pumps and pumping plants for irrigation system

Pumps and pumping plants for irrigation systemMd Moudud HasanLecturer Department Agricultural and Industrial Engineering Faculty of Engineering Hajee Mohammad Danesh Science and Technology UniversityDinajpur

Water liftsFour principles involved in pumping water

Atmospheric pressurePositive displacement Centrifugal forceMovement of columns of water caused by the difference in specific gravity

MM HASAN,LECTURER,AIE,HSTU

Classification of irrigation water lifts-


Low Head Water Lifts The depth to water surface from the ground does not exceed 1.2m Swing Basket

Ancient water liftsA basket or shovel like scoop to which four ropes are attached.Two person operate it Water filled basket is discharged into the field channel.


Low Head Water LiftsDon

Boat shaped troughClosed 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 leverOpen end is hinged to discharge pointDipped into water applying body weight and forceLifted by the counter-weight on the beam


Low Head Water LiftsDon


Low Head Water LiftsArchemedian screw

Consists of a wooden or metal drum with interior partition in the from of a screwRotated by means of a handle fixed to a central spindleLower end of the drum is placed in water with a angle less than 30˚Handle turned water moves up through the drumHalf of lower end is submerged in water


Low Head Water LiftsArchemedian screw


Low Head Water LiftsWater Wheel

Consists of small paddles mounted radially on a horizontal shaftFixed on a close-fitting concave trough Driven by a bullock-wheel driveRotating wheel pushes the water to the field surface through the trough


Low Head Water LiftsWater Wheel


Medium Head Water Lifts 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


Medium Head Water Lifts Persian Wheel:


Medium Head Water Lifts 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.


Medium Head Water Lifts 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 ropepasses 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.


Medium Head Water Lifts Rope and bucket lift with self-emptying bucket


Medium Head Water Lifts 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 aAt top , bucket is tilted automatically due to the loop on the rod.


Medium Head Water Lifts 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.


Medium Head Water Lifts 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

Or



Counterpoise-bucket lift:


High Head Water LiftsRope 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.


PumpA 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 pumps2. Variable-displacement pumps


Positive Displacement Pump 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 PumpsReciprocating 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.


Shallow Well Reciprocating PumpsReciprocating 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.


Animal powered reciprocating type pump: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.


Animal powered reciprocating type pump:

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.


Animal powered reciprocating type pump:

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.


Manually Operated Twin Treadle PumpA 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.


Manually Operated Twin Treadle PumpThe 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.

Manually Operated Twin Treadle Pump


Deep Well Reciprocating PumpBy 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.

Deep Well Reciprocating Pump


Types of reciprocating Pumps on Construction and Operating Features

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.

Air Vessel


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.

Then, connecting rod) (kg)w = specific weight of water (kglm8) Weight of water raised in one stroke= w a I = 1000 al


Force required to work a reciprocating pump:

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 = PhTherefore, P= 1000 al

Example 3.1A 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.

Solution:(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

Solution: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 kgTotal 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 kgDischarge 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/minTotal 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.


Rotary PumpsThough 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.

Rotary PumpsThey 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.


VARIABLE DISPLACEMENT PUMPSThe 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 PUMPSVariable 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.


Specific Speed of PumpsIt 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)


Specific Speed of PumpsIn 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)


Example 3.3. 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


Pump CharacteristicsA 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.


TerminologyCapacity 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.


TerminologyTotal 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.


TerminologyStatic 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).


TerminologyStatic 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.


TerminologyTotal 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).


TerminologyFriction 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


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


TerminologyPressure 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. Expressed mathematically,

in which, Hp = pressure head (m) p = pressure inside the vessel (kg/m2) w = specific weight of water (kg/m3)


TerminologyTotal 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.


TerminologyVelocity head is the pressure, expressed in meters of water, required to create the velocity of flow.

Expressed mathematically,

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)


TerminologyNet 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).


TerminologyMaximum 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)


TerminologyHf = 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.


Example 3.4Determine 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.


Solution: 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


Characteristic Curves

• 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.


Characteristic CurvesIt 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.


Characteristic CurvesSeveral 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.


Effect of Speed and Impeller Diameter on Pump Performance

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. Expressed mathematically,


HP

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. Expressed mathematically,

H


P

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.


CENTRIFUGAL PUMPSAmongst 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.


Principles of Operation of Centrifugal PumpsA 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).


Principles of Operation of Centrifugal PumpsThe 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.


Centrifugal Pump Classification 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.


Volute centrifugal pumpsThe 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.


Volute centrifugal pumps


Diffuser or turbine pump: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.


Single stage and multi-stage pumps: 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.


Types of impellersThe 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.


Single suction and double suction pumps:

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.


Horizontal centrifugal 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.


Vertical centrifugal pumps: 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 or unibuilt pumps 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.


Close-coupled or unibuilt pumps 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.


Close-coupled or unibuilt pumpsBelt-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.


Installation of Horizontal Centrifugal Pumps

• 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.


Foundation: • 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.


Alignment: 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.


Belt drive: • 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.


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


Suction piping: 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.


Suction piping: 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.


Delivery piping: 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.


Installation of Centrifugal Pumps in Deep Open Wells

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).


Installation of Centrifugal Pumps in Deep Open Wells


Pumping from canals and rivers: 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.


Portable Pumping Sets 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.


Portable Pumping Sets .


Tractor-driven centrifugal pumps: 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).


Electrical Connections 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.


Electrical Connections AC motors: The most common type of electric motors used in irrigation pumping are the 3-phase squirrel cage induction motors.

They are1. low in initial and running costs, 2. smooth in running and 3. have a long life, if maintained properly.


Electrical Connections 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.


Main switch: 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.


AC motor starters: 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).


Operation, Maintenance and Trouble-Shooting of Centrifugal Pumps

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.


Operation: 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.


Pump maintenance: 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.


Pump Troubles 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: 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.


Assignment-21) Prepare a check-list for centrifugal pump

troubles and their causes.

Hint: table 3.2


References 1. Irrigation: Theory and Practice by A. M. Michael, Vikas Publishing House Pvt Ltd, 2008 ISBN 8125918671, 97881259186772. Water Well and Pump Engineering by A.M. Michael al S.D. Khepar, Tata McGraw Hill Publishing Co.Ltd. New Delhi, 1992.