Jet, Submersible and Rotary pumps

7/27/2019 Jet, Submersible and Rotary pumps

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Course: Advanced Hydraulics Module 6: Pumps and Turbines

Lecture 13Jet, Submersible and Rotary pumps


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Jet Pumps

A Jet Pump is a type of impeller-diffuser pump, which is used to draw water from bothshallow and deep wells.

Jet pump consists of a single stage centrifugal pump at the top with a jet/nozzle located atthe suction screen in the well.

A portion of the water discharged by the pump flows down through the pressure pipe andit helps to improve the flow into the pump and up the discharge tube. At the end of the

pressure pipe the water is accelerated through a cone-shaped nozzle. Then the water goes

through a Venturi in the Suction Pipe.

Jet pump uses the Venture effectof a converging-diverging nozzleto convert thepressureenergy of the fluid to velocityenergy which creates a low pressure zone that sucks more

water through the intake.

The venturi has two parts: the Venturi Throat, which is the narrowest section of thesuction tube; and above that it is the part where the tube widens and connects to the

suction pipe.

The venturi speeds up the water causing a pressure drop which sucks in more waterthrough the intake at the very base of the unit. The water goes up the Suction Pipe and

through the impeller.

Jet can be either located at the on the surface in the pump casing or in the well. Jet pumps can handle sediment laden or muddy water and used for dewatering the

trenches and in mines.

Jet pumps are portable and light weight and are easy to install and maintain.


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Jet pumps are well suited for small wells and have low initial and maintenance costs

therefore they are widely used.

However, the efficiency of the jet pump is low and is upto 25%.

Figure 6.25. A typical jet pump

Submersible Pumps

As the name suggests the submersible pumps are submerged under the water in the well.Submersible pumps often used to pump water from the deep wells.

The submersible pumps remains suspended on the delivery or riser pipe. Hence the pumpmotor always remains submerged in the water.

The pumps are of multistage centrifugal type, the impellers and diffusers are arranged inseries.

Pressurepipe

Venturi

Suctionpipe

Nozzle


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The pumps delivers the water as such a pressure head which is sufficient to force the

water out of the well.

A check valve is provided at the outlet of the pump, which prevents the delivery pipefrom emptying when the pump is stopped.

Electric motor is connected to the power supply by special submarine type electricalcables.

The electric motor used for submersible can be classified as 'dry motor' and 'wet motor. The dry motors are sealed to prevent the entry of the water into it. Whereas, the wet

motor the water of the well can enter inside of the well and rotor and bearing revolves in

the well water.

Submersible pumps are easy to install, compact, requires lesser space and are economicalas they have low maintenance and installation costs.

No priming or lubrication is required and have high efficiency. However, they have disadvantages that the pump is not accessible for inspection and

during the repairs the pump needs to be lifted up from the well and sometimes it becomes

very difficult to lift the pump up in case of sludgy water.


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Figure 6.26. A typical submersible pump

ROTARY PUMPS

Rotary pumps are a type of positive displacement pumps. They are suitable for small discharge, which use rotary motion instead of reciprocating

motion as in reciprocating pump.

The rotary motion is obtained by means of two rotating elements called cams, gears orscrews fitted closely in casing and rotate in opposite direction.

Fluid is trapped in the space between the gear teeth and the casing and is moved from theinlet to the outlet.


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Flow from a rotary pump is more or less steady as compared to the pulsating flow from a

reciprocating pump. These pumps have a higher efficiency at low and moderate heads

with lower discharge.

These pumps are self-priming and do not requires valves. These pumps give a largercapacity and can operate at higher speeds.

However, these pumps are subjected to excessive slip and wear of the parts. These pumpsoften have higher initial and maintenance costs.

Rotary pumps are classified into four typesi. Gear pumps

ii. Rigid rotor vane pumpsiii. Rotary piston pumpsiv. Screw pumps

Gear pumps are the simplest type of rotary pumps, consisting of two gears laid out side-by-side with their teeth enmeshed. The gears turn away from each other, creating a

current that traps fluid between the teeth on the gears and the outer casing, eventually

releasing the fluid on the discharge side of the pump as the teeth mesh and go around

again. Many small teeth maintain a constant flow of fluid, while fewer, larger teeth create

a tendency for the pump to discharge fluids in short, pulsing gushes. Its geometric

dimensions and the rotative speed of the shaft determine the capacity of the pump. These

pumps are reversible in nature.

