Design steam drum and water drum is important step in design steam boiler due to mechanical design and performance design. Steam drum and water drum are two of pressure part in boiler. The steam drum should be furnished with internals which at least include hydraulic baffle, mesh dryer screens to meet the required steam purity, and blowdown connection. Also, the drum internals are protected from any possible attack by the chemicals and there is no chance of chemicals short-circuiting and being discharged through the continuous blowdown system.
The water drum size is dependent on the number of tubes deep in the boiler bank. The chemical feed connection could be located on the water drum. By placing the chemical feed on the water drum the chemicals combine with the circulating water and are better mixed before entering the steam drum.
The drums should be of fusion welded construction fabricated from carbon steel plate and should be equipped with 450 mm diameter manholes on both end plates of the drum to permit access into the drum interior. All faces between manholes and drum plates should be fabricated and machined so as to obtain a perfect seal.
Necessary welding end inlet and outlet connections and nozzles are provided to accommodate the required valves and accessories. Steam drum should be provided with the necessary connections, such as safety valves, continuous bIowdown, water level and pressure measuring instruments, vents, etc.
Sufficient thermocouples should be provided on the drum shell. The metal temperature should be measured as input data to the plant computer which should be furnished in the computer room.
All connection openings should be protected with caps before shipment. Drum should be capable of complete drainage so as to assure the dry preservation of boiler when not in service. Pipes carrying the saturated steam from the boiler drum to the superheater should be evenly spaced on the drum and designed so that an adequate distribution of steam will be obtained for the superheater.
Necessary drum supports, such as U-bolts and nuts, should be furnished. Continuous steel rings for insulation support around manholes should be welded on the drum. Two (2) multiple port type drum water level assemblies, two (2) level transmitter pressure gauges and other necessary accessories should be provided on the drum. Boiler water level should be indicated on operator monitor in the central control room.
Insulation and jacketing should be installed on boiler drum and necessary fittings should be assembled and welded in the manufacturer's shop before shipment. Field welding of parts or fittings should be minimized. A single pass - cross flow or multiple passes generating bank design should be furnished. The generating bank should employ a 400 mm central access cavity for maintenance.
The tubes are rolled during erection into the steam and water drums without any butt welds. The entire generating bank sidewall construction features finned tubes will eliminates the need for an inner casing. Where header/drum ends are attached by welding this should be by using a fully penetrating plain butt weld joint.
Drums and headers should be self draining and provided with drain valves. Design headers / design steam drum and water drum material should be in accordance with ASME Boiler & PV SectionII D.