BOILER MOUNTINGS AND ACCESSORIES
For efficient operations and safety, boilers are equipped with:
Boiler mountings
Boiler accessories
The details of boiler mountings and boiler accessories are given in this lesson and next lesson, respectively.
29. BOILER MOUNTINGS:
Boiler mountings are the fitting and devices which are mounted on the boiler for the safety of the boiler and for complete control of the process of steam generation.
The mountings which must be fitted on the boilers according to the Indian boiler regulations are described below:-
(a) Safety valves
(b) Water level indicators/Water Gauge
(c) Pressure gauge
(d) Fusible plug
(e) Steam stop valve or Junction Valve
(f) Feed check valve
(g) Blow off cock
(h) Manhole and Handhole
(i)Steam Scrubbers/Anti-priming pipe
(j) Air vents and vacuum breakers
(k) Soot blowers
From the above mountings, the safety valve, water level indicator, and fusible plug take care of the safety of the boiler and the remaining care for complete control of steam generation process.
29.1.Safety Valve
Function: The function of a safety valve is to prevent excessive pressure from building up in a steam boiler. If the steam pressure in the boiler drum exceeds the working pressure, it permits the steam in the boiler to escape to the atmosphere until the safe working pressure in the boiler is reached again. The safety valve also warns the boiler attendant as the steam escape through the safety valve. .
Principle: The principle of operation of safety valve depends on the fact that a valve is pressed against a seat through some external force. When the steam force corresponding to boiler pressure acting under the valve exceeds the external force, the valve gets lifted off its seat allowing some of the steam to escape until the working pressure is restored again.
Number and Location: As per boiler regulations, each boiler must be fitted with at least two safety valves. Safety valves are located above the steam space in the boiler.
Types: There are four types of commonly used safety valves as given below:
(i) Dead weight safety valve, (ii) Spring loaded safety valve,
(iii) Lever safety valve and (iv) High steam and low water safety valve.
The choice of a safety valve for a particular boiler depends on the type of the boiler and its safe working pressure.
29.1.1. Dead Weight Safety Valve:
In this valve, the steam pressure in the upward direction is balanced by the downward force of the dead weights acting on the valve.
Construction:
Refer Fig 29.1. It consists of a vertical cast iron or steel pipe through which the steam pressure acts on the valve. At the lower end of vertical pipe, a bottom flange is provided to bolt safety valve to the seating block on the boiler shell. At the upper end of the vertical pipe, a gun metal valve seat is screwed to it. Under normal working condition, the gun metal valve rests on this valve seat. This valve is free to move in discharge pipe and secured to a large weight carrier which hangs freely over the vertical pipe. To balance upward force of steam pressure, the dead weights of cast iron rings are placed on the weight carrier. Further, the dead weights and weight carrier are covered by a cast iron cover plate just to avoid any mishandling of the weights. A discharge pipe is used to discharge the steam to the atmosphere after the valve is lifted.
(a) (b) |
Fig. 29.1. Dead Weight Safety Valve in (a) closed and (b) open position |
Operation:
- When steam pressure in side the boiler is below or equal to working pressure:
The upward force exerted by boiler steam on the valve is balanced by the downward force due to total weight of the valve, weight carrier, dead weights and cover plate. Under these normal conditions, the valve lies on its seat and the steam will not escape from the boiler as shown in Fig. 29.1 (a).
For equilibrium condition,
where W = total weight of the valve, weight carrier, dead weights and cover plate, N; d = diameter of the vertical pipe, m;
p = upward steam pressure on the valve, N/m2
- When steam pressure inside the boiler is above the working pressure:
The upward force exerted by boiler steam on the valve is over powered the downward force due to total weight of the valve, weight carrier, dead weights and cover plate. Under this condition, the valve along with weight carrier is lifted up from its seat and the steam escapes through the discharge pipe as shown in Fig. 29.1 (b).
Uses:
This type of valve is used only on stationary boilers such as Lancashire boilers, or other low capacity boilers.
