Pumps and pumping plants for irrigation system

Md Moudud Hasan
Lecturer
Department Agricultural and Industrial Engineering
Faculty of Engineering
Hajee Mohammad Danesh Science and Technology University
Dinajpur

Four principles involved in pumping water
Atmospheric pressure
Positive displacement
Centrifugal force
Movement of columns of water caused by the
difference in specific gravity
MM HASAN,LECTURER,AIE,HSTU

The depth to water surface from the
ground does not exceed 1.2m
Swing Basket
Ancient water lifts
A basket or shovel like scoop to which
four ropes are attached.
Two person operate it
Water filled basket is discharged into
the field channel.

Don
Boat shaped trough
Closed at one end and open at the other
Closed end of the trough is tied with a rope to a
long wooden pole ( act as lever)
A weight is fixed to the shorter end of the lever
Open end is hinged to discharge point
Dipped into water applying body weight and force
Lifted by the counter-weight on the beam

Don

Archemedian screw
Consists of a wooden or metal drum with interior
partition in the from of a screw
Rotated by means of a handle fixed to a central
spindle
Lower end of the drum is placed in water with a
angle less than 30˚
Handle turned water moves up through the drum
Half of lower end is submerged in water

Archemedian screw

Water Wheel
Consists of small paddles mounted radially on a
horizontal shaft
Fixed on a close-fitting concave trough
Driven by a bullock-wheel drive
Rotating wheel pushes the water to the field
surface through the trough

Water Wheel

The height of the lift is within the range of 1.2
to 10 m .
Persian Wheel:
Consists of a chain and a row of buckets mounted
on an open-spoked drum and provided with a
suitable driving mechanism.
Capacity of the bucket ranges from 7 to 14 liters

Persian Wheel:

Chain pump:
Consists of an endless chain which is provided
with leather discs or washers spaced at intervals
of about 25 cm.
Chain passes over a notched wheel mounted on a
suitable platform fixed on top of the well.
On one side of the chain is a pipe of about 10 cm
diameter, having a flared opening at its bottom and
connected to a trough at the top.
The bottom of the pipe is submerged about 60 to
90 cm below the surface of water.
the discs have the same diameter as the inside of
the pipe.
When the wheel is turned, each disc brings up a
volume of water.

Rope and bucket lift with self-emptying bucket
Consists of a sheet metal or leather bucket, having a capacity of about
100 to 150 litres.
At the bottom of the bucket is fixed a leather tube or spout.
The bail of the bucket is attached to a heavy rope which passes over
a pulley.
A second lighter rope is fastened to the lower end of the spout.
This second rope
passes over a roller fixed to the lip of the-receiving through on
the ground surface.
Both the ropes are tied together and then attached to the bullock
yoke.
Their lengths are so adjusted that the spout folds up along the side of
the bucket while it is being raised from the well.

Rope and bucket lift with self-emptying bucket

Circular two-bucket lift:
Two buckets which are alternately
raised, emptied, lowered and filled.
While one bucket is filled and lifted,
the other is lowered empty into the
well.
A rope and pulley arrangement along
with a central rotating lever permit
reciprocating action while the bullocks
move in a circular path .
Flap valve at the bottom for filling a
At top , bucket is tilted automatically
due to the loop on the rod.

Counterpoise-bucket lift:
• This device consists of a long wooden pole which is pivoted as a lever
on a post.
• A weight, usually a large stone or a ball of dried mud or a basket full
of stones, is fixed to the shorter end of the pole.
• This weight serves as a counterpoise to a bucket suspended by a rope
or a rod attached to the long arm of the lever.
• To operate the lift, a man pulls down the rope or rod, using his body
weight and strength until the bucket is immersed in the water and
filled.
• The bucket is lifted up by the counter weight.
• As the bucket reaches the ground level, it is tipped into a trough.
• Alternatively, a rod can be fixed to the well wall on which the bucket is made
to slide. This enables the bucket to be emptied automatically into a
trough, as it reaches the top of the well.

Counterpoise-bucket lift:
The counterpoise is adjusted to balance the weight of the full bucket. The proper
weight of counterpoise can be computed as follows:
W1= weight of bucket (Kg)
W2= weight of water in the bucket (kg)
W3= weight of counterpoise (kg)
Z1= lift or motion of load (m)
Z2= motion of effort (m)
Velocity ratio
=
𝑀𝑜𝑡𝑖𝑜𝑛 𝑜𝑓 𝑒𝑓𝑓𝑜𝑟𝑡
𝑀𝑜𝑡𝑖𝑜𝑛 𝑜𝑓 𝑙𝑜𝑎𝑑
=
𝑍2
𝑍1
=
𝑎
𝑏
𝑁𝑜𝑤, 𝑊1 + 𝑊2 𝑏 = 𝑊3 𝑎
Or
𝑊1−𝑊2
𝑊3
=
𝑎
𝑏
=
𝑍2
𝑍1
𝑊3 = (𝑊1 + 𝑊2)
𝑏
𝑎MM HASAN,LECTURER,AIE,HSTU

Rope and Bucket lift:
The only indigenous water lift suitable for deep wells is
the rope-and-bucket lift operated by bullocks.
The device may be operated singly or in multiples of
two or more working simultaneously, depending on the
yield of the well and the requirement of Irrigation water.
It consists of a bucket or bag having a capacity of about
150 to 200 liters and made of leather or galvanized iron
sheet.
A pair of bullocks. hitched to the other end of the rope,
provide the power to lift the bucket.

