Boilers

Table of Contents
1. Boilers
2. Classification of Boilers
3. Common Boiler Terms
4. High-Pressure Boilers - Overview
5. Steam Generator Control
6. Types of High-Pressure Boilers
7. Boiler Mountings and Accessories
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Boilers

A boiler or steam generator is equipment used for producing steam. It may be defined as: a closed vessel in which steam is produced from water by the combustion of fuel.

Uses of the steam generated:

• Power generation (driving turbines or prime movers).
• Space and process heating.
• Industrial processes - chemical plants, sugar mills, textile, paper, and other industries.

Classification of Boilers

• By axis orientation - Horizontal, Vertical or Inclined: If the axis of the boiler is horizontal, it is a horizontal boiler; if vertical, a vertical boiler; if inclined, an inclined boiler. Horizontal boilers offer easier inspection and repair but occupy more floor area. Vertical boilers save floor area and are compact for small installations.
• By heat-transfer arrangement - Fire-tube and Water-tube: In fire-tube boilers the hot combustion gases flow inside tubes and water surrounds the tubes (examples: Cochran, Lancashire, Locomotive boilers). In water-tube boilers water flows inside tubes while hot gases surround them (examples: Babcock & Wilcox, Stirling, Yarrow).
• By furnace location - Externally-fired and Internally-fired: In externally-fired boilers the furnace is outside the shell (examples: Babcock & Wilcox, Stirling). In internally-fired boilers the furnace is within the boiler shell (examples: Cochran, Lancashire).
• By circulation method - Natural circulation and Forced circulation: In natural circulation boilers the movement of water and steam is due to density differences and convection currents (examples: Lancashire, some Babcock & Wilcox designs). In forced circulation boilers circulation is maintained by a pump (examples: Lamont, Velox, Benson).
• By operating pressure - Low-pressure and High-pressure boilers: Different texts use slightly different thresholds. Many classifications treat boilers producing steam at about 80 bar and above as high-pressure. In practice, boilers for power generation are designed and operated over a range (for example from about 30 bar up to over 160 bar) depending on plant requirements. Water-tube boilers are generally preferred for higher pressures and outputs; shell (fire-tube) boilers are used for lower pressures and smaller outputs.
• By mobility - Stationary and Portable: Stationary boilers are installed for continuous plant use. Portable boilers are small, mobile units used for temporary or site duties.
• By number of tubes - Single-tube and Multi-tube: Single-tube boilers have one main tube (example: Cornish, simple vertical boilers). Multi-tube boilers have several tubes (examples: Lancashire, many locomotive boilers).

Common Boiler Terms

• Shell: Cylindrical body of the boiler made of steel plates, riveted or welded to form the pressure vessel that contains water and steam.
• Setting: The brickwork, casing and arrangement that surround the boiler and furnace to confine heat and form a safe passage for flue gases.
• Grate: Platform of cast iron bars in the furnace on which fuel is burned (in solid-fuel-fired boilers).
• Mountings: Safety and controlling devices fitted on the boiler shell - for example stop (junction) valve, safety valves, water-level gauge, pressure gauge, fusible plug, blow-off cock, feed check valve.
• Accessories: Ancillary equipment that improves boiler performance and safety but is not essential to the boiler pressure part - for example economisers, superheaters, feed pumps and air preheaters.
• Foaming: Formation of stable bubbles or froth on the water surface in the boiler, caused by high surface tension and impurities. Foaming can cause water carryover with steam.
• Scale: Hard deposits (usually calcium and magnesium salts) formed on heating surfaces from dissolved minerals in feed water; scale reduces heat transfer and can cause overheating and tube failure.
• Blowing-off (blowdown): The controlled removal of water from the boiler to discharge accumulated solids, sludge and reduce concentration of dissolved impurities.
• Lagging (insulation): Layers of asbestos, mineral wool, magnesia or similar insulating material wrapped around the boiler shell or steam pipes to reduce heat loss. (The input term 'logging' appears to be used here synonymously with lagging; the correct usual term is lagging.)
• High-pressure boiler (discussion): Where steam is required at relatively high pressures, water-tube boilers are preferred because they can safely and economically operate at higher pressures and steam production rates. Modern high-pressure boilers employed for power generation are designed for steam capacities of tens to several hundreds of tonnes per hour, pressures up to and beyond 160 bar and temperatures up to about 540°C or higher for advanced cycles.

