Boilers

Boilers

A boiler in one form or another will be found on every type of ship.  Where the main machinery is steam powered, one or more large watertube  boilers will be fitted to produce steam at very high temperatures and pressures. On a diesel main machinery vessel, a smaller (usually firetube type) boiler will be fitted to provide steam for the various ship services. Even within the two basic design types, watertube and firetube, a variety of designs and variations exist.

A boiler is used to heat feed water in order to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water.

A drum must be provided where steam and water can separate. There must also be a variety of fittings and controls to ensure that fuel oil, air and feedwater supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler.

In the steam generation process the feedwater enters the boiler where it is heated and becomes steam. The feedwater circulates from the steam drum to the water drum and is heated in the process. Some of the feedwater passes through tubes surrounding the furnace, i.e. waterwall and floor tubes, where it is heated and returned to the steam drum. Large-bore downcomer tubes are used to circulate feedwater between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums. The steam is produced in a steam drum and may be drawn off for use from here. It is known as 'wet' or saturated steam in this condition because it will contain small quantities of water, Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and 'dried', i.e. all traces of water are converted into steam. This superheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. An 'attemperator', i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature.

The hot gases produced in the furnace are used to heat the feedwater to produce steam and also to superheat the steam from the boiler drum. The gases then pass over an economiser through which the feedwater passes before it enters the boiler. The exhaust gases may also pass over an air heater which warms the combustion air before it enters the furnace. In this way a large proportion of the heat energy from the hot gases is used before they are exhausted from the funnel. The arrangement is shown in Figure 4.1.

Two basically different types of boiler exist, namely the watertube and the firetube. In the watertube the feedwater is passed through the tubes and the hot gases pass over them. In the firetube boiler the hot gases pass through the tubes and the feedwater surrounds them.

The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.

Watertube boilers

The construction of watertube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising

Figure 4.2 Foster Wheeler D-Type boiler

process is much quicker. Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the watertube boiler has replaced the firetube boiler as the major steam producer.  Early watertube boilers used a single drum. Headers were connected to the drum by short, bent pipes with straight tubes between the headers.

The hot gases from the furnace passed over the tubes, often in a single pass,

A later development was the bent tube design. This boiler has two drums, an integral furnace and is often referred to as the 'D' type because of its shape (Figure 4.2). The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These waterwall tubes are connected either to upper and lower headers or a lower header and the steam drum. Upper headers are connected by return tubes to the steam drum. Between the steam drum and the smaller water drum below, large numbers of smaller-diameter generating tubes are fitted.

Figure 4.3 Foster Wheeler Type ESD I boiler

These provide the main heat transfer surfaces for steam generation.  Large-bore pipes or downcomers are fitted between the steam and water drum to ensure good natural circulation of the water. In the arrangement shown, the superheater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brickwork is used on the furnace floor, the burner wall and also behind the waterwalls. The double casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner, The need for a wider range of superheated steam temperature control led to other boiler arrangements being used. The original External Superheater 'D' (ESD) type of boiler used a primary and secondary superheater located after the main generating tube bank (Figure 4.3). An attemperator located in the combustion air path was used to control the steam temperature.

The later ESD II type boiler was similar in construction to the ESD I but used a control unit (an additional economiser) between the primary and secondary superheaters. Linked dampers directed the hot gases over the control unit or the superheater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low temperature superheating was required.

In the ESD III boiler the burners are located in the furnace roof, which provides a long flame path and even heat transfer throughout the furnace. In the boiler shown in Figure 4.4, the furnace is fully water-cooled and of monowali construction, which is produced from finned tubes welded together to form a gaslight casing. With monowali construction no refractory material is necessary in the furnace.

The furnace side, floor and roof tubes are welded into the steam and water drums. The front and rear walls are connected at either end to upper and lower water-wall headers. The lower water-wall headers are connected by external downcomers from the steam drum and the upper water-wall headers are connected to the steam drum by riser tubes.

The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over the primary superheater and the economiser before passing to exhaust. The dry pipe is located in the steam drum to obtain reasonably dry saturated steam from the boiler. This is then passed to the primary superheater and then to the secondary superheater. Steam temperature control is achieved by the use of an attemperator, located in the steam drum, operating between the primary and secondary superheaters.

