Most of the boilers are fitted with accessories. They either increase the efficiency of the boiler plants or help in their working.
The following accessories are attached to a modern boiler:
Boiler Accessory # 1. Economizers:
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The greatest item of heat loss in a boiler plant is the heat carried away in the flue gases up the chimney or stack. It is certain for the gases to be hotter than the water in the boiler.
Some of the heat being carried away by the flue gases may be recovered and sent back into the boiler in the feed water if an economizer is placed between the boiler and the chimney.
Various kinds of economizers are manufactured and installed with boiler plants but they work on the same principle. The vertical tube economizer is most popular and a suitable one for the mills and other industrial establishments in our country.
The vertical tube or Green’s economizer consists of a group or groups of vertical cast iron tubes erected in the main flow between the boiler and the chimney. The waste flue gases flow outside the economizer tubes and heat is transferred to the feed water which flows upwards inside the tubes.
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The external surfaces of the tubes are kept free from soot by scrapers which travel slowly and continuously up and down the tubes. The continuous mechanical removal of soot, which is a bad conductor of heat, maintains the efficiency of the economizer and is an important feature.
The economizer tubes which are available upto 4 metre in length are hydraulically pressed into top and bottom headers to form sections, 4, 6, 8, 10 or 12 tubes wide. The sections are erected side by side in the economizer chamber, which is an enlargement of the flue and are connected together by top and bottom branch pipes located outside the chamber.
Every tube is separately tested before assembly. The two ends of the tubes taper in opposite directions so that all the tubes in a section may be pressed into top and bottom headers at one operation. The tube ends and header sockets are accurately machined to give sound metal to metal joints. The completed sections are subjected to a pressure test and thorough examination.
The top headers are fitted with internal conical lids readily removable to permit cleaning and examination of the interior of the tubes and top headers. Each header is provided with a master lid itself being removable through the outlet branch of the top header.
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The feed water is distributed along the sections by branch pipes or manifolds coupled to the bottom headers. These branch pipes have a removable access lid opposite to each section to facilitate inspection and internal cleaning. Similar branch pipes, but without access lids are used to connect the outlet branches on the top headers.
The external surface is kept clean and free from soot by means of scrapers moving up and down the economiser tubes. As the scrapers are slowly raised the beveled cutting edges are forced against the tubes and scrape away soot which falls into the soot chamber the economizer.
They are supported and held in position by carriers and guards and are moved up and down the tubes by means of chains passing over chain sheaves carried in gearing frames above the economizer chamber. The arrangement is balanced and there is one chain sheave to each set of four sections of economizer tubes.
The reversal of the scraper mechanism is brought about by a clutch and a reversing lever, the latter having a moving weight to ensure positive action.
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Every economizer is provided with a safety valve, a drain valve and an air release valve. According to Indian Boiler Regulations economizer should be fitted with explosion doors. Thermometers are fitted to show the water temperatures at the economiser inlet and outlet. A pressure gauge is provided along with a control cock and a siphon pipe.
The advantages gained by installing an economizer are:
(ii) Long life of the boiler
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(iii) Increase in steaming capacity.
The saving of fuel effected by Green’s economizer is proportional to the amount of heat recovered in the feed water. The average percentage saving is approximately 1% for every 5.5°C increase in feed water temperature.
The heat required to convert water into steam must be reckoned from the original feed water temperature and can be calculated with the help of steam tables.
It will be seen that low hot well temperatures and high working pressures increase the scope for fuel saving by means of an economizer. To enable the economiser to be kept clean, perfectly dry conditions must be maintained in the economiser chamber. The temperature of the feed water entering the economiser must be high enough to prevent condensation of moisture form the flue gases on the tubes. The constituents of flue gases are SO2, water vapour, CO2, etc.
If the temperature of the water entering an economiser is less than the saturation temperature corresponding to the partial pressure of water vapour, the water vapur will condense from flue gases on the tubes. It will absorb SO2 and form sulphurous acid which will corrode the tubes from outside.
