Hydro Pnuematic Accumulators

The accumulator are pressure storage reservoir in which non compressible fluids are stored under pressure. accumulators are connected to pump discharge lines for compensating for loss of fluid pressure in the hydraulic circuit.

They are used for

1. Emergency operation

2. pulsation Damping

TYPES OF ACCUMULATOR

1. Bladder type accumulator

2. Diaphragm type accumulator

3.Piston type accumulator

Bladder Accumulator 

Fluids are not in compressible and cannot store pressure energy, but gas can be compressed so it is used for storing fluids in accumulator.The accumulator consists of a shell. The shell consist of fluid section and gas section. The gas section has a bladder which is filled with nitrogen gas. Surrounding the bladder is the fluid.The bladder can be expanded and compressed for storing and releasing the fluid.The fluid section is connected to the hydraulic section ie poppet valve for drawing the fluid into the accumulator when the fluid pressure is higher and compresses the bladder with gas.so the fluid is stored inside the accumulator.when the fluid pressure in the line drops the bladder expands and forces the stored fluid in the hydraulic circuit with the pump flow

The accumulator bladder is pre pressurised with nitrogen gas upto 2.3bar. The bladder gas pressure is set 70%  to 80 %of the hydraulic circuit pressure.

Diaphragm type accumulator

These accumulators have a rubber plate or diaphragm as the separating element. This element is welded or screwed together between two spherical shells (or compartments). The compartment above the diaphragm is filled with nitrogen. The compartment below is directly connected to the hydraulic circuit. Diaphragm accumulators are useful if the required fluid storage capacity is low (i.e. 4 litres or less).

Diaphragm accumulators have most of the advantages of bladder-type units, but can handle gas compression ratios of up to 8:1. However, they are limited to smaller volumes, and their performance can sometimes be affected by gas permeating across the diaphragm.

Special Equipment are available to check the pressure of the nitrogen and it should be check once per year. If nitrogen pressure is high it can be reduced. By using mobile nitrogen charging devices the pressure can be added or refilled

PUMP CAVITATION

CAVITATION
   
            When a liquid boils at a suction line of a pump it is said to be cavitation. As the liquid boils it forms cavity of gases in the suction line. this happens when liquid pressure is well below the vapour pressure of the liquid.Boiling starts when pressure of the liquid is reduced to vapor pressure of the liquid at the actual temperature.

Vapour pressure of water: 
The pressure at which the water turns into vapor at the given temperature.
for example: water turns into vapor at 40deg at pump suction pressure below 0.07 bar, so pump suction pressure should not go below 0.07 bar at 40 deg to avoid cavitation.

Causes of cavitation
Cavitation can causes serious damage to the pump in the form of vibration and abnormal sound. cavitation leads to bearing and mechanical seal failure, shaft breakage , pitting on the impeller.
The damage occurs when these bubbles pass to the high pressure region of the pump.The bubbles explode at the vanes of the pump.

Problems due to cavitation
Loud noise
reduced flow
pitting formation in the impeller

Net positive suction head
The Minimum pressure required at the suction port of the pump to keep the pump from cavitating

Net positive suction head available
The pressure available at the suction side of the pump and it should be calculated and it should be more than NPSH R

Net positive suction head required
It is the minimum pressure required at  the pump suction to keep the pump away from cavitation and to be provided by the manufacturer.

Net positive suction lift
If the pump is positioned above the tank it has some net positive suction head. if the pump is raised above the tank the net positive suction head reduces and at some point the head becomes zero and the fluid starts to evaporate.

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

.

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

TYPES OF BOILER TUBE FAILURE AND THEIR REASONS

Procedure For installing interference Fit Hubs

Introduction 

Carefully follow these instructions for optimum performance and trouble free service.

These instructions apply to interference fit type hubs. This type of installation is for straight shafts, with the exception that the hubs must be heated before they are
installed on the shaft.

Component Preparation

Clean the exposed surfaces of all components, hubs, sub-assemblies, spacers, etc., to remove any protective coating applied at the factory. All parts must be clean and free of any foreign materials before attempting installation or assembly, use a clean cloth dampened with a nonflammable solvent.
Check Hub Bores, Keyways and Shafts for any raised metal, nicks, burrs, dents, gouges, etc., dress if necessary.

Warning!
Before beginning coupling installation make sure the machinery is made safe. Discount all power. 

Straight Bore Interference Fit 
It is important when mounting inference hubs to make sure that clearance exists over the top of keys: otherwise, when the hub cools, it will rest on the key and produce high stresses in the hub that could cause it to fail. Expand the hub bore with a uniform heat source, Oil, Oven or Induction.
Oil bath heating is usually limited to approximately 350°F (177°C), or less than the flash point of the oil used. Special handling devices are required such as tongs, threaded rods placed in puller holes in the hub, etc.

Warning!
If an oil bath is used, the oil must have a flash point of 350°F (177°C) or greater. Do not rest hubs on the bottom of the container. 