Radiant-type boilers are a more recent development, in which the radiant heat of combustion is absorbed to raise steam, being transmitted 

Figure 4.4 Foster Wheeler Type ESD III monowall boiler

by infra-red radiation. This usually requires roof firing and a considerable height in order to function efficiently. The ESD IV boiler shown in Figure 4.5 is of the radiant type. Both the furnace and the outer chamber are fully watercooled. There is no conventional bank of generating tubes. The hot gases leave the furnace through an opening at the lower end of the screen wall and pass to the outer chamber. The outer chamber contains the convection heating surfaces which include the primary and secondary superheaters. Superheat temperature control is by means of an attemperator in the steam drum. The hot gases, after leaving the primary superheater, pass over a steaming economises This is a heat exchanger in which the steam—water mixture

is flowing parallel to the gas. The furnace gases finally pass over a conventional economiser on their way to the funnel.  Reheat boilers are used with reheat arranged turbine systems. Steam after expansion in the high-pressure turbine is returned to a reheater in the boiler. Here the steam energy content is raised before it is supplied to the low-pressure turbine. Reheat boilers are based on boiler designs such as the 'D' type or the radiant type.

The problems associated with furnace refractory materials, particularly on vertical walls, have resulted in two water-wall arrangements without exposed refractory. These are known as 'tangent tube' and 'monowall' or 'membrane wall'.

In the tangent tube arrangement closely pitched tubes are backed by refractory, insulation and the boiler casing (Figure 4.6(a)), In the monowall or membrane wall arrangement the tubes have a steel strip welded between them to form a completely gas-tight enclosure (Figure 4.6(b)). Only a layer of insulation and cleading is required on the outside of this construction.

The monowall construction eliminates the problems of refractory and expanded joints. However, in the event of tube failure, a welded repair must be carried out. Alternatively the tube can be plugged at either end, but refractory material must be placed over the failed tube to protect the insulation behind it. With tangent tube construction a failed tube can be plugged and the boiler operated normally without further attention.

Firetube boilers

The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed. The name 'tank boiler* is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler* are also in use.

Package boilers

Most firetube boilers are now supplied as a completely packaged unit. This will include the oil burner, fuel pump, forced-draught fan, feed pumps and automatic controls for the system. The boiler will be fitted with all the appropriate boiler mountings.

A single-furnace three-pass design is shown in Figure 4.7. The first pass is through the partly corrugated furnace and into the cylindrical wetback combustion chamber. The second pass is back over the furnace through small-bore smoke tubes and then the flow divides at the front central smoke box. The third pass is through outer smoke tubes to the gas exit at the back of the boiler.

There is no combustion chamber refractory lining other than a lining

Figure 4,7 Package boiler

to the combustion chamber access door and the primary and secondary quart.

Fully automatic controls are provided and located in a control panel at the side of the boiler.

Cochran boilers

The modern vertical Cochran boiler has a fully spherical furnace and is known as the 'spheroid' (Figure 4.8). The furnace is surrounded by water and therefore requires no refractory lining. The hot gases make a single pass through the horizontal tube bank before passing away to exhaust. The use of small-bore tubes fitted with retarders ensures better heat transfer and cleaner tubes as a result of the turbulent gas flow.

Composite boilers

A composite boiler arrangement permits steam generation either by oil firing when necessary or by using the engine exhaust gases when the ship is at sea. Composite boilers are based on firetube boiler designs.  The Cochran boiler, for example, would have a section of the tube bank separately arranged for the engine exhaust gases to pass through and exit via their own exhaust duct.

Other boiler arrangements

Apart from straightforward watertube and firetube boilers, other steam raising equipment is in use, e.g. the steam-to-steam generator, the double evaporation boiler and various exhaust gas boiler arrangements.

The steam-to-steam generator

Steam-to-steam generators produce low-pressure saturated steam for domestic and other services. They are used in conjunction with watertube boilers to provide a secondary steam circuit which avoids any possible contamination of the primary-circuit feedwater. The arrangement may be horizontal or vertical with coils within the shell which heat the feedwater. The coils are supplied with high-pressure, hightemperature steam from the main boiler. A horizontal steam-to-steam generator is shown in Figure 4.9.

