Saturday, June 19, 2010

Firetube Boilers

The firetube boilers is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes and in these cases water tube boilers can improve uneconomic. 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.

Of the variety proprietary designs of firetube or tank type boiler, many are composite, including sections for diesel exhaust gas heat recovery as well as for direct firing. In the Cochran boiler (above), the products of combustion and exhaust gases pass through separate sets of tubes immersed in the boiler water. These tubes are expanded into tube plates which form part of the boiler pressure shells. With the Aalborg AQ5 the gas streams pass horizontally over the outside of vertical tubes expanded into tube plates forming part of the boiler pressure vessel, in such a way that boiler water flows upwards throughout the tubes. Large downcomer tubes complete the circulation system.


A G Weser produced a boiler unit where the products of combustion pass through tubes surrounded by boiler water whilst diesel exhaust passes over tubes through which boiler water passes . A design similar in principle came from Howaldtswerke.


The Sunrod oil fired boiler combines a firetube and a watertube by arranging the latter inside the former. The watertube surface is extended by having steel pins electric resistance welded on its outer surface. The furnace is arranged either as a water cooled shell with a refractory floor or as a completely water cooled shell. In the largest sizes the furnace walls are of watertube construction. In each case a number of firetubes of large diameter extend upwards from the furnace top to a tube plate forming the top pressure shell. Inside each of these is arranged a watertube with extended surface. The top and bottom of each is connected through the wall of its firetube into the water space of the boiler.

Watertube Boilers

The construction of watertube boilers, which use small-diameter tubes and have a small drum, enable 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 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.


Modern D-type boilers have generating, superheating, feed and air heating surfaces in percentage areas and position in the boilers to suit the required operating conditions.

In the middle sixties practically all new vessels were propelled by diesel machinery. Reliable slow speed diesel engines were available which, burning heavy fuel, were economical, and being less complicated than a corresponding steam plant, were more easily automated.


The closure of the Suez Canal, however, caused tanker owners to consider the economies of transporting crude oil in greater bulk and this resulted in the design of 200,000 dwt tankers requiring 20,000 k W for propulsion. Such powers were higher than normally available from the oil engines of that period, and presented a great opportunity for the revival of steam propulsion. Boiler and turbine designers took advantage of the situation with the result that steam was once more adopted for the higher powers.


In the constant quest for lower overall costs, including initial and operating costs, turbine machinery installations have been designed with a single boiler for propulsion purposes. This generally being supplemented by some form of auxiliary power as a ‘get you home’ device, in the event of complete boiler failure.


The single boilers of such installation have, of necessity, to be as reliable as possible, and at the same time, must be capable of operating for long periods between shut downs for cleaning operations, etc.


The single boilers of such installation have, of necessity, to be as reliable as possible, and at the same time, must be capable of operating for long periods between shut downs for cleaning operations, etc.


Features embodied in boilers for this service include:

(a) Large furnaces with conservative heat release rates and ample flame clearances.

(b) Furnaces completely water-walled either with membrane-type walls or closely pitched tubes to cut down brickwork maintenance.

(c) Roof firing to give a more uniform heat release and improved gas flow through the boiler.

(d) Superheaters in lower temperature gas zones shielded from the furnace.

(e) Improved forms and materials for superheater supports.

(f) Improved methods of superheat control.

(g) Improved soot blowing arrangements.


An early development in watertube boilers was bent tube design. This boiler has two drums, an integral furnace and is often referred to as the ‘D’ type because of its shape . 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.

These provided the main heat transfer surfaces for steam generation. Large-bore pips 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 doubling casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner.


The early version of the D-type boiler were an important advance in their time but changes in refining methods and crude from various sources produced residual type fuel oils which began to reveal their shortcomings. The furnaces, being small and employing large amounts of refractory, operated at very high temperature. Flame impingement was not unknown and conditions generally for the refractories were severe and resulted in high maintenance. Refractories broke down requiring replacement. They were frequently covered in glass-like deposits, and on the furnace floor thick vitreous accumulation often required the use of road drill for removal.


In the superheater zone the products of combustion were still at high temperature and deposits from impurities in the fuel condensed out on the tubes, reducing heat transfer and steam temperature. Eventually, gas passages between the tubes would become so badly blocked that the forced draught fans would be unable to supply sufficient air to the burners, combustion became impaired and the fouling conditions accelerated. Sodium and vanadium compounds present in the deposits proved very corrosive to superheater tubes causing frequent repeated failure. Due to the fouled condition there was a loss of efficiency and expensive time-consuming cleaning routines were required.


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. 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 less superheating was required.


In the ESD II 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 above, the furnace is fully water-cooled and of monowall construction, it is produced from finned tubes welded together to form a gastight casing. With monowall 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 required to raise steam being transmitted by infra-red radiation. This usually required roof firing and a considerable height in order to function efficiently. The ESD IV boiler, shown above, 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 economiser. 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.



Boiler Mountings

Definition
Various valves and fittings are required for the safe and proper working of a boiler. Those attached directly to the pressure parts of the boiler are referred to as the boiler mountings.
Minimum requirements for boiler mountings
  • Two safety valve's
  • One steam stop valve
  • Two independent feed check
  • Two water gauge or equivalent
  • One pressure gauge
  • One salinometer valve or cock
  • One blowdown/scum valve
  • One low level fuel shut off device and alarm
Functions
SAFETY VALVE
Protect the boiler from over pressurization. DTI require at least two safety valve's but normally three are fitted ,two to the drum and one to the superheater. The superheater must be set to lift first to ensure a flow of steam through the superheater. These must be set to a maximum of 3% above approved boiler working pressure.