Problem 29.1: Determine the total weight of the valve, weight carrier and cover plate of the safety valve in order that the valve may be in equilibrium position. Assume that the diameter of the vertical pipe of dead weight safety valve is 7 cm and the dead weight on the weight carrier is equal to 2900 N. The steam pressure in the boiler is 8 bar.
Solution:
Given: Diameter of the vertical pipe of dead weight safety valve, d = 7 cm;
Weight on valve = 2900 N
Steam pressure in the boiler, p = 8 bar = 8 x 105 N/m2
Formula: For the valve in equilibrium position
weight on valve + weight of carrier + weight of the valve + weight of cover = A x p
where, A is Cross sectional area of vertical pipe; p is steam pressure in the boiler
or weight of carrier + weight of the valve + weight of cover = (A x p) – weight on valve
Answer: Weight of carrier + weight of the valve + weight of cover = (A x p) – weight on valve
= (3078.76 − 2900) = 178.76 N
29.1.2. Spring Loaded Safety valve:
In this valve, the steam pressure in the upward direction is balanced by the downward force of the spring.
Construction:
Fig. 29.2 (a) shows a Ram’s Bottom spring loaded safety valve. It has a cast iron body which is provided with two channels for the flow of steam to the atmosphere when the steam pressure in the boiler exceeds the working pressure. These two channels are connected to a valve chest, the flange of which is bolted to the seating block on the boiler shell. Each channel has separate valve. The necessary thrust for keeping the valves on their valve seat against the steam pressure is provided by pivot of the lever which is held down by the helical spring. During steam generation operation, the working of the valve is checked by pulling the projected end of the lever.
Operation:
- When the steam pressure in the boiler is equal to the working pressure:
The upward force exerted by boiler steam on the valve is balanced by the downward force of spring. Under this condition, the valve lies on its seat tightly and the steam will not escape from the boiler as shown in Fig. 29.2 (a).
- When the steam pressure in the boiler is more than the working pressure:
The upward force exerted by boiler steam on the valve is overpowered the downward force of spring. Under this condition, the valve automatically opens and the steam escapes to the atmosphere(Fig. 29.2 (b)) till the pressure falls back to the working pressure.
Fig. 29.2. Ram’s Bottom spring loaded safety valve in (a) closed (b) open position.
Uses:
These valves are suitable for
both low and high pressure boilers
- stationary and non-stationary boilers
Advantage:
Elimination of heavy weight.
Easy maintenance and examination.
Not effected by jerks and vibrations.
29.1.3. Lever Safety Valve:
In this valve, the steam pressure in the upward direction is balanced by the downward force of the lever system.
Construction:
Figure 29.3 shows the arrangement of a lever safety valve. It has a cast iron valve body of diameter ‘d’, the flange of which is bolted to the seating block on the boiler shell. On the opposite end of valve body, it has a valve seat on which the valve is held tightly by downward force of a lever having fulcrum at one end and movable weight at the other end. To prevent side movement of the lever, a guide is provided between the lever and the valve body. This guide also limits the lift of the valve. According to required steam working pressure in boiler, the position of the movable weight on the lever is adjusted.
Operation:
When the steam pressure in the boiler is equal to or under the normal limit of working pressure: The upward force exerted by boiler steam on the valve is balanced by the downward force of lever system. Under this condition, the valve remains on its seat tightly and the steam will not escape from the boiler as shown in Fig. 29.3 (a).
When the steam pressure in the boiler exceeds above the normal limit of working pressure: The upward force exerted by boiler steam on the valve is overpowered the downward force of lever system. Under this condition, the valve automatically lifts off from its seat and escapes the steam to the atmosphere (Fig. 29.3 (b)) until the pressure falls back to the working pressure.
Fig. 29.3. Lever Safety Valve (a) closed (b) open position.
The position of movable weight ‘W3’ suspended on the lever for a given steam pressure can be calculated as described under:-
Let, W1, W2 and W3 are the weight of the valve through its center of gravity, weight of lever through its C.G. and weight of movable weight, respectively and L1, L2, L3 are their respective distances from fulcrum.