Rigid vane pumps have moveable sealing elements in the form of non-flexing blades,rollers, buckets, scoops, and so on. These elements move radially inward and outward by


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cam surfaces to maintain a fluid seal between the open to inlet and open to outlet sectors

during pumping operation.

Figure 6.27. A rotary pump ( From: Chandramouli, et al., 2012)

Figure 6.28. A Gear Pump ( From: Chandramouli, et al., 2012)


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Objective Questions:-

1. The force exerted by a jet impinging normally on a fixed plate isa.

b. c.

d. 2. The force exerted by a jet impinging on a fixed plate inclined at an angle with the jet is

a.

b. sinc.

d. sin2

3.

The ratio of the normal force of jet of water on a plate inclined at an angleas compared

to that when the plate is normal to the jet, is

a.

b. c. 2d. 2

4. The force exerted by a jet of water in the direction of jet on a stationary curved plate isa. b. 2c. 1 c o s


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d. 1 sin

5. Maximum efficiency of a series of vertical plates isa. 66.67%b. 33.33%c. 50%d. 100%

6. Efficiency of the jet of water with velocity striking on a series of vertical plates movingwith a velocity is

a. b.

c.

d. 7. The water jet after striking the flat plate will be deflected at an angle of

a. 1100b. 600c. 900d. None of the above

8. In a centrifugal pump, the liquid enters the pumpa. At the centreb. At the bottomc. At the topd. From sides


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9. Overall efficiency of a centrifugal pump is the ratio of

a. Energy available at the impeller to the energy supplied to the pump by the primemover

b. Actual work done by the pump to the energy supplied to the pump by the primemover

c. Energy supplied to the pump to the energy available at the impellerd. Manometric head to the energy supplied by the impeller per Newton of water

10. Multi stage centrifugal pumps are used toa. Give high dischargeb. Produce high headsc. Pump viscous fluidsd. All the above

11. Discharge of a centrifugal pump isa. Directly proportional to Nb. Inversely proportional to Nc. Directly proportional to N2d. Inversely proportional to N2

12. A centrifugal pump is superior to a reciprocating pump becausea. It is a high speed pumpb. It is more economicalc. It gives smooth flowd. All the above

13. In a reciprocating pump, air vessels are used toa. Reduce the flowb. Increase the delivery head


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c. Smoothen the flowd. Reduce the acceleration head

14. The discharge through a single acting reciprocating pump isa.

b.

c. d. None of the above

15. A turbine is a device which convertsa. Kinetic energy in to mechanical energyb. Mechanical energy in to hydraulic energyc. Hydraulic energy into mechanical energyd. None of the above.

16. An impulse turbine is used fora. Low head of waterb. High head of waterc. Medium head of waterd. High discharge

17. Francis turbine isa. A reaction radial flow turbineb. An axial flow turbinec. A radial flow turbined. An impulse turbine

18. A draft tube is not required for a


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a. Francis turbineb. Kaplan turbinec. Pelton wheel turbined. None of the above

Answers:-

1(d) 2(c) 3(b) 4(c) 5(c) 6(b) 7(c) 8(a)

9(c) 10(b) 11(a) 12(d) 13(d) 14(b) 15(c) 16(b)

17(a) 18(c)

Subjective Questions:-

1 Derive an expression for force of jet of water exerted on a fixed vertical plate in the

direction of the plate.

2 Show that the force exerted by a jet of water on a inclined fixed plate in the direction of

the jet is given by,

Where, a= area of the jet; v= velocity of the jet; = inclination of the plate with the jet.

3 Explain the working principles of a centrifugal pump with sketches

4 Define the term: suction head, delivery head, static head and manometric head.

5 What is priming of a centrifugal pump? Explain clearly why priming is essential before

starting a centrifugal pump?

6 Define the term specific speed of a centrifugal pump and deduce an expression for it in

terms of the head H, discharge Q, and the speed N.

7 Differentiate between a single acting reciprocating pump and double acting

reciprocating pump.

8 What are the factors, which influence the speed of a reciprocating pump?

9 Differentiate between the turbine and pump.

10 State the difference between an impulse turbine and a reaction turbine.


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11 Define and explain hydraulic efficiency, mechanical efficiency and overall efficiency

of a turbine.

12 Define the terms unit power, unit speed and unit discharge with reference to a

hydraulic turbine.