High-Pressure Boilers - Overview

A high-pressure boiler is designed to operate at pressures significantly higher than those of small heating boilers; for classification many references use around 80 bar as a threshold, but in practice power-station boilers operate over a wide range (for example 30-160 bar and above) depending on design and duty. High-pressure boilers commonly use water-tube arrangements and are designed for high steam generation rates and high steam temperatures suitable for driving turbines.

Different Components

Steam drum

• The steam drum in a water-tube boiler performs several functions: it stores water and steam to meet varying load requirements; it assists circulation by separating the steam-water mixture returned from risers; it provides surface area for liquid-vapour disengagement; it houses internals such as baffles or cyclones to improve separation and reverse flow direction; it allows chemical treatment (phosphate dosing) and blowdown to control water chemistry.
• Cyclone separators inside the drum use centrifugal force to separate water droplets from steam.

Circulation in water-tube boilers

• Definition: Circulation is the flow of water and steam within the boiler circuit between headers, risers and downcomers. If circulation is driven by density differences it is called natural circulation. If a pump forces the flow it is called forced or controlled circulation.
• In many natural circulation designs, the riser (heated tubes) is placed inside the furnace and the downcomer (unheated insulated path) is outside the furnace; heated water-steam mixture in risers rises to the steam drum while denser water descends in downcomers to the lower headers.
• The driving pressure head available for natural circulation depends on the density difference between the hot mixture in the riser and the colder water in the downcomer; this difference falls as operating pressure increases, and at the critical pressure there may be no natural circulation because the densities become essentially equal.
• The circulation ratio expresses the mass flow of water circulating through the riser/downcomer circuit relative to the mass of steam generated; related expressions and derivations are commonly shown in standard references.

Notes on formulas and symbols

• Illustrative expressions for pressure-head, circulation ratio and related quantities are often shown in design references; these expressions were indicated in the original material and are represented here by the preserved image placeholders where the detailed formulae and diagrams appear.

Economiser

• An economiser recovers heat from flue gases to preheat feed water before it enters the boiler drum, thereby reducing fuel consumption and increasing plant efficiency. An economiser typically raises the feed-water temperature close to saturation temperature.
• The overall heat-transfer rate for an economiser depends on the heat transfer area, number of coils, and the overall heat transfer coefficient; the detailed formulae and coil geometry were illustrated in the original text and are preserved at the indicated places.

Superheater

• A superheater raises the temperature of saturated steam leaving the boiler drum to a higher, 'superheated' temperature. Superheating increases the thermal efficiency of the cycle and improves steam quality (dryness) at the turbine inlet.
• Superheaters are classified by their dominant heat-transfer mode as radiant, convective or combined superheaters depending on whether they receive heat primarily by radiation, convection, or both from furnace gases.

Steam Generator Control

The objective of steam-generator control is to supply the steam flow, pressure and temperature required by the turbine, while maintaining safe operation and correct drum level. Key controlled variables include steam flow, steam pressure, steam temperature (superheat), primary and secondary air flows, fuel firing rate, feed-water flow, drum water level, and electrical power output. Control systems act on measured signals to adjust fuel and air flows, feed pumps, forced draft/induced draft fans and other actuators to maintain desired conditions.

1. Feed-water and Drum-level Control

• Feed-water flow and the drum water level are controlled so that steam flow meets turbine demand while the drum level remains within narrow limits.
• If turbine steam consumption increases without a corresponding increase in feed water the drum level will fall; conversely, too much feed will raise the level. Typically a three-element control system (which uses drum level, steam flow and feed-water flow measurements) is employed to regulate feed-water valves or pump speed to achieve stable level control over load changes.

2. Steam Pressure Control (Boiler Master)

• The steam-pressure control system, often called the 'boiler master', maintains steam pressure at its set point by adjusting fuel and combustion air flows. When pressure falls, fuel and air are increased; when pressure rises, they are reduced.
• The pressure sensor signal is used to control fuel feed devices (for example pulverised-coal feeders and fuel-oil valves) and fans (forced-draft, induced-draft). Trim signals from fuel- and air-flow sensors or a direct steam-flow measurement are used to maintain the correct fuel-air ratio.

