Firetube & Water Tube Boilers

FIRE TUBE BOILER







































Fire Tube – hot gases inside the tubes and the water surrounds the tubes



Two types of fire tube boiler:


 EXTERNALLY FIRED : Locomotive Boilers, Lancashire Boilers,



Horizontal Return Tubular (HRT)


 INTERNALLY FIRED : Scotch-marine Boilers, Package Boilers

Horizontal Return Tubular Boiler (HRT)

Horizontal Return Tubular Boiler (HRT)
























Two Pass Dry Back Two Pass Wet Back



























Three Pass Wet Back Three Pass Dry Back

Simple Vertical Boiler










It consists of a cylindrical shell, the greater

portion of which is full of water which surrounds the


fire box also and remaining is the steam space. At the

bottom the fire box is grate on which fuel is burnt


and the ash from its falls in the ash pit.

The fire box is provided with two cross

tubes. This increases the heat surface and the

circulation of water. This ensures efficient circulation
of water. At the ends of each cross tube are provided

hand holes to give access for cleaning these tubes.

The combustion gases after heating the water and

thus converting it into steam escape to the
atmosphere through the chimney

Vertical Multi Tubular Boiler (Cochran Boiler)







Cochran boiler is one of the best types of vertical
multi-tubular boiler, and has a number of horizontal

fire tubes. Cochran boiler consists of a cylindrical
shell with a dome shaped top where the space is

provided for steam. The furnace is one piece
construction and is seamless. Its crown has a
hemispherical shape and thus provides maximum

volume of space. The fuel is burnt on the grate and
ash is collected and disposed of from ash pit. The

gases of combustion produced by burning of fuel
enter the combustion chamber through the flue tube
and strike against fire brick lining which directs them

to pass through number of horizontal tubes, being
surrounded by water. After which the gases escape to

the atmosphere through smoke box and chimney. A
number of hand holes are provided around the outer
shell for cleaning purposes.

Lancashire Boiler



























It is stationary fire tube, internally fired, horizontal, natural circulation boiler. This is a


widely used boiler because of its good steaming quality and its ability to burn coal of inferior


quality. These boilers have a cylindrical shell 2 m in diameters and its length varies from 8 m to

10 m. It has two large internal flue tubes having diameter between 80 cm to 100 cm in which the


grate is situated. This boiler is set in brickwork forming external flue so that the external part of

the shell forms part of the heating surface.

The main features of the Lancashire boiler with its brickwork shelling are shown in figure. The

boiler consists of a cylindrical shell and two big furnace tubes pass right through this. The brick
setting forms one bottom flue and two side flues. Both the flue tubes, which carry hot gases, lay
below the water level

Locomotive Boiler








1. Fire box 2. Grate 3. Fire hole
4. Fire bride arch 5. Ash pit 6. Damper
7. Fine tubes 8. Barrel or shell 9. Smoke box
10. Chimney 11. Exhaust steam pipe 12. Steam dome
13. Regulator 14. Lever 15. Superheater tubes
16. Superheater header 17. Superheater exist pipe 18. Smoke boxdoor
19. Feed check valve 20. Safety valve 21. Whistle



Locomotive boiler is a horizontal fire tube type


mobile boiler. The main requirement of this boiler is


that it should produce steam at a very high rate.

Therefore, this boiler requires a large amount of


heating surface and large grate area to burn coal at a

rapid rate. Providing provides the large heating


surface area a large number of fire tubes and heat


transfer rate is increased by creating strong draught

by means of steam jet.

FIRE TUBE BOILER






 SHIPS: Scotch, Lancashire Boilers


 TRAINS: Locomotive Boilers


 VERTICLE :Cochrane, Vynke, Tubeless, Cross Tube –



space constraint


 PACKAGED : Under Fired Multi-tubular, Reverse Flame


Boilers



 MODIFIED : From oil fuel to agricultural waste fuel :

packaged with external furnace –to cater for increased


amount of fuel to burn


 COMBINATION BOILER : water tube wall for the


external furnace

FIRE TUBE BOILER








ADVANTAGES



 Have greater reliability


 Low first cost – simple and rigid construction


 Steam can be quickly raise


 Compact - occupies minimum floor space



 Need of only unskilled labour


 Less draught required



 Excellent for engines operating with rapid changes in

load


 Larger volume of water, large steam space



 Failure in feed water supply does not cause damage to

boiler

FIRE TUBE BOILER





DISADVANTAGES






 Longer start up



 Slow in reaching operating pressure


 The explosion of the fire tube boilers become very


serious of its large water capacity


 Drum and tube plates exposed to furnace heat



 The transportation is very inconvenience

WATER TUBE BOILER










Water Tube – water inside the

tubes and the hot gases


surrounds the tubes
















Classification of water tube boiler:




1)Type of tube : Straight tube boiler & Bent tube boiler



2) Circulation : Natural circulation & Force Circulation

WATER TUBE BOILER







 In these boilers water is inside the
tubes and hot gases are outside the
tubes.


 They consists of drums and tubes.
They may contain any number of
drums.

 Feed water enters the boiler to one
drum. This water circulates through

the tubes connected external to
drums.


 Hot gases which surrounds these
tubes will convert the water in tubes

into steam.

 This steam is passed through tubes
and collected at the top of the drum
since it is of light weight. So the

drums store steam and water.

 Examples: Babcock and Wilcox,
Vertical, Cross Drum and Bent Tube

or Stirling Boiler

WATER TUBE BOILER





Bent tube water tube boiler







Type ‘A’- has two small mud
drums with a larger steam drum.















b) Type ‘D’- the steam drum is placed directly
above the mud drum. The furnace and boiler are
placed off to one side.













c) Type ‘O’- two drums, with
the burner position in the center of the
boiler.