13 A jet of water of 5cm diameter is discharging under a constant head of 80m. Find the

force exerted by the jet on a fixed plate. Take coefficient of velocity 0.9. (Ans.2491.0N)

14 A jet of velocity of 5 m/s strikes a flat plate inclined at 300with the axis of jet. If the

cross sectional area of the jet is 5 cm2, find the force exerted by the jet on the plate.

Also, calculate the components of the force in the direction normal to the jet. Find also

the ratio in which the discharge gets divided after striking the plate. (Ans. 6.25N, 5.41N

13.928)

15 A jet of water of diameter 0.20m moving with a velocity of 25m/s, strikes a curved

fixed symmetrical plate at the centre. Find the force exerted by the jet of water in the

direction of the jet, if the jet is deflected through an angle of 1200at the outlet of the

curved plate. (Ans. 29437.5N)

16 A jet of 0.20 m in diameter strikes tangentially on a curved vane. The tangent at the

outlet end makes an angle of 600with the x-axis and the velocity of the jet is 20 m/s.

find the force exerted by the jet on the vane in the direction of jet. (Ans. 6280.5N)

17 A centrifugal pump delivers 0.35 cumecs of water to a height of 20 m. If the pump runs

at 1000 rpm, find the specific speed. (Ans. 62.55)

18 The diameter of an impeller of a centrifugal pump at inlet and outlet are 20 cm and 40

cm respectively. Determine the minimum starting speed of the pump if it works against

a head of 50 m. (Ans. 1733.2 rpm)

19 A centrifugal pump delivers 25 liters of water per second against a head of 10 meters

and running at 1300 rpm requires 10 kW of power. Determine the discharge, head of

the pump and power required if the pump runs at 1500 rpm. (Ans. 0.0288cumec,

13.31m, 15.36kw)


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20 A single acting reciprocating pump having a bore of 150 mm and a stroke of 300 mm

length discharges 250 liters of water per minute at 50 rpm. Neglecting losses, find:

a. Theoretical dischargeb. Coefficient of dischargec. Slip of the pump.

(Ans 4.4 10, 0.9469, 5.3%)

21 A single acting reciprocating pump has its piston diameter as 0.15 m and stroke 0.25 m.

The piston moves with simple harmonic motion and makes 50 double -stroke per

minute. The suction and delivery heads are 5 m and 15 m respectively. Find the force

required to work the piston during the suction and delivery stroke. Assume the

efficiency of the suction and delivery strokes as 65% and 75% respectively. Determine

the power required by the pump. (Ans 1.001 KW)

22 The cylinder bore diameter of a single acting reciprocating pump is 150 mm and its

stroke is 300 mm. the pump runs at 50 rpm and lifts water through a height of 25 m.

The delivery pipe is 22 m long and 100 mm in diameter. Find the theoretical discharge

and the theoretical power required to run the pump. If the actual discharge is 4.2 L/s,

find the percentage slip. Also, determine the acceleration head at the beginning and

middle of the delivery stroke. (Ans. 0.0044175cumec, 4.92%, 1.083KW, 20.75m, 0)


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References:-

1. D.S. Kumar, Fluid Mechanics and Fluid Power Engineering, S.K. Kataria & sons,Publishers of Engineering and Computer Books, Delhi.

2. G.I. Krivchenko, Hydraulic machines turbines and pumps, MIR Publications, Moscow,1986.

3. K.R.Arora, Fluid mechanics, hydraulics and hydraulic machines, Standard PublishersDistributors, Nai Sarak, Delhi.

4. M. Manohar, and P. Krishnamachar, Hydraulic Machinery & Advanced Hydraulics,Vikas Publishing House PVT LTD, New Delhi.

5. P.N.Modi and S.M. Seth, Hydraulics and Fluid Mechanics, Standard Book House,Delhi.

6. Ojha, C.S.P., Berndtsoon, R., and Chandramouli, P.N., 2010. Fluid Mechanics andMachinery, Oxford University Press, India.

7. Chandramouli, P.N., Ojha, C.S.P., and Singh K.M. (In press). Fluid Machinery, OxfordUniversity Press, India.

8. S.K. Agrawal, Fluid Mechanics and Machinery, Tata McGraw- Hill PublishingCompany Limited, New Delhi.

9. R.C. Patel, A.D. Pandya, B.M. Patel, and S.S. Sikh, Elements of hydraulic machinery.C. Jamnadas & co. Educational& Law publishers, Bombay, India.

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Jet, Submersible and Rotary pumps