Oven heating offers some advantages over oil. Parts can be heated to higher temperatures, usually not exceeding 600°F (315°C) and the parts can be handled with heat-resistant gloves. Do not rest hubs on oven; place them on a rack.

Warning!
Do not use an open flame in a combustible atmosphere or near combustible materials. 

An Induction heater can be used as long as the temperature rise is controlled.
Open Flame Heating is not recommended. If an oxyacetylene or blow torch is used, use an excess acetylene mixture. Mark the hub body at the top, center and bottom of their length in several places with heat sensitive crayons, one 350°F (177°C) and one 450°F (232°C) melt temperature.

Elevate the hub with refractory bricks to allow the flame to flow through the hub. With a “Blue flame” or “Rose bud torch” direct the flame towards the hub bore using constant motion to avoid overheating an area. Once the heat sensitive crayons melt the hub is ready for mounting.

Caution! 
Do Not Spot Heat the Hub or Distortion May Occur. 

Regardless of method used, heat MUST be applied evenly to avoid distortion. This is especially important when using open flame heating. In any event, extreme care must be exercised when handling heated hubs to avoid injury to personnel.

Caution! 
Do Not Exceed 600°F (315°C) During the Heating of the Hub. Excessive Heat may soften the Hub, Reducing the Strength of the Steel and may affect the Performance Characteristics of the Hub. 

what is XRP 623 or XRS 623

Coal Bowl Mill Model Number XRP 623

X - Frequency of power supply (50 cycles)
R - Raymond name of the inventor
S - Suction type with exhauster after mill
P - Pressurized Type With PA fan Before Mill

62 - stands For bowl diameter in inches (BHEL series)

3- number of rollers

BOWL MILL WORKING PRINCIPLE

                          pulveriser to dry the coal, sufficient pressure on the rolls to pulverise it, and

                          proper setting of the classifier deflector vanes to obtain a fineness reasonably                                       close to that for which the pulveriser was designed.Operating Experience will 

                          help determine the pulveriser's optimun conditions for a given coal.

STEAM EJECTOR WORKING PRINCIPLE

 Function of ejector:- The main function of the ejector is to extract air and no condensate gasses from a closed system . It removes air from the condenser and provide vacuum. The amount of air extracted at the time of starting is more as compared to running unit.

Parts of ejector:- It generally consists of converging nozzle , diverging nozzle , diffuser throat , inlet and outlet pipes, Rota meter for measurement of air flow , cells with tube for regeneration heat transfer , Steam traps for drip control .

 parts of a steam ejector

             

                   

Working principle of ejector :- When a high pressure motive fluid( steam/liquid) enters the steam nozzle , this result in a decreasing pressure and increasing velocity of the fluid again the fluid enters in the diffuser which result in a increasing pressure and decreasing velocity of the fluid so due to pressure difference vacuum is created in between the nozzle and diffuser we can say venture throat . So due to pressure difference or vacuum it sucks no condensable air and fluid from the system. The mixture then enters to an inter condenser where the steam is condensate. The inter condenser are designed with a special inter condenser baffles of tubes where heat transfer takes place . The second stage ejector handles the non condensate gasses of first stage ejector and through a vent opening to the atmosphere. The air flow can be measured by the rota-meter. The condensate water of steam send  back to re use through steam traps.

Types of ejector :-
There are generally two types of ejector generally used as ,          

  Hogger ejector :- This ejector is known as the starting ejector which is used to pull the gasses at the time of starting of system. It has high capacity of air extraction. This ejector operates with parallel with the running ejector till the vacuum reaches 500-600 mm of HG column , then the hogger ejector is switched off and main ejector will remain in service. The main disadvantages of this ejector is the steam escaped to atmosphere and can't  reuse.

 Main ejector :- This is also known as the running ejector which evacuate the air continuously at the system running condition. This is a multi stage type ejector . The high velocity air -water mixture enters to the shell and cooled in the first stage of the shell by condensate. Steam is thus condensate and the steam air mixture volume reduced and allowed to second stage of the nozzle . In the second stage the mixture is completely condensate and the air is vented to the atmosphere. The drains are  provided with loops or siphoned to prevent ingress of air from the atmosphere.Here the condensate seam is again recovered in condenser and reuse.

STEAM DRUM INTERNAL PARTS

STEAM DRUM

A steam drum of the boiler is meant for performing the following.

1. 1. Steam  is  purified  by  the  steam  drum  internals  which  may  have  the  baffle  separators,  cyclone 

1. separators, Chevron separators, demisters / Screen driers.  

1. 2. Acts as reservoir and provides the necessary head for the circulation of water through evaporative 

1.  surfaces. 

1.3.Supplies water for a brief period in the event of stoppage of water supply.