A pump is a mechanical appliance
used to increase the pressure energy
of a liquid, in order to lift it from a
lower to a higher level.
 This is usually achieved by creating a low pressure
at inlet and a high pressure at the outlet ends of
the pump.
 Thus, the principle of working of a pump is
distinctly different from the indigenous water lifts
in which water is lifted by displacement through
buckets, water wheels or screws.
 Two basic groups of pumps
1. Positive displacement pumps
2. Variable-displacement pumps

Positive displacement pumps discharge the same volume of
water regardless of the head against which they operate.
This type of pump must be powered to meet the maximum
load resulting from its discharge capacity and the greatest
head under which it will operate.
As the capacity is small, these pumps are not very popular
in irrigation and drainage.
However, they are commonly used in home water supply,
well drilling and under special situations in irrigation
pumping.

Reciprocating pumps, sometimes called piston or displacement
pumps, function by means of a piston movement which displaces
water in a cylinder.
The flow is controlled by valves.
A piston or plunger is a cylindrical piece which moves backward and
forward inside a hollow cylinder.
The capacity of the reciprocating pumps depends on the size of the
cylinder chamber and the length and speed of the stroke.
Numerous forms of packings are used to prevent leakage past a piston.
Cup-leathers are usually used for packing in case of pump cylinders.
The pressure of the water acting inside the cup presses the
leather outwards against the cylinder, thus preventing leakage.

Reciprocating pumps for shallow wells are usually of the lift type, using
atmospheric pressure to raise water in the pump column.
The piston is in the pump body, near the handle.
The piston when moved up and down by the movement of the handle
displaces air from the pump column.
This creates a vacuum which permits the force of atmospheric pressure
to push the water from the well up into the pump column and from it
to the outside.
Theoretically, it can lift water from a height of upto one atmosphere or
10.33 m. But in practice, the height reached is only about 6.5 to 7 m
due to friction and other losses.

An animal powered duplex reciprocating pump developed
by Khepar et al. (1975) is specially suitable for pumping
water from shallow tube-wells.
The conventional bullock gear of the type commonly
used with Persian wheels is used to transmit the
power of the pair of bullocks or other draft animals to
operate the pump.
The pumping unit consists of a pair of ordinary piston pumps.
Each pump has a cylinder diameter of 30 cm with pistons
having a stroke of 11 cm.
The suction ends of the two pumps are connected by
bends to a T-joint to which a common suction pipe is
connected.

A foot valve is fixed at the bottom of the suction
pipe, as usual.
The suction pipe which is smaller in diameter than
the tube-well, is lowered into the well.
The power transmitted through the bullock gear is
applied to operate the piston through a flywheel to
which are connected the pump handles.
The linkage mechanism is designed to obtained the
desired piston stroke.

The drive provides one discharge stroke in each pump for
each revolution of the flywheel.
The suction and discharge strokes of the pair of pumps
alternate with each other, i.e., when there is suction in one
pump there is discharge in the other.
The flywheel provides the inertia required for the smooth
running of the pump.
The pump has a discharge of about 7 litres per second against
a head of 4 metres.

A foot-operated twin pump developed by RDRS, Bangladesh, called
twin treadle pump, has been a major contribution to small scale lift
irrigation through low-cost shallow wells (Fig. 3.19).
Over 20,000 units of the pump were manufactured and installed in northern
Bangladesh during 1981-84.
The main features of the pump are the twin plunger pump and the use of
the body weight of the operator in pumping.
The components of the pump are shown in Fig. 3.20.
The suction pipe at the bottom of the pump is connected to the water source,
such as an open well. Tube-well, stream, or pond.
The pump sucks water through the suction pipe into a manifold through
which are connected the two pump cylinders.
Each cylinder is equipped with a foot valve which prevents the back-flow of
water from the cylinder to the suction pipe during the return stroke of the
pump.
The foot valve could be a simple rubber flap made from a used inner tube
of a truck tyre.

The pump plunger consists of two round disks fastened
to a rod and a moulded rubber or PVC cup or bucket. The
upper disk has holes which allow water to pass through the
plunger during the downward return stroke.
On the upward stroke, the bucket is pressed against both the
lower disk and the cylinder wall.
This provides a seal that prevents water from passing by the
piston.
It also creates a partial vacuum that sucks water into the
cylinder from the manifold and the suction pipe.
The plunger assembly is easy to fabricate with simple tools.
It utilizes the buckets of standard domestic water hand
pumps, which are widely used in developing countries.

The plunger rods are connected to the treadles (generally bamboo
poles) by means of a hinged joint.
The pump cylinders are generally made from standard size of steel
pipes or of cast iron. It could also be made of mild steel sheet (16 gauge)
using sheet metal.
During operation, it is possible to adjust the position of the feet of
the operator to obtain maximum efficiency in Pumping.
The twin-treadle pump has a capacity of about 3 lit/sec for a lift of 3
meters and 2 lit/sec for a lift of 4 meters.

By introducing the pump cylinder with its plunger and valve into the
water in a well, water can be lifted to almost any height required in
practical use. The plunger is connected to the pump handle, or other
operating devices like a mechanically powered crank shaft, The details
of construction of a deep well reciprocating pump are illustrated in Fig.
3.21. As the plunger in which the upper valve is located moves upward,
the water on the top of the valve is forced upward through the delivery
pipe and another charge of water fills the space between the valves. The
cycle is repeated in each upward stroke.

