50-Interview questions & answers on centrifugal pumps

 

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? 

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• 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 

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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

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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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18-Reasons for Turbine bearings vibration

 

1. Unbalance & Bend shaft:

Unbalance is the uneven distribution of body mass in rotating machine.

Machine unbalance & shat bend or bow lead to the bearing vibrations in radial direction

2. Miss alignment:

Miss alignment is non-coincidence of shaft centers of two rotating mating machines.

Miss alignment lead to axial vibrations of bearings

3. Looseness in turbine base bolts or foundation bolts leads to radial vibrations

4. More bearing clearance:

More clearance in journal bearings leads to radial vibrations

5. Rotor rubbing:

If Turbine rotor rubbing with any other stationary parts leads to more radial & axial vibrations. If it is rubbing radially then radial vibrations increase & if it is rubbing in axial direction then there will be more axial vibrations.

Read what do you mean by turbine supervisory system???

6. Seal rubbing:

Seal rubbing with rotating parts like rotor or blades to vibrations in horizontal directions.

This rubbing may due to improperly mounted seals or eccentricity of seals.

7. Distortions in foundations & casing:

Distortion & deflection of foundation include base frame and structure on which turbine is rested. In such cases bearing vibrations are most likely in radial direction & in axial direction. Also vibrations on base frame & on foundations show more.

Similarly bearings vibrations increase in radial direction due to casing distortion. Also casing vibrations show more.

8. Defective bearings:

Bearings vibrations are high in radial direction if bearing is defective in respect to pitting, wear, cracks & more clearance.

9. Inadequate Rigidity in bearing Pedestals:

Bearings start to vibrate in horizontal, vertical & axial directions if pedestal rigidity is less in horizontal, vertical & axial directions respectively.

10. Damage of thrust bearings

This leads to the vibrations of journal bearings in axial directions

11. Defects in coupling:

Coupling defects like more run out, looseness or crack also lead to turbine bearings vibrations

Defective couplings lead to bearings vibrations in both axial & radial directions.

12. High lube oil temperature:

High lube oil temperature causes decrease in viscosity of oil, which in turn reduces the oil film between journal & bearings leading to high vibrations in radial directions.

13. Contaminants in oil:

Lube oil containing burs & other foreign materials can also lead to high bearing vibrations

14. Lube oil & Control oil pipe line forces:

Improperly aligned & non stress relieved pipe lines lead to bearing vibrations. Oil lines not having expansion bellows may transfer vibrations from pump & lines to turbine bearings

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15. Steam line forces & Aerodynamic forces:

Stresses in steam line, improperly supported steam lines transfer vibrations to turbine

16. Changes in steam parameters:

If the quality of steam changes for an example, reduced steam temperature, wet steam etc

17. Overloading the Turbine:

If the turbine is loaded beyond the allowable capacity for long time without consultation with OEM can certainly lead to bearings vibrations & subsequent failure.

18. Other probable reasons for bearings vibrations are

• Turbine resonance
• Operating the turbine in critical speed
• Wrong installation

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Turbine oil flushing procedure

15-Emergencies in Power plant

Why do turbines go into over speed trip???

How to calculate steam & water pipe line size???

 

What are the factors needed to calculate steam & water pipe line sizes?

Quantity of maximum & minimum flow through the line

Pressure & temperature of the fluid

Pressure drop allowed

Velocity of the fluid in pipe

Density & specific volume of the fluid

How do you measure the steam flow through the pipe?

Steam flow is measured with the help of orifice plate, flow nozzles etc

How do you measure the water flow through the pipe?

Water flow is measured with the help of orifice plate, vortex meter, Rota meter & turbine meters installed in pipe lines.

What is the importance for calculating pipe line size for particular fluid flow?

To provide correct required flow

To avoid pressure drop of fluid

To avoid starvation

Why the velocity is the important factor while calculating the line size?

Flow = Area of the pipe X Velocity

By looking at the above relation, velocity is the critical & important parameters, as miss judgment of velocity may lead to wrong result.

Fluid having high pressure will be having high velocity & hence require lesser pipe line size & vice versa

And also fluid having lower density will be having high velocity & hence lesser pipe size and vice versa

What are the assumed velocity for various fluids flow?

Velocity of water at the suction of pump = 0.7 to 0.9 m/sec

Feed water flow at pressure 87 kg/cm2 = 2 to 4 m/sec

Saturated steam = 25 to 50 m/sec

Super-heated steam = 30 to 70 m/sec

What will happen if a 6” pipe line carrying hot water at the rate of 100 TPH suddenly contracts to 4”?