3. Steam Temperature Control

The principal variables affecting superheat temperature are:

• Furnace temperature and fuel firing condition.
• Cleanliness of radiant and pendant superheater surfaces.
• Temperature of gases entering the convective superheater.
• Cleanliness and fouling of convective superheater surfaces.
• Mass flow rate of flue gases through the superheater and flow distribution.
• Feed-water temperature and steam flow rate.
• Variation in plant load and transient conditions.
• Reduction in steam temperature causes a loss in plant efficiency and can increase condensation-related losses.

4. Electrostatic Precipitator

• An electrostatic precipitator (ESP) removes fly ash from flue gases by charging particles in an electric field and collecting them on grounded plates. The principal components are grounded collection electrodes (parallel plates) and centrally-located discharge or emitting electrodes (wires) that create the corona discharge so particles become charged as gas flows between the plates.

Ash Handling System

• Large ash particles (bottom ash) are collected under the furnace in a water-filled ash hopper. Fly ash is collected upstream in dust collectors such as ESPs or baghouses.
• Typical pulverised-coal (PC) boiler ash partitioning is approximately 80% fly ash and 20% bottom ash, although actual proportions depend on fuel and combustion conditions.
• Three major considerations for ash disposal and handling are: plant site, fuel source, and environmental regulations. Requirements for water and land are important in selecting disposal systems.
• Common handling methods: sluice conveyors for bottom ash and hydraulic/vacuum conveyors for fly ash; chosen system depends on ash properties, plant layout and environmental controls.

Feed-water Treatment

• Make-up water and feed-water must be treated to prevent scale, corrosion and deposits that damage boiler tubes, reduce heat transfer and harm turbine blades.
• Objectives of feed-water treatment: prevent scale formation, eliminate or reduce corrosion, and prevent silica and other deposits from being carried into the turbine.
• Typical raw-water impurities: suspended solids, organic matter, hardness salts (calcium and magnesium), alkalinity (bicarbonates, carbonates), dissolved ions (sodium, sulphate, chloride), silica, and dissolved gases (oxygen and carbon dioxide).
• Deaeration: Deaeration removes dissolved gases (primarily O2 and CO2) by heating and venting the water so gases are less soluble at elevated temperatures. Deaerators (mechanical and thermal) are a standard part of feed-water treatment.
• Internal chemical treatment: Chemical dosing in the boiler (internal treatment) controls pH and precipitates hardness salts in a soft, removable form. For example, trisodium phosphate (Na3PO4) is used to increase alkalinity and react with calcium salts to form soft precipitates; monosodium phosphate (NaH2PO4) can be used to reduce alkalinity when required. Boiler water pH is commonly maintained alkaline (for example around pH 10.5) to minimise corrosion.
• Scale prevention: Two main steps are periodic or continuous blowdown to remove concentrated impurities, and chemical treatment (internal or external) to remove scale producers.

Types of High-Pressure Boilers

1. Lamont Boiler

• The Lamont boiler is a forced-circulation water-tube boiler in which circulation is maintained by a centrifugal pump driven by a steam turbine using steam from the boiler.
• Feed water passes through an economiser into the steam drum; from the drum it is drawn by the circulation pump and delivered to the evaporating tubes. The steam-water mixture from the evaporator returns to the drum where steam separates and is taken to the superheater; the superheated steam is then supplied to the turbine.

2. Loeffler Boiler

• The Loeffler boiler is an indirectly-heated, forced-circulation design intended to avoid deposition problems on boiler tubes by keeping tubes free of evaporated salts. It uses superheated steam from a reheater to evaporate feed water in an external evaporator, while furnace heat is mainly used for superheating.
• This design allows operation with higher salt concentrations in the circulating water and is compact, making it suitable for certain land and marine power applications.

3. Benson Boiler

• The Benson boiler is a high-pressure, once-through water-tube boiler operating at pressures near or above the critical point, where water does not undergo a distinct phase change in the evaporator. Because it is once-through, there is no steam drum and problems associated with bubble attachment to tube walls (which reduce heat transfer) are mitigated.
• Benson designs are compact and suited to high-pressure, high-temperature applications; they are widely used in modern power stations.

