Blind Flange Design

The italian rules for power boilers have a formula for calculating that. However it is too long (and misterious in its grounds) to be replicated here.
I would go for the calculation as follows:
1) check the cover thickness with the rule for a flat head using for the diameter the maximum diameter that can be inscribed between the ribs and the outer perimeter (the bolt circle if it is bolted)
2) check the ribs as simply supported beams wisely dividing the pressure load among the available ribs and using a composite section for the beam that includes the cover plate with a width of up to 10 thicknesses (or less if less is available: also here it is necessary to divide the plate among the ribs).
Note that ribs in form of a cross (as I understand from your post) is in my opinion not the best choice: one of the crossing ribs will have to be divided into two parts and to be full penetration welded to the other one in order to gain full collaboration. Two or three parallel ribs are a better choice. prex

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You might try considering the blind as as reinforced flat plate as suggested by PREX. ASME Section VIII, Div 1, Appendix 13 gives some good methods for reinforcing rectangular vessels. This appendix helps find the effective composite section for stiffening of the flat plate.

what r the methods to set a safety valve for a boiler pressure 25 bar and flow 120 tons /hour
vj

There's a rough way to design ribbed flat heads in D.S.Azbel & N.P.Cheremisinoff, "Chemical and Process Equipment Design: Vessel Design & Selection" [Ann Arbor Science, 1982] par 7.7.1

Basically they assume half the load is taken by the ribs.