For this reason the temperature of water entering an economiser should not be less than 40°C. If the feed water temperature is lower than this value, the hot water from the outlet branch of the economiser is mixed with cold feed water and the resultant temperature is raised to the required value.
Boiler Accessory # 2. Air Pre-Heaters:
An air pre-heater is installed between the economizer and the chimney and it extracts heat from the flue gases and transfers to air which is entering the furnace. The portion of the heat that otherwise would pass up the chimney to waste. The pre-heating results in faster combustion of fuel.
The greatly increased volume of air due to pre-heating, creates higher furnace temperatures, accelerates combustion and transmission heat, increases the percentage of CO2 in flue gases and gives a larger output of steam than would be possible without pre-heating.
The pre-heating air facilitates the burning of poor grades of fuel, thus permitting a reduction in excess air and so improving the efficiency. It is a pre-requisite when burning pulverized fuel, both to accelerate combustion and to dry the fuel.
The difference in effects between a feed water economizer and an air pre-heater lies in the fact that while the economizer extracts and returns to the boiler itself heat, that would otherwise be sent to waste in the chimney gases; the pre-heater both does this and exercises an additional fundamental influence in improving heat generation and transfer in the furnace by its accelerating effects in the chemical reaction taking place.
Against these important advantages it is necessary to take into account the capital cost of the alteration to the plant and the initial and running costs of an induced draught fan, and sometimes also that of a forced draught fan.
The induced draught fan is necessary because the reduction in the temperature of the flue gases will seriously impair the draught intensity which is dependent on the temperature in case of natural draught. The forced draught fan is necessary to force the air through the pre-heater and ducts into the furnace chamber.
When pre-heated air is employed in connection with stokers, difficulty is frequently experienced with clinkering of the fuel bed. This is used mainly to the increase in temperature of combustion.
The temperature to which combustion air may be heated depends on the fuel, bed. This is due, chiefly to the increase in temperature of combustion.
The temperature to which combustion air may be heated depends on the fuel, the type of the stoker and the rating at which the boiler and the furnace are operated. With pulverized coal, oil or gas higher temperatures are permissible, but with stokers the air pre-heat must be generally limited to a lower value to prevent the overheating of stoker parts. With stokers the air temperature ranges between 100°C to 200°C, while with pulverized coal, air temperature upto 300°C. to 350°C is used.
The increase in overall efficiency of the boiler plant varies from 2 to 10 percent when an air pre-heater is used. This increase depends on location, rating and on the fact that whether other heat saving devices are employed or not. Draught losses in the pre-heater varies between 60 to 100 mm of water at the usual maximum output of the boiler.
There are three types of pre-heaters:
(i) Tubular type.
(ii) Plate type.
(iii) Regenerative type.
Tubular heater consists of a large number of tubes rolled into tube sheets at each end and usually placed so that the flue gases pass through them. Air being heated is made to make number of passes across and around the outside of the tubes. Sometimes the gas flows outside the tubes and the air inside.
In plate type pre-heaters alternate gas and air passages are formed between closely spaced parallel vertical plates. This type of pre-heater is usually arranged so the gas passes vertically upward through the narrow passages and air enters at the side and passes downward in one or two passes.
The regenerative air pre-heater consists of concentric rings of alternate flat and corrugated plates arranged so as to form vertical passages for the upward flow of flue gas through one half and downward flow of air through the opposite half.
The elements are slowly rotated so that they are alternately heated by the flue gas and then cooled by the combustion air which in turn is heated. This type of pre-heater occupies less space for the same heat recovery than other types.
Boiler Accessory # 3. Superheaters:
This is one of the most important accessories of a boiler.
Superheated steam effects improvements and economy in the following ways:
(i) By reducing the steam consumption of the steam engine or steam turbine, by producing more work per kg of steam
(ii) By reducing the condensation losses in steam mains and in engine cylinder
(iii) By eliminating erosion of steam turbine blades
(iv) By increasing the capacity of the plant
(v) By reducing the friction of the steam in the steam ports, while entering and leaving the cylinder.
