T18 is the IMDG Code Chapter 6.7 portable tank instruction at 10 bar test pressure with 6 mm reference-steel shell, bottom outlet permitted only for solids, and a frangible-disc plus tell-tale-gauge relief regime. T18 sits between T17 (10 bar, normal PRV) and T19 (10 bar, no BO at all). Cargo population: high-pressure highly hazardous Class 3, Class 6.1, and Class 8 cargoes where a frangible disc adds a useful safety margin.
What T18 is built for
The IMDG Dangerous Goods List assigns T18 to specific UN entries that combine high vapour pressure with a hazard profile justifying the frangible disc. Some Class 3 PG II flammables with elevated vapour pressure ride T18 rather than T15 because the frangible disc protects against catastrophic over-pressure. Acrylic acid stabilised UN 2218 may ride T18 with the TP18 special provision (no reheating during transit; cargo must arrive between 18 and 40 deg C without re-heating).
Construction and materials
316L stainless cylinder, 6 mm reference-steel shell. ASME U-stamp standard. Top discharge through a dip-pipe; solids-only bottom outlet typically plugged on liquid service. Frangible disc plus tell-tale gauge mounted in series with the spring-loaded PRV at 4.4 bar.
When T18 is the right choice
T18 is the right tank when IMDG DGL Column 13 specifies T18 for the UN entry. The substitution rule of IMDG 4.2.5.2.5 lets a T18 cargo also ride T19, T20, T21, T22.
When T18 is the wrong choice
T18 is the wrong tank for any cargo that fits T11 / T14 (where the lower test pressure is sufficient and the operator fleet is bigger) or T20 / T22 (where the higher hazard demands the thicker shell).