While the basic principles of operation apply to all piston pumps,
there are many modifications in design, which adapt these pumps to
specific uses. Piston ppumps may be either single acting or double acting.
Single acting pumps (Fig. 3.18) have one discharge stroke for every two
strokes of the piston. Thus, the water is delivered during alternate
strokes of the piston. The flow through the delivery pipe is
therefore intermittent. An air vessel (Fig. 3.22) may be fixed over or near
the delivery valve to remedy this. During the delivery stroke of the
piston, the air in the air vessel is compressed to a greater pressure
than that corresponding to the head of water at the bottom of the
delivery pipe. During the suction stroke, the pressure of the air in the
vessel maintains the flow of water through the delivery pipe, thereby
ensuring nearly continuous discharge.

Double acting pump s are constructed with piston and valves
so arranged that water is pumped on both the inward and
the outward movements of the piston (Fig. 3.23). Though the
arrangement is more commonly used in lift pumps, it is also
sometimes incorporated in force pumps.
Duplex and triplex pumps consist of two or three pistons
respectively, and are designed to pump a continuous
stream of water with minimum pulsation, often against high
pressures.

Once the pump cylinder and pipes are fully charged with water
it is evident that, neglecting the volume of the pump rod, the
volume of water delivered during each upstroke of the
piston is equal to the volume swept by the piston in one stroke.
Let,
a = area of cylinder (m2)
l = length of stroke (m)
h = total height through which water is raised (m)
p =force required to lift the piston
W= specific weight of water (kg/m3)

Weight of water raised in one stroke = w al =
1000 al
Work done in one upstroke = 1000 a lh = Ph
Therefore, P= 1000 al

A single acting reciprocating pump has its piston
diameter 15 cm and stroke 25 cm. The piston
makes 50 double strokes per minute. The suction and
delivery heads are 5 m and 15m, respectively. Find, (i)
the discharge capacity of the pump in liters per
minute, (ii) the force required to work the piston
during the suction and delivery strokes if the
efficiency of suction and delivery strokes are
60% and 75%, respectively, and (iii) the H.P.
required by the pump for its operation.

(i) Area of piston,
a= π/4 *d2=3.15/4x 15/100 x 15/100 =0.0177m2
Volume swept by piston per stroke
= al = 0.0177 x 25/100= 0.0044
Discharge of pump = 0.0044 x 50 = 0.22 m3/min
= 0.22 x 1000 = 220 litres/min
(ii) Average force of suction
= (w x a x suction head) /efficiency stroke
= (1000 x 0.0177 x 5)/.75
= 147.5 kg

Average force of delivery
= (w x a x suction head)/efficiency stroke
= 1000xO.0177x15/ 0.75
= 352.66 kg
(iii) H.P. required by the pump
= ( Total force (suction + delivery) x distance moved in
metres/min)/ 4560
= {(147.5+ 352.66) x25x50/100 }/4560
= 1.371
How many kg-m/min in 1 horsepower
[international]?
The answer is 4562.41349441.

Cross-sectional-area of piston ,
a= π/4 *d2=3.15/4x 25/100 x 25/100 =0.049 m2
Cross-sectional-area of piston rod,
a1 = 3.14/4x 5/100 x 5/100 =0.00196m2
(a) Force required to work the piston during 'in' stroke
(i) For suction = w x a x suction head
=1000 x 0.049 x 4.5 = 220.50 kg
(ii) For delivery = w(a - a1) x delivery head
= 1000(0.49 - 0.00196) x 18
= 846.72 kg
Total force during 'in' stroke = 220.50 + 846.72
= 1067.22 kg

(b) Force required to work the piston during 'out' stroke
(i) For suction = w(a - a1) x suction head
= 1000(0.049 - 0.00196)4.5
= 211.680 kg
(ii) For delivery = w x a x delivery head
= 1000 x 0.049 x 18
= 882 kg
Total force during 'out' stroke = 211.68 + 882.00
= 1093.68 kg
Discharge during 'in' stroke = a x l x rpm
= 0.0049 x 35/ 100 x 60
= 1.029 rn3/min= 1029 litres/min

Discharge during 'out' stoke = (a - al) x 1x rpm
= 0.04704 x 35/100 x 60
= 0.98784 m3/min
= 987.84 liter/min
Total quantity of water raised by the pump= 1029 + 987.84=2016.84 litres/min
H.P. required by the pump= {Total force (kg) x distance moved (m/min) }/4560
= (1093.68 x 35 x 2 x 60 ) / (4560x 100 ) = 10.73.

Though commonly used in pumping lubricating oils, rotary pumps are
sometimes used as boosters in irrigation pumping.
Figure 3.24 illustrates the construction details of a rotary pump.
The designs commonly encountered are those using cams or gears.
The pump body is a plain housing with inlet and outlet pipes and
openings for shafts which carry the gears or cams.
One of the gears is the driving gear and is rotated by an outside source of
power.
The other is the 'idler' gear driven by the driving gear.
The gears are fitted closely" in the housing and mesh with minimum
clearance.

They rotate in the direction shown and force the water
out through the discharge opening.
This creates a partial vacuum and brings in a replacement
supply of water along the inlet side.
Such an operation creates an even, continuous flow.
The capacity delivered is constant, regardless of
pressure.
Due to the necessity for close clearance and metal to
metal contact, rotary pumps work best and last long
when pumping liquids having lubricating qualities.

The distinguishing feature of variable displacement pumps
is the inverse relationship between the discharge rate and
the pressure head.
As the pumping head increases, the rate of pumping
decreases.
Unlike positive displacement pumps, variable displacement
pumps require the greatest input of power at a low head
because of the increase in discharge as the pumping head
is reduced.

Variable displacement pumps of the impeller type,
including centrifugal, mixed flow and propeller pumps are
predominantly used in irrigation pumping.
They use a rotating impeller to
pump water. In general, they
range from pumps with small
discharges and high heads to large
discharges with low heads.