PARTS OF STEAM DRUM

1. Feed water distributor pipe

2. Safety valve

3. Chemical dosing pipe

4. Continuous Blow down pipe

5. Drum level gauge

6. Cyclone box Chamber

7.  Steam Purifier/ demister/ steam Driers

8. Saturated Steam Outlet Pipes

9.Down comer

10 intermittent Blow down Pipe

Feed water distributor pipe

Should distribute the fresh water throughout entire drum length , so that the fresh water mixes well with boiler water. The feed distributor should not disturb the drum water level. It should not obstruct the water entry into the down comer. Otherwise it may lead to circulation failure of evaporator section. The  pipe  should  not  direct  the  cold  water  to  drum  surface.  It  may  lead  to  differential  metal 

temperature  and  may  strain  the  metal. The  feed  distributor  pipe  shall  be  flanged  for  easy  removal  and  flanges  shall  have  proper machining done on the mating faces. Gaskets are not to be used as they fail over a period.

Safety Valve

The safety valve is to release the excess pressure created in the drum than the set point of the valve. It should be so located that the steam when vented out should not disturb the steam drawl by the   saturated steam links to super heater. It should not lead to starvation of Super heater. Hence it is 

generally located outside the steam drier unit.

Chemical Dosing Pipe

In order to maintain desirable boiler water chemistry, suitable chemical is added directly into the drum 

water. The chemical is dosed through the chemical distributing pipe.

Concentrated chemicals are being dosed in the boiler water.  Hence the piping shall be of non 

corroding material. The piping shall be of stainless steel construction. The chemical should be well distributed throughout the drum length. Or else it can be connected to feed water pipe itself so that it helps for better mixing. The chemical distribution piping should not get choked due to chemical. The hole diameter shall be 6 mm. Holes shall be pitched at 300 mm.  

The chemical should not get drained off through the blow down pipe. It should be so located that  the chemical gets mixed well with the fresh water before entering in to the down comers.

Continuous blow down Pipe (CBD)

The boiler water is evaporated continuously and the relative pure steam leaves the boiler drum. This mechanism is leads to concentration of boiler water with more dissolved solids. Since the solubility of various  dissolved  solids  is  limited  based  on  the  operating  pressure,  it  is  necessary  to  limit  the concentration of dissolved solids in the boiler water. There is always a carryover of dissolved solids of boiler water in to the steam. This is called vaporous carry over. The percentage of carryover of each constituent is a  function  of  operating  pressure  of  the  boiler.  Hence it  is  necessary  to  limit  the concentration of boiler water with dissolved solids. Silica is a bad water constituent that leads to hard deposits on turbine blades. The CBD pipe should be able to draw the boiler water from the entire length steam drum. Any localized sampling will lead to a wrong sample of the boiler water. It should blow down the high TDS boiler water only. In should not lead to blow down of fresh water.

Drum Level Gauge

The exact level of the Steam Drum water level is very important factor in the Power plant. Because if the drum level is high, then the quantity of the steam production is less and it may affect the turbine blades due to low quality of steam.​ if drum level is low then there is a chance of starvation of the boiler tube and it may change the metallurgy of the tubes and there by the tube will be damaged, prematurely.

Hence it is necessary to get the correct feedback from the field regarding drum level.

For a Boiler operating up to a pressure of 28 Kg/Cm2, use of only one gauge glass is recomended.but for the power Boiler which is operating above 28 Kg/Cm2,an combination of the following is used.

•  Two gauge glass in continue service and visible by the operator.
• Two independent remote transmitters with one single gauge glass.
• One independent remote transmitter and one single gauge glass(continuously in service) both of the instrument must be continuously visible/Monitored by the operator from control room.

Types of Level Gauge

There are mainly four types of water level gauges 

1.     Tubular Type

2.     Reflex Type

3.     Flat Glass Type

4.     Ported Type

Tubular type water level Gauge glass limited to 17Kg/Cm2 operating pressure.

Reflex type can be used for the pressure up to 24 Kg/Cm2.

Flat glass type can be used for pressure up to 140 Kg/Cm2.

Ported Type gauge glass can be used up to 210 Kg/Cm2.

Cyclone Box Chamber

When  the  boiler  is  fitted  with  cyclone  separators,  all  the  water  steam  mixture  is  collected  in  a chamber. The mixture is made to pass through multiple cyclones. The cyclones are mounted through 

flange system to the cyclone box chamber.  The cyclones separate the water and return to the water 

space of the drum. The steam leaves through the top chevron unit. The total system should meet the 

following requirements.

Steam Purifier / Demister

Demister can be two stage Chevron driers / demister / screen driers.  Its purpose is to trap the droplets and lead them back to drum water

Downcomer.

The water from the entire length of the drum is allowed to pass through the down comer pipe and circulates through the water wall panel through natural or forced circulation.

Intermittent Blow Down Pipe

It is apt to name it as emergency drain valve through which the water level is brought down in case of 

high level.  In  low  pressure  boilers,  the  intermittent  blow  off  valve  was  strategically  located  for removal of settled sledges generated by phosphates reaction with calcium and magnesium salts