Power plant Maintenance: basics questions & answers.

1-Define the term maintenance.

Maintenance is keeping something in proper condition or good order.

Or
Action taken to prevent machine from its failure.

Or

It is a process of repair

2- What is the necessity of maintenance?

Necessity of maintenance:

• To avoid machine failure/damage

• To avoid harm to man

• To avoid production loss

• To reduce unnecessary inventory in store

• To increase equipment or component service life

• To increase equipment effectiveness

3-What are the different maintenance practices adopted in power plants?

• Breakdown maintenance

• Preventive maintenance

• Predictive maintenance/Condition based maintenance (CBM)

4-What is meant by breakdown and preventive maintenance?

In breakdown maintenance, maintenance of equipment is taken on its failure only. Machine runs continuously till its failure.

Preventive maintenance is time based activity, maintenance activity is taken to avoid machine from failure.

5-What is meant by predictive maintenance/CBM?

It is the maintenance practice based on condition of the equipments. This type of maintenance continuously monitors the equipments condition thoroughly and plans maintenance during stoppage or idle time of plant.

6-What are the activities involved in Condition based maintenance (CBM).

1-Equipments bearing temperature monitoring.

2-Equipments bearing vibration monitoring.

3-Thickness checking (pressure parts, storage tanks, cooling water line).

4-Testing (relay testing, lubricating oil).

5-Nondestructive tests...etc

6-Monitoring equipment performance (flow, pressure etc.).

Turbine oil and transformer oil standard testing parameters

7-Differentiate between Preventive maintenance and Breakdown maintenance.

Sl No.	Preventive Maintenance	Breakdown maintenance
1	Time or running based activity	Carried out as and when required
2	Will not lead to generation stoppage	Leads to generation stoppage
3	Work activities are already known to working persons	Due to breakdown, work activities are not known to working persons
4	Requires less man power	Requires more man power
5	Semi-skilled man power can handle preventive maintenance	Requires skilled man power
6	Consumes less or no spares	Consumes spares
7	More opportunities for improvement	Less or no opportunities for improvement
8	Maintenance activity consumes less time	Maintenance activity consumes more time
9	Less inventory can be maintained	More inventories to be maintained
10	No repetitive works	Involves repetitive works
11	Does not require experts	May require experts if machine is not restored within time.

8-List the advantages of implementation of TPM in power plant

• Increased equipment productivity

• Reduced equipment downtime

• Increased plant capacity

• Lower maintenance and production costs

• Approaching zero equipment-caused defects

• Enhanced job satisfaction

• Increased Return on Investment

9-What are the different forms of screw threads used in power plants and process plants?

It's all about HP heaters

Forms of Screw Threads:

1. British Standard Whit Worth (B.S.W.) Threads: It is a symmetrical V thread in which the angle between the flanks is 55°.

2. British Association (B.A.) Thread: It is the B.S.W. thread with fine pitches having thread angle 47.5°

3. American Standard Threads: Included angle is 60°

4. Unified Standard Thread: This thread has rounded roots and crests with included angle 60°

5. Square Thread: Used in power transmission, spindles of valves etc.

6. Acme Thread: It is a modified square thread. It is much stronger than square thread.

7. Butress Thread: Used for transmission of power in one direction only.

8. Metric Thread: It is an Indian standard thread similar to B.S.W, but has included angle 60°.

10-What is the function of shaft key?

Key is a piece of mild steel inserted between the shaft and hub or boss of the pulley/coupling to connect these together in order to prevent relative motion between them.

11-What are the various types of shaft keys used?

• Sunk keys: Rectangular, square, parallel, gib head and feather keys

• Saddle keys

• Round key

• Spines

• Woodruff key

• Gib head key

• Dowel

Power Transformers QnA

12-What is strut?

A machine part subjected to axial compressive force is called strut. Strut may be horizontal, vertical or inclined.

13-What is meant by pillar or column?

A vertical strut is called as pillar or column.

14-What are short and long columns?

The columns which have lengths less than 8 times of their diameter are called short columns. And columns which have lengths greater than 8 times of their diameter are called short columns.

15-What are the structural materials used in power plants?

Structural Materials:

Beam, Channel, angles, girder, purlin, bracings, gussets, plain and chequered plates, flats, corrugated plates, truss, trestles etc.

16-What is meant by truss?

A truss is a structure that consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object. A two-force member is a structural component where force is applied to only two points. A sample truss is shown in below figure.

17-What is the function of beam and channels in structural engineering?