It expresses the relationship between speed, discharge and head.
The index, originally developed for FPS units, is the speed in
revolutions per minute at which a theoretically and geometrically
similar pump would run if proportioned to deliver one gallon per minute
against one foot total head at its best efficiency.
The specific speed of a pump, as originally developed, formula: is calculated
from the following
ns = (nQ ½) / H3/4
in which, ns = specific speed (rpm)
n = pump speed (rpm)
Q = pump discharge, US gallons (per min)
H = Total head (ft)

In metric units, specific speed may be defined as the speed of a
geometrically similar pump when delivering one cubic metre/second of
water against a total head of one metre (Church and Jagdish Lal, 1973).
Expressed mathematically,
ns =(n Q1/2)/H3/4
in which, ns = specific speed (rpm)
n = pump speed (rpm)
Q = pump discharge (m3/sec)
H = total head (m)

A centrifugal pump at its best point of
efficiency discharges 0.03 cubic metres of
water per second against a total head of 40 m
when the speed is 1450 rpm. Compute the
specific speed of the pump.
Specific speed, ns = 15.9 rpm

A pump operates most satisfactorily under a
head and at a speed for which it is
designed.
The operating conditions should therefore be
determined, as accurately as possible, to
select pumps well adapted to the particular
conditions of operation.

Capacity is the volume of water pumped per unit time.
It is generally measured in litres per second. Small
capacities, how ever, may be stated in litres per
minute or litres per hour and large capacities in cubic
metres per second.
Suction lift exists when the source of water supply is
below the centre line of the pump.
Static suction lift is the vertical distance from the free
suction water level to the centre line of the pump.

Total suction lift is the sum of static suction lift,
friction and entrance losses in the suction piping.
Suction head exists when the source of water
supply is above the-centre line of the pump, as is the
usual case in a turbine pump. There will, however,
be no suction head in a volute centrifugal pump,
unless it is operated as a vertical pump.

Static suction head is the vertical distance from the centre line
of the pump to the free level of water to be pumped.
Total suction head is the vertical distance from the centre
line of the pump to the free level of the liquid to be pumped
minus all friction losses in suction pipe and fittings, plus any
pressure head existing on the suction supply. (Total suction
head, as determined in a pump test, is the reading of a
gauge connected to the pump suction, expressed in metres of
water and corrected to the pump centre line, plus the velocity
head at the point of gauge attachment).

Static discharge head is the vertical distance
from the centre line of the pump to the
discharge water level.
Total discharge head is the sum of the
static discharge head, friction and exit losses in
the discharge piping plus the velocity head
and pressure head at the point of discharge.

Total static head is the vertical distance from
suction water level to discharge water level or
the sum of static suction lift and static
discharge head. (Note: If there is static suction
head, total static head is the static
discharge head minus the static suction
head).

Friction head is the equivalent head,
expressed in metres of water required to
overcome the friction, caused by the flow
through the pipe and pipe fittings

Example: 3.1,3.2,3.3
Problems: 3.1,3.2
Questions: 1, 2, 6,7

Pressure head is the pressure, expressed in meters of water, in
a closed vessel; from which the pump takes its suction or
against which the pump discharges.
𝐻 𝑝 =
𝜌
𝜔
in which, Hp = pressure head (m)
p = pressure inside the vessel (kg/m2)
w = specific weight of water (kg/m3)

Total head is the energy imparted to the water by the pump. It
is the sum of total discharge head and total suction lift
when suction lift exists. It is the total discharge head
minus the suction head where suction head exists.

Velocity head is the pressure, expressed in meters of water,
required to create the velocity of flow.
𝐻𝑣 =
𝑣2
2𝑔
in which, Hv = veolicity head (m)
v = velocity of water through the pipe (m/sec)
g = acceleration due to gravity (m/sec2)
(usual value 9.81 m/sec2)

Net positive suction head (NPSH) is the total suction head,
determined at the suction nozzle (corrected to pump centre
line) minus the vapour pressure of water at the pumping
temperature, both expressed in metres.
In the pumping of liquids, the pressure at any point in the
suction line must not be reduced to the vapour pressure of a
liquid.
The vapour pressure of a liquid at any given temperature is that
pressure at which it will vapourise if heat is added to the liquid
or, conversely, that pressure at which vapour at the given
temperature will condense into liquid, if heat is subtracted.
(Vapour pressure of water at different temperatures is given
in standard physical tables).

Maximum practical suction lift of pumps: For the operation of
a centrifugal pump without cavitation, the suction lift plus all
other losses must be less than the theoretical atmospheric
pressure.
The maximum practical suction lift can be computed by the
equation,
Hs = Ha - Hf- es - NPSH - Fs
in which,
Hs = maximum practical suction lift, or elevation of the pump
centre line minus the elevation of the water surface (m)
Ha = atmospheric pressure at water surface (m)

Hf = friction losses in the strainer, pipe, fittings, and valves
on the suction line (m).
es = saturated vapour pressure of water (m)
NPSH = net positive suction head of the pump
including losses at the impeller and velocity head (m)
Fs = factor of safety, which is usually taken as about 0.6 m.
The approximate correction of Ha for altitude is a reduction of 0.36 m for each 300
m of altitude.
Friction losses and suction lift should be kept as low as possible. For this reason
the suction pipe is usually larger than the discharge pipe, and the pump is placed
as close as possible to the water supply. The head loss equivalent due to the vapour
pressure of water must be considered to prevent cavitation but it does not add
to the total suction head when the pump is operating.

