2/3 Rule for Heat Exchangers

[bluemax89]

Can someone tell me if this rule still applies? I have heard that there has been an update to the rule, however that's all I know. If anyone can shed some light on this topic it would be greatly appreciated.

Regards,
Bluemax89

 

[ihg]

CAUTION! It is only valid if the hydrotest pressure of the lowpressure-rated side equals or exceeds the design pressure of the HP side. With a 150% hydrotest, which used to be the case, this became the 2/3 rule. BUT current standards (ref. ASME Sect. 8) have dropped the hydrotest pressure relative to design pressure so now the rule is likelier to be 3/4. Check the relevant code which is being used.

 

[TD2K]

The intent of the 2/3s rule (or whatever we have to call it now) is that in the event of tube rupture (which is quite rare) is that the lower pressure side is not pressurized above its hydrotest pressure. If you remember that, you'll be fine.

All, the rule applies to the entire system including upstream and downstream piping that would be pressurized to the tube rupture pressure (whatever you work that out to be), you can't just stop at the exchanger's flanges and ignore everything else.

 

[CHD01]

Yes, with the new allowable stress values in the code, what we called the 2/3's rule must be changed and we now need to verify the hydrotest pressure - assuming (that word again) the fabricator applied the code correctly. The more you learn, the less you are certain of.