Process Equipment and Plant Design
A cylindrical storage tank can have a self supported conical roof,

Irrespective of its diameter
If the thickness of the roof is more than that of the cylindrical shell
If its diameter is less than 15-20metres
If its diameter is more than 50 meters

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Process Equipment and Plant Design
Ponchon Savarit method is based on the use of enthalpy concentration diagram, which contains the bubble point curve (saturated liquid curve), dew point curve (saturated vapour curve) and equilibrium tie lines. As compared to McCabe-Thiele's method, this method

Is more accurate in finding the number of equilibrium stages
Facilitates direct calculation of heat load on reboiler & condenser from, the diagram used in this method
Accounts for the enthalpy changes in the process
All of these

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