Tools tackles used in power plant maintenance


Tools & Tackles are the main resources for power plant maintenance..Thsese are classified into Hand tools,machine tools, lifting tools & measuring tools.


Power plant maintenance tools & tackles

Powerplants of capacity 10 MW to 100 MW need following tools for routine preventive, predictive & breakdown maintenance works.
A. Hand Tools:
  • Spanners (Ring, flat, box, tubular and Allen types)
  • Files (Rectangle, square, triangular, round, half round)
  • Chisels 6", 12"
  • Screw spanner 6", 12"
  • Pipe wrench 12", 24"
  • Plumb
  • Hammer and mallets
  • Pliers (Nose and circlip pliers)
  • Shim cutter
  • Torque wrench
  • Pipe bender
  • Hole and letter punches
  • Screw drivers
  • Line testers
  • Bench vice
B. Machine Tools:
  • Single phase Hand drill machine
  • Hand angle grinding machine (AG-4, AG-5 and AG-7)
  • Pipe cutting machine (Chop saw machine)
  • Air blower
  • Vacuum cleaner
  • Hot gun
  • Bearing heaters
C. Lifting/Pulling Tools:
  • Chain blocks (1T, 2T, 3T, 5T and 10T)
  • Dee shackles (1T to 10T)
  • Eye bolts (0.5T to 10T)
  • Polyester/Nylon Lifting belts (1T to 10T)
  • Chain puller (1T)
  • Pulley block (0.5T to 5T)
  • Hydraulic jack (25MT)
  • Screw jack (2T to 10T)
  • Coupling puller
  • Wire ropes (1/2" and 1")
  • Slings (1/2", 1", 1.5")
D. Measuring Tool (Instruments):
  • Vernier caliper ( 0–150 and 0–300 mm LC: 0.02 mm)
  • Measuring tape (3 meter and 5 meter)
  • Inside and outside micrometer screw gauges (0–25 mm and 50–200 mm LC: 0.01 mm)
  • Dial gauge (LC: 0.01 mm)
  • Inside and outside calipers
  • Ultrasonic thickness checking machine
  • Infra-red temperature gun
  • Tachometer
  • Multimeter
  • Clamp meter
  • Earth leakage tester
  • Earth resistance checker
  • Insulation resistance checker
  • Pressure gauge calibrator
  • Balance weigher
  • Sound level meters


Constructional materials & welding electrodes used in power plant

What are the various materials used in power plants construction???
1-Low Carbon Steel IS 2062, IS 1239: For structural steels like plates, angles, channels, beams, platform, walkways & LP steam lines etc.

2-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.
3-Alloy Steel SA 213 Gr. P11/T11, P22/T22, P91/T91: Super heater coils and main steam pipe lines.
4-Cast Iron: Travelling grate materials, pulley, coupling etc.
5-Copper: Air conditioning cooling lines.
6-Brass: Surface condenser and oil cooler tubes.
7-Stain Less Steel SS 304, 316, 410 etc: Ejector tubes, surface condenser/oil cooler tubes, control valve stems & valve trim materials
8-Plastics: PVC pipe lines, valves, tanks etc.
9-Steam Turbine
  • ESV: Cast alloy steel
  • HP Casing: Cast alloy steel
  • LP Casing: Carbon steel
  • Rotor: Forged chromium, molybdenum and vanadium alloy steel
  • Blades: Alloy steel
  • Bearings: Liner Babbitt or white metals
  • Labyrinth: Phosphorous, lead and aluminum

What welding electrodes are generally used in power plants various welding works???