F is the upward force due to steam pressure ‘p, N/m2’ acting at the center of the valve at a distance L1 from the fulcrum
By taking the moment about the fulcrum i.e. clockwise moment of the weights is equal to anticlockwise moment of the steam pressure, we have
W1L1+ W2L2 +W3.L3 = F.L1
or W1L1+ W2L2 +W3.L3 = 
…………(29.1)
Calculate the position of movable weight ‘L3’ from above equation from other parameters of known values.
Problem 29.2: In the lever safety valve, the weight of valve is 10 N and has the distance of its center line from the fulcrum equal to 12 cm. The weight of lever is 60 N and the weight has its center of gravity 36 cm from the fulcrum.
Calculate the distance of movable weight of 450 N from the fulcrum if the valve is required to operate at a pressure of 6 bar and the valve diameter is 10 cm. Also, calculate the location of movable weight if the effect of valve and lever is neglected.
Solution:
Given: Weight on valve, W1= 10 N
Distance of its center line of valve from the fulcrum, L1 = 12 cm = 0.12 m
Weight of lever, W2 = 60 N
Distance of center of gravity of lever from the fulcrum, L2 = 36 cm = 0.36 m
Movable weight, W3 = 450 N
Valve diameter, d = 10 cm = 0.1 m
Steam pressure under valve, p = 6 bar
Determine the distance of movable weight from the fulcrum, L3
Formula: From equation (29.1), we have
W1.L1+ W2.L2 +W3.L3 =
or 
Answer: L3 = 
= 
= 1.2059 m
Determine the location of movable weight if the effect of valve and lever is neglected.
Formula: If the effect of valve and lever is neglected, then equation (29.1) is reduced to
W3L3 =
or 
Answer: L3 = 
= 
= 1.256 m
29.1.4. High Steam and Low Water Safety Valve:
This valve has a combined safety arrangement against
a) high steam pressure in the boiler and
b) low water level in the boiler.
A commonly used high steam low water safety valve consists of two valves, namely valve ‘U’ and valve ‘V’ as shown in Fig. 29.4 (a). Under normal condition, the valve ‘U’ rests upon its valve seat and the valve ‘V’ rests on valve ‘U’ which act as seat for valve ‘V’.
When valve act as safety against high pressure steam in the boiler:
Construction:
High steam and low water safety valve acts like a simple lever safety valve when it acts as safety against high steam pressure in the boiler. A lever ‘L1’,hinged at one end and loaded at the other end by a weight ‘W’, forces valve ‘U’ upon its seat through pivot ‘P’ as shown in Fig. 29.4 (a).
Operation:
When the steam pressure in the boiler is under normal limit of working pressure: The upward force exerted by boiler steam on the valve ‘U’ is balanced by the downward force of lever system through pivot ‘P’. Under this condition, the valve ‘U’, having valve ‘V’ seated on it, remains on its seat tightly and the steam will not escape from the boiler as shown in Fig. 29.4 (a).
When the steam pressure in the boiler exceeds above the normal limit of working pressure: The upward force exerted by boiler steam on the valve ‘U’ is overpowered the downward force of lever system. Under this condition, the valve ‘U’ having valve ‘V’ seated on it, lifts off automatically from its seat and escapes the steam through the passage between the valve ‘U’ and its valve seat to atmosphere as shown in Fig. 29.4 (b)until the pressure falls back to the normal working pressure.
Fig. 29.4. High Steam Low Water Safety Valve acts as safety against high pressure steam
When valve act as safety against low water level in the boiler:
Construction:
For prevention against low water level the high steam and low water safety valve is operated by a lever ‘L’ which is hinged at the fulcrum ‘F’ inside the boiler as shown in Fig. 29.4 (a). A weight ‘W1’ is attached to one end of the lever ‘L’ and a large floating earthware ‘E’ to the other end. By hanging a dead weight ‘W2’ on the lower end of the spindle ‘S’ of valve ‘V’, the hemispherical shaped valve ‘V’ is forced downwards on valve ‘U’ which act as a seat for valve ‘V’. The knife edge ‘K’ of lever ‘L’ is provided to push collar ‘C’ of spindle ‘S’ for lifting valve ‘V’ from its seat.
