Boiler Mountings: Types, Functions and working

Table of content:

1. Water level indicator

2. Pressure gauge

3. Steam stop valve

4. Feed check valve

5. Blow-off valve

6. Fusible plug

7. Safety valve

Boiler Mountings:

• These are different fittings and devices which are necessary for the operation and safety of a boiler. These devices usually mounted over the boiler shell.

1. Water lever Indicator:

Water level indicator

Function: The function of a water level indicator is to indicate the level of water in the boiler shell constantly.

Construction: Fig. shows the details of a water level indicator. It is fitted on all types of boiler. It is fitted at the front end of the boiler so that it is visible to the operator.

Working: When the cocks D and E are opened, steam rushes from the upper passage in B and water rushes from the lower passage in C to the glass lube F. Both of them adjust their levels. This will show the level of water in the boiler. In hollow gun-metal column G, the water also stands at the same level.

In case, glass tube F gets broken, the steam will rush from the upper passage and the water from the lower passage because of the pressure difference between boiler pressure and atmospheric pressure. This water carries the hollow gun-metal ball J from its normal position to the position shown by a dotted circle. This will close the passage of water to the glass tube. 

Now the water rises in the hollow gun-metal column G. This water aided by the steam rushing through the upper passage will carry the ball H from its seat and placed it to the new position shown by a dotted circle. Hence the supplies of both water and steam to the glass tube fare closed. The glass tube F can be replaced by closing two cocks D and E.

The water level indicator used for high-pressure boiler should be provided with guards made of thick toughened glass which cover the front and sides of the glass tube. The boiler, the attendant is then protected from the flying fragments of glass when glass tube bursts.

2. Pressure gauge:

Pressure gauge

Function: The function of the pressure gauge is to measure the pressure exerted inside the vessel.

Working: The movement of the free end of the elliptical tube is proportional to the difference between external and internal pressure on the tube. Since the outside pressure on the tube is atmospheric, the movement of the free end is a measure of the boiler pressure above atmospheric i.e. gauge pressure.

The pressure gauge is connected with the boiler's steam space through a U-tube siphon. The U-shaped siphon pipe contains water which fills the Bourdon tube. The pressure of the steam acting' through the water on the inside of the tube tries to make the tube circular.

One end is fixed, the free end moves. The movement at the free end is proportional to the difference in pressure as shown above. Because of pinion and gear arrangement, the slight movement of the Bourdon tube is magnified considerably and the point gives a maximum deflection which can be read on the scale. 

The water in the V-tube syphon prevents the steam from coming in contact with the spring tube. This will reduce overheating and thermal stresses in the spring tube. A three-way cock is provided for connecting the boiler pressure gauge to the steam space.

3. Steam Stop Valve:

Steam stop valve

Function: The function of the steam stop valve is to regulate the flow of steam from one steam pipe to the other or from the boiler to the steam pipe.

What is a Junction Valve? - The steam stop valve, when directly mounted on, the steam space of the boiler shell and connected to the steam pipeline which supplies steam to the prime mover. It is called a junction valve.

Working: When the hand-wheel is turned in the anticlockwise direction, the spindle is raised up. This will raise the valve from its seat. Thus, a passage for the steam from the clearance between the valve and valve seat is formed. In order to lower the valve, the handwheel is rotated in a clockwise direction. 

This rotation will close the passage for steam. This can be regulated by adjusting the position of the valve based on the requirements. Under normal working, condition the valve is open and steam flows from the boiler to the steam pipe.

4. Feed Check Valve:

Feed check valve

Function: It is used 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 the pump is stopped.

Construction: This is a non- return valve placed as near to the boiler as possible. It is shown in Figure. The lift of the valve is controlled by an extension of the spindle of the screw-down valve above it. It is the feed valve. It is the flange. It is bolted to the front end of the boiler shell at a point from which an internal pipe leads the feed water and distribute the same to the boiler. It is the handwheel. This is used to control the positions of the valves. The construction above valve is similar to that of the steam stop valve.

Working: Under the normal working condition, the pressure on the feed pump side (connected to elbow) is more than the boiler side pressure. This pressure difference lifts the check valve. To permit the feed water to enter the boiler, the feed valve is lifted manually. 

Hence, the feed water may enter the boiler. In order to control the supply of feed water to the boiler, the position of the feed valve is controlled. In the event of failure of the feed pump, the pressure on the water sump side reduces. The check valve will be closed due to the higher steam pressure.

This will stop the backflow of water from the boiler to the water sump. The check valve is to be replaced if it does not give satisfactory results. The stuffing box is provided in the valve to stop the leakage at the spindle.

5. Blow-off cock Valve:

Blow off cock valve

Functions:

(l) To discharge a portion of water when the boiler is in operation to blowout mud, scale or sediments periodically.

(2) To empty the boiler, when necessary for cleaning, inspection and repair. It is fitted on the boiler shell directly or to a short branch pipe at the lowest part of the water space. This pipe is known as blowdown pipe.

Working: In order to operate the valve, the rectangular slot is brought in line with the passage of the body. This is possible by rotating the plug with the help of a wheel. When the slot is placed in this position, the cock is opened and all the impurities, mud, sediments etc. start flowing out of the boiler and they are removed. When the slot is brought at right angles to the passage as shown in Fig. of the body, the clock is closed.

