T20 is the IMDG Code Chapter 6.7 portable tank instruction at 10 bar test pressure with 8 mm reference-steel shell (about 5.57 mm in 316L per the Lloyd’s formula in IMDG 6.7.2.4), no bottom outlet, and a frangible-disc relief regime. It is one of three “extreme hazard” liquid codes in the T1 to T22 ladder, sitting alongside T21 and T22. Cargo population is small and specific: bromine UN 1744 (lead-lined), titanium tetrachloride UN 1838, trichlorosilane UN 1295, bromine pentafluoride UN 1745.
What T20 is built for
The IMDG Dangerous Goods List assigns T20 to a short list of substances where the combination of toxicity, corrosivity, and reactivity demands the heaviest standard build that still allows top discharge. Bromine is the canonical T20 cargo: SG 3.10, attacks essentially every common shell metal except lead and tantalum, vapour pressure at 50 deg C around 0.5 bar but with extreme acute toxicity. The cargo population is small enough that operator fleet management is highly specialised; Eurotainer runs the largest commercial bromine fleet.
Construction and materials
Two distinct builds sit on the same T20 plate, picked by the cargo.
Bromine lead-lined build (UN 1744). Carbon-steel Q345R shell with 3 to 6 mm lead lining. The lead resists bromine corrosion indefinitely; everything else fails. Lining-build tare runs 6,300 to 9,500 kg, considerably above a T11. Shell volume runs 16,000 to 20,000 L on a 20 ft frame (about 18,000 L typical). Bromine cargo SG 3.10 means the 36 t MGW caps the practical fill long before the shell volume: weight cap = (36,000 minus 8,500) / 3.10 = 8,870 L of bromine. Operators size and load bromine tanks so the weight-limited fill lands above the IMDG 80% surge floor, which means most bookings ship 22 to 26 tonnes of bromine in an 18,000 L lead-lined shell at 49 to 58% fill (the weight cap, not the surge floor, is the binding constraint and the 20% to 80% surge band does not apply when the cargo is denser than 1.0 kg/L).
TiCl4 / trichlorosilane stainless build (UN 1838, UN 1295). 316L stainless cylinder with a PTFE-lined extension on the TiCl4 fittings, where TiCl4’s reactivity with moisture matters. Trichlorosilane (UN 1295) ships in dry 316L stainless with rigorous moisture exclusion. Stainless tare runs 4,500 to 5,500 kg. Shell volume runs 17,000 to 22,000 L typical, in line with the T19 / T22 stainless fleet. TiCl4 SG 1.73 and trichlorosilane SG 1.34 mean the shell volume is the binding fill constraint, not the weight cap.
When T20 is the right choice
T20 is the right tank for the specific UN entries assigned T20 in IMDG DGL Column 13: bromine, titanium tetrachloride, trichlorosilane, bromine pentafluoride, and a small number of similarly extreme cargoes. The substitution rule of IMDG 4.2.5.2.5 lets a T20 cargo also ride T22 (the only stronger code in the relevant axis combinations). It does not allow T20 cargo to ride T14 or weaker.
When T20 is the wrong choice
T20 is the wrong tank for any cargo not in its specific UN list. The over-engineered shell (8 mm reference steel + lead lining) carries dead weight that is wasted on cargoes that can ride T11 or T14. The fleet is small and the lead time runs 120 to 240 days; a wrong booking is expensive to correct.
A specific failure mode
A specialty-chemical buyer books bromine UN 1744 from Tianjin to Sydney. The operator confirms a T20 lead-lined tank with shell volume 18,000 L. At loading the actual cargo mass is 30,000 kg of bromine. The math: 30,000 kg / 3.10 SG = 9,677 L of cargo in an 18,000 L shell. Volumetric fill ratio = 53.8%, well below the IMDG 80% surge ceiling for liquids denser than water (the 20% to 80% surge band specifically targets cargo of comparable density to water; for SG above 2.0 the ullage rule is governed by weight cap rather than fill ratio under IMDG 4.2.1.9.5). The booking checks out. The operational variance to watch is tare creep: if tare drifts from 8,500 kg toward 9,500 kg through age and re-lining, the maximum cargo mass falls from 27,500 kg to 26,500 kg, and a booking quoted at 30,000 kg cargo no longer fits.
How to verify a T20 booking
Pre-loading inspection covers the standard plate stack (CSC, 5-year hydraulic, 2.5-year intermediate) plus a lead-lining condition check, a frangible-disc tell-tale gauge inspection, and the operator’s maintenance history showing lining service. Lead linings are gradually phasing out across the broader fleet because of disposal cost, but for bromine specifically there is no cost-effective alternative. Eurotainer is the operator most experienced in this niche.