Why Torispherical Head is Restricted upto Pressure of 6.86bar in Specification

I don't know of any Code consideration that would require this. You may need to ask whoever wrote the spec for their reason.

banu....

It has been my pressure vessel design experience, in the USA, that there comes a point where 2:1 elliptical heads are a more economical choice than torispherical heads.

As I recall, there are both pressure and diameter considerations.

Where the pressure exceeds ~100 psig and the diameter exceeds ~ 60 inches, the elliptical head is usually a more economical choice.

There are other criteria to be applied for comparison and selection of heads, notably the selection of 80:10 heads for expensive vessel materials

Banu, tell us more about your vessel design conditions, material selection and ultimate duty of your component