A beam is a structural element that primarily resists loads applied laterally to the beam›s axis. Its mode of deflection is primarily by bending

18-What is the function of gusset plates in structural materials?

Gusset plates are triangular plates of steel that are used to connect beams and girders to columns.

19-List down the different types of materials used in power plant construction.

Following are the various materials used in power plant construction:

Low Carbon Steel: IS 2062, IS 1239: For structural steels like plates, angles, channels, beams, LP steam lines etc.

High Carbon Steel: SA 106 Gr. B/C, SA210 Gr, B/C, SA 516 Gr. 70: For Boiler pressure parts like water wall panels, headers, economiser coils, down comers, feed water lines, steam drum etc.

Alloy Steel: SA 213 Gr. P11/T11, P22/T22, P91/T91: Super heater coils and pipe lines.

Cast Iron: Travelling grate materials, pulley, coupling etc.

Copper: Air conditioning cooling lines.

Brass: Surface condenser and oil cooler tubes.

Stain Less Steel: SS 304, 316, 410 etc: Ejector tubes, surface condenser/oil cooler tubes.

Plastics: PVC pipe lines, valves, tanks etc.

20-Define the term spring in mechanical members.

Spring is an elastic body, whose function is to distort when loaded and recover its original shape when load is removed.

21-What are the applications of springs in power plant?

Springs are used in following equipments:

•  Safety valves
•  Relief valves
•   Control valves
•   Soot blowers cams
•    Limit switches
•    Steam lines hangers and supports
•    Turbine HP and LP valves
•    Turbine Emergency stop valve (ESV)
•     Solenoid valves
•     Pressure switches

22-What are the different types of springs used in power plants?

• Helical springs (Tension and compression helical springs)
• Conical and volute springs
• Torsion springs
• Leaf springs
• Disc springs

23-Write down the MOC of Turbine journal bearing liner (Babbitt metal), roller bearings, Turbine HP and LP casings, Turbine blades, Turbine rotor, labyrinth, ESV, Reduction gear box gears.

• Journal Bearing Liner: A tin base alloy containing 88% of tin, 8% of antimony and 4% of copper.

• Roller/Ball Bearing: Hardened chromium steel.

• Turbine HP Casing: Alloy steel.

• Turbine LP Casing: Carbon steel.

• Turbine Blades: Alloy steel Mo-13–15%, Chromium and stainless steel.

• Turbine Rotor: Forged alloy steel Cr, Mo and Vanadium.

• Labyrinth Packing: Nickel, lead, bronze, Carbon and SS.

• ESV: Alloy cast steel (1.25 Cr 0.5 Mo).

• Reduction Gear Box Gears (RBG): Forged alloy steel.

 24-What is the surface hardness of carbon steel pipes?

It is in the range of 25 to 30 HRC.

25-How do you convert hardness 200 BHN to hardness HRC?

As a thumb rule HRC 5 BHN/10.8

Therefore 200 BHN 5 18.5 HRC

26-What is the hardness of hard facing electrodes used for hard facing of sprocket teeth, fan impeller liners etc?

It is around 45 to 55 HRC.

27-What is shaft coupling?

A coupling is a mechanical device, which is used to connect driver and driven shaft permanently or

semi permanently.

28-What are the main functions of coupling?

Functions:

• Connects the shafts of two units, which are manufactured separately.
• Transfers motion, power and torque.
• To reduce transmission shock loads from one shaft to another.
• Protection against overloads.
• Introduces mechanical flexibility.

 29-What is the function of a shaft?

A shaft is rotating machine element which is used to transmit power from one place to another.

30-Define the term bearing

A bearing is a machine element which reduces friction between two members, having relative motion with respect to each other. It’s a device used to support and guide rotating, oscillating or sliding shaft.

Also read the 50-Questions & Answers on Bearings

31-What are the functions of bearings?

• It reduces heat developed, sound and wear out of the components, which are having relative motion. Thus saves energy.
• Supports a load.
• Guide moving/rotating parts.

32-What are the major types of bearings?

Journal bearings and rolling contact bearings are the two main types of bearings.

33-Classifry the rolling contact bearings.

 Ball Bearings:

• Deep groove ball bearing
• Angular contact ball bearings
• Self-aligning ball bearing
• Thrust ball bearing

B. Roller Bearings:

• Spherical roller bearing
• Cylindrical roller bearing
• Taper roller bearing
• Thrust Roller and Needle Bearings.

34-What are the different parts of rolling contact bearings?

• Outer race
• Inner race
• Cage
• Rolling elements

 35-What are the reasons for bearing failure?

