Steam Generators

3 Water Tube Boiler

A water tube boiler is such kind of boiler where the water is heated inside tubes and the hot gasses surround them.

This is the basic definition of water tube boiler. Actually this boiler is just opposite of fire tube boiler where hot gasses are passed through tubes which are surrounded by water.

Types of Water Tube Boiler

There are many types of water tube boilers, such as

1) Horizontal Straight Tube Boiler.

2) Bent Tube Boiler.

3) Cyclone Fired Boiler.

Horizontal Straight Tube Boiler again can be sub - divided into two different types,

i) Longitudinal Drum Water Tube Boiler.

ii) Cross Drum Water Tube Boiler.

Bent Tube Boiler also can be sub divided into four different types,

i) Two Drum Bent Tube Boiler.

ii) Three Drum Bent Tube Boiler.

iii) Low Head Three Drum Bent Tube Boiler.

iv) Four Drum Bent Tube Boiler.

Advantages of Water Tube Boiler

There are many advantages of water tube boiler due to which these types of boiler are essentially used in large thermal power plant.

1) Larger heating surface can be achieved by using more numbers of water tubes.

2) Due to convectional flow, movement of water is much faster than that of fire tube boiler, hence rate of heat transfer is high which results into higher efficiency.

3) Very high pressure in order of 140 kg/cm2 can be obtained smoothly.

Disadvantages of Water Tube Boiler

1) The main disadvantage of water tube boiler is that it is not compact in construction.

2) Its cost is not cheap.

3) Size is a difficulty for transportation and construction.

Boiler mountings are the machine components that are mounted
over the body of the boiler itself for the safety of the boiler and for
complete control of the process of steam generation.
Various boiler mountings are as under:
1) Pressure gauge
2) Water Level Indicator
3) Fusible plug
4)Safety Valve
i) Lever Safety Valve
ii) Spring Loaded safety Valve
5) Steam stop valve
6) Feed check valve
7) Blow off cock

 

3.Bourdon's pressure gauge.

Function: 

1.To record the steam pressure at which the steam is generated in the boiler.
2. A bourdon pressure gauge in its simplest form consists of elliptical elastic tube bent into an arc of a circle
3. This bent up tube is called as BOURDON’S tube.
4. One end of tube gauge is fixed and connected to the steam space in the boiler.
The other end is connected to a sector through a link.

 

2) Water Level Indicator

• The function of water level indicator is to indicate the level of water in the boiler constantly.
• It is also called water gauge.
• Normally two water level indicators are fitted at the front end of every boiler.

3) Fusible plug

• Function: To extinguish fire in the event of water level in the boiler shell falling below certain specified limit.
• It protects fire tubes from burning when the level of the water in the water shell falls abnormally low and the fire tube or
crown plate which is normally submerged in the water, gets exposed to steam space which may not be able to keep it
cool.
• It is installed below boiler's water level.
• When the water level in the shell falls below the top of the plug, the steam cannot keep it cool and the fusible metal
melts due to over heating. Thus the copper plug drops down and is held within the gunmetal body by the ribs. Thus the steam space gets
communicated to the firebox and extinguishes the fire. Thus damage to fire box which could burn up is avoided.
• By removing the gun metal plug and copper plug the fusible plug can be put in position again by interposing the fusible
metal usually lead or a metal alloy.

 4)Safety Valve

Function : The function of safety valve is to release the excess steam when the pressure of steam inside the boiler exceeds
the rated pressure.
There are 4 types of safety valves:
i) Lever Safety Valve
The disadvantage of this valve is that it admits of being tempered with, and the effect of
a small addition to the weight is magnified considerably in its action on the value.
ii) Spring Loaded safety Valve
• For locomotives and marine engines both the lever and dead weight types are unsuitable for obvious reasons,
and the valve must be spring loaded, as such valve is unaffected by vibration or deviation from the vertical.
• Disadvantage :
One disadvantage of this valve is that the load on the valve increases as the valve lifts, so that pressure required 
just to lift the valve is less than that required to open it fully.
iii)Dead Weight Safety Valve
• It is mainly used for low pressures, low capacity, stationary boilers of the Cornish and Lancashire types.
• Merits:
1)Simplicity of design
2)Gives quite a satisfactory performance during operation.
3)It cannot be easily tempered from the pressure adjustment view.
• Demerits:
1)Unsuitable for use on any boiler where extensive vibration and movement are experienced( e.g. locomotive and marine work).
2)It is not suitable for high pressure boilers because a large amount of weight is required to balance the steam pressure.
iv) High steam and low water safety valve
• It serves the following purposes.
(i) The steam automatically escapes out when the level of water falls below a certain level.
(ii) It automatically discharges the excess steam when the pressure of the steam rises above a certain pressure.
Use : It is generally used on Lancashire or Cornish boiler.
It cannot used in mobile boilers.

 

5) Steam stop valve

• A valve is a device that regulates the flow of a fluid (gases, fluidized solids, slurries, or liquids) by opening,
closing, or partially obstructing various passageways.
• Function: to shut off or regulate the flow of steam from the boiler to the steam pipe or steam from the steam
pipe to the engine.
• When the hand wheel is turned, the spindle which is screwed through the nut is raised or lowered depending
upon the sense of rotation of wheel. The passage for flow of steam is set on opening of the valve.

6) Feed check valve
Function: The function of a feed check valve is to control the supply of water
to the boiler and to prevent the escaping of water from the boiler when the pump pressure is less or pump is stopped.
i) To allow the feed water to pass into the boiler.
ii) To prevent the back flow of water from the boiler in the event of the failure of the feed pump.
The feed check valve is fitted in the water space of the boiler slightly below the normal level of the water.

 

7) Blow off cock

• Function: To drain out the water from the boiler for internal cleaning, inspection, repair or other purposes.
• It may discharge a portion of water when the boiler is in operation to blow out mud, scale or sediments, periodically.
• It is fitted on the boiler shell directly or to a short branch pipe at the lowest part of the water space.

Manhole and mud box: 
Manhole provides opening for cleaning, inspection and maintenance
purpose. Mud box is a collection chamber (as shown in Babcock and Wilcox boiler) for collecting the Mud
.

 

Boiler Accessories:
(i) Air pre heater

(ii) Steam separator.

 

(i) Air Pre-heater:

 

The function of air preheater is to increase the temperature of air before it enters the furnace. It is generally placed after the economizer; so the flue gases pass through the economizer and then to the air pre-heater.
An air-preheater consists of plates or tubes with hot gases on one side and air on the other. If preheats the air to be supplied do the furnace. Preheated air accelerates the combustion and facilitates the burning of coal.


(ii) Steam Separator: The steam available from a boiler may be wet, dry; or superheated; but in many cases there will be loss of heat from it during its passage through the steam pipe from the boiler to the engine tending to produce wetness.
The use of wet steam in an engine or turbine is uneconomical besides involving some risk; hence it is usual to endeavor to separate any water that may be present from the steam before the latter enters the engine. The function of a steam separator is to remove the entrained water particles from the steam conveyed to the steam engine or turbine.