BOILER MOUNTING AND ACCESSORIES

Boiler mountings are used to run a boiler in a safe way ex: saftey valve,Water level indicators etc

Boiler accessories are used to improve the efficiency of a boiler ex: economiser,air preheater etc.

A number of items must be fitted to steam boilers, all with the objective of improving:

Operation. Efficiency. Safety.

While this Tutorial can offer advice on this subject, definitive information should always be sought from the appropriate standard. In the UK, the standard relating to the specification of valves, mountings and fittings in connection with steam boilers is BS 759.Several key boiler attachments will now be explained, together with their associated legislation where appropriate.

Boiler name-plate

In the latter half of the 19th century explosions of steam boilers were commonplace. As a consequence of this, a company was formed in Manchester with the objective of reducing the number of explosions by subjecting steam boilers to independent examination. This company was, in fact, the beginning of today's Safety Federation (SAFed), the body whose approval is required for boiler controls and fittings in the UK.

After a comparatively short period, only eight out of the 11 000 boilers examined exploded. This compared to 260 steam boiler explosions in boilers not examined by the scheme. This success led to the Boiler Explosions Act (1882) which included a requirement for a boiler name-plate. An example of a boiler name-plate is shown in Figure

The serial number and model number uniquely identify the boiler and are used when ordering spares from the manufacturer and in the main boiler log book.The output figure quoted for a boiler may be expressed in several ways, as discussed in previous Tutorials within this Block.

Safety valves

An important boiler fitting is the safety valve. Its function is to protect the boiler shell from over pressure and subsequent explosion. In the UK: In Europe, matters relating to the suitability of safety valves for steam boilers are governed by the European standard EN 12953. In the US and some other parts of the world, such matters are covered by ASME standards.

Many different types of safety valves are fitted to steam boiler plant, but generally they must all meet the following criteria:

The total discharge capacity of the safety valve(s) must be at least equal to the 'from and at 100°C' capacity of the boiler. If the 'from and at' evaporation is used to size the safety valve, the safety valve capacity will always be higher than the actual maximum evaporative boiler capacity.

The full rated discharge capacity of the safety valve(s) must be achieved within 110% of the boiler design pressure.

The minimum inlet bore of a safety valve connected to a boiler shall be 20 mm. The maximum set pressure of the safety valve shall be the design (or maximum

permissible working pressure) of the boiler. There must be an adequate margin between the normal operating pressure of the boiler

and the set pressure of the safety valve.

Safety valve regulations

A boiler shall be fitted with at least one safety valve sized for the rated output of the boiler - Refer to EN 12953 for details.The discharge pipework from the safety valve must be unobstructed and drained at the base to prevent the accumulation of condensate. It is good practice to ensure that the discharge pipework is kept as short as possible with the minimum number of bends, so that the allowable backpressure indicated by the valve manufacturer is not exceeded.

Boiler stop valves

Boiler stop valve

A steam boiler must be fitted with a stop valve (also known as a crown valve) which isolates the steam boiler and its pressure from the process or plant. It is generally an angle pattern globe valve of the screw-down variety. Figure shows a typical stop valve of this type.

In the past, these valves have often been manufactured from cast iron, with steel and bronze being used for higher pressure applications. In the UK, BS 2790 (eventually to be replaced with EN 12953) states that cast iron valves are no longer permitted for this application on steam boilers. Nodular or spheroidal graphite (SG) iron should not be confused with grey cast iron as it has mechanical properties approaching those of steel. For this reason many boilermakers use SG iron valves as standard.

The stop valve is not designed as a throttling valve, and should be fully open or closed. It should always be opened slowly to prevent any sudden rise in downstream pressure and associated waterhammer, and to help restrict the fall in boiler pressure and any possible associated priming. On multi-boiler applications an additional isolating valve should be fitted, in series with the crown valve. At least one of these valves should be lockable in the closed position.

Feedwater check valves

The feedwater check valve is installed in the boiler feedwater line between the feedpump and boiler. A boiler feed stop valve is fitted at the boiler shell.

The check valve includes a spring equivalent to the head of water in the elevated feedtank when there is no pressure in the boiler. This prevents the boiler being flooded by the static head from the boiler feedtank.