Double evaporation boilers

A double evaporation boiler uses two independent systems for steam generation and therefore avoids any contamination between the primary and secondary feedwater. The primary circuit is in effect a conventional watertube boiler which provides steam to the heating coils of a steam-to-steam generator, which is the secondary system. The complete boiler is enclosed in a pressurised casing.

Exhaust gas heat exchangers

The use of exhaust gases from diesel main propulsion engines to generate steam is a means of heat energy recovery and improved plant

efficiency.

An exhaust gas heat exchanger is shown in Figure 4.10. It is simply a row of tube banks circulated by feedwater over which the exhaust gases flow. Individual banks may be arranged to provide feed heating, steam generation and superheating. A boiler drum is required for steam generation and separation to take place and use is usually made of the drum of an auxiliary boiler.

Figure 4.10 Auxiliary steam plant system

The auxiliary steam installation provided in modern diesel powered tankers usually uses an exhaust gas heat exchanger at the base of the funnel and one or perhaps two watertube boilers (Figure 4.10).  Saturated or superheated steam may be obtained from the auxiliary boiler. At sea it acts as a steam receiver for the exhaust-gas heat exchanger, which is circulated through it. In port it is oil-fired in the usual way.

Auxiliary boilers on diesel main propulsion ships, other than tankers, are usually of composite form, enabling steam generation using oil firing or the exhaust gases from the diesel engine. With this arrangement the boiler acts as the heat exchanger and raises steam in its own drum.

Boiler mountings

Certain fittings are necessary on a boiler to ensure its safe operation. They are usually referred to as boiler mountings. The mountings usually found on a boiler are:

Safety valves. These are mounted in pairs to protect the boiler against overpressure. Once the valve lifting pressure is set in the presence of a Surveyor it is locked and cannot be changed. The valve is arranged to open automatically at the pre-set blow-off pressure.

Mam steftm stop valve. This valve is fitted in the main steam supply line and is usually of the non-return type.

Auxiliary steam stop valve. This is a smaller valve fitted in the auxiliary steam supply line, and is usually of the non-return type.

Feed check or control valve. A pair of valves are fitted: one is the main valve, the other the auxiliary or standby. They are non-return valves and must give an indication of their open and closed position.

Water level gauge. Water level gauges or 'gauge glasses' are fitted in pairs, at opposite ends of the boiler. The construction of the level gauge

depends upon the boiler pressure.

Pressure gauge connection. Where necessary on the boiler drum, superheater, etc., pressure gauges are fitted to provide pressure readings.

Air release cock. These are fitted in the headers, boiler drum, etc., to release air when filling the boiler or initially raising steam.

Sampling connection. A water outlet cock and cooling arrangement is provided for the sampling and analysis of feed water. A provision may also be made for injecting water treatment chemicals.

Blow down valve. This valve enables water to be blown down or emptied from the boiler. It may be used when partially or completely emptying

the boiler.

Scum valve. A shallow dish positioned at the normal water level is connected to the scum valve. This enables the blowing down or removal

of scum and impurities from the water surface.

Whistle stop valve. This is a small bore non-return valve which supplies the whistle with steam straight from the boiler drum.

Boiler mountings (water-tube boilers)

Watertube boilers, because of their smaller water content in relation to their steam raising capacity, require certain additional mountings:

Automatic feed water regulator. Fitted in the feed line prior to the main check valve, this device is essential to ensure the correct water level in.the boiler during all load conditions. Boilers with a high evaporation rate will use a multiple-element feed water control system (see Chapter 15).

Low level alarm. A device to provide audible warning of low water level conditions.

Superheater circulating valves. Acting also as air vents, these fittings ensure a flow of steam when initially warming through and raising steam

in the boiler.

Sootblowers, Operated by steam or compressed air, they act to blow away soot and the products of combustion from the tube surfaces.

Several are fitted in strategic places. The sootbiower lance is inserted, soot is blown and the lance is withdrawn.

The water level gauge provides a visible indication of the water level in the boiler in the region of the correct working level. If the water level were too high then water might pass out of the boiler and do serious damage to any equipment designed to accept steam. If the water level were too low then the heat transfer surfaces might become exposed to excessive temperatures and fail. Constant attention to the boiler water level is therefore essential. Due to the motion of the ship it is necessary to have a water level gauge at each end of the boiler to correctly observe the level.

