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