Water Tube Boilers

Water tube boilers convert heat from burning fuel within a central, boxlike open furnace chamber to generate either hot water or steam (often at very high pressure, temperature, and output capacities). 

However, contrary to FT boilers, the BW is held within one or more tubes, and heat from hot gases flows across the tube walls from the outside. As a consequence, a significant benefit of WT boilers is the fact, that for any given tube diameter, WT boiler tubes have a greater heating surface than FT boiler tubes. This is because the heating surface of WT 

Almost all designs of WT boilers have multiple banks of special-purpose tubes (water walls, boiler bank, tube bundles, etc.), although a particular type of WT boiler, the coil boiler, typically has only one coiled tube. 

Water tube boiler design and construction provide for much greater capacity, pressure, and versatility than FT boilers because of the subdivision of pressure parts and the ability to rearrange boiler components into a wide variety of configurations. As a result, steam output may be from under 1,500 lb/hr to several million lb/hr. Designers have, over the years, developed WT boilers for many diverse industrial process applications. 

As a general mle, WT boilers are safer from explosion than FT boilers because the dmm is not exposed to the radiant heat of combustion. If tubes rupture, there is only a relatively small volume of water that can instantly flash to steam. 

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The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

The steam for process heating is generated in either fire or water-tube boilers, using the most economical fuel available. The process temperatures required usually can be obtained with low pressure steam (tyq ically 25 psig), and steam is distributed at a relatively low pressure (typically 100 psig). Higher steam pressures are needed for high process temperatures. 

Cooling water required on site often is stored in towers storage tank problems or piping and valve malfunctions could cause loss of this component. If seawater is used, materials of construction must be more resistant to salt. Loss of steam purchased or generated in water tube boilers could result from boiler lube failure, turbine failure, or piping or valve malfunction. 

Waste-heat boilers. Waste-heat boilers can be designed to accept any grade of waste heat to produce steam or hot water. Designs can be based on water-tube boilers, shell and tube boilers, or a combination of the two. 

There is limited application for CO trim systems which are widely used on utility and other large water-tube boilers. The principle of operation is for an infrared beam to traverse the flue from emitter to sensor. The absorption of the infrared radiation is proportional to the CO content. 

Table 17.7 Water tube boilers - examples of low-pressure boiler water standards...

Water tube boilers are installed in all manner of commercial and institutional buildings, smaller industries, large industrial processors, and power generators. (In fact, the utility power industry is the single largest user of WT boiler capacity in the world.)... 

Water tube boilers have a pressure gauge, vent cock, and drum safety valve on the top of the steam drum. Where superheaters are fitted, the steam takeoff line leads to the superheater, which is followed by a superheater safety valve, automatic nonreturn valve, and stop valve with a pressure-equalizing line and valve. 

NOTE Water tube boiler water-wall headers should not be blown down while the boiler is under load because it disturbs the natural circulation and may result in an overheated and bulging or ruptured tube. Usually header blowdown valves are locked closed and are only blown down when the boiler goes offline. 

Water tube boiler plants lacking a deaerator... 

The values for pressure at the boiler outlet given in the BSI table of recommended water characteristics for fired water tube boilers... 

In the table of recommended water characteristics for nonfired water tube boilers, BSI proposes that plants containing copper alloys in either the feed or condensate system should have a FW pH limited to 8.5 to 9.2. If the feed system is completely ferrous, the pH should be limited to 9.2 to 9.5. For plants utilizing the FW for spray attemperation or desuperheating, the pH should be controlled with volatile alkalis only. 

Hinchley (1975) discusses the design and operation of waste heat boilers for chemical plant. Both fire tube and water tube boilers are used. A typical arrangement of a water tube boiler on a reformer furnace is shown in Figure 3.12 and a fire tube boiler in Figure 3.13. The application of a waste-heat boiler to recover energy from the reactor exit streams in a nitric acid plant is shown in Figure 3.14. 

Figure 3.12. Reformed gas waste-heat boiler arrangement of vertical U-tube water-tube boiler (Reprinted by permission of the Council of the Institution of Mechanical Engineers from the Proceedings of the Conference on Energy Recovery in the Process Industries, London, 1975.)...

Figure 3.15a. An arrangement similar to a conventional water-tube boiler. Steam is generated in cooling pipes within the reactor and separated in a steam drum. 

Field erected steam boiler (Water-tube boiler) CS Steam generation (kg-h-1) 20,000 3.28 x 105 10,000-800,000 0.81... 

FIG. 24-40 A D-type shop-assembled water-tube boiler. Combustion Engineering, Inc.)... 

Other of such standards which are useful are the EN 764-7 and EN 12952-10 Water-tube boilers and auxiliary installations - Requirements for safeguards against excessive pressure. Although its Annex ZA shows that it supports the PED clauses 2.11 and 7.3, this standard does not give any indication on the set pressure and the overpressure of the Safety Relief Valves, in which case one can refer back to the above. 

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