Boiler check valve

Under normal steaming conditions the check valve operates in a conventional manner to stop return flow from the boiler entering the feedline when the feedpump is not running. When the feedpump is running, its pressure overcomes the spring to feed the boiler as normal.

Because a good seal is required, and the temperatures involved are relatively low (usually less than 100°C) a check valve with a EPDM (Ethylene Propylene) soft seat is generally the best option.

Location of feed check valve

Boiler water quality control

The maintenance of water quality is essential to the safe and efficient operation of a steam boiler. The measurement and control of the various parameters is a complex topic, which is also covered by a number of regulations. It is therefore covered in detail later in this Block. The objective of the next few Sections is simply to identify the fittings to be seen on a boiler.

TDS control

This controls the amount of Total Dissolved Solids (TDS) in the boiler water, and is sometimes also referred to as 'continuous blowdown'. The boiler connection is typically DN15 or 20. The system may be manual or automatic. Whatever system is used, the TDS in a sample of boiler water is compared with a set point; if the TDS level is too high, a quantity of boiler water is released to be replaced by feedwater with a much lower TDS level. This has the effect of diluting the water in the boiler, and reducing the TDS level.

On a manually controlled TDS system, the boiler water would be sampled every shift.

A typical automatic TDS control system is shown in Figure

Typical automatic TDS control system

Bottom blowdown

This ejects the sludge or sediment from the bottom of the boiler. The control is a large (usually DN25 to DN50) key operated valve. This valve might normally be opened for a period of about 5 seconds, once per shift.

Figure illustrates a key operated manual bottom blowdown valve whereas Figure 3.7.8 illustrates an automated bottom blowdown valve and its typical position in a blowdown system.

Key operated manual bottom blowdown valve

Typical position for an automated bottom blowdown valve

Pressure gauge

All boilers must be fitted with at least one pressure indicator.The usual type is a simple pressure gauge constructed to EN 12953.The dial should be at least 150 mm in diameter and of the Bourdon tube type, it should be marked to indicate the normal working pressure and the maximum permissible working pressure / design pressure.

Pressure gauges are connected to the steam space of the boiler and usually have a ring type

siphon tube which fills with condensed steam and protects the dial mechanism from high temperatures. Pressure gauges may be fitted to other pressure containers such as blowdown vessels, and will usually have smaller dials as shown in Figure.

Typical pressure gauge with ring siphon

Gauge glasses and fittings

All steam boilers are fitted with at least one water level indicator, but those with a rating of 100 kW or more should be fitted with two indicators. The indicators are usually referred to as gauge glasses complying with EN 12953.

Gauge glass and fittings

A gauge glass shows the current level of water in the boiler, regardless of the boiler's operating conditions. Gauge glasses should be installed so that their lowest reading will show the water level at 50 mm above the point where overheating will occur. They should also be fitted with a protector around them, but this should not hinder visibility of the water level. Figure 3.7.10 shows a typical gauge glass.

Gauge glasses are prone to damage from a number of sources, such as corrosion from the chemicals in boiler water, and erosion during blowdown, particularly at the steam end. Any sign of corrosion or erosion indicates that a new glass is required.

When testing the gauge glass steam connection, the water cock should be closed. When testing the gauge glass water connections, the steam cock pipe should be closed.

To test a gauge glass, the following procedure should be followed:

Close the water cock and open the drain cock for approximately 5 seconds. Close the drain cock and open the water cock.

Water should return to its normal working level relatively quickly. If this does not happen, then a blockage in the water cock could be the reason, and remedial action should be taken as soon as possible.

Close the steam cock and open the drain cock for approximately 5 seconds. Close the drain cock and open the steam cock.

If the water does not return to its normal working level relatively quickly, a blockage may exist in the steam cock. Remedial action should be taken as soon as possible.

The authorised attendant should systematically test the water gauges at least once each day and should be provided with suitable protection for the face and hands, as a safeguard against scalding in the event of glass breakage.

Gauge glass guards

The gauge glass guard should be kept clean. When the guard is being cleaned in place, or removed for cleaning, the gauge should be temporarily shut-off.

