You may see the statement in this post:
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=11452 I will self-quote my opinion from another post:
"I would add that for me it makes sense to follow NC-3658.1(b) "the Design Pressure used for the calculation of H […] shall be replaced by a flange design pressure =P+Peq", rather than consider P+Peq as design pressure.
The reason of my opinion is based on Kellogg book that mention “[…] analysis and experience which indicates that, with a properly preightened flange, the bolt load changes very little when a moment is applied, whereas the gasket loading changes appreciably.”
That means Kellogg team expectations were to have external loads transmitted to gasket loads rather than to affect W preightening load, and their Peq concept is a measure of this effect.
Indeed, including Peq only in H (or better said only in HD and consequently in H) means indirectly to consider
HG=W-H= W-PI/4*G^2*(P+ 16*M/PI/G^3+4*F/Pi/G^2)=W-PI/4*G^2*P-F-4*M/G
which is in line with Kellogg approach/ theory of plates. "
Best regards.