4. Velox Boiler

• The Velox boiler is a high-rate, high-heat-transfer boiler that uses high gas velocities and pressurised combustion. As gas velocity approaches or exceeds sonic speeds in parts of the heat-exchange path, heat transfer rates increase substantially, enabling large heat transfer from a relatively small surface area.
• Velox-type designs utilise pressurised combustion and forced circulation and are used where rapid response and compactness are required.

5. Supercritical Boilers

• Subcritical boilers consist of separate preheater, evaporator and superheater sections because evaporation occurs at a distinct saturation condition.
• Supercritical boilers operate above the critical pressure of water (where there is no distinct boiling point) and therefore eliminate a separate boiling section; they typically employ high pressures and temperatures (for example designs in the range of 125-300 bar and 510-660°C in various modern plants) and offer improved thermal efficiency over subcritical plants.

6. Fluidised Bed Boilers

• Fluidised-bed boilers generate steam by burning fuel in a fluidised bed of granular material (such as sand or ash) through which air flows from below. Pulverised or particulate fuel is fed into the bed; when sufficient air is passed upward the bed behaves like a fluid and becomes well mixed.
• Good intermixing produces uniform temperature and efficient combustion. The air velocity at which bed particles become suspended is called the fluidisation velocity.
• Two main types: Bubbling Fluidised Bed (BFB) and Circulating Fluidised Bed (CFB).
• Advantages of CFB boilers: fuel flexibility; high combustion efficiency; efficient sulphur capture (with limestone addition); low NOx emissions; simple fuel handling; and high availability.
• Disadvantages of CFB boilers: potential corrosion of reactor walls; attrition (wear) of particles; and complexity of hydrodynamics and solids handling systems.

Boiler Mountings and Accessories

Boiler Mountings

Mountings are devices attached to the pressure part of the boiler essential for safe operation and control. Typical mountings fitted on a boiler shell include:

• Safety valve: Releases excess steam automatically when pressure exceeds the set limit to prevent overpressure.
• Steam stop (junction) valve: A valve placed on the steam outlet to isolate the boiler from the steam main or turbine. Larger valves sometimes called junction valves; smaller sizes called stop valves.
• Vent valve: A small valve used for venting steam space during starting or shutdown and for controlled blowing down.
• Pressure gauge: Measures internal steam pressure; common types include Bourdon-tube and diaphragm gauges.
• Water-level indicator (gauge glass): Shows the water level in the boiler and is normally fitted with two independent indicators for safety.
• Feed check valve: Controls supply of feed water to the boiler and prevents backflow if the feed pump stops; fitted below normal water level.
• Fusible plug: A safety device that melts at a specified high temperature if water level falls too low, releasing steam into the furnace to alert operators and reduce overheating risk; usually made of a low-melting-point alloy or gunmetal.
• Blow-off cock (blowdown valve): Allows periodic removal of sludge and sediments from the boiler lowest water space and can be used to empty the boiler for inspection.

Boiler Accessories

• Feed pumps: Pumps that deliver feed water to the boiler at the required pressure and flow. Multiple pumps and redundancy are common to ensure reliable feed-water supply.
• Injector: A steam-powered device that uses steam to entrain and feed water into a boiler; suitable for small boilers where space or simplicity is important but generally not used for large, high-pressure plants.
• Economiser: Recovers heat from flue gases to preheat feed water, improving fuel economy and reducing thermal shock to boiler metal by supplying warmer feed water.
• Air preheater: Raises the temperature of combustion air by extracting heat from flue gases (usually placed downstream of the economiser), improving combustion efficiency.
• Superheater: Raises steam temperature above saturation; advantages include reduced steam consumption for a given output, reduced cylinder or blade condensation losses, less erosion of turbine blades and improved plant efficiency.
• Steam separator: Removes entrained water droplets from steam to improve steam dryness before it reaches engines or turbines; typically installed close to the steam outlet.
• Steam trap: Automatically drains condensate from steam lines and steam-using equipment while preventing live steam from escaping.

Advantages of fitting suitable accessories and maintaining correct mountings: improved thermal efficiency, reduced fuel consumption, longer component life (by preventing scale and corrosion), safer operation, and better steam quality for downstream equipment.