In accordance with the Indian boiler Regulations the following mounting should be fitted to boiler:
Two safety valves, two water level indicators, a pressure gauge, a steam stop valve, a feed check valve, a blow off cock, an attachment for inspector’s test gauge, a man hole and mud holes or sight holes. Boiler of Lancashire and Cornish type should be fitted with a high pressure and low water safety valve. All land boilers should have a fusible plug in each furnace.
Equipment # 1. Safety Valves:
Safety valves are placed on top of the boiler and fitted on mountings or blocks directly on the boiler. It is generally seen that in fire-tube boilers it is placed on the top of the shell above the furnace or the fire-box.
The function of the safety valve is to blow off the steam when the pressure of the steam in the boiler exceeds the working pressure. In case of high steam and low water safety valve it blows off steam when the pressure of the steam exceeds the working pressure as well as when the water level in the boiler falls too low to an unsafe extent. In each case of the escape of steam.
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There are four types of safety valves:
(a) Dead weight safety valve.
(b) Spring loaded safety valve.
(c) Lever loaded safety valve.
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(d) High steam and low water safety valve.
The above-mentioned classification of the first three safety valves is based on the method of loading the valve, while in the fourth case it is based on its mode of operation.
In this safety valve the weight, in the form of cylindrical cast iron discs, is placed directly in the valve. The valve is made of gun-metal and it rests on a gun metal seat secured on the top of a vertical cast steel pipe bolted to the mounting block, which is riveted to the top of the shell.
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The valve is secured by means of a nut to a large cast iron casting which hangs freely over the vertical pipe like a cover and the weight of the valve itself. When this load is greater than the force due to steam pressure acting on the valve the steam will not escape; when it is equal, it is just on the point of blowing; when it is less the valve is lifted up from its seat and the steam will escape to the enclosed discharge pipe which is connected to the discharge casing from where it is directed outside the boiler house.
The lift of the valve is controlled by the set screws, the heads of which project into the slots formed into the interior of the casing. The discharge casing is drained by a pipe connected at its bottom. The centre of gravity of the dead weight is well below the point of suspension and this ensures that the weight hangs vertically.
A dead weight safety valve is suitable for stationary boilers and not for marine boilers. It has got an advantage that it cannot be readily tampered with because any change in weight made by any unauthorized person is easily noticed. The objection to a dead weight safety valve in marine practice is the great weight which has to be carried and reduction in effective weight when the ship is on the slope of the wave.
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In this safety valve, the valve is loaded with a spring instead of a dead weight. The springs used are helical in form with round or square wires. The spring is made of steel. The load due to steam pressure acts along the axis of the spring and thus it may be subjected to an axial tensile or compressive load, and the material of the spring wire will be subjected to torsional shear stress.
Generally the spring is subjected to an axial compressive load but in Ramsbottom safety valve the spring is subjected to a tensile load. Fig. 5-2 illustrates a Ramsbottom safety valve, which is mostly used on the Locomotives. It consists of a cast iron casing, which has two valve chests.
There are two valves of the same size and they have their seats in the upper ends of the hollow valve chests. The two chests are united by a bridge and a base. The base is flanged which is bolted to a mounting block placed on the top of the boiler over the fire-box. The valves are held down on their seats by a helical spring and a lever.
The lever has two pivots- one is pinned to the lever while the other is forged on the lever. The pivots rest on the centres of the valves as shown in fig. 5-2. The lever is pulled downward by the spring and one-half of this force acts on each of the valves because the upper end of the spring is fixed to the lever midway between the valves.
The lower end of the spring is fixed to a shackle which is secured to the bridge by a nut. Either lifting or bearing down of the lever relieves the valves. The valves are usually made of gun-metal.
The spring loaded safety valves possess the following advantages over the dead weight and the lever loaded safety valves:
(a) Elimination of heavy weight
(b) Easy examination and maintenance
(c) In case of locomotive and marine boilers the load on the valve is not affected due to jerks, pitching and rolling. Hence steam will not escape unnecessarily and waste the pure feed water which is very valuable at least in ships.
