/s at 6 m.
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Delivery Pipe and Coupling
What type of pipe should the pump use? Pipes come in many different standard materials and sizes, often rated by
nominal diameters (DN). When selecting the appropriate pipe, the following
should be considered:
What environmental conditions will the pump be
operated and stored in?
How long does the pipe have to last?
How available and affordable is the pipe?
How will it connect to the discharge casing,
welded or bolted?
Can this connection support the weight of a fully filled pipe?
How much flowrate does the pump have to deliver? How much friction loss,
or pressure drop, is allowable? This will dictate the diameter needed, as
calculated by the empirically based Hazen-Williams equation.
Pipe Type Suitability Weight, kg/m
Cost, taka/m
Recommended Velocity, m/s
Recommended Flowrate, l/s
Engine Power Needed at 3m lift
and 30% eff
Mild Steel 1 mm 6” DN 8” DN
Rusts in 1 to 2 years in humid conditions.
2.5 3.0
130 155
0.9 to 1.4 15 to 20 20 to 25
2.0 kW or 2.7 hp 2.5 kW or 3.4 hp
UPVC 4” DN 6” DN 8” DN
Doesn’t rust. —— 3.2 8.0
—— 540 610
1.5 to 1.7
10 to 25 25 to 35 30 to 45
2.5 kW or 3.4 hp 3.0 kW or 4.0 hp 4.3 kW or 5.8 hp
S = hydraulic slope
hf = head loss, m
L = length of pipe, m
Q = volumetric flow rate, m3/s
C = pipe roughness coefficient
d = inside diameter, m
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Power Transfer
How is the pump connected to the engine? There are two main types of power couplings available to farmers (in
Bangladesh). The first is a standard V-belt drive system, which uses different
diameter sheaves allowing for RPM to be easily stepped up or down. The second
is a direct coupling using three tire bands.
Belts are classified to fit different
profiles, horsepower ratings, and
sheave diameters.
These systems can provide up to
90% power transfer, but only if
manufacturer recommendations
are followed.
Each belt can only provide a
limited amount of horsepower.
The number of belts needed for
full power can be calculated.
V-Belt Direct Coupling
The tire band connections are
common for centrifugal pumps.
Can provide up to 99% power
transfer with a stable platform.
The radial planes and center axes
of the pump and flywheel needs
to be as aligned possible.
Any damaged bands need to be
replaced immediately. Failure
during running is very dangerous
and can irreparably damage the
pump or operator.
Ha = allowable power, per belt
K1 = angle-of-wrap correction factor
K2 = belt length correction factor
Htab = rated power, hp
Source: Gates Rubber Co., Denver, CO
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Bearing Housing
What size bearings should the pump use? When selecting the bearing mount configuration for the head shaft of an AFP/
MF irrigation pump, the most economically sensible option is to use one from an
existing pump, often a centrifugal pump. The selection criteria for an
appropriate bearing housing is as follows:
What is the operational rating of the existing bearing housing, bearings and
shaft?
RPM and dynamic load capacity of the bearings?
Direct coupling or V-belt?
Maximum applied force?
Maximum applied torque?
What is the operational rating of the existing pump?
Kilowatt or horsepower rating?
Flowrate or pressure capacity? If these are known, along with the
vectorization profile in the existing pump, the system capacity of the
whole housing can be reverse engineered.
Bearing Dynamic Load Cap., kg
Max RPM Cost, taka
Thrust Bearing 25mm x 52mm
Radial 830 Thrust 530
—— 1,000
Ball Bearing 25mm x 52mm
1,430 17,000 200
Ball Bearing 30mm x 62mm
1,900 14,000 300
Bearing Housing Theoretical Applied Torque, Nm
Cost, taka
Weight, kg
3” DN Centrifugal 800 2,220 4.5
6” DN Centrifugal 2500 13,400 30.7
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Discharge Casing
What discharge casing configuration is needed? When designing the discharge casing, 1. the way it is mounted to the bearing
housing, 2. the coupling method to the delivery pipe, and 3. the redirection
needed for the flow are the primary constraints for the body of the casing.
Additionally, the following needs to be considered:
The discharge casing should be the same diameter as the delivery pipe to
reduce pressure losses. If the diameter is altered, a constricted discharge
will increase pressure delivery, while a expanded discharge will increase
flowrate.
The material chosen will dictate the thickness needed to support the
connection to the bearing housing, the thrust from the discharge, and the
weight of the pump. This in turn will determine the weight of the casing.
What angle does it need to be? Does it need a smooth transition? A 45º
discharge will have less pressure losses and might be easier for farmers to
use. However, 90º bend is lighter, cheaper, stronger, and easier to
manufacture.
Will the discharge outlet couple to a flexible hose pipe or flanged fitting? Flanged
fittings are heavier, but are necessary for pressurized systems such as ones with
delivery manifolds.
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Line Shaft and Bushings
What kind of bushings does the pump need? Bushings are important for stabilizing the line shaft, which transfers energy from
the head shaft to the impeller. Bushing spacing is important to support the line
shaft. This helps to 1. provide a low friction contact surface, 2. reduce shaft
deflection, and 3. reduce vibration on the system.
What diameter should the line shaft be? Solid shaft or hollow pipe? These
decisions are based on applied torsion, allowable deflection, allowable
displaced volume, and resistance from the turning impeller.
The length/diameter ratio needs to be between 0.5 to 2.0. The distance
between bushings should not exceed 2 meters.
Metal bushings will last longer than plastic bushings, but are more
expensive and may have anodic reactions with the mounting.
Manufactured bushings will have ratings that are important for selecting
the right material and diameter needed for the pump.
The load at velocity (PV) rating is a combined of pressure and
rotational speed capacity.
Environmental ratings including temperature.
These factors can be used for a time-wear equation, which will
predict when the bearing will fail or need to be replaced.
t = time before needing replacement, hr
L = length of bushing, in
D = diameter of bushing, in
w = allowable wear, in
f1 = motion related factor
f2 = environmental factor
K = wear factor, in3*min/lbf/ft/hr
V = radial velocity, ft/min
F = radial load, lbf Source: Oiles America Corp., Pymouth, MI 48170
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Impeller
What type of impeller does the pump use? For these high flow, low head irrigation pumps, there are two types of impellers
to use: the axial flow and mixed flow impeller.
The axial flow impellers only
impart axial forces on the water
flow (higher flow, lower
pressure).
These should only be used if the
lift is 3 meters or less.
RPM range: 1500 to 1800
Three blades will use less
energy, but deliver a lower
flowrate than four blades.
High angulation blades (20º
and 25º off the radial plane) will
deliver higher pressure, but
lower flowrates than low
angulation blades (10º and 15º).
Axial Flow Impeller Mixed Flow Impeller (open or shrouded)
The mixed flow impeller has a
conical shape, which imparts
axial and radial force on the
water flow (lower flow, higher
pressure).
These should be used if the lift is
between 2 and 8 meters.
RPM range: 1000 to 1300.
Thinner blades are more
efficient, but will reduce
strength.
Geometries can be complex. A
30º cone with a 6 bladed helical
structure, with intake parallel to
radial and discharge normal to
the cone, works well
Rotating Shaft
Radial Direction
Axial Direction
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