BABCOCK & WILCOX BOILER Straight tube water tube boiler




























CROSS DRUM BOILER WATER TUBE BOILER (VERTICAL)

WATER TUBE BOILER






ADVANTAGES



 Generation of steam is much quicker and reaching in steam

temperature in short time


 Large evaporation capacity


 High steam pressure range


 The boiler can easily transported



 Capable of burning any available fuel with high efficiency


 Smaller diameters provide greater heating surfaces, better

heat transmission and rapid response to steam demands


 Thicker and smaller diameter tubes allow higher pressures


 System of tubes provide better natural circulation



 Failure of a small tube will not result in a disastrous

explosion

WATER TUBE BOILER









DISADVANTAGES


 Require careful attention



 Maintenance cost are higher


 Difficulty in cleaning water side


 Strict water treatment required



 More auxiliaries; complex control

COMPARISON BETWEEN FIRE



TUBE & WATER TUBE BOILER





No Particular Fire tube boiler Water tube boiler

1. Position of water and hot Hot gases inside the tube and Water inside the tube and
gases water outside the tube hot gases outside the tubes


2. Operating pressure Limited to 16bar High pressure to 100bar

3. Rate of steam production Lower Higher

4. Suitability Industrial purposes Large power plant


5. Risk on bursting Less risk on explosion due to More risk on bursting to
lower pressure high pressure


6. Floor area More floor area Less floor area

7. Construction Difficult Simple


8. Transportation Difficult Simple

9. Shell diameter Large for same power Small for same power

10. Treatment of water Not necessary Necessary

11. Accessibility of various Not so easily accessibility More accessibility

part

12. Requirement of steam Less skill More skill

OTHER TYPES OF BOILERS








COMBINATION BOILERS







 Compact fire tube boiler but


with a water wall at the rear




















 With a larger external


 furnace, with



 water wall tubes

CHOICE OF BOILER

STEAM DRUM

Steam Drum + BFW pipe

STEAM DRUM








 Risers discharge into the drum a mixture of


water, steam, foam and sludge




 Drum use to separate steam from the mixture



 Any moisture carried with steam contains


dissolved salts




 No vapour bubble should flow along with


saturated water from the drum – will reduce



density difference

STEAM DRUM INTERNAL

STEAM DRUM INTERNAL






Baffle


- primary separator


- change or reverse steam flow direction



Demister (screen)


- Made of wire mesh act as secondary separator


- Wire attract and intercept the fine droplet – just like filter



Cyclone


- Separation two phase mixture


- Steam goes to zigzag path, called dryer to remove the


last traces of moisture.

BOILER MOUNTING

BOILER MOUNTING












 2 or more safety valves


 2 water gauges



 A steam pressure gauge


 Main steam stop valve


 A blow down valve



 2 feed pumps, same output pressure


 Feed check valve


 A low water alarm



 A low water fuel cut-out


 (liquid and gaseous fuel)

BOILER MOUNTING

BOILER MOUNTING

BOILER MOUNTING

BOILER MOUNTING

BOILER MOUNTING

BOILER BREAKDOWN









 LOW WATER CONDITIONS
 BULGING OF FURNACES

 CRACKS ON TUBE ENDS AND TUBE PLATES

 SCALES

 PITTINGS

 EROSION

 CRACKS ON REINFORCEMENT

 STEAM STARVATION
 BAFFLES IN STEAM DRUM

 FURNACE EXPLOSIONS

 OTHERS

 BOILER REPAIRS

LOW WATER CONDITIONS







 Low water conditions may be caused by :


- Faulty water level controls and alarms

- Failure of Boiler or any of its parts


- Human error with boiler on manual control

- Excessive blowdown


- Pump, non-return valve or water supply failure





 Early warning systems include the installation of

the fusible plugs. This can be found on Cochrane

firetube boilers and Fraser watertube boilers.

Fusible plugs are exempted on boilers fired with

liquid or gas fuels, where the fuel feeding can be

stopped immediately

BULGING OF FURNACES






Firetube boiler furnaces have often become

“pregnant”due to





 Flame impingement on to the furnace wall
on the fireside, caused by :

 Burner misalingnment,


 Wear and tear of burner assembly

 Improper atomization of heavy fuel oil, due

to inadequate preheating of the fuel





 Low water




 Overheating due to scale deposition on the
water side :


 There has been cases of scales initiated by
fist-sized bolts left behind during

maintenance

CRACKS ON TUBE ENDS




AND TUBE PLATES








 Cracks on tube plates and tube ends
are peculiar to Fire Tube Boilers, as

tube plates and tube ends are exposed

to hot flue gases.

 Cracks on tube plates and

reinforcement structures occur possibly
due to gross overheating, cyclic loads,

overpressure or maybe even

manufacturing defects.

 Cracks are normally indicated by leaks.

A hydrostatic test and an NDT such as
MPI or at least DPT is required to

confirm existence of cracks.

SCALES








 Scales deposit in boilers when the
feedwater is not adequately treated. If

blowdown is not regularly done, the

deposits have time to harden and stick to
heating surfaces and reduce effective

heat transfer from the flue gas to the

water. This results in reduced efficiency,
and overheating of the metal.


 If the scales choke a boiler tube, the tube
can no longer have adequate cooling, and

the tube will fail, sooner or later.

 Scales on the furnace often lead to

“pregnant”boilers, leading to expensive

repairs