Thermal Power plant interview questions and answer

1.What are boiler mountings?
   Ans: boiler mounting are mounted on the boiler itself and mandatory required for safe and proper operation of boiler
1.water level indicator
2.pressure Gauge
3.safety valve
4. Blowdown valve
5.Feed Check valve
6.Main steam Stop valve

2.what are boiler accessories?
Boiler accessories are components that are attached to boiler and used for working of boiler and increasing its efficiency
1.Feed pump
2. Economizer
3. Air preheater
4.Fans
5. super heater
6.Ash handling system

3.what type of boiler does not need steam drum?
   Ans: Super critical Boiler

4.Why it is unsafe to have high water condition in drum?
    Ans: High Drum Level does not allow efficient steam water separation and makes water to carry over with steam to turbine.

5.What are the causes for boiler tube failure?
   Ans: Due to long term overheating due to scales and deposits formed on the outer surface of tubes
          Short term overheating , occurs due to flow restriction in the tube
          Thermal stress and shock  , occurs due to frequent start and stops
           Erosion, occurs due to coal , oil, ash
           corrosion
           Creep occurs due to constant stress of material at high temperature

6.Why boiler is purged every time before starting?
    To remove any left over fuel in boiler which leads to explosion

7. What are the uses of oil firing?
    1.warming up of boiler due to cold start up
    2.increase the furnace core temperature to switch to coal firing
    3. Stabilizing flame when interruption occurs to coal firing
    4.Useful during low load operation

8. What is the pressure and temperature of super critical boiler?
    Boiler operating pressure is more than critical pressure (221.2 bar and 375 deg temp) is called super         critical boiler. At this point both steam and water have same density.

9.how to find incomplete combustion in boiler
    high carbon oxide content at flue gas at exit

10 The efficiency of Rankine cycle is in the range of
     35-45 %

11. On which factor does calorific  valve of coal depend upon?
      Ash content

12.What is Thermal Energy?
     The ratio of heat energy to the turbine to heat of coal combustion
   

Impulse cum Reaction Turbine

Steam Turbine is considered as the the heart of the turbine. Turbine is used to Convert the Thermal energy to Mechanical Energy. Thermal Energy is the Steam and Mechanical Energy is used to rotate the steam turbine.

Thermal Energy = kinetic Energy in steam + pressure energy in steam + temperature of steam
Enthalpy = pressure energy + temperature of steam

so, Thermal Energy = kinetic energy + entalpy of steam

Impulse Turbine

Impulse Turbine is the first stage in the steam turbine. Its is the control stage. Here the blades will like cup shaped. The steam enters through the inner casing nozzle at constant pressure and temperature and enters the stationary blades. The stationary blades acts as a nozzle, it increases the kinetic energy and decreases the pressure of the fluid. Fluid with high velocity impacts the rotor and moves it. As the fluid strikes the moving blades in the rotor, the kinetic energy decreases and pressure remains constant because here the kinetic energy is used to convert into mechanical energy

Reaction Turbine

The remaining stages in the turbine are reaction turbine. The steam after leaving the impulse stage enters the reaction stationary blades. Here both stationary and rotary blades acts as nozzle. when steam enters stationary blades velocity increases and pressure decreases.The fluid with increased velocity enters the rotary blades , here the pressure decreases ad the velocity also decreases as both pressure and kinetic Energy are required to convert into Mechanical Energy.The entalpy and kinetic energy decreases through the stages

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Impulse Turbine	Reaction Turbine
In stator stages velocity increases and pressure decreases	In stator stages velocity increases and pressure decreases
In Rotary Stages velocity decreases and pressure remains constant	In Rotary stages velocity decreases and pressure decreases
Here Kinetic Energy is only required to convert to Mechancial Energy	Here Both pressure Energy and Kinetic Energy is used to convert into Mechanical Energy
Here stationary blades only acts as nozzle	Here  both stationary and rotary blades acts as nozzle

Gear box backlash

The Gear box backlash is the clearance or gap between the two mating gear tooth.
The backlash can be measured with dial gauge , feeler gauge or lead wire.The Dial Guage pointer is kept at Driven gear face and the drive gear is locked. Then the driven gear is moved which shows the backlash valve in the dial gauge.
Another Method is keeping the lead wire in between the two faces of the mating gear and allowed to mesh.After the lead wire compresses it is measured with the micrometer which shows the backlash reading.

Reasons for Gear Backlash increase
Due to gear face wear after running so many hours
Due to bearing Radial wear
If the center distance between two gear is high

Back Lash and Gear box Clearance are different . Gearbox clearance is the space between the addendum circle of one gear to the deddendum circle of another gear

Backlash valve for normal gearbox 0.18-0.3mm for heavy load < 3mm
There should be sufficient backlash for allowing lubrication to gears and compensate mialignments

GEAR RUN OUT

Gears can be eccentricity. eccentricity is caused due to the gear teeth profile. All gear teeth cannot be manufactured accurately with proper dimension. they can be +or- 0.5 mm. so due  to this eccentricity happens. Eccentricity can cause vibrations.

A badly Eccentric tooth may cause abrupt gear failure. so the run out must be checked. Dial gauge is used for checking the rut out valve.

The Dial gauge Tip is inserted between the tooth and the gear is rotated tooth by tooth, The high eccentricity point will shoe high valve and its is noted.