Depending upon the boiler operating pressure, one of two basically different types of water level gauge will be fitted.

For boiler pressures up to a maximum of 17 bar a round glass tube type of water level gauge is used. The glass tube is connected to the boiler shell by cocks and pipes, as shown in Figure 4.11. Packing rings are positioned at the tube ends to give a tight seal and prevent leaks. A guard is usually placed around the tube to protect it from accidental damage and to avoid injury to any personnel in the vicinity if the tube shatters. The water level gauge is usually connected directly to the boiler. Isolating cocks are fitted in the steam and water passages, and a drain cock is also present. A ball valve is fitted below the tube to shut off the water should the tube break and water attempt to rush out.

For boiler pressures above 17 bar a plate-glass-type water level gauge is used. The glass tube is replaced by an assembly made up of glass plates within a metal housing, as shown in Figure 4.12. The assembly is made

up as a 'sandwich' of front and back metal plates with the glass plates and a centre metal plate between. Joints are placed between the glass and the metal plate and a mica sheet placed over the glass surface facing the water and steam. The mica sheet is an effective insulation to prevent the glass breaking at the very high temperature. When bolting up this assembly, care must be taken to ensure even all-round tightening of the bolts. Failure to do this will result in a leaking assembly and possibly shattered glass plates.

In addition to the direct-reading water level gauges, remote-reading level indicators are usually led to machinery control rooms.

It is possible for the small water or steam passages to block with scale or dirt and the gauge will give an incorrect reading. To check that

passages are dear a 'blowing through' procedure should be followed.  Referring to Figure 4.11, close the water cock B and open drain cock C. The boiler pressure should produce a strong jet of steam from the drain.  Cock A is now closed and Cock B opened. A jet of water should now pass through the drain. The absence of a flow through the drain will indicate that the passage to the open cock is blocked.

Safety valves

Safety valves are fitted in pairs, usually on a single valve chest. Each valve must be able to release all the steam the boiler can produce without the pressure rising by more than 10% over a set period.

Spring-loaded valves are always fitted on board ship because of their positive action at any inclination. They are positioned on the boiler drum in the steam space. The ordinary spring loaded safety valve is shown in Figure 4.13. The valve is held closed by the helical spring

whose pressure is set by the compression nut at the top. The spring pressure, once set, is fixed and sealed by a Surveyor. When the steam exceeds this pressure the valve is opened and the spring compressed.

The escaping steam is then led through a waste pipe up the funnel and out to atmosphere. The compression of the spring by the initial valve opening results in more pressure being necessary to compress the spring and open the valve further. To some extent this is countered by a lip arrangement on the valve lid which gives a greater area for the steam to act on once the valve is open. A manually operated easing gear enables the valve to be opened in an emergency. Various refinements to the ordinary spring-loaded safety valve have been designed to give a higher lift to the valve.

The improved high-lift safety valve has a modified arrangement around the lower spring carrier, as shown in Figure 4.14. The lower

spring carrier is arranged as a piston for the steam to act on its underside. A loose ring around the piston acts as a containing cylinder for the steam. Steam ports or access holes are provided in the guide plate. Waste steam released as the valve opens acts on the piston underside to give increased force against the spring, causing the valve to open further. Once the overpressure has been relieved, the spring force will quickly close the valve. The valve seats are usually shaped to trap some steam to 'cushion' the closing of the valve.

A drain pipe is fitted on the outlet side of the safety valve to remove any condensed steam which might otherwise collect above the valve and stop it opening at the correct pressure.

Combustion is the burning of fuel in air in order to release heat energy. For complete and efficient combustion the correct quantities of fuel and air must be supplied to the furnace and ignited. About 14 times as much air as fuel is required for complete combustion. The air and fuel must be intimately mixed and a small percentage of excess air is usually supplied to ensure that all the fuel is burnt. When the air supply is insufficient the fuel is not completely burnt and black exhaust gases will result.