Operation:
When the water level in the boiler is at normal level: The floating earthen ware ‘E’ remains in water and whole level system ‘L’ with its weights is balanced. Under this condition, the valve ‘V’ is seated tightly on valve ‘U’ due to the downward force of dead weight ‘W2’ and the steam will not escape from the boiler as shown in Fig. 29.4 (a).
When the water level in the boiler go down below the normal level: When the level of the water falls, the floating earthen ware ‘E’ is partly uncovered thus its weight increases due to decrease in buoyancy force on partly uncovered earthenware ‘E’. With this increase in weight of earthen ware ‘E’ as compared to Weight ‘W1’, the lever system ‘L’ becomes unbalanced and the earthen ware float ‘E’ moves in a downward direction. With this movement of lever the knife edge ‘K’ provided on lever push the collar C with the spindle and valve ‘V’ in the upward direction leaving valve ‘U’ on its seat and the steam gets the passage to escape to atmosphere between the valve ‘V’ and valve ‘U’ as shown in Fig. 29.5. The escaping of steam causes a loud noise as it passes through a specially constructed passage and alert the boiler attendant regarding low water level.
Fig. 29.5. High Steam Low Water Safety Valve in open position due to low water level
Uses:
This valve is used in internally fired boilers such as Lancashire and Cornish in which there is always a chance of overheating of the fire tubes.
29.2. Water Level Indicator/Water Gauge
Function: The purpose of this fitting is to indicate the level of water in the boiler and to enable the attendant to regulate the supply of feed water in the boiler to maintain correct level.
Number and Location: Normally two water level indicators/water gauge are fitted in front of the boiler.
Construction and operation: A commonly used water level indicator is shown in Fig. 29.6. It consists of a gauge glass tube having guard cover fitted between two hollow gun metal casting tubes of which one of it is fixed with steam space and the other with water space of the boiler. As steam comes from upper tube and water comes from lower tube in gauge glass, so the water level inside the boiler will be the same as seen in the gauge glass tube. It has a steam cock and a water cock to keep or shutoff the glass tube in connection with steam space and water space in the boiler, respectively. It has also one drain cock to discharge water at regular intervals to avoid accumulation of foreign element in the path from boiler to glass tube. During operation, for the observation of the water level in the boiler, the steam and water cocks are opened (in vertical position) and drain cock is closed (in horizontal position) as shown in Fig. 29.6. The various screw caps provide access to different passages for cleaning purposes.
Safety against if gauge glass tube gets broken:
An arrangement of hollow metal tube and two small metal balls inside the upper and lower metal casting tubes, as shown in Fig. 29.6, is used for an automatic shut off the steam and water supply to the gauge glass in case the gauge glass tube gets broken.
Under normal condition: The balls are at rest as pressure exerted on all side of it remains the same, as shown by Fig. 29.6 (a).
Under condition when the glass tube breaks: Then the pressure at the gauge glass tube will be much less than the pressure in the boiler. This pressure difference causes the both ball to move towards the ends of the gauge glass tube and fit into the steam and water openings as shown in Fig. 29.6 (b) and thus closes the gauge glass tube ends. As a result steam and water may not escape any more. Now by closing the steam cock and water cock (in horizontal position), the glass tube may be replaced.
Fig. 29.6. Water Level Indicator/Water gauge (a) under normal condition (b) under condition when the glass tube breaks
29.3. Pressure Gauge
Function: With pressure gauge steam pressure of boiler can be measured.
Number and Location: All boilers must be fitted with at least one pressure gauge and it is usually mounted at the front top of the boiler shell or drum so that attendant can easily read the pressure reading .
