BOILER MOUNTINGS AND THEIR FUNCTIONS :

Such of those fittings and appliances which are essential for the functioning of a boiler are called 'mountings'. They are intended for the complete control of the process of steam generation. A few important mountings are :

Steam stop valve

Water level indicator

Pressure gauge

Fusible plug

Safety valve .

Stop Valve :

A stop valve is placed directly over a boiler and is connected to steam space at one end. The exit end of stop valve is connected to a steam pipe leading to a steam engine or turbine to any point of use.

It consists of a valve which is attached to a spindle. The spindle is connected to a hand wheel and passes through a stuffing box to prevent leakage of steam. Using a hand wheel the spindle is raised or lowered thereby seating or opening the valve. If steam is to be admitted into stop valve from boiler valve is lifted of its seat and then closed by lowering the spindle.

Water Level Indicator:

The Hopkinson's Water Level Indicator indicates the level of water surface inside the boiler. It is fitted to the front end of the boiler. It has two horizontal tubes (T, T), one in contact with steam space and the other with water in the boiler as shown. When the cocks (C, C) are in vertical position they are in operation and the water level in the glass tube (G) corresponds with water level in the boiler. When water goes on evaporating into steam, its level decreases continuously. For the proper working of boiler, the water should always be kept at safe level. If level of water falls below the safe level and production of steam continues without addition of feed water, great damage like crack and leak can occur. This can result in the stoppage of steam generation and bursting of parts uncovered by water. In such case Water Level Indicator or Water Gauge as it is sometimes called works as a safety device also as follows.

Under normal working condition, cocks being vertical, the balls (B.B) remain in the positions shown, closing passage to metal tube (M) Incase, the glass tube breaks due to the above reason, the steam rushes through horizontal tubes pushing the balls to the positions shown dotted, thus closing the passage to glass tube. Then steam and water will not escape out. They simply enter the hollow metal tube. Broken tube can be replaced. The drain cock (DC) serves to blow out water periodically to avoid accumulation of sediments.

Pressure Gauge :

A pressure gauge is fitted to the boiler to measure the steam pressure. It simply shows the pressure on its dial. Bourdon's pressure gauge is shown in figure.

An important element of the gauge is an elliptical spring tube (2) (Its section is an ellipse). It is made of phosphor bronze and is called Bourdon's tube. Gauge is fitted in combination with a U-tube syphon which communicates with steam space in the boiler. Steam that enters the U-tube condenses into water and fills the spring tube. As pressure is applied by steam, the water flows into the tube and tends to assume a circular shape. In doing so it pulls the link attached to the end of tube. This in turn moves a toothed sector. A pinion mating with toothed sector rotates thus moving the pointer over a graduated scale on the front face of the dial. A red mark on the gauge shows the danger mark. Pressure should not exceed this limit.

Safety Valve :

Function of a satety valve is to permit the steam in the boiler to escape out when its pressure exceeds designed safe limit. It is located above the steam space. Escaping steam produces whistling sound thus raising alarm to the boiler operator. There are four common types of safety valves, namely 1. Lever safety valve. 2. Spring Loaded safety valve. 3. Dead weight safety valve 4. High steam and low water safety valve.

A lever safety valve contains a valve 'V' made of bronze. It is seated accurately on a valve seat (not shown in sketch) which is screwed to the body 'B'. A lever ‘L’ having a fulcrum 'F' has an adjustable weight 'W' as shown. The downward thrust of this lever-load is transmitted to the valve through the strut `S' and valve is held in position. Steam entering the valve body exerts upward pressure on the valve. When steam pressure and downward load are equal, lever remains horizontal and valve remains seated on valve seat. Once the steam pressure exceeds rated value, the valve is lifted and radial clearance is formed between valve and valve seat. Steam escapes through it with a whistling sound. As soon as pressure falls and reaches designed value, the valve drops on to proper seating. This type of safety valves is extensively used on stationary boilers.

Fusible Plug :

It is also a safety device. When level of water inside the boiler falls below the specified limit, the shell becomes overheated. Fusible plug puts off the fire in such eventuality.

It consists of a hollow gunmetal plug 'P' (Figure). A second hollow plug 'Q' is screwed to 'P’. There is also a third plug 'R' separated from 'P' by a ring of fusible metal M as shown. The plug is fitted to the crown of the fire box. It is rather screwed into the crown, C of fire box or over the combustion chamber at the lowest permissible water level.

The fusible metal is made of tin or lead which has a low fusing point. Plug R is usually made of copper.

Under normal working conditions, the fusible plug is covered with water and temperature of fusible metal is below its melting point. When water level falls below the specified level, the fusible plug is uncovered from water and is exposed to hot steam. Plug gets overheated and fusible metal melts. The plug 'R' falls down, then a stream of water-steam mixture rushes into the furnace and extinguishes the fire. Thus the danger of explosion of the shell due to overheating is avoided. A new plug is used afterwards.