Sl No.
Particular Material and Grade
Type of Welding
Welding Electrodes Used
Application
1
Low carbon steel IS 2062/1239
Arc
E 6013
General welding works like structures, non IBR pipe lines plates etc.
2
Carbon steels SA 106 Gr.B/C, SA 210 Gr.A/C, SA 516 Gr.70, SA 234, etc.
TIG
Arc
ER 70 S2
E 7018
Boiler water wall panels, economizer coils, evaporator headers, process steam lines etc.
3
Low alloy steel SA 213 Gr.T11/P11
TIG
Arc
ER 80S B2
E 8018 B2
Primary super heater coils, headers, soot blower lines etc.
4
High alloy steel SA 213 Gr.T22/P22 and T91/P91
TIG
Arc
ER 90S B3/90S B9
E 9018 B3

Secondary and radiant super heater coils, main steam line etc.

Why thermal expansion is necessary in Boilers



Understanding the term expansion & contraction


                                

When a body is heated it will expand & when it cooled it contracts. So body expands & deforms when heated & cooled. Change in temperature of a free body causes body to expand & contract without inducing stress. When the deformation of the body is restricted by means of any external force, there will be huge chances of stress induction. Such induced stresses are called temperature stresses. These may be tensile or compressive in nature.





Here L = Original length of the steel bar

∆t = Change in metal temperature deg C

ɑ = Coefficient of thermal expansion

Temperature strain e = Free deformation / Original length = ∆L / L

E = L ɑ ∆t / L = ɑ ∆t

Temperature stress σ= Young’s modulus X Strain = E X e

Temperature stress σ = E ɑ ∆t

Stress induced in a rigid or constrained body




P = Force exerted by a rigid support of constraint

We have σ = P / A

P = E ɑ ∆t A

Expansion in Boilers
Boiler is made up of plates, tubes, pipes and simple steel of various grades depending upon the duty conditions. Depending on the service such as cold air/hot air/cold flue gas/hot flue gas/cold water/hot water/saturated steam/super heated steam, thermal expansion movement of steel materials takes place to different extent in Boiler. Ignorance of thermal expansion movement of boiler components in design/installation may lead to failure of boiler components. The damage to boiler components can be costly affecting human life in some cases.
There are two types of expansions in Boilers

Absolute Expansion: Boiler whole mass expands.
Differential Expansion: Individual parts expansion. There are places where there is a relative expansion movement, which can cause stress in those parts.
In boilers expansion pointers are attached to all the pressure parts header to under stand the direction & value of expansion.
Expansion pointers are used for verifying the expansion movement of the boiler. These are attached to the drum ends/bottom or top header ends. When the boiler is under commissioning stage the expansion must be monitored. Depending on the anchor points in X-axis and Y-axis, the expansion is predicted by designers. The same is counter checked at site. Deviations in the form of non-uniform expansion should be checked.
Modern boilers expand towards bottom, during start ups & shutdown it is very important to observe the Boiler expansion.
Expansion of the metals depends on;
  • Change in metal temperature
  • Length & area of the materials
  • Coefficient of expansion of the materials

Provisions for thermal expansions in boilers:

  • Boilers all headers bottom space should be free from obstacles
  • For air & flue gas ducts expansion bellows may be of metal or fabric should be provided
  • Expansion loops for all steam lines should be provided
  • Spring supports & hangers for steam line & hot water lines
  • Rocker washers for steam drum & super heater headers supports
  • Provision of stay bolts

 Precaution to be taken for free expansion:
  • After shutdown, ensure all foreign material from Boilers & ductings have been removed completely
  • Ensure all temporary supports & platforms have been removed from boilers & ducting
  • Ensure there is no welding between any pressure part & hot ducts with platform or other or beams which can restrict the expansion
  • Ensure enough space is available for all bottom headers for free expansion
  • Ensure steam line drains line are free to expand with steam lines
  • Ensure there are no any uneven expansions

Examples:

A square rod of size 20mm X 20 mm in cross section & 2000 mm in length is allowed to expand by fixing both of its ends. Determine the force developed if the rod is heated from 25 degree c to 150 deg C.

