Fluid mechanics with engineering applications, Volume 1

7/30/2019 Fluid mechanics with engineering applications, Volume 1

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Irrigation Pumps

Prof. S.K. SondhiPunjab Agricultural University ,Ludhiana


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Variable Displacement pumps

Centrifugal Mixed flow Propeller Jet Air Lift

Volute Diffuser Turbine

Single stage

Multi stage

Deep well turbine

Submersible

Variable Displacement pumps

Centrifugal Mixed flow Propeller Jet Air Lift

Volute Diffuser Turbine

Single stage

Multi stage

Deep well turbine

Submersible

Classification of Pumps


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Determination of Operating

Conditions The source of water

* Surface water

* Ground water

The required pumping flow rate The total suction lift

The total dynamic head Net positive suction head available


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Determination of flow rate

Q = 27.78 AY/ RT

Q = Discharge capacity of the pump (lps)

A = area (ha) under crops

Y = depth of irrigation (cm)

R = rotation period (days)

T = duration of pumping (hrs/day)


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The total dynamic head

Ht = Hs + Hd + Hl + Hv

Ht=total dynamic head of the system (TDH)

Hs=static head (static lift + static dischargehead)

Hd=drawdownHl=friction loss head

Hv=velocity head


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Pump Performance Parameters

Capacity

Head

Power Requirements

WHP = Discharge (lps) x Total head (m)

76 BHP = Water Horse Power (WHP)

Pump efficiency X Drive efficiency


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Efficiency

Efficiency = WHP x100BHP

Net Positive Suction Head (NPSHr

)

Net positive suction head. This is the pressure apump requires for cavitation-free operation.

Specific Speed

Ns = N x Q1/2

H3/4Ns = specific speed rpm

N = pump speed rpm

Q = Discharge in m3/secH = Total Head (m)


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Working principle of centrifugal pump


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Centrifugal pump


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Net Positive Suction Head available

NPSHa = BP - SH - FL VP

BP=barometric pressure

SH=suction head or lift

FL=friction losses in the intake pipe

VP=water vapor pressure at a given temperature

The NPSHr versus Q curve can be used. The

NPSHa must be greater than NPSHr at agiven Q to avoid pump cavitation.


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Pump Characteristic curves


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Determination of the operating point for a given centrifugal pumpand water system


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Effect of Speed Change on Pump Performance

(N2/N1) x Q1 = Q2

(N2/N1)2 x H1 = H2

(N2/N1)3 x BHP1 = BHP2

N1 = Initial rpm

N2 = New rpm desired

Q =Discharge (liters per second)

H =Total Head

BHP = Brake Horsepower

C it ti i t if l


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Cavitation in centrifugal pump

The term cavitation implies a dynamic process offormation of bubbles inside the liquid, their growth

and subsequent collapse as the liquid flows

through the pump.General symptoms

*Reduction in capacity of the pump*Abnormal sound and vibrations

*Fluctuating pump motor current

Effects of cavitation

*Degraded pump

*Excessive pump vibration*Damage to pump impeller, bearings, seals


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Cavitation

Cavitation occurs when a void forms within

a pump when discharge of water is faster than intake.

Air is taken into pump

Cavitation can usually be heard- sounds

like rocks passing through pump


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Preventing cavitation

Decrease the static lift by placing the

pump near to water surface Decrease the temperature of the liquid

being pumped

Increase the diameter of suction pipe,

reduce the number of bends, fittings in the

suction pipe


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Installation of centrifugal pump

Location

Foundation

Mounting unit on foundation

Grouting unit on foundation

Alignment


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Operation of centrifugal pumps

Check the alignment of the pump

Make sure that the engine or motor will drive thepump in the direction indicated on the pumpbody.

Make sure that the gland is tightly and evenlyadjusted and the pump shaft revolved freelywhen turned by the hand.

Check the air tightness of suction pipe andleakage.

Fill the suction line and pump with water and

remove the air from pump casing. Attend the lubrication requirements


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Energy Conservation in Pumping systems

Various reasons for low efficiency of pumping1. Excessive suction lift

2. Use of sharp bends in piping system

3. Excessive height of delivery pipe from groundlevel

4. Improper selection and inferior quality of pump

5. Leakage in joints6. Lowering of undersized pipe in the suction line

7. Improper matching of RPM of pump and

motor/engine8. Use of poor quality driving belts

9. Use of poor quality reflux valve

10. Misalignment of pump and motor/engine pulley


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Example: Calculate pump capacity

Season Crop Area to be

irrigated

(ha)

Irrigation

depth per

irrigation(cm)

Rotation

period in

days

Period of

work

hours/day

Winter

(rabi)

Wheat 3 7.5 15 10

Summer

(kharif)

Maize 1 7.5 20 10

Paddy 2 5.0 2 10


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Discharge required for winter crop:

Q = 27.78(3 x 7.5)/ (15 x 10) =4.17 l/s

Discharge required for summer crop:Q = 27.78((1 x 7.5)/ (20 x 10) + (2

x 5.0)/ (2 x 10))

= 14.93 l/sTo allow for conveyance losses between the

pump and the field, increase the pumpcapacity by 20 per cent.