Steam consumption of turbines is reduced about one per cent for each 5.5°C of superheat. This saving is partly due to increase in volume of steam but mostly because absence of moisture decreases friction losses. Saving of steam consumption by use of superheated steam in engines is about 1.5 to 2% for each 5.5°C of superheat.
This is due to greater volume of superheated steam and therefore it requires less mass of steam to fill the cylinder upto the point of cut-off and due to which there is a decrease in cylinder condensation. The saving mentioned above drops off slightly as the amount of superheating is increased.
The temperatures of superheated steam are being standardized. These temperatures begin with 150°C and increase by increments’ of 25°C. Higher the pressure of the boiler, higher will be the temperature of superheated steam.
The present practice in power plant engineering is indicated in the table below:
Now-a-days the trend is towards the temperature of 600°C. The velocity of steam in the superheater tubes should lie between 12 to 20 metre/sec. The pressure drop in superheater should not exceed 5% of the boiler steam pressure.
Methods of Superheating Steam:
Steam generated in a boiler is never dry because it is in contact with water. It is superheated by adding heat to it after it is removed from contact with water.
This may occur in:
(a) The shell of a boiler as in the steam space surrounding the tops of the fire-tubes in large vertical boilers.
(b) Special forms of superheaters located in the path of the furnace gases and known as attached superheaters.
(c) Superheaters, with separate furnaces, known as separately fired or portable superheaters.
Most commonly used superheaters are attached ones. They are used with water tube and fire tube boilers, and with special construction are adopted to marine and locomotive boilers. The surface of the superheater tubes is -either smooth or extended.
In extended surface type of superheaters the cast iron fins are shrunk upon a smooth tube to increase the heating surface.
Superheaters may also be classified as convection type and radiation type depending upon their location in the boiler.
Convection superheaters are located:
(a) Overdect
(b) Interdeck and
(c) Intertube.
The radiant type superheaters are placed in one or more walls of the furnace of a steam boiler where the superheater tubes receive heat by direct radiation from fire and re-radiation from refractory walls behind them.
The superheaters tubes are usually 5 cm in diameter. Generally carbon steel superheaters are used but in any section where inside metal temperature exceeds 510°C tubes of chrome nickel or chrome molybdenum alloys are used. These are available to withstand inside metal temperature as high as 650°C.
Methods of Control of Superheat:
Exact control of superheat becomes necessary as steam temperature goes higher and safety margins become less.
In superheaters heated by convection steam temperature rises with load. If the load doubles, twice as much steam passes through superheater but this is more than offset by higher gas temperature and doubled gas velocity. In superheaters heated by radiation steam temperature falls with load.
When the boiler load is doubled radiant heat delivered to the superheater does not double. So steam temperature in case of a radiant superheater falls with increase in load. Therefore one method of controlling the superheat is to connect radiant and convection superheaters in series and there by produce a practically flat temperature load curve. More common practice is to use a single superheater placed to receive part of its heat by radiation and part by convection.
The temperature control of superheated steam with convection superheaters can be carried out by any one of the following methods:
(i) By connecting two superheaters in series with a desuperheater in between them. A thermostatically controlled by-pass around the desuperheater, which reduces the temperatures of superheat, maintains constant temperature at the outlet of the second superheater.
(ii) By controlling the gas flow by special baffles and dampers whereby any desired part of the gas flow can be by-passed around the superheater.
Smooth Tube Hairpin Type Superheater:
This superheater is used in most of the low pressure boilers which are used for steam generation for power purposes, viz., Lancashire, Cornish, Babcock and Wilcox and other water tube boilers. The degree of superheat obtained by this type of superheater is not as high as that obtained with multiple loop superheaters.
The position of the superheater in some boilers is given below:
Boiler:
1. Lancashire and Cornish
2. Locomotive
3. Babcock and Wilcox water tube
4. Scotch Marine
Position of Superheater:
1. In downtake
2. In internal flue-tubes
3. In overdeck section
4. In internal flue-tubes or in smoke box or in uptake.
A smooth tube hairpin type superheater which is used in a Lancashire boiler is shown in fig. 5-13, which also gives the position of the superheater in the boiler. The superheater is placed in the downtake of the boiler and it receives heat from the hot gases passing through the downtake.
