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.