A lever loaded safety valve consists of a cast iron casing bolted or riveted to the top of the boiler shell. It has a gun-metal valve and a valve seat. The valve seat is secured inside the casing. The valve is loaded by a wrought iron or mild steel lever loaded by a weight at one end and having a fulcrum at the other end.
The thrust is applied to the valve through a rod placed very near to the fulcrum. A lateral guide to the lever is provided by a forked piece fixed in the casing opposite to the fulcrum. Sometimes the fork is bridged over at the top for preventing the blowing away of the valve in case the lever is broken, though this is not a good practice. The weight is firmly secured to the lever by a pin and is locked so that an unauthorized person cannot tamper with it.
In this safety valve weight acts at the longer arm of the lever so a smaller weight can give a larger thrust at the valve.
High Steam and Low Water Safety Valve:
Lancashire and Cornish type of boilers are generally provided with this type of valve. It has two valves combined in one. One of it is lever loaded safety valve while the second one operates by blowing of steam with loud noise when water level in the boiler becomes too low.
Fig. 5-4 illustrates the principle of operation of low water safety valve. The lever safety valve which is not shown in fig. 5-4 is like the one which is described before; but in the centre of the valve, which is called the main valve, a seat is formed for a hemispherical valve for low water operation.
The valve is loaded directly by the dead weight which is attached to the valve by a long rod. Inside the boiler there is a lever which is having fulcrum and which has a float connected at the other. The balance weight balances the weight of the float when it is fully immersed in water.
When float end of the lever comes down as the float is heavier than the balance weight, the lever has a projection which comes in contact with a collar fixed on the rod. When it turns due to low water, the hemispherical valve is lifted and the steam escapes through it with a loud noise warning the attendant.
When the hemispherical valve is closed, the main valve acts as an ordinary lever safety valve but the load on the valve is partly due to lever weight and partly due to weight attached on the hemispherical valve.
Equipment # 2. Water Level Indicators:
Water level indicators are two in number. They are mostly placed in front of the boiler from where they are easily visible to the attendant.
The function of the water level indicator is to show the level of water in the boiler. To carry out the function its upper end opens in the steam space while the lower end opens in the water space.
Fig. 5-5 illustrates a commonly used water level indicator. This gauge provides an arrangement for automatically closing off the water and steam supply to the gauge glass if the latter gets broken. It consists of a strong glass tube. The ends of the tube pass through stuffing boxes formed in the hollow castings.
These castings are flanged and bolted to the boiler. It has three cocks two of them control the passages between the boiler and the glass tube, while the third one, the drain cock, works as a blow-through cock. In the working boiler, for the proper functioning of the indicator, the blow through the cock remains closed and the other two are open and these positions are obtained by keeping their handles vertical as shown in fig. 5-5.
The upper and lower castings are connected by a second metal tube. Two balls are provided and their positions are shown in normal working condition in the figure. When the water gauge glass tube is broken, the water rushes out from the bottom casting and the steam rushes out from the top column.
The flow of water carries the bottom ball into the dotted position thus shutting off water. The rush of steam and water from the metal tube will then carry the top ball in the dotted position as shown in the figure, thereby stopping the steam. The operator can then easily shut the cocks and replace a new glass tube.
Water gauges are provided with guards made of tough glass which covers the front and the sides of the glass tube. This will protect the operator from the flying pieces of gauge glass when broken. Water gauge glasses are also provided with plugs at suitable places. They are meant for cleaning purposes.
Equipment # 3. Pressure Gauge:
This is used to measure pressure of steam inside the boiler. It is fitted in front of a boiler in such a position that the operator can conveniently read it. It is connected to steam space by a siphon pipe. Its function is to indicate the pressure of steam in a boiler. The most commonly used pressure gauge is the Bourdon pressure gauge.