The superheater consists of two headers. They are connected by a number of hairpin type (U-shaped) tubes. The ends of these tubes are expanded in the headers. The steam from the boiler is led to a header called a saturated steam header of the superheater by a short pipe and the steam then passes to the discharge header through the hairpin tubes.
The superheated steam then passes to the steam pipe connected to one end of the discharge header. The flow of steam in the saturated header is in the opposite direction to superheated steam in the discharge header to ensure equal pressure drop across all the tube elements.
The steam suffers a slight pressure drop while passing through the superheater but for ordinary calculations this can be neglected and the pressure of steam in the superheater can be taken the same as that of the boiler, while steam is passing through the superheater its volume and temperature are increased.
To prevent the overheating of the superheater tubes in a Lancashire boiler when steam is first being raised and / or when the superheater is to be put out of action, a balanced damper is provided and it can be operated by a handle. When the damper is horizontal the hot gases pass round the superheater tubes but when it is vertical, the hot gases will directly pass to the bottom flue.
Fig 5-13 also shows the arrangement of steam pipes to pass the steam through the superheater or direct to the main steam pipe as may be necessary.
The superheater is fitted with a safety valve so as to avoid overheating of the superheater tubes when steam flow through the tubes stops suddenly when the engine stops or the turbine trips.
They consist of many elements; each element being a continuous tube bent backs on itself a number of times between an inlet and an outlet header. Ordinarily each tube is bent to lie in a single plane and is inserted vertically between tube banks. This type of superheater gives higher degree of superheat than that of the hairpin type.
Boiler Accessory # 4. Feed Pumps:
They are the appliances used for delivering the feed water into steam boilers. Commonly employed appliances are reciprocating pumps, rotary pumps and injectors. Rotary pumps are generally of the high speed centrifugal type. They are driven by a small steam turbine or by electric motor and are used when a large quantity of feed water is to be supplied to the boiler.
There are many designs of reciprocating pumps; the difference in design being largely due to the way in which the pump plunger or piston is driven. The pump may be driven through levers or cranks and connecting rods from the main engine, but generally the pump has its own cylinder and is worked independently of the main engine.
The reciprocating pumps are single or double acting. The double acting pumps deliver water during each stroke. The most commonly used form of independent reciprocating feed pump is that in which the steam cylinder is directly opposite to the water cylinder. The piston rod of the steam cylinder is directly connected to the rod or to the piston of the water cylinder.
Commonly used duplex feed pump which is an independent reciprocating feed pump, is described below:
The duplex feed pump is a double acting pump. It has two pumps mounted side by side. It is direct acting in that the pressure of the steam acts directly on a piston to force the water. This pump is a positive action pump, that is, a definite amount of fluid is supplied per stroke under definite pre-determined conditions. The amount of water delivered is, however, reduced by leakage and slippage.
Each pump has one steam and one water cylinder. The arrangement of cylinders is such that the rocker arm of one operates the steam valve of the nearby cylinder, Fig. 5-15 is a cross-section of a duplex feed pump. The steam cylinders in the pump are fitted with pistons that contain self-adjusting iron pistionrings.
Above each steam cylinder are four ports, the two outside ones are called ‘Steam ports’; and the two insideones are called ‘Exhaust ports’. A D-slide valve is used to control the admission and exhaust of steam.
An inside guide is provided for the valve rod. Packing glands are provided for the piston rod and the valve rod where they enter the cylinder head and the valve chest respectively. These glands prevent leakage of steam at these points. One rocker arm has a motion direct, and the other indirect; one valve rod moves in the same direction as the piston that is operating it; the other moves in the opposite direction.
Midway on the piston rod is mounted a cross-head, to which is attached the rocker arm. On the other end of the piston rod is located a piston on the water cylinder which has a renewable liner. The water piston is actuated by the steam piston which is fitted with a removable follower using fibrous or metallic packing rings and moves back and forth in a cylinder. The ends of the cylinder are fitted with drain plugs similar to those of the steam cylinder.
