I've not actually seen any official industry practice that references the "10/13" rule but I suspect that it is just a "revised" form of the previously referenced "2/3" rule which is an informal reference to API RP-521 (see Section 3.18.2, 4th edition, March 1997).
All of this has to do with whether or not you can exclude tube failure as a case for relief system design and is related to ASME Section VIII, Div 1 code requirements for the standard hydrostatic test pressure for vessels. Previously, ASME required a test pressure of 1.5x MAWP, the current version of ASME allows a test pressure of 1.3x MAWP.
The current version of API RP-521 (4th edition) is written based on the older version of ASME and considers that...
"Since standard hydrostatic test pressure is 150 percent of the equipment design pressure, equipment failure, in other words, loss of containment of the low-pressure side to atmosphere, is unlikely to result from a tube rupture where the low pressure side (including upstream and downstream systems) is designed for at least two-thirds of the design pressure of the high pressure side."
and
"Pressure relief for tube rupture is not required where the low-pressure exchanger side (including upstream and downstream systems) is designed at or above this two-thirds criteria."
In other words an exchanger with a shell rating of 100 psig is not likely to lose containment to atmosphere due to tube failure when the tube side is rated for 150 psig or less because the shell side would have been tested at 150 psig (100 x 1.5) anyway.
Now I believe everyone is applying the "10/13" rule in anticipation of a change in API RP-521. Personally, since the change in the ASME code, I no longer like to think in terms of 2/3 or 10/13 but always look to see what the test pressure of the low side is compared to the high side rating. This way you don't have to worry about which ASME code was being used. I have heard that the next version of API RP-521 will not specifically reference the test pressure multiplier (1.5x or 1.3x) except maybe by example.
It is also important to note the following...
"For new installations, increasing the design pressure of the low-pressure side may reduce risk. Upstream and downstream piping and equipment systems must be thoroughly evaluated when this containment approach is taken."
In other words, having an exchanger with a shell rating of 300 psig when the tube side is rated to 450 psig has no benefit for the shell side piping or other equipment connected to the exchanger shell side if they are rated for only 150 psig.
Can I understand from your perfect explanation that the 10/13 or 2/3 rule should be applied for all heat exchangers where the LP side is still lower than the design pressure of the high side even after considering the hydrotest factor?
In other words, shall this rule be applied for all heat exchangers regardless of the situation and the possibility of having blocked isolation of the LP side?
I think by installing PSV(s) designed for tube rupture the 10/13 rule is not any more required.
You are correct, the 10/13 rule replaces the need for sizing your relief valve based on a tube rupture case. You need to look at the economics of both options.