Determine the maximum practical suction lift for a pump
having a discharge of 38 litres per second. The water
temperature is 20°C. The total friction loss in the 10 cm
diameter suction line and fittings is 1.5 m. The pump is
operated at an altitude of 300 m above sea level. The
NPSH of the pump, as obtained from the characteristic
curve supplied by the manufacturer is 4.7 m.

es at 20°C = 0.24 m (From standard physical tables)
Fs is assumed to be 0.6
Atmospheric pressure = 10.33 - 0.36 = 9.97 m
Hs = Ha - Hf - ef -NPSH - Fs
= 9.97 -1.5 - 0.24 - 4.7 - 0.6 = 2.93 m

Water horse power (WHP) is the theoretical
horse power required for pumping. It is the
head and capacity of the pump expressed in
terms of horse power.
WHP = (Discharge in litres per sec x total head in
metres )/76
=(Discharge in cubic metres per hour x
total head in metres )/273

Shaft horse power is the power required at the pump shaft.
Shaft horse power = Water horse power / Pump efficiency
Shaft horse power is used up in the pump in water horse power,
disc friction, circulation losses, stuffing box and bearing friction
and hydraulic losses (friction, shock and turbulence). It is always
greater than water horse power.

Efficiency is the ratio of the power output to power input.
Pump efficieney= Water horse power / Shaft horse power

Brake horse power is the actual horse power required to be
supplied by the engine or electric motor for driving the pump.
(i) With direct driven pump (drive efficiency 100%):
Brake horse power = Shaft horse power
(ii) With belt or other indirect drives:
Brake horse power= Water horse power/(Pump efficiency x
drive efficiency )
Horse power input to electric motor =
Brake horse power x 0.746
Motor efficiency
Kilowatt input to electric motor =
Brake horse power x 0.746
Motor efficiency

• The characteristic curves, also called performance
curves,
• show the interrelations between capacity, head, power
and efficiency of a pump.
• The knowledge of pump characteristics enables one to
select a pump which is best adapted to particular
conditions of operation and thus obtain a relatively
high value of efficiency with low operating cost.

It is usual to plot the head, the power input and efficiency as
ordinates against capacity as abscissa at a constant pump speed, as
shown in Fig. 3.26. The net positive suction head, when shown, is
also plotted as ordinate.
About 6 to 12 values are taken during the pump test to plot the
points. Smooth curves are drawn joining the points.
The head-capacity curve shows how much water a given pump will
deliver at a given head. As the discharge increases, the head
decreases. The resulting efficiency is observed to increase from 0
when the discharge is 0 to a maximum and then decreases.

The brake horse power curve for a centrifugal pump usually
increases over most of the range as the discharge increases,
reaching a peak at a somewhat higher rate of discharge than that
which produces maximum efficiency. The curves vary with the
speed of the pump.
Hence, speed must be considered while selecting a pump to obtain
maximum efficiency. Each of the curves also varies with the type of
pump.

Several curves, representing different pump speeds or impeller
diameters, may be drawn on the same graph. This type of graph
shows a number of head-capacity curves for one impeller
diameter and different speeds or head-capacity curves for
different impeller diameters and one speed (Fig. 3.27). A curve
of this type is known as a composite characteristic curve. When
this is done, iso-efficiency lines are plotted by joining points of
equal efficiency on the head-capacity curves.

The performance of centrifugal pumps can be changed by
changing the impeller diameter or speed.
Effect of change of pump speed: When the speed of a
centrifugal pump is changed, the operation of the pump is
changed as follows:
(i) The capacity varies directly as the speed.
(ii) The head varies as the square of the speed.
(iii) The brake horse power varies as the cube of the speed.
𝑄 = 𝑄1
𝑛
𝑛1

H= 𝐻1
𝑛
𝑛1
2
P= 𝑃1
𝑛
𝑛1
3
𝑛
𝑛1
=
𝑄
𝑄1
=
𝐻
𝐻1
=
3 𝑃
𝑃1
in which, n = new speed desired (rpm)
Q = capacity at the desired speed n (litres/sec)
H = head at the desired speed n for capacity Q (m)
P = BlIP at the desired speed n at H and Q
N1 = speed at which the characteristics are known (rpm)
Q1 = capacity at speed nI (litres/sec)
H1 = head at capacity QI and speed nl (m)
P1 = BHP at speed nI at HI and QI

Effect of change of impeller diameter: Changing the impeller
diameter has the same effect on the pump performance as
changing the speed.
Therefore, the following relationships apply:
(i) The capacity varies directly as the diameter.
(ii) The head varies as the square of the diameter.
(iii) The brake horse power varies as the cube of the diameter.
𝑄 = 𝑄1
𝐷
𝐷1
H= 𝐻1
𝐷
𝐷1
2

P= 𝑃1
𝐷
𝐷1
3
𝐷
𝐷1
=
𝑄
𝑄1
=
𝐻
𝐻1
=
3 𝑃
𝑃1
in which,
D = changed diameter of impeller (mm)
D 1 = original diameter of impeller (mm)
The other terms are analogous to those used in equation (3.9).
When pumps are driven by belts, or variable speed drivers, it is
possible to change the operating speed.
Many pumps are directly coupled to electric motors and must
run at constant speed. In this case, it is necessary to change the
impeller diameter to alter pump performance.

Amongst modern pumps, centrifugal pumps are most widely
used in irrigation practice.
1. They are simple in construction, easy to operate, low in initial
cost and produce a constant steady discharge.
2. The wearing parts are few.
3. They are adapted to direct motor or engine drives without the
use of expensive gears.
4. This type of pump is well adapted to usual pumping services
such as irrigation, water supply and sewage service. Having no
valves, the pump can handle liquids having solids in suspension,
provided it is constructed to suit such conditions.

