1. What are the centrifugal pumps?
Centrifugal
pumps are the mechanical devices which pump or transport various fluids by
converting their rotational kinetic energy into hydrodynamic energy.
2. Why the name centrifugal pump?
A centrifugal pump uses
centrifugal force
3. Where the centrifugal pumps find
applications in power plants?
Read reference books for power plant O&M
- Boiler feed water
pump
- Auxiliary & main
cooling water pumps
- Raw water transfer pumps
- Condensate extraction
pump,
- Deaerator & feed
water tank make up pumps
- Firefighting water pumps
- UF & RO feed water
pumps
- MGF feed pump
- Degassed water transfer
pumps
- Sometimes lube &
control oil pumps
4. How do you specify the centrifugal
pumps?
Centrifugal pumps are
specified as bellow
- Flow in M3/Hr or
M3/sec
- Head or discharge
pressure in meter or bar or kg/cm2
- Shutoff head
5.What are the various parts of centrifugal
pumps?
Centrifugal pumps have
following parts
- Pump casing or
diffuser
- Impeller
- Wear ring
- Shaft
- Lantern ring
- Stuffing box
- Inlet vertex
- Mechanical seal or gland
packing
- Shaft sleeve
- Bearings
6.What are the energy conversions take place
in centrifugal pumps
In centrifugal pumps
hydraulic energy is being converted into kinetic energy
7.What types of reducers are used at pump
suction & discharge ends?
Suction side: Eccentric
type & Discharge side: Concentric
8.What are the two main types of centrifugal
pumps?
Axial flow & Radial
flow
9.What is the function of impeller in
centrifugal pumps?
It converts kinetic
energy of pump into hydrodynamic energy by rotary motion
10.What is the function of pump casing?
Casing converts velocity
head from impeller into pressure head & also guides the flow to the
discharge end.
11. What are the types of pump casing?
Volute & diffusers
are two different types of pump casing
12. What do you mean by volute?
A volute is a
spiral-like geometry with an increasing through-flow area, reducing the
velocity of the fluid and increasing the static pressure
13. What are the different types of volutes?
Single volute &
Double volute
15. Write down the working principle of
centrifugal pumps
In centrifugal pumps,
fluid enters the impeller through inlet eye & exists along the
circumference between the vanes of impeller. This impeller is connected to
shaft & in turn to motor, this rotary motion of the impeller converts
kinetic energy of the fluid into hydrodynamic energy.
16.What are the types of impellers?
Open impeller: As its
name suggests, an open impeller has vanes that are open on both sides without
any protective shroud. These are structurally weak.
These are used for low
flow & low head applications. Generally used for pump solids or sludge.
These require much NPSH.
Semi open impeller: Semi-open
impellers have a back-wall shroud that adds mechanical strength to the vanes.
Closed impeller: Are
very robust & require low NPSH
Impellers are also
classified as single suction & Double suction
17.What are the rotary & stationary parts
of the pumps?
Rotary parts:
- Shaft
- Impeller
- Shaft sleeve
- Bearings
Stationary Parts
- Pump casing
- Gland packing or
mechanical seal
- Lantern ring
18.Why eccentric reducers are used at pump
suction side?
To avoid air locking
& cavitation eccentric reducers are used at suction side
19. What do you mean by the NPSH in
pumps?
It is the net positive
head required at pump suction to avoid cavitation
20. What do you understand by the term
cavitation?
Cavitation is the
formation & collapsing of vapor bubbles at pump’s suction
21. How the cavitation does affect the pump’s
life?
- Cavitation causes
- Vibrations in pump
- Damage of
impellers
- Heavy noise
22. What are the factors considered for
centrifugal pumps design?
- Flow required
- NPSH available &
NPSH required
- Total head
- Pump efficiency
- Fluid used
23. What are the materials used for pump
casing?
Generally cast steel or
cast iron are used for single stage centrifugal pumps
24. What are the materials used for
Impellers?
Impellers are made up of
cast iron, gun metal & stain less steel
25. What is the function of wear ring?
As the name indicates it
protects the wear & tear of impeller
26. What do you mean by static suction head in
pump?
Therefore, the static
suction head is the vertical distance from the center line of the pump to
the free level of the liquid to be pumped.
27. What do you mean by static suction head in
pump?
Static discharge head is
the vertical distance between the pump centerline and the point of free
discharge or the surface of the liquid in the discharge tank.
28. What do you mean by total static head?
Total static head is
the vertical distance between the free level of the source of supply and the
point of free discharge or the free surface of the discharge liquid.
29. What do you mean by total head?
It is total dynamic
discharge head plus total dynamic suction head
Note: If source water
level is below the pump center line, then
Total head = Discharge
head Suction lift
If source Water level is
above the pump suction line, then
Total head = Discharge
head-Suction head
30. What are the problems associated with
centrifugal pumps?
Following are the common
problems associated with pumps
- Low discharge
pressure
- Low delivery
- Cavitation
- High vibrations
- Pump seize
- Over load
- More suction lift
- Air locking & No
priming
31. What are the reasons for no delivery or no
discharge in centrifugal pumps?
- Probable reasons are
- Air lock in pump suction
- Suction valve closed
- Low tank level
- 32. What are the reasons
for low delivery?
