Q.) Draw the characteristics of centrifugal pump?
Ans.)
Q.) Why Q drops when H increases?
Ans.)Q is directly related to flow velocity and following Bernoulli’s equation, if H increases, kinetic energy decreases i.e flow velocity decreases and so Q drops
Q.) What is critical temp. in pump?
Ans.) Reciprocating pump : 94 degC, loses suction
Centrifugal pump : 75 degC, loses suction
In such case, pressure head to be increased
Q.) How is axial thrust in multistage centrifugal pump developed and taken care of?
Ans)it arises in the centrifugal pumps due to their asymmetry. The clearances between casing cover and impeller back shroud, casing and impeller front shroud are filled with fluid at delivery pressure. This pressure acts on the impeller shrouds. As the back shroud is having larger surface area than front shroud, a net thrust acts on the impeller in the direction opposite to that of the incoming flow. The force contributed by the change in the momentum of incoming flow is also to be considered.
As shown in Fig. above the resultant unbalanced axial thrust is vector summation of the following forces–
1. Force acting on front shroud due to liquid of delivery pressure entrapped between pump casing and front shroud. (F1)
2. Force acting on back shroud due to liquid of delivery pressure entrapped between casing cover and back shroud. (F2)
Force acting in the direction of the liquid flow due to its momentum change. (Fm)
The axial thrust can be safely carried out:-
1. By a thrust bearing
2. By Providing wear rings and balancing holes
One of the method which is usually employed to reduce or eliminate axial thrust in single stage pumps in a chamber on the back of the impeller is provided with a closely fitted set of wearing rings and suction pressure is admitted to this chamber either by drilling holes through the impeller back shroud into the eye or by providing a special channel connecting the balancing chamber to the suction nozzle. The use of drilled holes
through the impeller shroud to the balancing chamber is inferior to the arrangement using a special channel to connect the balancing chamber with the suction nozzle because leakage through the holes is directed against the flow in the impeller eye, causing disturbances. The balance by this method is never complete.
From 10 to 25 per cent of the axial thrust always remains depending on the size of the holes. For a complete balance the diameter of the wearing rings of the balancing chamber should be greater than that at the impeller eye.
3. Balancing with radial back vanes
In this method, radial ribs are used on the back shroud to reduce the pressure in the space between the impeller, and the pump casing. Figure 5 shows the schematic of an impeller housed inside pump casing the radial back vanes provided at the back shroud of the impellers acts as auxiliary impeller which restricts the entry of liquid into the clearances between impeller back shroud and casing cover. It is evident that the
balancing with wearing and holes method introduces the leakage loss of the pump which, in turn, increases as wearing rings are worn. Balancing with ribs method requires some additional power which, however does not change with time. In addition it is cheaper and more effective than the first method. In this method the power consumed is much less than the power loss due to leakage through balancing holes under normal conditions.
In multi stage pumps:
1. opposite impeller configuration half stages can have impellers oppositely oriented in design
2. using balance piston and balance ring at the end of last stage impeller
Q.) Boiler water feed pump type, v/v type. reasons for that.
Ans.) 2 or more impeller in series for higher pr. impellers are opposite to cancel axial thrust
Q.) How axial thrust is taken care of in boiler feed water pump?
Ans.) boiler feed pump is a multistage pump, thus axial thrust can be controlled by using
opposite impellers (usually in 2 stage pump) balancing chamber (if more than 2 stages)
Q.) What takes the radial thrust in centrifugal pump ?
Ans.) The bearings take the radial thrust. To minimise radial thrust:
1. Double volute casing
2. Diffuser type pump
Q. What is lantern ring? Where it is located?
Ans.)Lantern ring are the split rings. They are provided in centrifugal pump between the packing ring so as to direct the clean fluid between gland packing and seal and lubricate the gland.
Q. What are the advantage of axial flow pump over centrifugal pump?
Ans.) 1.pump is reversible.
2. pump will idle and offer little resistance when flow is induced through it by external means.
3.under low head and high discharge an axial flow pump with a higher speeed that equally matched centrifugal pump can be used. the electric motor of the pump will be of small size.
Q.)Why main LO pump of main engine is not positive displacement type?
Ans.) in centrifugal pump with increase in temperature of oil head decreases but there is an appreciable increase in discharge with little decrease in head.
Q. What is inducer and diffuser in a centrifugal pump?
Ans.) Inducer:-is a one which is fitted in suction side to ensure the supply of fluid to impeller is at suffiecient pressure to avoid cavitation at impellor suction. it enables to operate the pump with lower NPSH required.
Diffuser:-is used to increase pressure.it is the static part around the impeller. It decreases the velocity of fluid which rotates by impeller and increases performance.
Q.)How is the weight supported in vertical hung type centrifugal pump?
Ans.) 1. by the bearing.
2. by the upthrust provided by the water on underside of the impellor
Q. What are the causes of wear down of wear ring in centrifugal pump?
Ans.) 1.erosion caused by the liquid leaking through the small clearance between the wear ring and impeller.
2. worn out of bearing
3. misalignment in pump
4. vibration due to radial thrust
5. tight clearance causes small amount of water to flow from discharge side to suction side, hence causing erosion of wear ring
6. erosion due to foreign particles in water strainer
7. cavitations
8.unbalanced impeller
9. high temp. of water
Q. How is axial thrust accomodated in screw pump?
Ans.) 1. one left handed and other right handed, cancel each other
Thrust Balancing of screw pump : Hydraulic balancing : Radial force is balanced by hydrodynamic film formed by the idle rotor while rotating in the housing..Axial force is hydrostatically balanced by the differential pressure in the power rotor and idle rotor…..
Axial thrust is taken care of by thrust collar of screw p/p shaft
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