A centrifugal pump may be defined as one in which an
impeller rotating inside a close-fitting case draws in the
liquid at the center and by virtue of centrifugal force
throws out the liquid through an opening at the side of
the casing.
A centrifugal pump is a rotary machine consisting of
two basic parts-
1. the rotary element or impeller and
2. the stationary element or casing (Fig. 3.28 to 3.30).

The impeller is a wheel or disc mounted on a shaft and provided with a
number of vanes or blades usually curved in form. The vanes are arranged in
a circular array around an inlet opening at the center.
In some pumps, a diffuser consisting of a series of guide vanes or blades,
surrounds the impeller (Fig. 3.29). The impeller is secured on a shaft
mounted on suitable bearings.
The shaft usually has stuffing box or seal where it passes through the casing
wall (Fig. 3.30).
Stuffing box packings are generally made of materials such as asbestos or
organic fiber.
The casing surrounds the impeller and is usually in the form of a spiral or
volute curve with a cross-sectional area increasing towards the discharge
opening.

Priming: While positive displacement pumps
can move and compress all fluids, including air,
centrifugal pumps are very limited in their
capacity to do so. Hence, they are to be primed,
or filled with water upto the top of the pump
casing to initiate pumping.

Many devices and techniques are used for priming centrifugal
pumps and maintaining the pruned condition. In general; they
involve one or a combination of the following:
(i) a foot valve to hold the water in the pump (Fig. 3.32),
(ii) an auxiliary piston pump to fill the pump casing and suction line
with water,
(iii) connection to an outside source of water under pressure for
filling the pump,
(iv) use of a self-priming construction. The self-priming construction
retains water for priming in an auxiliary chamber which forms a
part of the pump body.

The foot valve retains water in the pump and suction line. The need
for priming each time the pump is started is therefore eliminated.
The valve is installed at the bottom of the suction pipe. A strainer is
usually fixed to the foot valve to prevent foreign material from
entering the pump.

1. Type of energy conversion
(a) Volute (b) Diffuser or turbine.
2. Number of stages
(a) Single stage (b) Multi-stage.
3. Impeller types
(a) Open (b) Semi-open
(c) Closed (d) Non-clog.
4. Type of suction inlet
(a) Single suction (b) Double suction
5. Construction of casing
(a) Vertically split (b) Horizontally split
6. Axis of rotation
(a) Horizontal (b) Vertical
7. Method of drive
(a) Direct connected (i) Coupled (ii) Close-coupled or Urn-built
(b) Belt-driven.

The volute type pump (Figs. 3.31 and 3.33) has a casing made in the
form of a spiral or volute curve. The volute casing starts with a
small cross-sectional area near the impeller periphery and
increases gradually to the pump discharge. The casing is
proportioned to reduce the velocity of water gradually, as it flows
from the impeller to the discharge, thus changing velocity head into
pressure head. Most of the irrigation pumps are of the volute type.

In the turbine type pump, the impeller is surrounded by diffuser
vanes. The diffuser vanes have small openings near the impeller
and enlarge gradually to their outer diameter where the liquid
flows into the chamber and around to the pump discharge. A
major part of the conversion of velocity into pressure takes
place between the diffuser vanes. The diffuser vane casing was
introduced from water turbine practice where diffusion vanes
are indispensable. Hence, these pumps are often called turbine
pumps.

The choice between volute type and turbine type
pumps varies with the conditions of use. Ordinarily, the volute type
pump is preferred for large capacity, low head applications. Turbine
pumps are usually used for high head conditions. Turbine pumps
are most popular in deep tubewells because of its design advantage
where the diameter of the pump is small.

A single stage pump is one in which the total head is developed by
a single impeller.
A multi-stage pump has two or more impellers on a common shaft,
acting in series in a single casing (Fig. 3.34).
For a given type of impeller, the characteristics exhibited by a multi-stage pump
are as follows:
1. The head and power requirement increase in direct proportion
to the number of stages (impellers).
2. The discharge capacity and efficiency are almost the same for a
single stage of the pump operating alone.

The design of the impeller greatly influences the efficiency and operating characteristics
of centrifugal pumps. Centrifugal type impeller used in irrigation practice may be open,
semi-open, or closed (Fig.3.31). An open impeller consists essentially of a series of vanes
attached to a central hub. It is used to pump water with a limited amount of small solids.
A semi-open or semi-enclosed impeller has a shroud or side wall on one side only, usually
on the back. It can be used to pump water having some amount of suspended sediments.
In an enclosed impeller, the vanes are enclosed between shrouds or side walls on either
side. It is designed to pump clear water. Enclosed impeller develops higher efficiencies,
especially in high pressure pumps.
For pumping ordinary water, centrifugal pump impellers may be made of bronze or cast
iron. To handle brackish or salt water, gun metal impellers are commonly used.
Non-clog impeller (Fig. 3.35): Non-clog impellers are specially designed for sewage
service. They have vanes which are well rounded at their entrance ends and have large
passage-ways between the vanes. They can handle sewage water containing solid
particles, rags and other impurities.

In a single suction pump, the liquid enters the impeller from one
side (Fig. 3.33). In a double suction pump, it enters from both sides.
The double suction impeller is similar to two single suction
impellers cast back to back. They are theoretically in axial hydraulic
balance making a thrust bearing unnecessary. However, due to
manufacturing difficulties, double suction pumps are not as
common as single suction pumps.