It consists of a curved spring tube of oval section. It is solid drawn out tube of a special quality bronze. One end of the tube is plugged while the other end is secured to a hollow block threaded at the bottom where it is connected to the siphon pipe. The 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 section circular. This action moves the free end of the tube outward. The motion of the end is proportional to the difference between the internal and external pressure on the tube. This motion is magnified by a mechanism which consists of a connecting link and a lever carrying a toothed sector pivoted by a pin.
The sector gears with a small pinion mounted on the spindle which carries the pointer. The pointer indicates, on the dial, the pressure in bar. Since the pointer motion is obtained by the difference of pressure in bar. Since the pointer motion is obtained by the difference of pressure acting on the tube and the external pressure being atmospheric, the pointer indicates pressure above atmosphere.
The siphon is filled with water to prevent the contact of hot steam with Bourdon tube; otherwise due to overheating of the tube its accuracy is permanently affected.
Equipment # 4. Attachment for Inspector’s Test Gauge:
In front of a pressure gauge of a boiler an extension pipe is provided, the end of which is plugged. The pipe is provided with a cock. This is called an attachment for inspector’s test gauge.
Its function is to provide an easy fitting for the boiler inspector’s standard pressure gauge; thereby the accuracy of the boiler gauge can be tested while the boiler is working.
When the inspector’s pressure gauge is to be fitted the cock is closed to disconnect the plug from steam pressure. The plug is opened and the gauge is fitted. When the cock is opened, the inspector’s pressure gauge will give the correct reading which can be compared with that of the boiler pressure gauge. Then by closing the cock the inspector’s pressure gauge can be removed and the plug can be placed in position.
Equipment # 5. Steam Stop Valve:
This valve is placed on the highest part of the steam space of a boiler and is connected to a steam pipe which supplies the steam to outside. The function of a stop valve is to stop or to allow the flow of steam from the boiler to the steam pipe. It may be operated automatically or by hand.
A hand operated stop valve consists of a valve chest made of cast iron. The chest has two flanges at right angles. One flange is bolted to the mounting block riveted on the boiler and the second flange is bolted to the steam pipe.
A gun-metal valve seat is secured in the chest and a valve of the steel metal rests on it. The valve is connected to the spindle by a nut, the lower end of which comes in contact with a collar on the lower end of the spindle. By this construction the spindle can rotate free in the valve but at the same time it carries the valve with it when raised or lowered.
The spindle has a hand-wheel at the top end by which it is rotated. The spindle passes out of a gland and stuffing box formed in the cover of the body. The upper portion of the spindle has screw threads which pass through a nut in a yoke. The yoke is carried by two pillars fixed to the body.
Sometimes a cast iron pipe called anti-priming pipe is fixed inside the boiler and is connected to the mounting block on which the steam stop valve is fixed. Its function is to prevent the water passing with the steam from the boiler, through steam stop valve, due to priming in the boiler.
The pipe is perforated on the upper side and steam is made to turn, while passing through the pipe, due to which water particles are thrown out of the steam. The water particles collect at the bottom of the pipe from which they drain in the boiler through holes provided at the ends of the pipe.
Equipment # 6. Feed Check Valve:
It is fitted to the boiler slightly below the working level of the water in the boiler. It is connected to the boiler end of the delivery pipe from the feed water pump.
It consists of two valves combined in one valve. One is the feed valve and the other is the check valve. The feed valve is operated by hand and its function is to allow or to stop the supply of water to the boiler. The check valve is automatic in operation and its function is to prevent the water escaping from the boiler in case of a failure of a feed pump.
A commonly used feed check valve is shown in fig. 5-8. It consists of a check valve whose lift is controlled by the lower end of a spindle. The valve rests on its seat and is operated by the difference of pressure of water acting on its top and bottom side. The valve can be kept in a closed position by pressing it down by the spindle which can be lowered or raised by a hand-wheel.
So, in this type of valve the check valve also carries out the function of a feed valve with the help of the spindle. This type of valve has one disadvantage that the check valve cannot be inspected or cleaned while the boiler is working. Therefore, another type of feed check valve is also employed where a separate feed valve is placed over and above the check valve and the feed valve is connected to the spindle.