Air supply

The flow of air through a boiler furnace is known as 'draught'. Marine boilers are arranged for forced draught, i.e. fans which force the air through the furnace. Several arrangements of forced draught are possible. The usual forced draught arrangement is a large fan which supplies air along ducting to the furnace front. The furnace front has an enclosed box arrangement, known as an 'air register', which can control the air supply. The air ducting normally passes through the boiler exhaust where some air heating can take place. The induced draught arrangement has a fan in the exhaust uptake which draws the air through the furnace. The balanced draught arrangement has matched forced draught and induced draught fans which results in atmospheric pressure in the furnace.

Fuel supply

Marine boilers currently burn residual low-grade fuels. This fuel is stored in double-bottom tanks from which it is drawn by a transfer pump up to settling tanks (Figure 4.15). Here any water in the fuel may settle out and be drained away.

The oil from the settling tank is filtered and pumped to a heater and then through a fine filter. Heating the oil reduces its viscosity and makes it easier to pump and filter. This heating must be carefully controlled otherwise 'cracking' or breakdown of the fuel may take place. A supply of diesel fuel may be available to the burners for initial firing or low-power operation of the boiler. From the fine filter the oil passes to the burner where it is 'atomised', i.e. broken into tiny droplets, as it enters the furnace. A recirculating line is provided to enable initial heating of the oil.

Fuel burning

The high-pressure fuel is supplied to a burner which it leaves as an atomised spray (Figure 4.16). The burner also rotates the fuel droplets by the use of a swirl plate. A rotating cone of tiny oil droplets thus leaves the burner and passes into the furnace. Various designs of burner exist, the one just described being known as a 'pressure jet burner' (Figure 4.16(a». The 'rotating cup burner' (Figure 4.14(b)) atomises and swirls the fuel by throwing it off the edge of a rotating tapered cup. The 'steam blast jet burner', shown in Figure 4.16(c), atomises and swirls the fuel by spraying it into a high-velocity jet of steam. The steam is supplied down a central inner barrel in the burner.

The air register is a collection of flaps, vanes, etc., which surrounds each burner and is fitted between the boiler casings. The register provides an entry section through which air is admitted from the windbox. Air shut-off is achieved by means of a sliding sleeve or check.  Air flows through parallel to the burner, and a swirler provides it with a rotating motion. The air is swirled in an opposite direction to the fuel to ensure adequate mixing (Figure 4.17(a)). High-pressure, higb-0i»tput marine watertube boilers are roof fired (Figure 4.17(b)). This enables a long flame path and even heat transfer throughout the furnace.

The fuel entering the furnace must be initially ignited in order to burn.

Once ignited the lighter fuel elements burn first as a primary flame and provide heat to burn the heavier elements in the secondary flame. The primary and secondary air supplies feed their respective flames.  The process of combustion in a boiler furnace is often referred to as 'suspended flame' since the rate of supply of oil and air entering the furnace is equal to that of the products of combustion leaving.

Purity of boiler feedwater

Modern high-pressure, high-temperature boilers with their large steam output require very pure feedwater.

Most 'pure* water will contain some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer, which can result in local overheating and failure of the tubes. Other salts remain in solution and may produce acids which will attack the metal of the boiler. An excess of alkaline salts in a boiler, together with the effects of operating stresses, will produce a condition known as 'caustic cracking'. This is actual cracking of the metal which may lead to serious failure.

The presence of dissolved oxygen and carbon dioxide in boiler feedwater can cause considerable corrosion of the boiler and feed systems. When boiler water is contaminated by suspended matter, an excess of salts or oil then 'foaming' may occur. This is a foam or froth which collects on the water surface in the boiler drum. Foaming leads to 'priming' which is the carry-over of water with the steam leaving the boiler drum. Any water present in the steam entering a turbine will do considerable damage.

Common impurities

Various amounts of different metal salts are to be found in water. These include the chlorides, sulphates and bicarbonates of calcium, magnesium and, to some extent, sulphur. These dissolved salts in water make up what is called the 'hardness' of the water. Calcium and magnesium salts are the main causes of hardness, The bicarbonates of calcium arid magnesium are decomposed by heat and come out of solution as scale-forming carbonates. These alkaline salts are known as 'temporary hardness'. The chlorides, sulphates and nitrates are not decomposed by boiling and are known as 'permanent hardness*. Total hardness is the sum of temporary and permanent hardness and gives a measure of the scale-forming salts present in the boiler feedwater.

Feedwater treatment deals with the various scale and corros