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Chapter 4 : Centrifugal Pumps

Chapter 1 : General Concepts Chapter 2 : Pelton Turbine Chapter 3 : Francis and Kaplan Turbine

Q. 1. Classify hydraulic pumps.

Chapter 4 : Centrifugal Pumps Chapter 5 : Similarity Relations and Performance Characteristics

Ans. Pumps may bi placed in one of the two general

Chapter 6 : Reciprocating Pumps

(i) Dynamic pressure pumps: centrifugal pump, jet

Chapter 7 : Hydraulic devices and Systems

categories. pump, propeller, and turbine. (ii) Positive, displacement pump: Piston plunger, gear, lab, vane, screw etc. Q. 2. Distinguish between the positive and nonpositive displacement pumps. Ans. Positive displacement pump: It causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe. E.g. Lobe, gear, gear, screw, vage pump etc. Non-positive displacement pump (rotodynamic pump) - It is pump in which the dynamic motion of a fluid is increased by pump action. e.g. centrifugal, turbine, propeller pr opeller etc. Q. 3. What is the principle of working of a Centrifugal Pump? Ans. It is very clear that the principle used for centrifugal pump is the centrifugal force in the form of dynamic pressure which is generated by rotary motion of one or more rotating wheels called the impellers. Q.

4.

Sketch

layout

installation and label it. Ans.

of

a

centrifugal

pump

Q. 5. Distinguish between volute casing and vortex casing in a centrifugal pump. Ans. Volute casing—This casing—This is a spiral shaped whose area of cross-section gradually increases towards the delivery pipe. The velocity of liquid decreases as area increases along the path of flow. This arrangement converts K.E. into pressure energy. The efficiency of this casing is less than others. Larger amount of energy is lost due to formation of eddies.

Fig. Different types of casings. Vortex casing—When casing—When a circular chamber is introduced between the impeller and casing, the casing is known as vortex casing. In this case, the liquid from the impeller enters into the vortex chamber and then through the

Q. 5. Distinguish between volute casing and vortex casing in a centrifugal pump. Ans. Volute casing—This casing—This is a spiral shaped whose area of cross-section gradually increases towards the delivery pipe. The velocity of liquid decreases as area increases along the path of flow. This arrangement converts K.E. into pressure energy. The efficiency of this casing is less than others. Larger amount of energy is lost due to formation of eddies.

Fig. Different types of casings. Vortex casing—When casing—When a circular chamber is introduced between the impeller and casing, the casing is known as vortex casing. In this case, the liquid from the impeller enters into the vortex chamber and then through the

volute chamber, in this arrangement, the eddy loss is considerably reduced and the efficiency of conversion from K.E. into pressure energy is increased as compared with volute casing. casing. Q. 6. Explain different types of impellers used in centrifugal pump with neat sketch. Ans. (1) Closed, semi—closed semi—closed and open impellers in the closed impellers the vanes are covered with side plates (shrouds) on both sides. The back shroud is mounted into shaft and front shroud is coupled to the former by the vanes. The arrangement provides a smooth passage for the liquid; wear is reduced to minimum. This ensures full capacity operation with high efficiency for a prolonged running period.

Fig. (,) Open (ii) Semi-open (iii) Enclosed Semi-open impeller has a plate only on back side. The design is adopted to industrial pump problems which required a rugged pump to handle liquids containing fibrous material such as paper pulp, sugar molasses and sewage water etc. in open impeller, no shroud or plate is provided on either side i.e., the vanes are open on both sides. Such pumps are used where the pump has a very rough duty to perform i.e. to handle abrasive liquids etc. (ii) Axial, Radial and mixed flow impellers—In the axial flow pumps, the head is developed by the propelling or lift action of the vanes on the liquid which enters the impeller axially and discharges axially

Fig. Axial radial and mixed flow impellers. In radial flow impellers, the head is developed by the action of centrifugal force upon the liquid which enters the impeller axially at the centre and flows radially to the periphery.

In

the

mixed

flow

impeller

there

is

a

combination of axial and radial flows. The head is developed partly by the action of centrifugal force and partly by axial propulsion. Q. 7. Draw velocity triangles for (i) Curved forward vanes. (ii) Radial vanes (iii) Curved backward vanes. How Euler’s head varies with discharge? Also draw variation of power with discharge. Ans.

Euler’s head variation with discharge: Is negative, so with increase in flow rate the Euler head rises. For radial vanes

= 90° and cot

= 0, so the head

remains constant with variation in discharge. For curved backward vanes

90° leads to backward curved

90° means radial blades and

900 (140—160°) are used. Q. 13. Discuss the influence of exit blade angle on the performance and efficiency of a centrifugal pump. Assume radial flow at entrance. Ans. Blade angle

affects

the

efficiency of

the

centrifugal pump When

> 90° efficiency - 75% 90° efficiency - 80% to 85% 90° the speed is slow = 90° the speed is medium