Boiler drum

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The drum, also known as the boiler drum, is the heaviest pressure-bearing component and the largest piece of equipment in boilers with natural circulation or controlled circulation. The drum of modern subcritical pressure boilers is 25–28 meters long, 1700–1800 mm in diameter, and 130–200 mm in thickness. The drum is provided with numerous tube openings along its length and perimeter, to which tube seats are welded. During installation, it is connected to the water supply pipe, downcomer, steam-water mixture inlet pipe, steam outlet pipe, and other auxiliary fittings respectively. The drum is equipped with many devices or components that perform different functions, among which the steam-water separation equipment is the most numerous and also the most complex. In natural circulation and controlled circulation boilers, the steam drum plays an important role. (1) The drum receives water from the economizer and a steam-water mixture from the water wall; it supplies water to the circulation loop through the downcomers and provides saturated steam of appropriate quality to the superheater through the steam outlet tubes. It serves as a connecting link that ensures the proper operation of the boiler’s main heating surfaces (economizer, water wall, and superheater). Internally, it facilitates thorough separation of steam and water, thereby serving as an important factor in ensuring that the superheater and turbine receive steam of high quality. Among these, ensuring the proper operation of the circulation loop and the production of steam of high quality is of utmost importance. (2) The drum has a large volume and can store a large amount of water, thus possessing a certain heat storage capacity; this provides some buffering effect when the boiler load changes. (3) To thoroughly separate the steam-water mixture from the water wall, multiple stages of steam-water separation equipment are installed in the drum. Figures 5-6 show the drum structure of a control-cycle boiler, which is equipped with 110 turbine-type cyclone separators and corresponding corrugated plate-type secondary separators as the primary separation devices for steam and water. Before the steam is sent out of the drum (to the superheater), it passes through a corrugated plate separator and porous flow equalizing plates for the final separation of fine water droplets. The separation efficiency is as high as 99.7%–99.9%. (4) Appropriate chemical treatment is applied to the circulating boiler water to prevent the deposition of impurities such as scale in the circulation loop; therefore, dosing pipes are installed inside the drum. The feed water that enters the boiler (condensate from the turbine plus a small amount of make-up water) is all treated pure water. During the treatment process, the scaling substances are converted into suspended sludge, and after multiple stages of treatment, the vast majority of the contaminating impurities in the water are removed. To prevent trace suspended contaminants from deteriorating the quality of the steam, a continuous drain pipe is also installed in the drum to remove the suspended impurities near the water level. At the lowest point of the circulation loop (at the bottom of the lower water wall header), there is also a regular blowdown device designed to remove impurities such as scale and rust.