Reasons for Bearings Failure:

• Lack of lubrication
• Excessive vibrations
• Dirt in lubrication
• Misalignment
• Overload
• Fatigue, repeated jerk loads
• Excessive clearance or interference fit

36-List down the Do’s and Don’ts for bearings handling and storing.

Following are the Do’s and Don’ts for bearing:

DO’s

• Keep the bearings in protective packing till they are mounted.
• Cover the assembled bearings appropriately to avoid dust ingress.
• Store the bearings in dry area.
• Use tubes, Puller, Hydraulic nuts, and other tools for removal/fixing of the bearings.
• Use correct amount of specified lubrication at right time with correct procedure.

Don’ts

• Over lubrication is a silent killer for bearings. Do not over lubricate.
• Never allow welding at bearings that will damage the contact surfaces.
• Don’t let the bearing stand upright, store them flat on their sides.
• Do not use water to clean the bearings.

37-What is meant by bearing clearance?

Bearing clearance is the clearance or gap between outer race or inner race and rolling elements. And in journal bearings it is the clearance between shaft and bearing liners.

38-How do you calculate the bearing clearance of bearing having bore diameter D?

As a thumb rule Bearing clearance minimum = 0.00185 X D

Bearing clearance maximum =0.00254 X D

39-What is the hardness of rolling contact bearings?

Bearings are usually made up of hardened chromium steel having hardness 55–60 HRC.

40-What are the reasons for bearing seize?

• Lack of lubrication
• Less clearance
• Over speed
• Contaminant in lubricant

41-What are the reasons for bearing scoring and smearing?

• Incorrect lubrication
• Contamination
• Misalignment
• Sharp speed ramp up

42-What is the major difference between a deep groove ball bearing 6205 ZZ and 6305 ZZ?

A deep groove ball bearing 6305 ZZ is bigger in size having more outer diameter and width, whereas 6205 ZZ is smaller in size with less OD and width. Both the bearings have same bore diameter.

 43-What is the shaft size on which a deep groove ball bearing of size 6215 C3 fits?

For deep groove ball bearing

Shaft size = 15 X 5 = 75 mm…(Last two digits of bearing number X 5)

Note: In above bearing 6 indicates type of ball bearing (deep groove, angular contact etc), 2 indicates the size of a bearing (OD and width) and last two digits indicate the bore of bearing. In any bearing last two digits indicate the bore diameter.

In tapered bore spherical roller bearings 22222 K/C3,

Shaft size 5 (Last two digits) 22 X 5 + Sleeve thickness

44-If a machine has higher vibration in only vertical/horizontal/axial direction then what does it indicate?

• If there is more vertical vibration and other direction vibrations are minimum, then indicates looseness in machine components.
• If there is more horizontal vibration and other direction vibrations are minimum, then indicates unbalance of rotating part.
• If there is more axial vibration and other direction vibrations are minimum, then indicates misalignment.

45-Define the term lubrication

Lubrication is any procedure that has the effect of reducing friction and wear

46-What is the purpose of lubrication?

Purpose of Lubrication:

• To reduce friction
• To minimize wear & tear
• To cool the parts
• To disperse contaminants
• Acts as sealant
• To transfer the power
• Provides electrical isolation rotor to ground

47- What are the main contents of lubricants?

•    90% Base oil Synthetic or mineral oil
•     7–8% Additives
•    2–3% Thickeners 

48-What are the different types of lubrication?

Following are the some lubrication types:

• Boundary lubrication
• Full film lubrication
• Hydrodynamic lubrication

49-What are the different properties of lubricants especially of turbine lube oil which is very must to check before use?

• Viscosity
• Flash point
• Pour point
• Moisture content
• Acid No
• Neutralization value
• Sediments content
• Mineral test
• Demulsibility
• Dissolved gas analysis

 50-A 6205 2Z bearing has OD 52 mm and width 15 mm, what amount of grease is required for first lubrication?

We have,

Bearing lubrication quantity in grams = Bearing OD (D) X Width (B) X 0.05

                                                                   = 52 X 15 X 0.05 = 39 grams.

51-What are the effects of misalignment?

• Effects of misalignment
• Excessive vibration and noise
• Bearings failure
• Seals failure
• Excessive heat generation
• Higher power consumption
• No equipment reliability

52-What are the possible reasons for misalignment?

• Loose/broken foundation bolts
• Loose shim packs/dowel pins
• Loose coupling bolts
• Jerk loads due to frequent start/stop

Online leakage arresters & sealants