T6 is the IMDG Code Chapter 6.7 portable tank instruction for Class 3 packing-group II flammable liquids. Min test pressure 4 bar, MAWP 2.65 bar, 6 mm reference-steel shell (4.18 mm 316L), bottom outlet allowed, normal pressure-relief valve. Cargo population: methanol UN 1230, ethanol UN 1170, acetone UN 1090, MEK UN 1193, toluene UN 1294, xylene UN 1307, isopropanol UN 1219, the broad middle of solvent and feedstock chemistry that ships in 20 to 30-tonne lots from Chinese petrochemical complexes.
What T6 is built for
Class 3 PG II covers flammables with a flash point below 23 deg C and an initial boiling point above 35 deg C. The IMDG Dangerous Goods List Column 13 typically assigns T6 or T7 to these cargoes (T7 carries the additional “solids only” bottom-outlet restriction; T6 allows liquid bottom outlets). In practice most Class 3 PG II ships in T11 rather than T6, because the substitution rule of IMDG 4.2.5.2.5 allows a T6 cargo to ride any stronger code through T22, and the operator fleet of T11 dwarfs any T6-only fleet. A new buyer rarely sees a quoted T6 build; the same physical equipment is usually quoted as a T11.
Construction and materials
Mechanically T6 is essentially T11 with a lower test pressure (4 bar vs 6 bar) and a lower PRV setting. 316L stainless cylinder, 6 mm reference-steel shell, polyurethane foam insulation 50 mm under aluminium or GRP cladding. Top fittings: DN500 manlid, 3 inch top discharge, 1.5 inch air-inlet, sample valve, thermometer well. Bottom fittings: 3 inch foot valve, butterfly secondary, flanged outlet with dust cap, the standard three-closure stack. Indicative new ex-China price USD 16,000 to 22,000, modestly below T11 because of the lower test-pressure rating.
When T6 is the right choice
T6 is the right tank when the assigned T-code is T6 or weaker and operator economics favour the lower-pressure build. In practice this is rare for ocean carriage: the price difference is small, fleet availability is thin, and the substitution rule lets the buyer pick T11 without penalty. T6 builds appear more often in dedicated shipper fleets (a chemical major running its own methanol fleet from a captive source) where the pressure rating is matched to the cargo and the fleet rotates predictably.
When T6 is the wrong choice
T6 is the wrong tank for any cargo IMDG DGL Column 13 assigns to T7 through T22. The substitution rule runs one way: stronger codes can carry weaker cargoes, not the other way around. T6 is also the wrong tank for non-DG food-grade or pharma-grade cargo, where the cleaning and certification regime matters more than the IMDG rating; pick a T11 food-grade or T11 pharma-grade build instead.
How a T6 booking compares to a T11 booking
The two are visually identical at the dock. Both are 20 ft 1CC frames with stainless cylinders and the standard top and bottom fittings. The plate stamps differ: a T6 plate reads “UN T6 / IMDG / 4 bar test” while a T11 plate reads “UN T11 / IMDG / 6 bar test”. Both carry the CSC plate, the 5-year hydraulic plate, and the 2.5-year intermediate plate. For a buyer the practical question is whether the operator fleet has ready T6 inventory at the loading port. Most operators consolidate fleet management around T11 for the substitution-rule reason; insisting on a T6 specifically usually adds lead time without adding value.