8.4.7 Shell and dished end thickness
The pressure range over which LP heaters operate is from maximum bleed point pressure to full vacuum. If the shell thicknesses were determined by differential pressure alone, the thickness of the lower pressure heater shells would be insufficient to withstand loads from pipework or being lifted into place during the course of plant erection. In practice, shell thickness is not made less than about 12 mm but this again depends on individual manufacturer's practice. External stiffening rings are sometimes also used to permit the use of slightly thinner shells. The dished end thickness is chosen to suit the shell thickness.