T21 is the IMDG Code Chapter 6.7 portable tank instruction at 10 bar test pressure with 10 mm reference-steel shell (about 6.96 mm in 316L), no bottom outlet, normal pressure-relief valve (no frangible disc; the cargoes do not require one). T21 differs from T22 only on the relief regime: T21 uses a normal PRV while T22 adds a frangible disc. The cargo population is pyrophorics and water-reactives that ignite or react violently with air or water but where the kinetics don’t justify a frangible disc.
What T21 is built for
The IMDG Dangerous Goods List assigns T21 to a small list of organometallic and reactive substances. Triethylaluminium UN 3394 (TEAL), used as a co-catalyst in polyethylene production, is the canonical T21 cargo: pyrophoric on contact with air, reacts violently with water, transports under nitrogen blanket in dedicated equipment. Boron trifluoride diethyl etherate UN 2604 (BFEE), methyl iodide UN 2644 (a common alkylation reagent in pharma synthesis), and Ziegler-Natta polymerisation catalysts UN 2924 round out the list.
Construction and materials
316L stainless cylinder, 10 mm reference-steel shell, with built-in nitrogen-blanket capability: a top-mounted N2 line maintains positive inert pressure inside the tank to prevent any air ingress that would ignite the cargo. The fittings stack is similar to T22 (top discharge via dip-pipe, sample valve, thermometer well, manlid with PTFE gaskets) but with the relief regime simplified to a single spring-loaded PRV.
The “no frangible disc” choice for T21 reflects the chemistry: TEAL and similar pyrophorics react quickly with any oxygen ingress, so a frangible disc that would vent a large quantity of cargo on a transient over-pressure is more dangerous than a normal PRV that closes after relieving. The IMDG drafters made this distinction case by case in Table 4.2.5.2.6.
When T21 is the right choice
T21 is the right tank when IMDG DGL Column 13 specifies T21 for the UN entry. The substitution rule allows T21 cargo to also ride T22 (the only stronger alternative). In practice the dedicated T21 fleet is very small (low double digits of tanks worldwide) and operates as captive equipment for the polyolefin industry’s catalyst supply chain. A new buyer rarely sees a T21 booking on the spot market.
When T21 is the wrong choice
T21 is the wrong tank for any cargo not assigned T21 in IMDG DGL Column 13. The over-engineered shell and the N2-blanket capability are dead weight on cargoes that don’t need them. T21 is also the wrong tank when the cargo specifically requires a frangible-disc regime (escalate to T22 or T20).
How a T21 booking is verified
Pre-loading inspection covers the standard plate stack (CSC, 5-year, 2.5-year) plus the N2-blanket integrity (positive-pressure check at the manlid), the operator’s history showing dedicated TEAL or organometallic service (no cross-contamination from other cargoes), and the lining or alloy clad certificate where the build deviates from bare 316L. The fleet rotates between specific catalyst manufacturers (Mitsui, ExxonMobil Chemical, LyondellBasell, Chevron Phillips) and specific polyolefin plants on captive routes; operator selection is highly constrained by chemistry-supply-chain history.