The high run out reading also indicate that the bearing clearances have increased and must be replaced.
The problem in the bearing can be found out by run out inspection

Run out value <0.2mm for medium duty gears and 0.4mm for heavy duty gears

Balancing line in vacuum pumps

A balancing line is provided in suction of vacuum pumps and connected to condensate storage tank. This line is used to remove external air continuously and ensure that pump has enough suction ie NPSH, to drag water from the condenser with out cavitation.

The balancing line is to maintain the suction pressure in negative even though if any air leak is happening through. Its acts as a vent to remove air pockets while charging the pump.

BOILER FEED WATER BALANCING LINE

In any centrifugal pump, each impeller tends to produces some amount of thrust because of different pressures and different geometries on the two sides of the impeller. In a high pressure multi-stage pump (such as BFW) the number of impellers is high, thus the net thrust would be large unless something is done to balance it out.

In Multistage centrifugal pump operation, Suction side will have less pressure and discharge side will have high pressure. Due to this pressure difference the impeller with pump shaft and bearing will move from discharge side to suction side. This movement is called axial trust. The axial trust will Damage the pump internals and bearings.The two main ways to reduce the net thrust are to oppose the impellers or to use a balance disk/drum.For axial split pumps, it is usually most economic to oppose the impellers.  About half of the stages are oriented with the suction pointing toward the coupling and the rest are oriented with the suction toward the thrust bearing.  The thrust of the stages pointed in opposite directions tend to cancel out.  The net thrust that the thrust bearing must take is much smaller than it would be if they all pointed in the same directionBut axial split cases tend to have an upper pressure limit.  At very high pressures, barrel pumps are used since they can handle the very high pressures better.  With a barrel pump, it is much more difficult to find a good way to direct the flow path through a set of opposed impellers.  So instead, they point all the impellers in the same direction and use a balance disk or drum on the end.The balance disk is just after the last stage so it has full discharge pressure on one side.  A line is routed from the other side of the balance disk back to the suction. Since there is a very high pressure differential across the balance disk/drum, it is critical that the clearances are correct or else excessive flow will be diverted back to suction and the thrust balancing force will be lost.  In other words, if the balance drum/disk fails, a thrust bearing failure is likely to follow.  

Balancing Line and Balance Drum  is used to balance the axial trust . The balance Drum is attached to the shaft in the Non drive side The balancing chamber will be in the Discharge cover. The discharge Cover will have the balance sleeve or bush. There will be gap between the Balance Disk and the Balance Bush.Balance line  is a tapping  taken from the pump discharge line and connected to the pump suction.

During operation the water from the last stage of the impeller passes between the drum and drum bushes and enters the balance chamber at pressure equal to the suction pressure. there will be two pressures. pressure before the balance drum and after the balance drum. the difference in pressure causes the balance drum with shaft to move and create a clearance between bush and  the drum. 

If the axial trust on the impeller is higher the shaft will move to non drive side reducing clearance between the drum and bush. so that the amount of leakage to balance chamber also reduced causes pressure in balance chamber to be reduced. Thus lower the pressure in balance chamber causes difference in pressure and the disk tends to move away from the disk head causing more clearance and the pressure builds up in the balance chamber and moves the disk towards the drum.

by this way the shaft of the pump is hydraulically balance under tension by the balance drum. The small amount of residual forces will be taken by the trust bearing 

Boiler Bank Tube Leakage attending

Bank Tube:

• Bank tubes are provided in Bi Drum Boiler. Boiler Bank tubes have steam drum and mud drum. 
• Bank tubes are provided between these two drums.  
• It acts as a downcommer and raiser between theses drums. 
• It supports the whole weight of the mud drums and downcommers.
• Any failure on these tubes Damage the near by tubes. so it must be attended immediately 

Bank tube and Drum connection

• The bank tubes are held inside the Drum holes by expanding it with a expander. while expanding the tube expands and deform plastically and the drum being in elastic condition exerts force to hold the tube against pressure. 
• while expanding the bank tube, its thickness reduces 7 to 11 % this indicates the good joint.
• Normally these joints are good upto pressure of 120 kg/cm2
• sometimes bank tubes are seal welded with the drum to avoid any leakages during operation

 

Bank Tube Expansion:

Reason for bank tube Failure

• The main reason for bank tube failure is due to inadequate expansion. More than 1000 bank tubes will be there and its difficult to measure the tube thinning for all the tubes after expansion. tube thinning measurement is done randomly for 200 tubes after expanding all the tubes.the requires torque is applied using expander to all the tubes and its taken for granted that all the tubes are expanded
• Expanded joints also fail due to large number of start up and shutdown which causes thermal shock to the tubes. 
• Internal deposit leading to long term overheating Failure
• Blockage resulting short term overheating failure
• Errosion and Corrosion of the tube Surface

Bank Tube Leakage attending procedure

• If bank tube failure occurs at expanded joint. This Leakage can be attended by plugging in both top and bottom drum.This is mainly due to the reason that failed tubes cannot be reached which could be between large number of tubes.
• spool piece welding should not be done in bank tubes as this can cause failure in bank tubes
• The expansion leakage can also attended by re expanding all the tubes but the tube thinning must be limited to 13 to 15 %. More than that the tube cause brittle and leads to failure
• Seal welding is permitted after all the bank tubes are expanded.

Replacing all the bank tubes

There are at times all the bank tubes must be replaced due to lot of plugging. for this bottom drum must be perfectly supported and the bank tubes can be removed . and after removing the drum holes must be checked for dimension . if dimension deviate due to steam and water corrosion it must be corrected by weld buildup. Don't seal weld the bank tube without proper expansion