Sunil,
Exactly the same as your question was asked from India by Vishal Tailor on November 3, 2010 and Tom (Tom@codeware) answered with nice explanation. So I am copying here Tom's answer
Tom says
This is an interesting question, one that I had some correspondence about with TUV some 1-2 years ago.
ASME standard B16.5 (2003 Edition) provides dimensions and ratings in metric units (these are the primary tables, the US units are secondary in Annex F). But the bolt sizes remain specified in US units ("inches") rather than converted or transformed to metric size. For example, Table 10, page 78, for Class 300 flanges shows metric dimensions for drilling but shows bolt sizes (and holes) as inches.
Paragraph 6.10 addresses the flange bolting dimensions and Table 1C (page 12) refers to ASME B18.2.1; this standard addresses only "inch" dimensions. Thus it appears that B16.5-2003 requires the use of the "inch" sized bolts; using metric bolts might disqualify the flanges from being used on the B16.5-2003 basis (could be analyzed by ASME Section VIII Division 1, Appendix 2, but many standard flanges do not pass this analysis).
Then there is a materials issue. For example, a 24" Class 300 flange requires 1.5" bolts. This converts to 38.1 mm. Not that you can buy a 38.1 mm bolt, I don't think that even a 38 mm bolt can be obtained. But in any case, if you want to use SA-193 B8 Class 2 bolt material from the Metric Edition of ASME Section II Part D this material is listed only for up to 38 mm diameter bolting.
It seems that ASME needs to tweak these standards further.
The final answer... consult with your Authorized Inspection Agency.
See this site
http://www.flangevalid.com/en/downloads and obtain the paper "Metric Bolts in Imperial Flanges" for a useful discussion.
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