6.2 Construction of high pressure heaters
The construction of the pressure envelope of vertical and horizontal HP heaters is very similar and the only differences are the arrangement of the heater internals, support feet, etc.
The water header is very similar in both, although the tubeplate tube hole pattern may differ to allow for the different arrangements of the drain cooling and desuperheating sections. In all cases, BS5500  is used to determine the thickness of the water header, tubeplate, shell, etc. The U-tube wall thickness can be found by application of BS1113: 'Design and manufacture of water tube steam generating plant' , as BS5500  makes no provision for minimum bend radius of the thinning effect when forming a U-tube.
The materials normally used in the construction of HP heater headers are low carbon steel for the tube-plates with a forged steel or cast steel hemispherical head (hemi-head). The tubeplate and hemi-head are welded together to form the feedwater header. An alternative which has been used in the past is a one-piece forged low carbon steel water header of the 'bottle' type with welded-on feedwater branches. Typical examples of these methods of construction are shown in Figs 3.47 and 3.48. The tubes used are low carbon manganese steel suitable for fusion welding to the back of the tubeplate. The heater shell, dished end, baffle plates, etc., are all made from mild steel plate.
The following sections outline how the design codes are used to determine the physical size of a typical HP feedwater heater.