MAIN STEAM STOP
Mounted on supherheater outlet header to enable boiler to be isolated from the steam line if more than one boiler is connected. Valve must be screw down non return type to prevent back flow of steam from other boiler into one of the boilers which has sustained damage (burst tube etc) valve may be fitted with an emergency closing device.

AUXILLIARY STOP VALVE
Similar to main stops but connected to the auxiliary steam line

FEED CHECK VALVE'S
a SDNR valve so that if feed pump stops the boiler water will be prevented from blowing out the boiler. The main check is often fitted to the inlet flange of the economiser if no economiser fitted then directly connected to the boiler. The Auxiliary feed check is generally fitted directly to an inlet flange to the drum with crossovers to the main feed line. Usually fitted with extended spindles to allow remote operation which must have an indicator fitted.

WATER GAUGES
Usual practice is to fit two direct reading and at least one remote for convenient reading.

PRESSURE GAUGES
Fitted as required to steam drum and superheater header

SALINOMETER COCKS OR VALVE'S
Fitted to the water drum to allow samples to be taken. Cooling coil fitted for high pressure boilers.

BLOWDOWN COCK
Used to purge the boiler of contaminants.Usually two valve's fitted to ensure tightness . These valve's lead to an overboard valve.

SCUM VALVE
These are fitted where possibility of oil contamination exists. They are designed to remove water and/or contaminants at or close to normal working level.

Basic Boiler Contruction

Introduction

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating application.

Components

Steam drum
In the early designs the drums were riveted or solid forged from a single ingot, but for modern boilers the drum is generally fabricated from steel plate of differing thicknesses and welded. The materials used are governed by classification society rules. Test pieces must be provided. The cylindrical drum is normally constructed from four plates. Two dished End plates, a thick wall tube plate ( thicker to accommodate the holes drilled in it without increased stress) and completed with a thinner wrapper plate. Construction takes the form of rigidly clamping the descaled, bent wrapper and tube plates together. In addition test pieces cut from the original material are attached to the construction in such away that the longitudinal weld extends either sided of the join. These pieces are later removed and shaped test shapes cut out from specified areas including across the weld. The longitudinal weld is critical ( taking twice the circumferential stress) and is normally carried out by specialised automatic machinery using submerged arc techniques. The dished end pieces are accurately aligned and welded. On completion the construction is cleaned and non-destructive testing- such as x-ray photography, carried out. Final machining is carried out and any stub pieces and doublers attached. The now complete drum is heat treated at 600 to 650'C. The final process is hydraulic testing to classification requirements. Natural circulation within a boiler is due to the differing specific gravities of the water at the differing temperatures, the steam drum provides a reservoir of cool water to give the gravitational head necessary for natural circulation. Cool water entering the steam drum via the feed lines provides the motive effect for the circulation distributing it to the downcomers. Also the space within the drum provides for the separation of the steam and water emulsions formed in the water walls and the generating tubes. Water droplets entrained with the separated steam are removed by separating components fitted in the drum as well as the perforated baffle plates fitted at the water line. The space above the water line provides for a reserve steam space needed to maintain plant stability during manoeuvring conditions. Also fitted are the chemical injection distributing pipe and the scuming plate. The smaller the drum is made, the less thickness of material that is required. However, the limitation to how small is that sufficient space must be allowed for the separation of water from the steam before passing out to the superheater space otherwise dryers must be used. Also, due to the smaller reserve of water, larger fluctuations in water level occur during manoeuvring.

Water drum
Distributes feed water from the downcomers to the headers and generating tubes. Provides a space for accumulating precipitates and allows them to be blown down. Water drum size is limited to that required to receive the generating tubes, for modern radiant heat boilers with only a single bank of screen tubes and no generating tubes between the drums, the water drum has been replaced by a header and the downcomers fed straight to the waterwall headers. With system blow down is done at the steam drum. Too small a water drum can cause problems of maintaining ideal water level and little steam reserve.

Headers
These have a similar purpose to the water drum but are smaller in size. Due to their reduced size they may have a square cross section without resorting to exceptional thickness.

Generating tubes
Consists of a large number of small diameter tubes in the gas flow, more commonly found in boilers of an older design For roof fired boilers the generating bank may consist of one or two rows of close pitched tubes. For a modern radiant heat boiler the generating bank has been omitted to allow the replacement of the water drum by a distribution header, a bare tube economiser is fitted generating 5% of the steam capacity. The generation bank is normally heated by convection rather than radiant heat. For a set water circulation the tube diameter is limited to a minimum as the ratio of steam to water can increase to a point where the possibility of overheating could occur due to the lower heat capacity of the steam. The number of tubes is limited to prevent undercooling of the gas flow leading to dew point corrosion.

Screen tubes
These are larger bore tubes receiving the radiant heat of the flame and the convective heat of the hot gasses. The large diameter keeps the steam/water ratio down hence preventing overheating. There main duty is to protect the superheater from the direct radiant heat. On a modern marine radiant heat boiler the screen wall is formed out of a membrane wall.

Waterwall tubes
Contains the heat of the heat of the furnace so reducing the refractory and insulation requirements. Comes in three designs ;
  1. Water cooled with refractory covered studded tubes
  2. Close pitched exposed tubes
  3. Membrane Wall
Downcomers
These are large diameter unheated i.e. external to the furnace, their purpose is to feed water from the steam drum to the water drum and bottom headers.

Riser/Return tubes
These return steam from the top water wall headers to the steam drum.

Superheater tubes
These are small diameter tubes in the gas flow after the screen tubes. Due to the low specific heat capacity of the saturated steam they require protection from overheating in low steam flow conditions, say when flashing.

Superheater support tubes
These are large diameter tubes designed to support part of the weight of the superheater bank of tubes.