8.4.1 Tube thickness
As the LP heater U-tubes are roller-expanded into the tubeplate, the tube wall thickness is determined by the need to have a long term leak-free joint between the tube and the tubeplate.
Experience has shown that tubes expanded into a mild steel tubeplate, as used for LP heaters, need a tube wall thickness of at least 1.2 mm to prevent the tube wall relaxing under conditions of thermal cycling.
While the thickness of 1.2 mm is greater than is needed for feed water pressure requirements, it nevertheless allows for corrosion damage and also makes the U-tubes easier to handle while the heater is being manufactured.
The tubes are fully expanded into the tubeplate for a depth of approximately 50-75 mm from the water side of the tubeplate and then lightly expanded for the rest of the depth of the tubeplate. The light expansion is to close the gap between the tube and tubeplate, where water could penetrate and possibly cause crevice corrosion. The mechanism of roller-expanding tubes into tubeplates is described in depth in Chapter 4, Section 3 which deals with the methods of fixing tubes in condenser tube sheets.