7.3 De-aerator construction
The de-aerating heads and tanks shown in Figs 3.56 to 3.58 are cylindrical pressure vessels, with dished ends constructed of mild steel. The spray nozzles and trays are made of stainless steel to prevent corrosion attack.
The spraywater distribution system and tray support structure, shown in Figs 3.56 and 3.57, are made of mild steel and the perforated stainless steel trays are supported and held by this structure. The trays have to be fixed firmly, as they are subjected to the forces created by the steam as it flows across the trays.
Because the head on the de-aerator illustrated in Fig 3.56 is supported on steelwork, the connections to the tank are all provided with bellows joints to allow for relative movement. The vent condensers are similar in construction to the LP heaters described in Section 8 of this chapter. They have mild steel tubeplates with stainless steel tubes fixed by expansion into the tubeplate. The shell and other details are made of mild steel.
De-aerators ordered prior to 1976 were designed and constructed in accordance with BS1500 and after 1976, to BS5500 . The thickness of the tank walls, dished ends, sizes of branch compensations, allowable size and positions of support feet, etc., are all determined by use of BS5500 .
Appendix J of BS5500  is used to determine relief valve capacity.
As already indicated, the de-aerator tank is a large vessel and therefore cannot be transported and erected in one piece. The tank is made in segments rolled from mild steel plate, with the axial joints made by welding. When the segments are joined to form the tank, these joints are positioned so they do not coincide. This prevents a continuous line of weld in the horizontal direction and the crossing of two welds. The tank is site-erected in segments and welded together when in place.