A horizontal centrifugal pump has a vertical impeller mounted on a
horizontal shaft (Fig. 3.30). This type of pump is most commonly
used in irrigation. It costs less, is easier to install and is more
accessible for inspection and maintenance. The pump should be
installed so that it is always above the water surface, but as close to
it as possible. For satisfactory operation, the suction lift of the
pump should not exceed 4.5 to 6 m.

The vertical centrifugal pump has a horizontal impeller mounted on
a vertical shaft (Fig. 3.36). This type of pump has the advantage that
it can be lowered to the depth required to pump water and the
vertical shaft is extended to the surface where the power is applied.
The volute type vertical centrifugal pump may be either submerged
or exposed. The exposed pump is set in a sump at an elevation that
will accommodate the suction lift. In the submerged pump, the
impeller and suction entrance remain submerged below the water
level. Thus, the pump does not require priming. However, the
arrangement is not popular in irrigation practice due to the
difficulty in lubricating the bearings. Volute type vertical centrifugal
pumps are usually restricted to pumping heads upto about 15
metres and are commonly used to pump from sumps or pits.

Close-coupled pumps are built with a common shaft and bearings for the pump
and driver so as to form a single compact unit (Fig. 3.33). They are commonly
used with electric motor driven pumping sets of small to medium capacity.

Direct connected pump: In this type, the pump is mounted on a base plate and
is connected directly to its driver through a flexible coupling (Fig. 3.37). Flexible
couplings are most commonly used to connect the pump shaft to motor or
engine shaft. They permit minor misalignment of the shafts.

Belt-driven pump: The pump is provided with a pulley head for belt drive (Fig.
3.37). It is suitable when the power source is located away from the pump.
Usually a set of two pulleys-one fast pulley and a loose pulley-are provided to
facilitate engaging and disengaging the pump.

• The pump is installed as close to the water
surface as possible.
• It is located at an easily accessible place in
clean, dry, well ventilated surroundings.
• To ensure maximum capacity, the site selected
should permit the use of the shortest and most
direct suction and discharge pipes.

• The pump is installed on a foundation rigid enough to absorb all
vibrations (Fig. 3.39 and Fig. 3.41).
• A cement concrete base of adequate dimensions, steel girders,
and wooden beams are used as pump foundations.
• Pumps driven by electric motors do not require any special
foundation but are bolted securely to support and anchor the
pump independent of the piping.
• The support to which the motor-driven pumping set is bolted
should be stiff and rigid enough to prevent bending.
• Direct connected pumping sets are always mounted in a
horizontal position on a level foundation. Concrete foundations,
with foundation bolts imbedded in the concrete, arc in general,
a satisfactory arrangement.

The pump and driver must be carefully aligned.
The correct method of aligning flexible couplings is shown in Fig.
3.40.
Parallel alignment can be checked by placing a straight edge across
the coupling halves.
They must raise evenly on both halves at four positions placed at
approximately 90° intervals around the coupling.
Angular alignment can be checked with a feeler gauge placed
between the coupling halves at 4 points at approximately 90°
intervals around the coupling.

• On belt-driven units, the pump and driver shafts must be parallel.
(Figs. 3.37 and 3.43).
• The pulleys also must be properly aligned.
• An angle of 45° or less between the line of shaft centers and the
horizontal is desirable.
• Normally, the belt speed should not exceed about 1,500 meters per
minute.
• The ratio of the diameters of the pulleys should not exceed 5 to 1.
• The belt tension is adjusted just sufficient to prevent slippage.
• Excessive tension overloads the bearings.

Suction and delivery pipes should be supported
independently of the pump without being
strained into position (Fig. 3.41).

1. The suction piping should be as direct and short as possible.
2. It should have a minimum of fittings so as to avoid excessive
friction losses.
3. Sharp angle bends should be avoided.
4. Particular care is taken to see that the suction pipe and its joints
are absolutely air-tight.
5. The size of the suction pipe should be equal to or larger than
the suction opening of the pump.
6. The foot valve is located at the bottom of the suction pipe with
a minimum submergence of 60 cm below the pumping water
level.

7. The size of the suction pipe is so selected that the velocity of
water on the suction side does not exceed 3 metres per second.
Suction velocities in excess of this may cause cavitation.
8. There should be ample openings on the strainer which is fixed
below the foot valve.
9. The combined area of the strainer openings should be about
three to four times the area of cross-section of the suction pipe.
10. It is necessary that the strainer is kept at least about 1 metre
above the bottom of the well so as to prevent its choking by
mud and other material accumulated at the bottom of the well.

A pipe of suitable size to carry the normal discharge of the pump
without excessive frictional resistance should be selected.
Use of bends, elbows, tees, and other fittings is kept to the
minimum to reduce head loss in the discharge line.
However, when pumping to distant places or under high heads, it is
desirable to have a reflux or non-return valve fixed on the delivery
side close to the pump.
This will overcome water hammer which may occur at the time of
stopping the pump.
Sometimes, a sluice valve is fitted immediately after the reflux
valve. The sluice valve helps in regulating the discharge rate and
thus creates controlled working conditions.
It is also useful for disconnecting the pump for inspection and
repairs without disturbing the delivery pipeline.

The pumping set is lowered to the lower platform when the water
table falls.
When the depth of water column in the well exceeds the pump
suction limit substantially, it may be necessary to install the
centrifugal pump on a float and provide a minimum length of
flexible pipe on the delivery line.
The centrifugal pump is not efficient in deep-wells when it is driven
by an engine, as it involves excessive power loss with long vertical
belt drives used in transmitting power from the engine placed at
the ground surface to the pump installed in the well (Fig. 3.43).