Therefore, the discharge rate required by

the pump = 14.93 x 1.2 = 17.92 l/s


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Determine the horse power requirements


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Determine the horse power requirements1.Discharge of the pump = 18 l/s

2. Dia. Of suction and delivery pipe = 100 mm3. Depth of water table = 3m4. Height of delivery pipe above ground surface =

1 m5. Horizontal section of delivery pipe after bend =2 m

6. Horizontal section of suction pipe betweenbend and pump = 1 m7. No. of reflux valve = 18. No. of long radius bend = 2

9. Drawdown = 3 m10. Friction loss for 100 mm GI pipe of dischargeof 18 l/s = 9m/100 m length of pipe


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Estimation of TDH

Friction losses in pipe and fittings:Total length of pipe above pumping water level =

3+3+1+9+1+2 = 19 m

The equivalent length of straight pipe for 100 mmreflux valve = 8.23.m

The equivalent length of straight pipe for 100 mm

two long radius bends = 2 x 2.13 = 4.26 mThe total friction loss in pipe and fittings =

(19+8.23+4.26)x9/100 =2.83 m

Velocity head for discharge of 18 l/s through 100

mm pipe = 0.27 m

Therefore, total dynamic head (TDH)=3+3+9+1+2.83+0.27=19.10 m


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Head calculations

Vertical turbine pump


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Motor and pump

Submersible pump

Selecting an Irrigation Pump


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Selecting an Irrigation Pump

1. Centrifugal Pump Advantages

1. High efficiency over a

range of operatingconditions

2. Easy to install.

3. Simple, economical andadaptable to manysituations.

4. Electric, engine or tractorpower can be used

5. Does not overload with

increased TDH.

Disadvantages

1. Suction lift is limited. It

needs to be within 4m ofthe water level.

2. Priming required.

3. Loss of prime candamage pump.

4. If the TDH is muchlower than design value,the motor may overload.

V ti l T bi P


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Vertical Turbine Pump

Advantages

1. Adapted for use in wells.

2. Provides high TDH and

flow rates with high

efficiency.

3. Electric or engine power

can be used.

4. Priming not needed.

5. Can be used where water

level fluctuates.

Disadvantages

1. Difficult to install, inspect

and repair

2. Higher initial cost than

centrifugal pump.

3. Repair and maintenance

is more expensive than

centrifugal pump.

Submersible Pump


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Submersible Pump

Advantages

1. Adapted for use in deep

wells

2. Priming not needed.

3. Can be used in crooked

wells

4. Easy to install.

5. Smaller diameters are

less expensive than

comparable vertical

turbine pumps

Disadvantages

1. More expensive in larger

sizes than verticalturbine pumps.

2. Only electric power can

be used.

General maintenance


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The suction lift should be periodically checked

The gland packing in the pump should be checked. The watershould drip through the packing at a rate of 15 to 30 drops perminute.

The rpm of the prime mover should be at the rated value. The alignment of the pump and motor shaft should be checked

periodically.

A record of the pump running hours, problems, servicing,maintenance and repairs should be kept in a logbook.

The inlet screen, foot valve/ reflux valve and pipe threadsshould be checked, and any corroded or damaged threads re-

cut.

The reflux valve should be checked and rubber gasket may bereplaced if worn out.

Trouble shooting in centrifugal pumps


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Trouble shooting in centrifugal pumps

1. No water delivered

Pump not properly primed

Speed too low Discharge head too high

Suction lift too high Impeller or suction pipe completely plugged

Wrong direction of rotation

Air pocket in suction line

Air leak in suction line or stuffing box

Insufficient NPSH available

2 Not enough water delivered


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2. Not enough water delivered

Air leak in suction line or stuffing box Speed too low

Discharge head higher than anticipated

Suction lift too high

Impeller or suction pipe partially plugged

Wrong direction of rotation Insufficient NPSH available

Foot valve too small Insufficient submergence of suction inlet

Wearing rings worn

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3. Not enough pressure developed

Speed too low

Excessive amount of air or gas in liquid

Wrong direction of rotation Viscosity of liquid higher than anticipated

Wearing rings worn

Impeller diameter too small

4. Pump works for a while and then loses


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4. Pump works for a while and then loses

prime Air leak in suction line or stuffing box

Excessive amount of air or gas in liquid

Air pocket in suction line

Water seal tube clogged

Water seal ring improperly located Suction lift too high

Insufficient submergence of suction inlet

5. Pump requires excessive power


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5 u p equ es e cess e po e

Speed too high Head lower than anticipated

Specific gravity or viscosity too high

Wrong direction of rotation

Misalignment

Stuffing box too tight Rotating element tubing binding

Bent shaft Wearing rings worn

6. Stuffing box leaks excessively


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g y

Packing is worn or improperly lubricated Packing improperly installed

Incorrect type of packing

Shaft sleeve scour

Bent shaft

7. Pump noisy or vibrates


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p y

Suction lift too high Insufficient NPSH available

Impeller or suction pipe partially plugged

Misalignment

Foundation not rigid

Lack of lubrication Bearing worn

Rotating element out of balance Bent shaft

Pumping do's and don'ts


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Pumping do s and don ts

Do: site the pump as close as possible to the water

make sure suction and delivery pipes do not put a strain on

the pump casing check that all pipe connections are tight

use a strainer recommended by the pump manufacturer

anchor the pump securely so that it doesn't move duringoperation

work the pump within its limits

provide ventilation for the motor or engine keep the pump and motor connection aligned

make sure the pump is primed before starting

keep the strainer clean service the pump regularly.


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Don't:pump corrosive liquids

operate the pump without wateroperate the pump if the discharge valve is closed

operate the pump if the strainer is blocked

operate the pump if it is vibrating excessivelyinstall the suction pipes so that air can build up in them

forget to do regular maintenance.

Documents

Fluid mechanics with engineering applications, Volume 1