Above the water cylinder are located two decks of valves, the lower deck group being the suction valves and the upper deck group being the discharge valves. The lower valves are set over the suction inlet and the discharge goes directly to the discharge pipe through the upper ones. An air cock is mounted on the top of the chamber and is called a vent. This is used to remove entrapped air from the pump while starting.
The slide valve is very simple and is operated by direct lever connection. Motion is imparted to the valve by a rocker arm actuated by the piston rod on the nearby cylinder. After admitting steam to the other cylinder the piston completes its stroke and waits for its own valve to be operated on by the other piston, so that it may return on the next stroke. One of the valves is always open and so the pump can be started from any position.
The lost motion between the lugs on the back side of the D-slide valve and the nut which is placed on the valve rod for adjusting the lost motion, does not permit the valve to move until the piston that actuates it, has travelled some distance. The lost, motion usually allowed in the valve mechanism permits the piston the travel one half of its stroke before operating the valve.
The amount of lost motion permitted enables the valves to close slowly and quietly before the pump reverses the stroke. The flow of water, however, is not interrupted. While one piston is being slowly brought to a stop, the other continues in action. This prevents fluctuations in the pressure and ensures a uniform flow.
The difficulties may be encountered when operating duplex pumps particularly when handling hot water. Hot water, which comes to the pump by suction, is difficult to lift as it vaporizes when the pressure is reduced below atmospheric. As a result, the expansion of the vapour fills the suction chamber and pump cylinder.
This vapour is compressed and re-expands, the pump thus failing to discharge a full cylinder of water. This results in considerable fluctuation in flow and pressure. Approximately 65°C is the maximum temperature at which water can be raised by suction.
Pumps should be provided with a strainer and a foot valve on the suction end of the line.
The steam consumption of the horizontal duplex pump is equivalent to about 5% of the water discharged by the pump. In order to give satisfactory result about 5% of the water should be discharged by the pump.
In order to give satisfactory service a boiler feed pump should be capable of discharging at least double the quantity of water required by the boiler per hour and it is more economical to install a pump of sufficient capacity to allow it to run at regular and moderate speeds than one that will have to work at abnormal and intermittent speeds.
The power to drive a pump can be found from the following equation:
Boiler Accessory # 5. Steam Drier or Separator:
The steam drier or separators are provided in the boilers to remove water particles carried along with the steam when it goes to the engine. It is generally placed near to the steam engine or turbine. A simple form of steam separator is shown in fig. 5-16.
The water separates out from steam because of its greater inertia, when the steam is made to change its direction of flow. The water is collected and is drained out periodically. A glass water gauge is also provided for viewing the water level in the separator.
Boiler Accessory # 6. Steam Trap:
Steam trap is provided to collect and automatically discharge the water resulting from partial condensation of steam without allowing any steam to escape.
The steam traps are of two types:
i. Expansion Trap:
The expansion traps work on the principle of expansion of metals under heat. Expansion steam trap consists of hollow spring tube made up of nickel steel which contains liquid. This liquid becomes a gas at temperature which the steam has above its lowest temperature. The water enters in the trap and is at a lower temperature than steam which causes hollow spring tube to contract and resulting in opening of a valve.
When all the water is discharged and steam enters the trap, the increased temperature converts the liquid in the hollow spring tube in to the gas. This tube tends to straighten itself and resulting in closer of valve. The steam trap is provided with adjusting screw which allows to arrange the discharge either continuously or intermittently. The end of the tube is held against the adjusting the screw by the spring which presses against the lugs fixed to the casing. This allows adjustment of the discharge from the trap.
ii. Bucket or Float Steam Trap:
Bucket trap collects the condensed steam and discharge is controlled by float or bucket. The steam condensed is collected in the casing. The water overflows into the bucket when the water level in the casing is above the edge of bucket. This causes increase in the weight of bucket and resulting in dipping of bucket.
When bucket dips down, the valve opens and water is forced through the guide tube. The valve is closed when whole of the water collected in the bucket and it is forced out. When the bucket is nearly emptied, it again floats and the valve is closed automatically.