Centrifugal pumps are commonly used for river
and canal pumping (Fig. 3.44).
The pump, coupled to an engine or electric motor,
may be fixed on a permanent foundation.
Engine- operated pumping sets may also be
installed on a float and used as a portable unit.

Centrifugal pumps may be used as portable units
for pumping from several wells, one 'after the
other, or from streams, canals or rivers.
The essential requirement for portable units is
that the pumping water level should not be
deeper than about 6 meters from the ground
surface.

Tractor power can be used with advantage to
operate centrifugal pumps.
Direct shaft drive or belt drive may be used for
power transmission from tractor to pump (Fig.
3.45 and 3.46).

Alternating current, 50 cycles, 400-440 volts, 3-phase
electric power supply is usually available on the farm.
However, single phase 230 volts line could also be tapped
from the 3-phase system for lighting, running single phase
motors and, other uses.

AC motors: The most common type of electric motors
used in irrigation pumping are the 3-phase squirrel cage
induction motors.
They are
1. low in initial and running costs,
2. smooth in running and
3. have a long life, if maintained properly.

The following accessories are used in the
electrical connections using 3-phase motors:
(i) Energy meter,
(ii) Volt meter and ammeter,
(iii)Indicator lamps,
(iv)Main switch, and
(v) Starter.

 The main switch connects the motor to the supply lines.
 It also includes fuses of appropriate capacity to protect the
supply lines and instruments from faults in the motor.
 The switch contains a moving system connected to an insulated
lever.
 In a single phase switch, two fuses are provided, one on the
phase wire and the other on the neutral.
 In a 3-phase switch, fuses are provided for the three phase
wires.

If an AC motor is started on full line voltage, it will draw about
four times its normal running current.
This may damage the motor and cause line disturbances
affecting the operation of other motors on the same line.
For single phase small motors, a hand-operated switch can be
used to control the motor(Fig. 3.47).
On three phase motors it is necessary to insert a starter in the
line which will reduce the starting current.
Direct-an-line or push button starters are commonly used for
motors up to 5 H.P.
Star-delta starters are used for motors above 5 H.P. (Figs. 3.48
and 3.49).

Starting:
Prior to starting the pump for the first time special attention is
paid to the following points:
1. Check the alignment of the pump. Any misalignment is
corrected by placing shims under the pump or driver.
2. Make sure that the engine or motor will drive the pump in the
direction indicated on the pump body.
3. Make sure that the gland is tightly and evenly adjusted and the
pump shaft revolves freely when turned by hand.
4. Check the air-tightness of the suction pipe and any leakage in
the foot valve.

Starting:
5. Fill the suction line and pump with water and remove the air
from the pump casing.
6. Attend to lubrication requirements. If ring oil bearings are
fitted, fill the bearings with good quality engine oil as
recommended by the manufacturer. However, if ball bearings
are fitted, no initial attention is required as they are properly
lubricated before leaving the factory.

Proper lubrication of the bearing and adjustment of the glands are
usually the only things which need attention from the operator.
The centrifugal pump must be stopped promptly if no liquid is
being pumped. Running a pump dry will result in excessive wear to
moving parts that depend on water for lubrication.
Sluice valves, when provided, are kept closed at the time of
starting. This will allow the motor or engine to be started free from
load.
When the pump reaches full speed, the sluice valve is opened
gradually, until the desired quantity of water is being delivered.
Care is taken not to run the pump for a long period with sluice valve
closed, as this may overheat the pump.

1. Every month: Check bearing temperature. Bearing may run hot due to lack of
lubrication or excess lubricants.
2. Every three months: Drain lubricants in oil ring bearings and wash out oil wells
and bearings with kerosene. In case of sleeve bearing, check to see that oil rings
are free to turn with the shaft. Refill with the lubricant recommended by the
manufacturer. Check the wear in the bearings and replace, if excessive.
3. Every six months: Replace gland packing. Check alignment of pump and driver
and add shims, if required. If misalignment occurs frequently, the entire piping
system may have to be checked and corrected.
4. Every year: Thoroughly inspect the unit once a year. Remove bearings, clean,
and examine for flaws. Clean bearing housings. Remove packing and examine the
wear in the shaft sleeve or shaft. Disconnect coupling valves and check
alignment. Inspect foot valve and check valves.

Cavitation: The term cavitation refers to the formation of cavities
filled with liquid vapour due to a local pressure drop and their
collapse as soon as the vapour bubbles reach regions of high
pressure.
To prevent cavitation, the following precautions are taken in
operating a centrifugal pump:
1. Avoid operating the pump at heads and capacities much
lower than the design value.
2. Excessive suction lifts and high values of water velocity on
the suction side are avoided.
3. Pump speed is not allowed to exceed the manufacturer's
recommendation.

Water hammer or hydraulic shock occurs when water flowing
through a pipe undergoes a sudden change in velocity.
The kinetic energy of the flowing liquid, under a sudden change in
the velocity of flow, is converted to a dynamic pressure wave which
may produce a substantial impact in rebounding back and forth in
the pipeline.

1) Prepare a check-list for centrifugal pump
troubles and their causes.
Hint: table 3.2

1. Irrigation: Theory and Practice by A. M.
Michael, Vikas Publishing House Pvt Ltd, 2008
ISBN 8125918671, 9788125918677
2. Water Well and Pump Engineering by A.M.
Michael al S.D. Khepar, Tata McGraw Hill
Publishing Co.Ltd. New Delhi, 1992.

Pumps and pumping plants for irrigation system

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