Make sure there is a satisfactory water level before shutting off the gauge and take care not to touch or knock the gauge glass. After cleaning, and when the guard has been replaced, the gauge should be tested and the cocks set in the correct position.

Maintenance

The gauge glass should be thoroughly overhauled at each annual survey. Lack of maintenance can result in hardening of packing and seizure of cocks. If a cock handle becomes bent or

distorted special care is necessary to ensure that the cock is set full open. A damaged fitting should be renewed or repaired immediately. Gauge glasses often become discoloured due to water conditions; they also become thin and worn due to erosion. Glasses, therefore, should be renewed at regular intervals.

A stock of spare glasses and cone packing should always be available in the boiler house. Remember:

If steam passes are choked a false high water level may be given in the gauge glass. After the gauge has been tested a false high water level may still be indicated.

If the water passages are choked an artificially high water level may be observed due to steam condensing in the glass. After testing, the glass will tend to remain empty unless the water level in the boiler is higher than the top connection, in which case water might flow into the glass from this connection.

Gauge glass levels must be treated with the utmost respect, as they are the only visual indicator of water level conditions inside the boiler. Any water level perceived as abnormal must be investigated as soon as it is observed, with immediate action taken to shut down the boiler burner if necessary.

Water level controls

The maintenance of the correct water level in a steam boiler is essential to its safe and efficient operation. The methods of sensing the water level, and the subsequent control of water level is a complex topic that is covered by a number of regulations. The following few Sections will provide a brief overview, and the topic will be discussed in much greater detail later.

External level control chambers

Level control chambers are fitted externally to boilers for the installation of level controls or alarms, as shown in Figure

External level control chamber

The function of the level controls or alarms is checked daily using the sequencing purge valves. With the handwheel turned fully anticlockwise the valve is in the 'normal working' position and a back seating shuts off the drain connection. The handwheel dial may look similar to that shown in Figure. Some handwheels have no dial, but rely on a mechanism for correct operation.

Purge valve handwheel

The following is a typical procedure that may be used to test the controls when the boiler is under pressure, and the burner is firing:

Slowly turn the handwheel clockwise until the indicating pointer is at the first 'pause' position. The float chamber connection is baffled, the drain connection is opened, and the water connection is blown through.

Pause for 5 to 8 seconds. Slowly move the handwheel further clockwise to full travel. The water connection is

shut-off, the drain valve remains open, and the float chamber and steam connections are blown through. The boiler controls should operate as for lowered water level in boiler i.e. pump running and / or audible alarm sounding and burner cut-out. Alternatively if the level control chamber is fitted with a second or extra low water alarm, the boiler should lock-out.

Pause for 5 to 8 seconds. Slowly turn the handwheel fully anticlockwise to shut-off against the back seating in the

'normal working' position.

Internally mounted level controls

Level control systems with sensors (or probes) which fit inside the boiler shell (or steam drum) are also available. These provide a higher degree of safety than those fitted externally. The level alarm systems may also provide a self-checking function on system integrity.

Because they are mounted internally, they are not subject to the procedures required to blow down external chambers. System operation is tested by an evaporation test to '1st low' position, followed by blowing down to '2nd low' position.

Protection tubes are fitted to discourage the movement of water around the sensor.

Internally mounted level controls

Air vents and vacuum breakers

When a boiler is started from cold, the steam space is full of air. This air has no heat value, and will adversely affect steam plant performance due to its effect of blanketing heat exchange surfaces. The air can also give rise to corrosion in the condensate system, if not removed adequately.

The air may be purged from the steam space using a simple cock; normally this would be left open until a pressure of about 0.5 bar is showing on the pressure gauge. An alternative to the cock is a balanced pressure air vent which not only relieves the boiler operator of the task of manually purging air (and hence ensures that it is actually done), it is also much more accurate and will vent gases which may accumulate in the boiler. Typical air vents are shown in Figure

Typical air vents and vacuum breakers

Safety valve

An oxygen safety valve ND250-safety valves

A safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits.