- Suction valve partially
opened
- Reverse rotation of pump
- Low speed of pump
- Suction strainer is
chocked
33. What are the reasons for over load of
pump?
- More flow
- High speed
- Reverse rotation of pump
- Pump discharge kept open
to atmosphere
- Internal friction in
impeller & wear ring or impeller & casing
- More tightened gland
packing
- No lubricant in bearing
or bearing seized
34. What are the potential reasons for pump
vibrations?
- Overloading of pump
- Reverse rotation of pump
- Impeller rubbing inside
the casing
- Misalignment
- Damaged bearing
- Shaft run out
- Shaft imbalance
35. Too much noise coming from pump inside,
what does this mean?
- Air lock in pump
- Overloading of pump
- Pump discharge line is
less than actual required
- Cavitation
- No lubricant in bearings
36. What are the common mistakes done during
pump installation?
- Choosing poor foundation
- Note: Pump foundation
weight should be 3 to 4 times the pump weight
- Lesser size suction pipe
line
- Lesser size discharge
pipe line
- Interchanging concentric
& eccentric reducers
37. What are the safety protections &
interlocks given for a centrifugal pumps?
- Over load
- Low load
- High bearing vibrations
- High bearing temperature
- High suction DP
- Source water level low
38. How do you increase the head & flow of
pump by modifying impeller size?
By increasing the
impeller diameter head & flow can be increased
By increasing the
impeller width flow can be increased
39. What are the reasons for reduction of pump
efficiency?
- Operating the pump at
lower capacity
- Operating the pump at
higher load
- Throttling the discharge
valve
- Increase in impeller
& wearing clearance
- Lower suction head
- High suction lift
Calculation part
40. How do you calculate NPSHA?
NPSHA is Net positive
suction available
NPSHA = Atmospheric
pressure + static head - vapor pressure - pressure loss in the suction piping -
pressure loss due to the suction strainer.
41. A centrifugal pump of rated capacity 75
M3/Hr & total head 35 meter is supplying water to fill a tank in 2 hours,
calculate the total power consumption. Consider pump & motor efficiency 50%
& 85% respectively
Power consumption = Pump
flow in m3/sec X Pump total head in meter X fluid density X g / (1000 X Pump
eff. X Motor eff)
Power consumption = (75/3600)
X 35 m X 1000 kg/m3 X 9.81m/s2 / (1000 X 0.5 X 0.85)
Power consumption =
16.83 KWH
Power consumption in 2
hours = 16.83 X 2 = 33.66 KW
42. A centrifugal pump having hydraulic power
22 KWH, discharge & suction head 55m & 12m respectively
Calculate the pump flow
in m3/hr, assume density of water 990 kg/m3
Pump flow = Pump
hydraulic power X 1000 / (Pump total head X density of fluid kg/m3 X 9.81 m/s2)
Pump flow = 22 X 1000 /(
(55-12) X 990 X 9.81)
Pump flow = 0.052 m3/sec
Pump flow in M3/hr =
0.052 X 3600 = 189.6 M3/hr
43. A centrifugal pump having hydraulic power
15KWH & pump efficiency 65% calculate the pump shaft power
Pump shaft power = Pump
hydraulic power / Pump efficiency = 15 / 0.65 = 23 KW
44. A centrifugal pump produces flow 20M3/hr
(Q1) flow at rated speed 1500 RPM (N1) , then calculate the flow of pump at 1000
RPM(N2)
We have pump affinity
law
Q1/Q2 = N1/N2
20 / Q2 = 1500 / 1000
Q2 = 13.33 M3/hr
45. A centrifugal pump consumes power of 25KW
(P1) at speed of 1500 RPM (N1), after reducing certain RPM its power
consumption reduces by 5 KW (P2), calculate that speed
We have pump affinity
law
P1/P2 = (N1/N2)3
25 / 5 = (1500 / N2)3
N2 = 877.2 RPM
46. A centrifugal pump produces 150 m (H1)
head at 3000 RPM (N1), calculate the head produced if its speed reduced to 50%
We have pump affinity
law
H1 / H2 = (N1/N2)2
N2 = N1 X 50% = 3000 X
0.5 = 1500 RPM
150 / H2 = (3000 / 1500)2
H2 = 37.5 meter
47. A centrifugal pump having impeller diameter
250 mm produces flow 250 M3/hr, calculate the diameter of impeller to produce
flow 300 M3/hr
We have
Q1 / Q2 = D1 / D2
250 / 300 = 0.250 / D2
D2 = 0.35 m = 350 mm
48. A centrifugal pump having impeller diameter
300 mm produces 250 m head & what could be the diameter if we want to
reduce the head by 30m
Reduced head = 250 – 30 =
220 m
We have
H1 / H2 = (D1/D2)2
250 / 220 = (300 / D2)2
D2 = 281.4 mm
49. A centrifugal pump having impeller diameter
150 mm (D1) consumes 15 kw (P1), what is the size of impeller if we want reduce
power by 4 KW
P2 = P1-4 = 15-4 = 11 KW
We have
P1 / P2 = (D1 / D2)3
15 / 11 = (150 / D2)3
D2 = 135.2mm
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