Following shall be observed

Velocity in the pipe will increase suddenly

Head or pressure loss will occur

Energy required to pump the water will increase

What are the factors that can cause the pressure drop in a pipe line?

Friction factor of pipe (pipe internal wall roughness)

Length of the pipe

Diameter of the pipe (size of the pipe)

Velocity of fluid in the pipe line

Pipe line fittings like valves, bend, tee etc. present in the pipe line

What are the effects of over sizing the pipe lines?

The cost of pipe lines & related fittings like valves, bend, Tee etc. will increase accordingly

Higher installation cost including pipe line supports & insulations

For higher sized steam pipe lines more condensate will tend to form & hence more number of steam traps are required

For higher sized steam pipe lines, there is more possibility of carryover wet steam to end user

More heat loss due to more exposed heat/hot surface area

What are the effects of under sizing the pipe lines?

For under sized pipe lines low pressure will be available for end user

In steam lines more pressure drop may cause starvation in pipe lines

Chance of erosion

Chance of water hammer & noise

Calculations:

1. Calculate the pipe line size required to pump 100 m3/hr of water at pressure 85 kgg/cm2 to the Boiler.

As discussed in the above theory part, velocity of the feed water at pressure 85 kg/cm2 is around 3 m/sec

Then flow, Q = Area of pipe line in M2 X Velocity in meter

(100/3600) m3/sec = (3.142 X D²/4) X 3 m/sec

D = 0.108 m = 108 mm

Looking at the above value, the pipe line size required should have internal diameter 108mm.

Then pipe line size = Pipe ID + 2 X thickness

For feed water pipe line having above pressure needs minimum 80 schedule, so refer carbon steel pipe line chart & select the required schedule & thickness.

2. Calculate the main steam pipe line size required for connecting Boiler out let steam to distribution header. Maximum steam flow is 125 TPH at pressure 110 kg/cm2 & temperature 540 deg C, assume velocity of steam in pipe line is 52 m/sec

Steam flow = 125 TPH = 125000 kg/hr

Density of steam at pressure 110 kg/cm2 & 540 deg C = 32 kg/m3...Refer steam table

Steam flow in m3/sec = 1250000 / (32 X 3600) = 1.08 m3/sec

We have,

Q = AV

1.08 = A X 52

A = 0.02 M2

A = 3.142 X D²/4

0.02 = 3.142 X D²/4

D = 0.159mm = 160 mm

So need pipe line of internal diameter 150 mm

Note: Outer diameter of the pipe line is standard, need to select schedule based on operating pressure & temperature to get desired line size.

3-Calculate the velocity of 55 TPH saturated steam flowing in 500 NB pipe line at pressure 1.7 kg/cm2 & 135 deg C

Density of steam at pressure 1.7kg/cm2 & temperature = 1.5 kg/m3

Steam flow in m3/sec = 55 X 1000 / (1.5 X 3600) = 10.18 m3/sec

We have,

Q = AV

10.18 = (3.142 X 0.9²/4) X V

V = 15.99 m/sec

Following are the various cases taken as case study for pipe line size

 Case-1

Pressure (kg/cm2)	2.7
Temperature (deg c)	135
Density (kg/m3)	1.4
Flow TPH	135
Flow M3/sec	26.79
Line size-mm	800
Area M2	0.50
Velocity m/sec	53.28

 Case-2

Pressure (kg/cm2)	2.7
Temperature (deg c)	135
Density (kg/m3)	1.4
Flow TPH	160
Flow M3/sec	31.75
Line size-mm	900
Area M2	0.64
Velocity m/sec	49.9

 Case-3

Pressure (kg/cm2)	2.7
Temperature (deg c)	135
Density (kg/m3)	1.4
Flow TPH	80
Flow M3/sec	15.87
Line size-mm	600
Area M2	0.28
Velocity m/sec	56.13

 Case-4

Pressure (kg/cm2)	2.7
Temperature (deg c)	135
Density (kg/m3)	1.4
Flow TPH	40
Flow M3/sec	7.94
Line size-mm	450
Area M2	0.16
Velocity m/sec	49.90

 

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 Questions & Answers on steam blowing

 

How do you calculate the attemperator water consumption in Boilers?

 

1-What do you mean by attemperation?

Attemperation is a method employed for controlling the super-heated steam temperature

Attemperators are used to control the main steam (super-heated steam) temperature in Boilers.