It is part of a bigger set named pressure safety valves (PSV) or pressure relief valves (PRV). The other parts of the set are named relief valves, safety relief valves, pilot-operated relief valves, low pressure safety valves, vacuum pressure safety valves. Safety valves were first used on steam boilers during the industrial revolution. Early boilers without them were prone to accidental explosion.

Function and design

A cross-section of a proportional-safety valve

The earliest and simplest safety valve on the steam digester in 1679 used a weight to hold the pressure of the steam, (this design is still commonly used on pressure cookers); however, these were easily tampered with or accidentally released. On the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to sudden accelerations used a spring to contain the steam pressure, but these (based on Salter spring balances) could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase performance of a steam engine. In 1856 John Ramsbottom invented a tamper-proof spring safety valve which became universal on railways.

Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat exchangers. The term safety valve should be limited to compressible fluid application (gas, vapor, steam).

The two general types of protection encountered in industry are thermal protection and flow protection.

For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.

Flow protection is characterized by safety valves that are considerably larger than those mounted in thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline. This protection can alternatively be achieved by installing a high integrity pressure protection system (HIPPS).

Technical terms

A steam locomotive boiler safety valve belonging to 60163 Tornado

In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power generation, food, drinks, cosmetics ans pharmaceuticals industries, the term safety valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and relief valve. The generic term is Pressure relief valve (PRV) or pressure safety valve (PSV) It should be noted, as most people think PRV and PSV are the same thing. Is that PSV's have a manual lever to open the valve in case of emergency.

Relief valve (RV): automatic system that is actuated by static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure.

Safety valve (SV): automatic system that relieves the static pressure on a gas. It usually opens completely, accompanied by a popping sound.

Safety relief valve (SRV): automatic system that relieves by static pressure on both gas and liquid. Pilot-operated safety relief valve (POSRV): automatic system that relieves by remote command from a

pilot on which the static pressure (from equipment to protect) is connected. Low pressure safety valve (LPSV): automatic system that relieves static pressure on a gas. Used when

difference between vessel pressure and the ambient atmospheric pressure is small. Vacuum pressure safety valve (VPSV): automatic system that relieves static pressure on a gas. Used when

the pressure difference between the vessel pressure and the ambient pressure is small, negative and near the atmospheric pressure.

Low and vacuum pressure safety valve (LVPSV): automatic system that relieves static pressure on a gas. The pressure is small, negative or positive and near the atmospheric pressure.

RV, SV and SRV are spring operated (even said spring loaded). LPSV and VPSV are spring operated or weight loaded.

Legal and code requirements in industry

In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves. Also in most countries, equipment design codes such as those provided by the ASME, API and other organizations like ISO (ISO 4126) must be complied with and those codes include design standards for relief valves.

Today, industries such as food, drinks, cosmetics, pharmaceuticals and fine chemicals industries ask for hygienic safety valves, fully drainable and Cleanable In Place; most of them are stainless steel made; Hygienic norms are mainly 3A in the US and EHEDG in Europe.

Types

Steam locomotive No. 46229, Duchess of Hamilton lifts her boiler safety valve after hauling the Welsh Marches Pullman charter.

There is a wide range of safety valves having many different applications and performance criteria required to cover different areas. In addition, national standards are set by many kinds of safety valve.

ASME I tap - a safety valve in accordance with the requirements of Section I of the application code ASME pressure vessel, which opens 3% and 4% of the pressure. Will rule on two rings serving and supported by a National Seal V defined.

ASME VIII valve - safety valve in accordance with the requirements of Article VIII of the ASME code for pressure vessel applications, which is within 10% overpressure that opens and closes in 7%. Characterized by a National Board UV stamp.

Low-lift safety valve - the current position of the disc around the drain valve. Full lift safety valve - the region of the exemption is not determined by the position of the disc. Full-flow relief valve - A valve which is expected in the hole and lift the valve a sufficient measure of the

minimum area for each position or under the seat to make the control panel.

Water heaters

Temperature and Pressure safety valve on a water heater.

They are required on water heaters, where they prevent disaster in certain configurations in the event a thermostat should fail. There are still occasional, spectacular failures of older water heaters that lack this equipment. Houses can be leveled by the force of the blast.

