Today in 1972, US Patent 3685508 A was issued, an invention of Le Roy W. Heilmann, for his “Tank Construction.” Here’s the Abstract:

A tank bottom having spaced inner and outer members, said inner member being made of relatively thin material having good heat transfer, said spaced inner and outer members forming part of a pressure chamber for heating the contents of said tank, said pressure chamber being subjected to relatively high pressures and temperatures, said inner bottom having strengthening means connected to the outer surface thereof, said strengthening means not being connected to said outer member, said inner member being strong enough to hold the contents of the tank but not strong enough to withstand the cyclical pressures within the pressure chamber over a period of use without said strengthening means.