L = 2000 mm
A =20 X 20 = 400 mm2
E = 2 X 105 Mpa
ɑ  = 12 X 10-6 0C
∆t = 150-25 = 125 0C
We have             P = σA
P = E ɑ ∆t A
P =2 X 105 X 12 X 10-6 0C X 125 X 400 = 30000 N
Force developed at the end P = 30 KN

What is the expansion of Boiler side water wall panel , whose total length is 18 meters & metal temperature is 295 deg c. Consider atmosphere temperature 30 deg c &  ɑ  = 12 X 10-6 0C
Expansion of side water wall panel ∆L =L ɑ ∆t = 18 X = 12 X 10-6  X (295-30) =0.068mm = 68mm


Restricted expansion in Boilers or steam lines will lead to..
  • Damages to the restricted part
  • Damage to the refractory & sealing
  • Leakages
  • Secondary failure
  • Tube/pipe puncture
  • Explosions


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Why does Boiler back fire?


Why  does boiler back fire?

Following potential situations may cause back fire in biomass or bagasse fired boilers


1-Sudden trip of ID fans or closing of fans suction or discharge dampers:

If there is no interlock on ID fans trip to FD fans, a very huge back fire occurs in boiler which has potential to cause damages to the Boiler & operators.

Sudden closure of ID fans suction or discharge dampers will cause same back fire as above.

2-Malfunctioning of pneumatic dampers positioner may cause closure of ID fans dampers resulting into huge back fire in Boiler

3-APH tubes jam : Area required  for the flue gas flow is equals to Boiler capacity/9. Due to accumulation of ash in tubes, the flue gas flow area gets reduce. This situation causes backfire
This situation occurs if Boiler is run at lower load for long time without bypassing APH for FD/SA air, as at lower load flue gas temperature will be less, which get condense in APH tubes.

4-Higher moisture in the fuel: If the fuel has higher moisture that required, then there will be more requirement of combustion air, while maintaining this air fuel mixture back fire may occur.

5-Frequent variation in fuel moisture : This is experienced in sugar plants due to variation in bagasse moisture at mill outlet due to variation in imbibition water added to extract juice from cane.

Upon continuous variation of fuel moisture it become very difficult for operator to maintain balanced draught in furnace.

6-Improper spreading of fuel : Due to this furnace may experience bed heap combustion of fuel, due to this there will not be uniform distribution of bottom combustion air causing boiler back fire.
7-Sudden drop of boiler load: Due to this boiler air fuel ratio & draught system gets disturbed causing boiler back fire.

8-Fouling in Super heater coils : Higher moisture in fuel cause accumulation of bagasse & unburnt on super heater coils, which blocks the flue gas path causing boiler back fire

9-Accumulation & deposition of ash in ESP fields: This situation causes more resistance to flow of flue gas.

10-Improper air fuel mixture during start up & shutdowns may cause boiler back fire

11-Air leakage into & out of the system : Due to the air ingress & flue gas leakages, it becomes very  difficult for operator to maintain balanced draught.

12.Leakage or puncture of water wall tubes: Leakage or puncture of water wall tubes will result into high furnace pressure



Question & Answers on ESP troubleshooting

                                 ESP TROUBLESHOOTING GUIDE

1-What are the potential reasons for large dust collection on collecting plates & discharge electrodes?
  • Higher dust load on ESP
  • Improper rapping
  • Low velocity of flue gas
  • Higher resistivity of ash
  • ESP running under capacity
2-Why there is sparking in the fields?
  • Excessive dust formation on plates
  • Very less gap between collecting plate & emitting electrodes
  • Improper alignment of electrodes
  • Obstacle of foreign material in the field & touching the discharge electrode
  • Improper charge ratio
3-What are the reasons for higher emission at stack outlet?
  • Under size ESP or wrong design
  • Improper rapping
  • Higher spark rate
  • Improper distribution of flue gas
  • Transformer not working or under capacity
4-What are the reasons for damage/corrosion of ESP electrodes & casing plates?
  • Operation of ESP at lower flue gas temperature <120 deg C continuously
  • More ash deposition on plates
  • Leakages in ESP, due to which air ingress & cause condensation of flue gas on electrodes & other internals
  • No insulation or using lesser thickness insulation material
  • Wrong operation of rapping system
  • Not cleaning of ash in shutdowns

5-Why there is a difference in hopper temperatures of ESP fields?