6. Fusible Plug:

Fusible plug

Function: Its function is to protect the boiler against damage due to overheating for low water level.

Where it is fitted? - It is fitted in the crown plate of the furnace or firebox at the appropriate place.

Working: In the normal working conditions of the boiler, the fusible plug is full, submerged underwater. This is so because, under normal conditions, the proper water level is maintained and can be checked by a water level indicator. Under the circumstances, the heat from the fusible plug is being conducted to water. This keeps the fusible metal at an almost constant temperature, much below its melting point. 

When the water level falls below the fusible plug, the plug gets uncovered from water. The upper portion of the plug gets exposed to the steam space. The steam cannot keep the plug cool. This will overheat the fusible metal. The plug falls down along with the fusible metal making a hole. The steam and water, being under pressure immediately reach to the firebox and extinguish the fire.

The attendant is cautioned against overheating and burning of the fire tubes etc. Before starting the boiler again, the operator fixes a new fuse. The fusible plug should be renewed generally after a period of about two years as they are liable to become defective over a long period of use. One plug is provided for each furnace.

7. Safety Valves:

Function: It is a device, attached to the steam space of the boiler shell, which opens automatically to discharge some steam and to prevent the steam pressure inside the boiler to exceed.

There are four types of safety valves generally fitted on a boiler:

(a) Dead-Weight Safety Valve

(b) Lever-Weight Safety Valve

(c) Spring Loaded Safety Valve

(d) High Steam and low-water safety Valve

The spring-loaded safety valve is used with locomotive and marine boilers. The other type of valves is used with stationary boilers.

(a) Dead-weight Safety Valve:

Deadweight safety valve

This valve is used only with the stationary type of boilers. It is the most elementary type of safety valve.

Working: The valve will stick to its seat, as long as the pressure exerted on the valve is less than the pre-determined value. When the pressure exerted by the steam on the valve becomes more than the predetermined value, the valve is lifted from its seat. 

The pre-determined value of pressure is adjusted by the force acting from above the weights etc. Because of the lifting of the valve, the steam will go to the atmosphere through the discharge pipe. 

The valve comes back to its seat when the upward steam pressure is less than the downward force exerted by the weights etc. This will help to maintain constant pressure inside the boiler shell.

Merits of dead weight safety valve:

l. Simple in design.

2. Give quiet and satisfactory operation.

3. From the pressure adjustment viewpoint it cannot be easily tempered.

Demerits:

l. Unsuitable for any boiler subjected to vibration and movement (i.e. Locomotive boiler).

2. Because of the requirement of a large amount of weight to balance steam pressure this valve is not suitable for use on the high-pressure boiler.

(b) Lever loaded safety valve:

Lever loaded safety valve

For larger size safety valve lever loaded safety valve is used. Weight is not put directly on the valve but provided at the end of the lever. In this type of valve, a pivot pin is put on fulcrum which has a lever. As shown in Fig. the valve is located with another pivot pin. At the far end of the lever, weight is suspended. When steam pressure exceeds the limiting pressure dead weight with a lever is raised and steam exits from boiler shell until boiler pressure and limiting pressure becomes the same.

The force acting by a steam on valve = P✕𐍀/4✕D*2

                                                            = a/b✕m✕g

where,

P = Max. working pr.

V = Diameter of valve sheet opening

a/b = leverage 

m = mass suspended at the end of lever

This type of safety valve is used in stationary boilers.

(c) Spring-loaded safety valve:

Spring-loaded safety valve

Deadweight safety valve and lever loaded safety valves are not used in marine and locomotive boiler because of vibrations. Fig. shows Rams bottom spring-loaded safety valve. Body of valve is of C.I and Y shaped. Valve sheets are located on both ends of the pipe. Spring ends are fitted on shackle provided in the valve body. The upper end of spring is fitted with lever pivot. The lever is forged, both valves are fitted with the lever as shown in the figure. Both valves remain fit on valve sheet due to the spring tension force.

When steam pressure exceeds the limiting pressure then upward force exceeds limiting pressure(force) of spring & valve lifts from valve sheet & excess steam come out until steam pr. (boiler pressure) becomes equal to spring pressure. Then valve fits on the valve sheet. Boiler attendant checks valve position & working by operating lever up and down manually. Links provided on both sides of spring keep the lever in position during breakage of spring.

(d) Combined low water level and high steam safety valve:

Combined low water level and high steam safety valve

It provides safety to the boiler in the following two conditions.

(I) When working pr. of steam boiler exceeds limiting pressure then the valve operates and excess steam rushes outside.

(2) When the water level in a boiler decreases then safety level, in this condition valve operates & steam comes out with a hissing noise, which makes attention to the operator.

As shown in Fig. one lever loaded safety valve is given in such a construction & another valve is provided in the main valve. When pressure exceeds then the main valve (high-pressure valve) lifts on the sheet & excess steam rushes outside.

When the water level goes down than a safe level in a boiler drum, then balancing disturbs between float weight and balancing weight & dead weight. Hence float weight goes towards the downside. In such condition, the lever end of balancing weight side goes up. The upward force is created due to projection. Hence low water raises from sheet & steam comes out from the drum with a hissing noise, which warns boiler attendants for low water level.

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