There are two types of pressure gauges:
Bourdon tube pressure gauge and
Diaphragm type pressure gauge.
The common type of pressure gauge for steam boilers is Bourden’s pressure gauge, construction and operation of which is described as follows:
Construction:
The Bourden type pressure gauge, as shown in Fig. 29.7, consists of a C shaped spring tube ‘C’ of elliptical in cross-section. One end ‘A’ of this spring tube is closed and movable while the other end is fixed to a hollow block ‘D’. To have link with steam space of a boiler, the hollow block of Bourden tube is connected to the boiler through a water filled U-shaped siphon tube. The water in the siphon tube prevents the steam from entering and coming in contact with the spring tube when for pressure measurement three way lever handle cock ‘S’ is opened to the boiler pressure otherwise without U-tube steam from boiler could overheat the spring tube which may ultimately spoil the accuracy of pressure gauge. To have reading of pressure in bar on the dial of the gauge, the dial mechanism is attached to the movable closed end of the spring tube. In this mechanism, the closed end of tube is connected to a toothed sector wheel ‘T’ through a link ‘R’. Further, the toothed sector hinged at point ‘H’ is meshed with a pinion ‘P’ fixed to the spindle of the pointer ‘N’.
The accuracy of the pressure gauge during working of boiler can be tested by connecting the inspector’s steam pressure test gauge to normally plugged connection ‘Q’ provided on U-tube siphon. Three way lever handle cock ‘S’ is used to isolate the gauge from the boiler when the test gauge is to be attached/removed. The three way cock ‘S’ is also used to isolate the gauge from the boiler in the event of repair or replacement of the gauge. For cleaning the siphon, plug ‘Z’ is provided on U-tube.
Operation:
When the three way lever handle cock ‘S’ is opened to sense the steam pressure of boiler, the water in U-tube fills the spring tube ‘C’ and cause the spring tube straightens out depending upon steam pressure in the boiler as the steam pressure acting through water on the inside of the spring tube tries to change the cross-section of elliptical spring tube ‘C’ to circular. Since one end of the spring tube is fixed, this makes closed end ‘A’ of tube to move in outward direction. The movement of closed end of spring tube is magnified considerably through the dial mechanism attached at the closed end and the pointer rotates over a circular graduated dial and pressure can be easily read against the pointer on the circular graduated dial in bars.
Fig. 29.7. Bourden’s pressure gauge with water filled U-tube siphon attachment
29.4. Fusible Plug
Function: The function of the fusible plug is to protect heating surface area of the boiler against damage of overheating when the water level in the boiler falls below the safe limit.
Location: The fusible plug is generally inserted in the crown of the furnace or in the combustion chamber at the lowest permissible water level. (Refer Figures 27.4, 28.1 and 28.2)
Construction:
A fusible plug as shown in Fig. 29.8 is made from three parts which plug the whole in crown plate of the furnace and thus stop the steam or water of boiler from entering into the furnace. First part of it consists of a hollow gunmetal body ‘A’ which is screwed into the crown plate of the furnace with the help of a hexagonal flange. The second part consists of another hollow gun metal plug ‘B’ which is screwed into the upper portion of the hollow gunmetal body ‘A’ and the third part consists a solid copper plug ‘C’ which is kept and held firmly in the hole of plug ‘B’ by setting low melting point fusible metal between plug ‘A’ and plug ‘B’. The fusible metal may be of tin or lead which has a lower melting point than plug ‘A’ and plug ‘B’.
Operation:
Under normal water level condition: Under this condition, the fusible plug fixed on furnace is always covered with water. As for hot gases in furnace the water is always available for heat transfer, the temperature of fusible metal remains below its melting point. This keeps the solid copper plug in its position as shown in Fig. 29.9 (b) and normal steam generation process is carried out in boiler.