It is due to the different load on different fields. Field which discharge more ash has more temperature & field discharging low ash is having low hopper temperature
6-What do you mean by Back Corona effect?
A phenomenon that occurs when the gas within a high resistivity dust layer becomes ionized, which causes heavy positive ion back flow, which neutralizes negative ion current and reduces voltage levels.
7-Why there is fluctuation in ESP current & Voltage
  • Movement or swaying of discharge electrodes & creating close contact with collecting plates
  • Close contact of foreign material with discharge electrodes
  • Loose connection of cables which are connected to control panel
  • Leakage current & Induction
  • High ash level in hopper & touching discharge electrodes
8-What are the reasons for failure of support insulators?
  • Higher operating temperature
  • Higher dust collection & electrical arcing
  • Shock loads
9-Why there is more ash collection at ESP 1st Field as compared to other fields?

In ESP 1st field there is huge drop of flue gas velocity that is from 12 m/sec to around 1 m/sec, which causes more ash collection in 1st field. So current for ESP 1st field is kept lesser as compared to other fields.
10-Why ESP field voltage reduces when the current increased?
It is due to the back corona effect due to higher resistivity of ash. To get rid of this increase rapping frequency.
11-Why explosion occurs always in ESP? How do you avoid it?
Generally ESP has low velocity zone, where there is possibility of collection of explosive gases & unburnt fuel. The explosive gas namely carbon monoxide associated with unburnt fuel settles at ESP corners or at collecting plate rapping system, when the cold air or hot unburnt particle comes in contact with this gas it suddenly explodes.
So in order to avoid this need to take care on following points
  • Maintain balanced draught in furnace to achieve good combustion as well as to avoid carryover of unburnt particles
  • Arrest all lair leakages in ESP
  • Avoid charging ESP at flue gas temperature < 125 deg C
  • DO not bypass any of the ESP interlocks
  • Ensure proper operation of rapping system
  • Ensure healthiness of explosion vent provided at ESP inlet & outlet cones

12-How the air ingress affects on ESP performance?
  • Air ingress lowers the flue gas temperature, there by corrosion of ESP internals
  • Pressure drop across ESP increases
  • Load on ID fans increases
  • Leads secondary combustion of unburnt particles in ESP
13-What operation parameters affect on ESP performance?
  • Flue gas temperature
  • Flue gas volume
  • Flue gas composition
  • Inlet dust load
  • Density of ash
14-How do you calculate the ESP efficiency?

ESP efficiency = ( Dust in –Dust out) X 100 / Dust in

It is around 99% for best performing ESPs

15-What do you mean by the term Emission?
Emission is the release of pollutants into the air from a source.
16-What do mean by Aspect ratio? How doe it affect on dust collection?
It is the ratio of collecting plates length to the height. It should be around 0.5 to 2
If collecting plates height is more than their length, then there is a possibility of carryover of dislodged dust particles with flue gas

Question & answers on vacuum troubleshooting in steam turbines



Understanding  the term Vacuum.