The feed valve is placed first near the boiler. Therefore, when it is closed, the boiler water pressure cannot act on the check valve and hence when the feed valve is closed the check valve can be removed for inspection or cleaning.
As shown in fig. 5-8, on the check valve the boiler pressure acts from the top side while the feed water pressure acts from the bottom. In normal working the latter pressure is more than the boiler pressure and the valve remains open but in case the feed water pressure becomes less than the boiler pressure due to greater pressure on the top of the valve, the valve closes automatically and thus reverse flow of water from boiler to feed pipe is prevented.
A cover is bolted to the body of the feed check valve on the top side. The cover has stuffing box formed on its top side. The spindle passes through the stuffing box which prevents the leakage of water. The stuffing box gland can be tightened by nuts in case leakage takes place and the leakage can be stopped. At the lower end of the spindle square screw threads, which work in the nut, are provided. The hand-wheel is fixed on the top end of the spindle.
Equipment # 7. Blow-Off Cock:
It is fitted at the lowest part of a boiler. It can be directly fitted to the boiler shell or to a short pipe which is fitted to the boiler.
Its function is to remove periodically the sediments collected at the bottom of the boiler while the boiler is working and to empty the boiler while it is to be cleaned or inspected. The sediment-removal is carried out by discharging a portion of the water by opening the blow off cock. The water, which is under pressure, rushes out carrying the sediments with it when the blow off cock is opened.
A commonly used blow off cock consists of a conical, hollow, gun-metal plug type valve which fits accurately into a corresponding hole in the casing. The plug valve has a hole which when brought in line with the hole in the casing, by turning the plug, water will flow out of the boiler. The flow of water can be stopped by turning the plug such that its solid part comes in line with the hole of the casing.
The plug valve shank projects out of the stuffing box gland. The stuffing box is provided to prevent the leakage of water at the shank. The gland can be tightened by the nuts provided. A set screw is provided under the plug to force ‘it off its seat if jammed.
A guard is provided over the spindle. This prevents the handle from being removed unless the cock is connected to the boiler by one of the flanges. The blow off cock is connected to the boiler by one flange and the other flange is connected to the pipe carrying the blow off water out of the boiler house.
Equipment # 8. Manhole:
It is provided on the boiler shell at a convenient position so that a man can enter through it inside the boiler. Its function is to provide an opening from which a man can enter in a boiler for cleaning and inspection purposes.
The manholes are mostly oval shaped. They are generally 400 mm x 300 mm in size and the opening is closed by a cover, called manhole door, fitted from inside of the manhole, the fitting of the door from inside is possible due its oval shape. It is secured in position by bolts and bridge bars generally two in number.
As the manhole is cut in the shell plate, the latter gets weaker and so it is strengthened by a plate riveted round the manhole on the outer side of the boiler shell.
Equipment # 9. Fusible Plug:
It is fitted in the crown plate of a furnace or a fire-box. Its function is to extinguish the fire in the furnace of a boiler when the water level in the boiler falls to an unsafe extent thereby preventing an explosion which may take a place due to overheating of the furnace plate.
There are many designs of the fusible plug. One which is commonly used in Lancashire and other large boilers consists of a gun-metal body screwed onto the top of the furnace of the boiler. It has a hexagonal flange for fixing it in position by the help of a spanner. The body has a gun-metal plug screwed into it which has a second hollow gun metal plug place in the corresponding hole in the first plug.
They are kept together by pouring fusible metal between them. The fusible metal is protected from fire by the flange on the inner plug. When the boiler has normal working level of water the fusible plug is covered by water. So though combustion takes place in the furnace, the temperature of the plug is not increased much.
When the water level falls too low in the boiler so that the fusible plug is uncovered out of water the heat of the furnace heats the plug and the fusible metal in it melts, thereby the inner plug falls down creating a hole in the plug through which water and steam rush in the furnace and the fire gets extinguished. Before starting the boiler again a new fusible plug should be fixed in place of the used one.