2-What are the two basic types of attemperators?

Spray type attemperator

Surface type attemperator

3-What is the main differences between attemperator & Desuper heaters?

Sl No.	Attemperator	Desuper heater
1	An attemperator controls steam temperature	Desuperheater removes whatever superheat there is in steam and reduces the temperature to a point at or nearly at saturation temperature
2	Attemperators are generally found in and/or associated with boiler steam, in zones where too high temperature affects something downstream of that point	Desuperheaters are used for downstream use of saturated steam
3	Outlet of the attemperation will be superheated steam only	Outlet of desuperheater will be saturated steam
4	Used in super-heated lines	Used in MP or LP steam lines

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4-What type of attemperation method is used in modern high pressure boilers?

In modern high pressure boilers variable nozzle and spray type attemperators are generally used

5-What is the percentage of attmepration used in High pressure Boilers?

Generally varies from 8 to 15%

 6-What is the reason behind using stainless steel sleeve inside the attemperator header?

Generally SS sleeves are fitted inside the attemperator header, this sleeve performs following functions.

 

Acts as a thermal barrier, separates hot and cold working elements to mitigate the intensity of thermal cycles experienced by critical components.

Protects steam pipe from thermal shock, helps to improve secondary atomization.

 

7-What precautions should be taken during attemperator liner or sleeve design?

•  Minimum length of straight pipe upstream of the liner should be three times the pipe diameter.
• Length of the liner downstream from the spray nozzles should be between 3 and 6 ft, depending on the particular installation.
• Length of straight pipe downstream of the liner should allow a residence time of 0.067 second for spray water to evaporate before the first elbow.
• Location of the temperature sensor should be at a distance downstream of the liner that allows 0.2 seconds of residence time to ensure complete mixing of the evaporated water and superheated steam. However, if the mass flow of spray water is greater than 15% of the mass flow of superheated steam, the residence time should be increased to 0.3 seconds.

 8-What factors are considered for designing attemperator?

Following factors are considered:

• Feed water pressure, flow rate, and temperature at the spray water control valve during various load conditions
• Locations of temperature sensors at the upstream & downstream ends
• Water chemistry
• Residence time of steam & water mixture for sensing temperature at downstream end
• Type of attemperator spray nozzle
• Rate of atomization & size of droplets

9-What is the distance of temperature sensors from attemperator spray nozzles?

For proper controlling of steam temperature, the upstream & downstream distance of sensors should be minimum of 5D & 20D respectively for straight pipe, where D is the diameter of attemperator header

 How do you calculate the Attemperator water consumption?

1. An attemperator is used to control the 95 TPH super-heated steam temperature from 425 deg C to 395 deg C by using 110 deg C feed water. Consider the main steam & feed water pressure 87kg/cm2 & 100 kg/cm2 respectively. Calculate the quantity of attemperator water

Given data,

Mass of steam, Ms = 95 TPH

Enthalpy of steam before attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H1 = 762.41 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 395 deg C, H2 = 741.81 kcal/kg

Feed water enthalpy at temperature 110 deg C, Hf = 111.65 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

95 X (762.41-741.81) = Mw X (741.81-111.65)

Mw = 3.1 TPH

2. A 125 TPH Boiler having variable type attemperator control valve for controlling main steam temperature from 495 deg C to 425 deg C at pressure 67 kg/cm2. The feed water is used for attemperation is 105 deg C, calculate the quantity of water required for de-superheating.

In the above example, Boiler feed pump having head 1000 meter & efficiency 62% supplies attemperator water, then calculate the extra power consumption for attemperation. Consider motor efficiency 95%

Given data,

Mass of steam, Ms = 125 TPH

Enthalpy of steam before attemperation at pressure 67 kg/cm2 & temperature 495 deg C, H1 = 812 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H2 = 771 kcal/kg

Feed water enthalpy at temperature 105 deg C, Hf = 106 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

125 X (812-771) = Mw X (771-106)

Mw = 7.7 TPH

For calculation of power required for pumping 7.7 TPH of water, we have

Motor input power = Flow in m3/sec X Total head X Density of water X 9.81 / 1000 X Pump efficiency X Motor efficiency)

Density of attemperator water at temperature 105 deg C = 960 kg/m3

Attemperator flow in m3/sec = 7.7 X 1000 X / (960 X 3600) =0.022 m3/sec

Now,

Motor power = 0.0022 X 1000 X 960 X 9.81 / (1000 X 0.62 X 0.95)

Motor power = 35.17 KW

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