The earth's atmosphere exerts pressure upon us known as atmospheric pressure which can be measured in number of ways & methods.At the seal level the pressure measured is 760 mm of Hg, 1 Torr or 14.7 PSIa or 1 bar.Because the barometric pressure varies above seal level pressure is used as reference point.
As there is 1 bar pressure exerted on our body and also there is 1 bar pressure within our body. So as per Newton's law action &* reaction are equal and opposite, we do not feel any discomfort. That is there is no any pressure difference between inside & out side of the body.
Evacuating air from a closed volume develops a pressure differential between the volume and the surrounding atmosphere. If this closed volume is bound by the surface of a vacuum cup and a work piece, atmospheric pressure will press the two objects together. The amount of holding force depends on the surface area shared by the two objects and the vacuum level.
Because it is virtually impossible to remove all the air molecules from a container, a perfect vacuum cannot be achieved. Of course, as more air is removed, the pressure differential increases, and the potential vacuum force becomes greater.
The vacuum level is determined by the pressure differential between the evacuated volume and the surrounding atmosphere
Is it possible to have a negative absolute pressure?
No, absolute pressure is measured with reference to a perfect vacuum so it is impossible for it to go negative. You can only measure negative pressure between two different pressures. For example if you allow atmospheric air to gradually flow into a vacuum vessel and measure pressure inside relative to outside it will show a negative pressure reading.

What type of problems do you face in steam turbines related to vacuum?
Problems such as:
  • Low vacuum
  • High exhaust pressure
  • High exhaust temperature
  • Higher specific steam consumption
  • More cooling water circulation
  • Hot well level variation
How do you create vacuum in steam condensers?

Vacuum is created in condenser by steam jet ejectors, where high pressure 8–12 kg/cm2 steam is passed through nozzle which is connected to air line from condenser. This creates high negative pressure there by evacuating air from condenser.

Generally there are Two Types of Ejectors:
Hogger Ejector: Initially this ejector is used for pulling vacuum. It has steam and air lines connections, steam is vented directly into atmosphere. It consumes more steam than main ejectors. It requires 20–30 minutes to create 85% of operating vacuum.

Main Ejector: It comes with first stage and second stage. Air line from surface condenser is given to 1st stage then again air from 1st stage is collected and discharged into 2nd stage. 2nd stage ejector has air vent line.
It consumes less steam than hogger ejector. Generally an ejector come with 1W + 1S i.e. one working and one stand by.
Also vacuum pumps called liquid ring vacuum pumps are used to create vacuum in condensers. Which consume less energy than steam jet air ejector
How does low vacuum affect on turbine speed?
Lower vacuum creates back pressure on turbine blades and rotors. So in emergency, vacuum breaker valve is opened to bring down the turbine speed to zero in minimum time to avoid any further damages.
What is the effect of low vacuum & high exhaust pressure on steam turbine performance?
Low vacuum or high exhaust pressure & high exhaust temperatures lead to more steam consumption to generate unit power.

Steam condenser,vacuum & calculations....

What are the potential reasons for lower vacuum in steam condenser?
  • More condenser load than design
  • Lesser amount of cooling water circulation in condenser
  • Higher atmosphere temperature
  • Location of the steam condenser at higher elevations.
  • More exhaust temperature
  • Air leakages in the system
  • Lesser efficiency of steam ejector or vacuum pump
  • Ejector inter condense (1st stage) condensate seal break
  • Lesser pressure & temperature of motive steam at ejector inlet
  • Worn out ejector nozzles
  • Improper quality of motive steam
  • Variation in condenser inlet & outlet cooling water temperatures
  • Operation of Turbine at lower load
  • Lower gland seal steam pressure


What are the effects of air leakage in condenser?

Following are the major effects due to air leakage into condenser:

Lower Thermal Efficiency: The leaked air in the condenser results in increased back pressure on the turbine this means there is loss of heat drop consequently thermal efficiency of plant will decrease.

Increased Requirement of Cooling Water: The leaked air in the condenser lowers the partial pressure of steam due to this, saturation temperature of steam lowers and latent heat increases. So it requires more cooling water to condense more latent heat steam.

Reduced Heat Transfer: Due to poor conductivity of air heat transfer is poor.

Corrosion: The presence of air in the condenser increases the corrosion rate.

What is the function of vacuum breaker valve?

Vacuum breaker valve is used to bring down the turbine speed quickly to zero in case of emergency trip of turbine. Valve can be manually or auto opened.

What are the different conditions on which vacuum breaker valve opens?

