Introduction Watertube boiler construction Steam generation process Watertube boiler classification Raising steam

Boiler = a closed pressure vessel in which feed water is heated in order to produce steam

Watertube boiler Firetube boiler

Watertube boiler is a type of boiler in which water circulates in tubes heated externally by the fire.

A. Smoke uptake

B. Economisera heat exchanger that transfers heat from boiler flue gases to boiler feedwater

C. Steam outletsaturated steam from the steam drum to the superheater

D. Cyclonea device inside the drum that is used to prevent water and solids from passing over with the steam outlet

E. Stay tubefor superheater

F. Staysfor superheater tubes

G. Superheated steam outlet

H. Superheatera bank of tubes, in the exhaust gas duct after the boiler, used to heat the steam above the saturation temperature

I. Superheater headersdistribution and collecting boxes for the superheater tubes

J. Water Drum

K. Burner

L. Waterwall headerdistribution box for waterwall and downcomers

M. Footing

N. Waterwalltubes welded together to form a wall

O. Waterwall headerdistribution box for waterwall and downcomers

P. Back side waterwall

Q. Boiler hood

R. Waterwall headercollecting box for waterwall and risers

S. Riserthe water-steam emulsion rises in these tubes toward the steam drum

T. Downcomera tube through which water flows downward; these tubes are normally not heated, and the boiler water flows through them to supply the generating tubes

U. Steam Drumseparates the steam from the water

V. Economiser headerdistribution box for the economizer tubes

furnace or combustion chamber

air is supplied to the boiler furnace - to enable combustion of fuel to take place

burning of fuel (LNG, liquid fuels, etc.) to release its energy - heat is developed

the energy released by the burning fuel in the boiler furnace is stored (as temp. and pressure) in the steam produced

feed water circulates from the water drum to the steam drum and is heated in the process

some of the feed water passes through tubes surrounding the furnace, i.e. waterwall and floor tubes, where it is heated and returned to the steam drum

steam is further heated and 'dried‘ (i.e. all traces of water are converted into steam) in the superheater

temperature of superheated steam will be above that of steam in the drum

an 'attemperator', i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature

superheated steam then leaves the boiler for use in system

Where main propulsion is steam powered, one or more large watertube boilers will be fitted to produce steam at very high temperatures and pressures.

On a vessel with diesel propulsion, a smaller (usually firetube type) boiler may be fitted to provide steam for additional ship services.

uses small diameter tubes, has a small steam drum, enables the generation or production of steam at high temperatures and pressures

weight of the boiler is much less than an equivalent firetube boiler

steam raising and steam generation process is much quicker

design arrangements are flexible, efficiency is high

the design facilitates good natural circulation of feed water

these are some of the many reasons why the watertube boiler has replaced the firetube boiler as the major steam producer

CLASSIFICATION ACCORDING TO: TYPES

Application

Pressure

Number of drums

main boiler – auxiliary boiler (donkey boiler)

low-pressure b. , intermediate-pressure b. , high-pressure b.

single-drum b., two- drum b., three-drum b.

CLASSIFICATION ACCORDING TO: TYPES

Tube inclination

Arrangement of gas flow

horizontal - vertical

single-flow b., double-flow b.

the forced draught fan should be started and air passed through the furnace for several minutes to 'purge' it of any exhaust gas or oil vapours

the air slides (checks) at every register, except the 'lighting up' burner, should then be closed

the operating burner can now be lit and adjusted to provide a low firing rate with good combustion

the fuel oil pressure and forced draught pressure should be matched to ensure good combustion with a full steady flame

the superheater header drains may be closed once steam issues from them

the boiler must be brought slowly up to working pressure in order to ensure gradual expansion and to avoid overheating the superheater elements and damaging any material

the main and auxiliary steam lines should now be warmed through then the drains closed

the water level gauges should be blown through and checked for correct reading

when the steam pressure is about 300kN/m2 (3 bar) below the normal operating value the safety valves should be lifted and released using the easing gear

once at operating pressure, the boiler may be put on load and the superheater circulating valves closed

all other vents, drains and bypasses should then be closed

the water level in the boiler should be carefully checked and the automatic water regulating arrangements observed for correct operation