77% rule for heat exchangers

[picasa]

What exactly is the 77% rule for heat exchangers? What is the engineering basis for it? Is it followed for all shell and tube heat exchangers? What if it is not followed? Does any of the applicable codes mention it?

 

[codeeng]

Its taken from API 521. Basically if you satisfy this rule then you need not take into account a tube rupture case when sizing your relief valve. It assures some mechanical integrity from the hydrotest pressure 1.3 x Design Pressure (formally 1.5 x DP or 2/3 rule). Your Process group should consider this when establishing the exchanger design criteria.