30-things you must know about steam Turbines

On following emergency or fault cases vacuum breaker valve will get open
  • High bearing vibrations
  • High bearing temperature 
  • High axial displacement of rotor
  • High differential expansion
How can you identify the air leakage into the system?

If there is air leakage into the system, then this should be vented out though ejector system. Rota meter of ejector shows the increase in air quantity than the normal air flow.

How do the motive steam pressure & quality affect on ejector performance?

If the motive steam pressure is below design by more than 5%, or above design by 20%, poor performance may occur with a resulting increase in the condenser pressure.
Motive steam quality - Wet motive steam will cause poor performance as well as ejector wear. Super heated steam having a temperature greater than 10° C above the saturation temperature will also cause poor performance if not considered in the design.

What will happen to hot well level, if condenser vacuum drops suddenly?

Hot well level rises up
What are the common problems associated with steam jet ejectors related to vacuum?
Common problems are:
  • Low or high motive pressure due to improper sized nozzles:If the cross section area at those locations is greater than 7% above the design values, performance problems are likely.
  • Wet motive steam
    • Failure of vacuum trap
    • Larger pressure drop at shell side: If the shell side pressure drop is greater than 5% of the absolute operating pressure, then either shell side fouling or flooding of the condenser could be present. Check the trap or loop seal on the condensate outlet for proper drainage
    • Breaking of 1st stage condenser U loop seal
    • Air leakages through safety valve & flanges
    • Non operational rota meters
    Why U loop and float valves are used in steam ejector 1st stage (inter condenser) and second stage (after condenser)?
    U loop and float valves are used for sealing purpose between 1st stage and 2nd stage ejectors and condensers. As there is a pressure difference between these two and turbine steam condenser.

    U loop is around 2.5 to 3 meter, it depends on pressure difference between 1st stage and steam condenser. If there is pressure difference of 0.25 kg/cm2 between 1st stage and steam condenser then the U loop height should be 2.5 meter. So it is very must to seal the U seal (filling DM water in loop) before pulling vacuum.

    A steam Turbine's exhaust steam temperature gauge is showing 60 Deg C & vacuum gauge is showing pressure -0.75 Kg/cm2, then what do you think, is the pressure gauge showing  correct reading?

    As discussed earlier, condenser vacuum depends on the atmospheric pressure, as the atmospheric pressure is more, vacuum can be maintained more. Hence the steam condenser installed at higher elevation have lower vacuum than that of condensers installed at lower elevations.

    In this case at temperature 60 deg & considering atmospheric pressure 1.033 kg/cm2 the gauge  pressure in the condenser should be around 0.81 kg/cm2.

    There might be error in vacuum gauge or might be installed at some higher elevation around 600 mm causing  lower pressure due to head difference.

    A steam power plant is installed 580 meters above  the seal level, then what will be the atmospheric pressure in that area?

    Atmospheric pressure = P = 1.033 X (1-2.2557 X 10-5 X 580 m)5.2558
    Atmospheric pressure = P = 0.9638 kg/cm2

    What are the potential reasons for a Steam jet ejector consuming more steam for creating particular vacuum in steam condenser?

    Potential reasons are:
    • Improper design of ejector
    • Improper pipe line layout from & to the ejector
    • Worn out steam nozzles
    • Steam quality is wet
    • Higher steam pressure
    • Air leakage into the system
    • Steam line leakages
    • Fouling in ejector shell
    • Insufficient quantity of cooling water

    • Water tubes leakage
    What can cause,If ejector's motive steam pressure & temperature are higher than design?
    • Ejector capacity gets reduce
    • Ejector performance gets reduce
    • Steam wastes
    What will be the hogger ejector capacity as compared to main ejectors?

    Hogger ejector should create 60-70% vacuum in 15-20 minutes

    What is the steam steam consumption for Hogger ejectors as compared to main steam jet ejectors?

    It is usually 30-40% more than main ejectors








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