Pressure cookers

Pressure cookers are pots for cooking with a pressure-proof lid. Cooking at pressure allows the temperature to rise above the normal boiling point of water (100 degrees Celsius at sea level ) which speeds up the cooking and makes the cooking more thorough.

Pressure cookers usually have two safety valves. One is a hole upon which a weight sits. The other is a sealed rubber grommet which is ejected in a controlled explosion if the first valve gets blocked.

The term safety valve is also used metaphorically.

Safety for different boilers

Historically, boilers were a source of many serious injuries and property destruction due to poorly understood engineering principles. Thin and brittle metal shells can rupture, while poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized steam. Collapsed or dislodged boiler tubes could also spray scalding-hot steam and smoke out of the air intake and firing chute, injuring the firemen who loaded coal into the fire chamber. Extremely large boilers providing hundreds of horsepower to operate factories could demolish entire buildings.

A boiler that has a loss of feed water and is permitted to boil dry can be extremely dangerous. If feed water is then sent into the empty boiler, the small cascade of incoming water instantly boils on contact with the superheated metal shell and leads to a violent explosion that cannot be controlled even by safety steam valves. Draining of the boiler could also occur if a leak occurred in the steam supply lines that was larger than the make-up water supply could replace. The Hartford Loop was invented in 1919 by the Hartford Steam Boiler and Insurance Company as a method to help prevent this condition from occurring, and thereby reduce their insurance claims.

Superheated steam boilers

A superheated boiler on a steam locomotive.

Most boilers produce steam to be used at saturation temperature; that is, saturated steam. Superheated steam boilers vaporize the water and then further heat the steam in a superheater. This provides steam at much higher temperature, but can decrease the overall thermal efficiency of the steam generating plant because the higher steam temperature requires a higher flue gas exhaust temperature. There are several ways to circumvent this problem, typically by providing an economizer that heats the feed water, a combustion air heater in the hot flue gas exhaust path, or both. There are advantages to superheated steam that may, and often will, increase overall efficiency of both steam generation and its utilisation: gains in input temperature to a turbine should outweigh any cost in additional boiler complication and expense. There may also be practical limitations in using wet steam, as entrained condensation droplets will damage turbine blades.

Superheated steam presents unique safety concerns because, if any system component fails and allows steam to escape, the high pressure and temperature can cause serious, instantaneous harm to anyone in its path. Since the escaping steam will initially be completely superheated vapor, detection can be difficult, although the intense heat and sound from such a leak clearly indicates its presence.

Superheater operation is similar to that of the coils on an air conditioning unit, although for a different purpose. The steam piping is directed through the flue gas path in the boiler furnace. The temperature in this area is typically between 1,300–1,600 degrees Celsius (2,372–2,912 °F). Some superheaters are radiant type; that is, they absorb heat by radiation. Others are convection type, absorbing heat from a fluid such as a gas. Some are a combination of the two types. Through either method, the extreme heat in the flue gas path will also heat the superheater steam piping and the steam within. While the temperature of the steam in the superheater rises, the pressure of the steam does not: the turbine or moving pistons offer a continuously expanding space and the pressure remains the same as that of the boiler. Almost all steam superheater system designs remove droplets entrained in the steam to prevent damage to the turbine blading and associated piping.

Supercritical steam generators

Supercritical steam generators (also known as Benson boilers) are frequently used for the production of electric power. They operate at "supercritical pressure". In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi/22.06 MPa or 220.6 bar) that actual boiling ceases to occur, and the boiler has no water - steam separation. There is no generation of steam bubbles within the water, because the pressure is above the "critical pressure" at which steam bubbles can form. It passes below the critical point as it does work in the high pressure turbine and enters the generator's condenser. This is more efficient, resulting in slightly less fuel use. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" actually occurs in this device.