The water flowing through the guide tube impinges on the spirally formed vanes and thus make the spindle and bucket to rotate. Also the condensed water enters into the casing tangentially, which accelerates the rotation of bucket and spindle. This rotation of spindle keeps the valve surface clear from the depositions. The trap is also provided with a knob at the top by which depression of the bucket can be tested by pressing it.
Boiler Accessory # 7. Injectors:
An injector is a device to lift and force water into a boiler which is operating under pressure. It consists of a group of nozzles so arranged that steam expanding in these nozzles imparts its kinetic energy to a mass of water.
The injector offers several advantages, especially with smaller plants, although its use is not limited to these. In addition to occupying the minimum of space the injector is comparatively inexpensive in cost and everyday maintenance.
Though the steam used to operate the injector is much more than that in the feed pump for an equivalent duty, the injector has the advantage that practically the whole of the heat of the steam is returned to the boiler and so thermally it is very efficient. The injector feed water is supplied hot to the boiler and thereby reduces the thermal stresses in it.
The maximum temperature of feed water, that an injector is capable of handling, is about 65°C. Increased lift should be accompanied by a decrease in temperature. Since an injector cannot handle very hot water and the present day practice favours high feed water temperatures, this method of feeding boilers has become impracticable.
Though when operating at steady load and uniform pressure, the injector is entirely satisfactory, it is unreliable when the operation is irregular and the injector has to operate with fluctuating pressures.
In addition to live steam operated injectors there are designs which employ exhaust steam, but they are rarely used in industrial boiler houses, although they recover and return to the boiler a large percentage of heat which might otherwise be wasted.
The essential parts of the injector are the steam jet, the suction jet, the combining and delivery tube and the overfloand discharge tubes.
In operation, steam expands in the steam nozzle and thereby drops in pressure but gains in velocity. As the steam passes across space between the steam and the suction nozzles a vacuum is created in the suction chamber. Therefore, water is drawn into the suction chamber from the feed tank.
The high speed steam jet picks up the water which is sucked and crosses the intervening space between the steam jet and the suction jet, and it forces the water along with the steam into the combining tube and finally into the delivery tube. Here the steam is condensed and the delivery tube receives the water and the condensate.
The delivery tube is so designed that a considerable amount of kinetic energy of the jet changes itself into pressure energy which is sufficient to force the water in the boiler against the boiler pressure. The thermal energy in the expanding steam gives the feed water sufficient energy to force itself into the boiler and also in addition, heats the water.
In starting injector water and steam escape out of the overflow valve but when sufficient pressure to force water in the boiler is built up in the injector the overflow valve is automatically closed. The gap at the combining cone communicates with the overflow branch. When the injector is working normally, the steam and water being in proper ratio, there is no escape of steam or water through the overflow passages.
In order that the injector may act properly there is a definite relation between the quantity of steam and water entering the injector. The relation is different with different steam pressures; if the ratio of steam to water is greater, the pressure of steam will be lower.
Boiler Accessory # 8. Pressure Reducing Valve:
The function of pressure reducing valve is to maintain constant pressure on its delivery side. The demand of steam from the boiler is fluctuating and therefore the pressure will be continuously varying. In order to control the pressure the pressure reducing valve is essential boiler mounting.
The pressure reduction is achieved by throttling of steam while passing through the pressure reducing valve. This is normally used in low capacity boilers where it is difficult to maintain constant delivery pressure because of the fluctuating demand. In these boilers steam is generated at high pressure than the required pressure by prime mover. This pressure can be reduced by pressure reducing valve installed in the steam supply pipe.
The high pressure steam enters the steam inlet through a throttle valve and flows to steam outlet. The throttle valve is actuated by a spring and valve rod mechanism. When the steam goes through throttle valve, it is throttled and the pressure of the steam is reduced.
The force exerted by the spring can be adjusted by the adjusting screw. This results into variation in opening of throttle valve which allows adjustment of exit pressure. The diaphragm of the valve is protected by the condensing steam at the bottom of the valve.