Heat Exchanger Ruling for 2/3 rule eliminate PRV?

[dumbdonn]

I have a heat exchanger with 1050psi feedwater into the tubes with a 1050MAP rating. The shell is heated with 215psi Steam and has a 700 MAP. Am I required to protect the shell with a Pressure Relief Device as per ASME VIII-div.1 UG-133 (d). Heat exchangers shall be protected with a pressure relief device. I hear some type of ruling is out there for a 2/3 rule that eliminates the PRV for the shell?

 

[samuelliu]

I don't know the rule at all but I am sure you have to install one PRV not only based on ASME BPVC, but also other standards/codes, such as HEI, API etc. It should be noted that the shell PRV selection is a very complex process for FWH due to high temperature feedwater leakage would result in flashing.

 

[EGT01]

Take a look at the Technical Inquiries section of the API web site and see if it answers your question about the 2/3's rule. I would add the 2/3's rule applies only when the low side has been pressure tested at 150%. The rule doesn't necessarily eliminate the need for a PRV but it does provide reasoning to exclude tube rupture from relief system sizing basis.

The 2/3's rule would also have to extend to all of equipment/piping on the low pressure side, not just the exchanger shell. Samuelliu is correct about accounting for flashing. There may be additional transient pressure (water hammer) type concerns if the low pressure were liquid full but with steam on the shell this may not be an issue.

http://www.api.org/techinq/PRS/521/521_inquiries.htm#top