Hydronic boilers

Hydronic boilers are used in generating heat for residential and industrial purposes. They are the typical power plant for central heating systems fitted to houses in northern Europe (where they are commonly combined with domestic water heating), as opposed to the forced-air furnaces or wood burning stoves more common in North America. The hydronic boiler operates by way of heating water/fluid to a preset temperature (or sometimes in the case of single pipe systems, until it boils and turns to steam) and circulating that fluid throughout the home typically by way of radiators, baseboard heaters or through the floors. The fluid can be heated by any means...gas, wood, fuel oil, etc., but in built-up areas where piped gas is available, natural gas is currently the most economical and therefore the usual choice. The fluid is in an enclosed system and circulated throughout by means of a motorized pump. The name "boiler" can be a misnomer in that, except for systems using steam radiators, the water in a properly functioning hydronic boiler never actually boils. Most new systems are fitted with condensing boilers for greater efficiency. These boilers are referred to as condensing boilers because they condense the water vapor in the flue gases to capture the latent heat of vaporization of the water produced during combustion.

Accessories

Boiler fittings and accessories

Safety valve: It is used to relieve pressure and prevent possible explosion of a boiler. Water level indicators: They show the operator the level of fluid in the boiler, also known as a

sight glass, water gauge or water column is provided. Bottom blowdown valves: They provide a means for removing solid particulates that condense

and lie on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates out.

Continuous blowdown valve: This allows a small quantity of water to escape continuously. Its purpose is to prevent the water in the boiler becoming saturated with dissolved salts. Saturation would lead to foaming and cause water droplets to be carried over with the steam - a condition known as priming. Blowdown is also often used to monitor the chemistry of the boiler water.

Flash Tank: High pressure blowdown enters this vessel where the steam can 'flash' safely and be used in a low-pressure system or be vented to atmosphere while the ambient pressure blowdown flows to drain.

Automatic Blowdown/Continuous Heat Recovery System: This system allows the boiler to blowdown only when makeup water is flowing to the boiler, thereby transferring the maximum amount of heat possible from the blowdown to the makeup water. No flash tank is generally needed as the blowdown discharged is close to the temperature of the makeup water.

Hand holes: They are steel plates installed in openings in "header" to allow for inspections & installation of tubes and inspection of internal surfaces.

Steam drum internals, A series of screen, scrubber & cans (cyclone separators). Low- water cutoff: It is a mechanical means (usually a float switch) that is used to turn off the

burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastrophic failure.

Surface blowdown line: It provides a means for removing foam or other lightweight non-condensible substances that tend to float on top of the water inside the boiler.

Circulating pump: It is designed to circulate water back to the boiler after it has expelled some of its heat.

Feedwater check valve or clack valve: A non-return stop valve in the feedwater line. This may be fitted to the side of the boiler, just below the water level, or to the top of the boiler.

Top feed: A check valve (clack valve) in the feedwater line, mounted on top of the boiler. It is intended to reduce the nuisance of limescale. It does not prevent limescale formation but causes the limescale to be precipitated in a powdery form which is easily washed out of the boiler.

Desuperheater tubes or bundles: A series of tubes or bundles of tubes in the water drum or the steam drum designed to cool superheated steam. Thus is to supply auxiliary equipment that does not need, or may be damaged by, dry steam.

Chemical injection line: A connection to add chemicals for controlling feedwater

Steam accessories

Main steam stop valve: Steam traps: Main steam stop/Check valve: It is used on multiple boiler installations.

Combustion accessories

Fuel oil system: Gas system: Coal system: Soot blower

Other essential items

Pressure gauges: Feed pumps: Fusible plug: Inspectors test pressure gauge attachment: Name plate: Registration plate:

Controlling draught

Most boilers now depend on mechanical draught equipment rather than natural draught. This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as the chimney height. All these factors make proper draught hard to attain and therefore make mechanical draught equipment much more economical.

There are three types of mechanical draught:

Induced draught: This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet oriented in the direction of flue gas flow induces flue gasses into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draught fan (ID fan) which removes flue gases from the furnace and forces the exhaust gas up the stack. Almost all induced draught furnaces operate with a slightly negative pressure.

Forced draught: Draught is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure.

Balanced draught: Balanced draught is obtained through use of both induced and forced draught. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draught fan works in conjunction with the forced draught fan allowing the furnace pressure to be maintained slightly below atmospheric.