Question: “We want to install a bottom discharge valve on our T20 tank container. What do we need to do to change from T20 to T12? Is a design certifying engineer needed?”
Tank Guru: No, you do not need a DCE. Although you will technically have to re-rate the vessel, you are down-rating the MAWP and Test Pressure rather than trying to increase the pressure ratings beyond what the original manufacturer certified.
The reason you will have to downrate the tank is because most bottom discharge valve assemblies commercially available for ISO tanks are only rated for 4 bar MAWP / 6 bar TP, so you will likewise need to downgrade the tank to 4 bar MAWP and then install a 4.4 bar PRV in place of the existing PRV that had a higher set-to-discharge pressure. This will make it a T11, or if a rupture disc is fitted, a T12.
With a change like this you are also supposed to request approval for the modification as per 49 CFR § 178.273 (e). The reason I say this is because the tank was originally designed with the relief valve(s) that is on it, and installing a PRV with a lower set pressure means that you now have less venting capacity than before. The approval agency should look at the specs for the new PRV to be used and confirm that the venting capacity is adequate. When we do modifications like this in our Houston shop, we take care of that paperwork on behalf of the customer.
Here is a link to the relevant section of 49 CFR: https://www.ecfr.gov/current/title-49/subtitle-B/chapter-I/subchapter-C/part-178/subpart-H/section-178.273
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