Under condition when the water level falls below the lower limit in the boiler:In this condition, the fusible plug gets uncovered out of water and is exposed to steam only. As now the water is not available as heat transfer medium for hot gases of furnace, the both plugs ‘B’ and ‘C’ gets overheated. The overheating of plugs result melting of fusible metal first which make the copper plug ‘C’ is released and falls as shown in Fig. 29.9 (b). This opens a way between steam space and furnace and steam rush into the fire box and extinguish the fire and stop the steam generation process.
Fig. 29.8. A fusible plug (a) Under normal condition (b) Under condition when water level low
Before starting the steam generation process again in the boiler, the plug ‘C’ is fixed back in plug ‘B’ with fusible metal.
29.5.Steam Stop Valve or Junction Valve
Function: The function of the steam stop valve or junction valve is to stop or regulate the flow of steam from the boiler to the steam pipe or from one steam pipe to another steam pipe or from the steam pipe to the turbine/engine.
Location: This valve is usually mounted over the boiler drum (Fig 29.1), in the steam pipe leading to-the steam turbine/engine, and between the steam pipes to stop or regulate the flow of steam.
When this valve is mounted over the boiler drum it is called a junction valve. If this valve is mounted in the steam pipe leading to-the steam turbine/engine or between the steam pipes, it is called a steam stop valve. The steam stop valve and junction valve are basically the same valves.
Construction:
It consists of a valve body made of cast iron having two flanges at right angles as shown in Fig. 29.9. When the valve is used to stop or regulate the flow of steam from the boiler to the steam pipe, its inlet end flange is attached to the seating block at the highest point of the steam space of the boiler shell while its outlet end flange is bolted to the steam pipe. It also consists of a gun metal valve in the form of disc and a valve seat made of gun metal. The valve seat is screwed on the valve body whereas the valve seat is connected to hand wheel through a spindle. To prevent any leakage of steam from valve body to atmosphere, the spindle is passed through a gland. Due to rotation of hand wheel, the valve may move in downward or upward direction to close or open the passage fully or partially for the flow of steam.
Its flange is attached to the delivery end of the feed pump is attached to the pipe which leads to the boiler.
Operation:
Under normal working condition of boiler: The valve is open and steam flows from the boiler to the steam pipe as shown in Fig. 29.9 (a).
Under conditions when boiler is shut down or steam is not required from boiler: The valve is closed by operating the hand wheel as shown in Fig. 29.9 (b) and steam flow from the boiler to the steam pipe stops.
Fig. 29.9: Steam stop valve (a) open and (b) close positions
29.6.Feed Check Valve
Function: The function of the feed valve is
To control the supply of feed water to the boiler from the feed pump and
To prevent any water escaping back from the boiler in the event of failure of the feed pump or the pump pressure less than the boiler side.
Location: It is fitted over the boiler shell slightly below the normal water level of the boiler.
Construction:
It consists of a valve body having two flanges at right angles as shown in Fig. 29.10. Its inlet end flange is attached to the delivery end of the feed pump while its outlet end flange is attached to the pipe which leads to the boiler. The body of feed check valve contains two valves, one is the feed valve to control the supply of feed water to the boiler and other is the check valve to prevent any water escaping back from the boiler.
The feed valve is operated manually by a hand wheel similar to steam stop valve and it may move in downward or upward direction to close or open the passage fully or partially for the flow of water. While the check valve operates automatically due to pressure difference of water on the pump side and the boiler of the check valve and it may either in open or close position to allow the feed water flow in one direction only i.e. from feed pump to boiler side only.
Operation:
Under normal working condition of boiler when the feed pump is working properly: The supply of water in the boiler is controlled by manually raising or lowering the feed valve from its seat by means of a hand wheel as shown in Fig. 29.10 (a). In this condition, the more water pressure on feed pump side of the check valve cause the check valve to lift from its seat and allow the water to enter into the boiler to maintain a constant water level.
Under conditions when there is failure of the feed pump or less pump pressure than the boiler side: The feed valve is in open position as shown in Fig. 29.10 (b). But due to failure of feed pump or less pump pressure than the boiler side, the less water pressure on feed pump side of the check valve cause the check valve to press on its seat and prevents the back flow of water from the boiler to water pump.
