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Can any body explian the term "Minimum Allowable Thickness after Forming" (M.A.F) for dished end of heat exchangers?

Forming operation of heads or other shell sections cause reduction of thickness at certain locations. That's why the term "Minimum allowable thickness after forming" is used to specify that the reduction of thickness during forming operation should not go beyond this at any location.

Normally, there are two types of allowance which are added to the calculated pressure thickness of any component as per code. Corrosion allowance and the fabrication allowance; corrosion allowance is included by the designer and then he specifies the minimum thickness of any component. This thickness for dished ends become the minimum thickness required after forming operation.

Now the drawings come to the fabricator who has the fabricate the pressure components (shell, dished ends) and it's his responsibility to add the fabrication allowances (for rolling, forming etc.) and take the suitable thickness plate so that the thickness after forming operation remain within the thickness specified by the designer.