Axial and Mixed

Flow Pumps: A

Simplified

Engineering

Design Manual

CIMMYT-Bangladesh

Eric Lam

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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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