Fig. 29.10. Feed check valve (a) under normal condition (b) under condition when there is failure of the feed pump
29.7. Blow-off Cock
Function: The blow off cock serves three purposes:—
(i) To blow out sediments, precipitated sludge, loose scale or other impurities periodically when the boiler is in operation.
(ii) To empty the boiler when necessary for cleaning, repair and inspection.
(iii) To permit rapid lowering of water level in the boiler if accidentally it becomes too high.
Location: The blow off cock is fitted at the lowest water space of a boiler shell.
Construction:
Blow-off Cock consists of a casing having two flanges at their ends as shown in Fig. 29.11. One of the flanges of the casing is attached to the boiler while the other flange is attached to a pipe which takes the blow off water out of the boiler. It also consists of a conical plug which fits perfectly into a matching hole in the casing. For the discharge of water from boiler a rectangular hole is provided in the plug.
Operation:
During working of boiler when blow out of sediments is not desired: The position of plug is such that the solid part of plug comes in line with the hole in the casing as shown in Fig. 29.11 (a). In this position, there is no discharge of water from boiler.
During working of boiler when blow out of sediments is desired: The plug of blow off cock is turned in such a way that the hole in the plug comes in line with the hole in the casing. In this position, the water from boiler rushes out of the boiler.
After required time of blow out, the discharge of water is again stopped by turning plug in previous position.
Fig. 29.11. Blow off cock (a) close (b) open
29.8.Manhole and Handhole
Function: Man holes and hand holes are required for cleaning, inspection and repairing of the boiler. The manhole is provided for the entrance of a man inside the boiler shell whereas hand hole is for a hand.
Location: The manhole is provided on the boiler shell at a convenient place.
Construction: Generally they are elliptical in shape and cover is provided. The size of the man hole is usually 400 mm x 300 mm.
29.9.Steam Scrubbers/Anti-priming pipe
Function: The steam in contact with the water surface always contains water particles. A steam scrubber also known as steam drier is used in a boiler to remove the water particles contained in steam.
Location: It is fitted inside the boiler drum or shell just before steam outlet of boiler.
Construction and operation:
Refer Fig. 29.12. Anti-priming is a cast iron box which is fitted under the mounting block on which the steam stop valve is to be bolted. When the steam with water particles passed between closely fitted corrugated plates or the perforation made in the upper half of the anti-priming pipe, the heavier water particles separate out and are collected at the bottom of the anti-priming pipe. The water thus collected is later on drained to the boiler through the small holes ‘O’ provided at the bottom of the anti-priming pipe.
Fig 29.12: Anti-priming pipe mounted in boiler
29.10. Air vent/Vacuum breaker:
Function: It is used to eliminate air from steam space of the boiler.
Location: Air vent/vacuum breaker is located above the steam space in the boiler Fig 29.12.
Construction and operation:
When a boiler is started from cold, the steam space is full of air. As this air blankets the heat exchange surfaces and it has no heat value, the performance of steam plant is adversely affected. This air also gives rise to corrosion in the condensate system, if not removed adequately.
So, to increase the performance and reduce the corrosion of boiler, the air is purged from the steam space by air vent using a simple cock during start up of boiler. Normally this is done by leaving the air vent open until a pressure of about 0.5 bar is achieved inside the boiler.
29.11. Soot blowers:
Function: It is used to blow soot and the combustion products from the tube surfaces.
Location: It is located near the water tubes inside the boiler as shown in Fig 29.13.
Construction and operation:
The soot layer on the tubes and shells of the furnace of the boiler acts as a heat insulator. This not only causes the boiler efficiency to be lowered, but a more serious problem can also occur. The soot can also catch fire. Therefore it is important to remove the soot without stopping the steam generation process of the boiler. This is done by soot blower as shown in Fig. 29.13. It is operated by steam or compressed air.
Fig. 29.13. Soot blower
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