Unit -5
Centrifugal Pumps
Q1) Classify roto-dynamic pumps.
Ans.
- Single stage pumps:
- It is known as single impeller pump.
- It is simple in design and easy in maintenance.
- It is ideal for large flow rates and low pressure installations.
- Two stage pump:
- It has two impellers operating side by side.
- It is used for medium use applications.
- Multistage Pumps:
- It has three or more impellers in series.
- They are used for high head applications.
- Axial split:
- In these types of pumps the volute casing is split axially and split line at which the pump casing separates is at the shaft’s center – line.
- Radial split:
- In it pump case is split radially, the volute casing split is perpendicular to shaft centre line.
- Single suction:
- It has single suction impeller which allows fluid to enter blades only through a single opening.
- Double Suction:
- It has double suction impeller which allows fluid to enter from both the sides of blades.
- They are most common types of pumps.
- Single volute pump:
- It is usually used for low capacity pumps, as it has small volute size.
- Double volute pump:
- It has two volutes which are placed 180 degrees apart.
- It has a good capability of balancing radial loads.
- Horizontal Centrifugal pumps:
- It is suitable for low pressure.
- Vertical Centrifugal pumps:
- It can easily withstand higher pressure loads.
- It is more expensive than horizontal pumps.
Q2) With neat sketch explain construction & working of centrifugal pump.
Ans.
1. Impeller
2. Casing
a) Volute casing:
b) Vortex casing
Vortex Casing Casing with Guide Blades
c) Casing with guide blades or turbine pump
3. Suction pipe with a foot valve and strainer
4. Delivery pipe.
Q3) State the different types of casing used in centrifugal pump. Explain any one with neat sketch.
Ans.
d) Volute casing:
e) Vortex casing
Vortex Casing Casing with Guide Blades
f) Casing with guide blades or turbine pump
Casing with guide blades is more efficient because of gradually reduction in velocity through guide blades.
Q4) What are the different types of heads associated with the centrifugal pump?
Ans.
2. Delivery head
3. Static head
4. Manometric head
It is given by the following expressions
a)
=
=
b)
Vertical height of the outlet of the pump from datum line and
Corresponding values of pressure head, velocity head and datum head at the inlet of the pump,
(c)
Where Suction head
Delivery head
Frictional head loss in suction pipe
Frictional head loss in the delivery pipe and
Velocity of water in delivery pipe
The term is a small quantity as compared to other terms. Generally it is neglecting then
Q5) Draw & explain velocity triangles of centrifugal pump.
Ans.
Let, N= speed of the impeller in r.p.m
Diameter of impeller at inlet
Tangential velocity of impeller at inlet=
Diameter of impeller at Outlet
Tangential velocity of impeller at outlet=
Absolute velocity of water at inlet
Relative velocity of water at inlet
= Angle made by absolute velocity at inlet with the direction of motion of vane
= Angle made by relative velocity (at inlet with the direction of motion of vane and are the corresponding values at outlet.
A centrifugal pump is the reverse of a radially inward flow reaction turbine. But in case of radially inward flow reaction turbine, the work done by the water on the runner per second per unit weight of water striking per second is given by equation
Work done by the impeller on the water per second per unit weight of water striking per second = - [work done in case of turbine]
Work done by impeller on water per second
Where W = weight of water=. ×g×Q
Where Q= Volume of water
And, Q= Area × velocity of flow =
Where are width of impeller at inlet and outlet and velocity of flow at inlet and outlet
Q6) State the effect of blade angle on efficiency of pump.
Ans.
From the outlet velocity triangle
Then …………………. (1)
Under the ideal condition assumed above the manometric efficiency of the pump will become
………………….. (2)
Q7) What is cavitation? Write a note on Thoma’s cavitation factor.
Ans.
but we have
Q8) Explain NPSH in detail.
Ans.
NPSH= Total head at inlet of the pump - vapour pressure head
…………………. (1)
Substituting this value in equation (1) we get
NPSH=
=
Where atmospheric pressure head vapour pressure heat
……………………….. (2)
Q9) What is priming in case of centrifugal pump?
Ans.
2. With vacuum pump
3. With jet pump
4. With separator
Q10) Define specific speed & derive expression for same.
Ans.
Expression for specific speed for a pump
The discharge Q for a centrifugal pump is given by the relation
Q= Area ×velocity of flow
D= diameter of the Impeller of the pump
B = width of the impeller
We know that BD
From equation (i) we have
We also know that tangential velocity is given by
Now the tangential velocity (u) and velocity of flow are related to the manometric head as
Substituting the value of u in equation (iii) we get
Substituting the values of D in equation (ii)
Where K is constant of proportionality
If,
Substituting these values in equation (v) we get
Substituting the value of K in equation (v) we get
Q11) Explain performance characteristics of centrifugal pump.
Ans. Characteristic curves of centrifugal pumps
Main characteristic curves
Operating characteristic curves
Constant efficiency curves
Q12) Explain pump in series.
Ans.
Let. n = number of impellers mounted on the same shaft.
= developed by each impeller
Then total head developed=
The discharge passing through each impeller is same.
Q13) Explain pump connected parallelly.
Ans.
Let n=number of identical pumps arranged in parallel
Q=discharge from one pump
Total discharge= n × Q
Q14) Draw & explain system resistance curve.
Ans.
Q15) Explain selection of pump.
Ans.
Q16) Explain model analysis of centrifugal pump.
Ans.
(Ns)m = (Ns)p
=
2. Tangential velocity also
=
3. We have,
=
4. Power of the pump
=
Q17) Derive equation for minimum starting speed for centrifugal pump.
Ans.
Where =Tangential velocity of impeller at outlet= and
=Tangential velocity of impeller at inlet =
Head due to pressure rise in impeller =
The flow of water will commence only if
Head due to pressure rise in impeller
For minimum speed we must have …………………. 1
But from we have
Substituting this value of in equation 1
and
Substituting the values of and in equation
Dividing by we get
Above equation gives the minimum starting speed of the centrifugal pump.
Q18) Derive equation for maximum suction lift.
Ans.
Let =Suction lift
…………………………. 1
Where =Atmospheric pressure on the free surface of liquid
=Velocity of liquid at the free surface of the liquid =0
=Height of free surface from datum line =0
=Absolute pressure at the inlet of pump
=Velocity of liquid through suction pipe=
=Height of inlet of pump from datum line =
=Loss of head in the foot valve, strainer and suction pipe=
substitute the above values in equation 1
…………………… 2
where = vapour pressure of the liquid in absolute units
Now the equation 2 becomes as
……………..3
=Atmospheric pressure head =
=Vapour pressure head =
Now equation 3 becomes as