ISO tank · Polymerizing substances · IMDG Code Amendment 42-24

Where a monomer is stabilized chemically, special provision 386 of the IMDG Code requires the person offering it for transport to ensure the stabilization is sufficient to prevent dangerous polymerization at a bulk mean temperature of 50 °C, or 45 °C in a portable tank. The animation below shows a 20-foot tank container in which the stabilization fails, in section and as stowed in hold 4 of MSC Flaminia, where three such tanks were carried in 2012.

What the inhibitor does

Monomers such as styrene and divinylbenzene form free radicals continuously. The inhibitor, usually 4-tert-butylcatechol (TBC), captures radicals before polymer chains can grow. TBC needs dissolved oxygen to work, and in a closed tank the oxygen can run out before the TBC does.

Each radical captured consumes inhibitor, and the rate rises with temperature. At 40 °C the inhibitor is used about seven times faster than at 25 °C.

Heat balance and onset

Once the inhibitor or its dissolved oxygen falls too low, monomer units link into chains and each link releases heat. This can start locally in the tank while some inhibitor remains elsewhere. Reaction rate rises exponentially with temperature. Heat loss through the shell rises only in step with the temperature difference to the outside air. When total heat input exceeds total heat removal, the cargo warms and the reaction speeds up further.

What the relief valve does

The relief valve starts to discharge hot flammable vapour at its set pressure. Pressure can run above the setting if vapour is produced faster than the valve can pass it, and venting does not stop the reaction. On MSC Flaminia the valves operated and no tank rupture was reported. Once a valve lifts, the hazard is also outside the tank. On deck the vapour can disperse. Under deck, if ventilation is insufficient, it can accumulate in the hold until it reaches a flammable mixture.

MSC Flaminia, 14 July 2012

Three tank containers of divinylbenzene 80 % were filled on 21 June 2012 and stood on the New Orleans terminal for ten days, with daily highs up to 37 °C (98 °F). They were loaded on 1 July into hold 4, which lies between heated fuel oil wing tanks to port and starboard. Heated tank containers of diphenylamine (DPA) were stowed on top of, beside and diagonal to them, and the hold’s mechanical ventilation was not running. The cargo was shipped as Class 9 at the time, which allowed stowage under deck.

On the morning of 14 July, in mid-Atlantic, a white cloud of venting vapour set off the alarms. The relief valves had operated as designed. The crew, believing there was a fire, opened an access to hold 4 to insert a hose. The United States District Court found that this produced the spark that ignited the vapour. Three crew members died.

What the Code requires now

Amendment 38-16 added polymerizing substances to Class 4.1 as UN 3531 to UN 3534, with the criteria in 2.4.2.5 and special provision 386. Temperature control applies where the self-accelerating polymerization temperature is 50 °C or less in a packaging or IBC, or 45 °C or less in a portable tank.

Under 7.1.3.2, category C is on deck only. Category D is on deck only on cargo ships and on passenger ships within the passenger limit, and prohibited on other passenger ships. The under-deck stow in the animation is the 2012 MSC Flaminia stow of divinylbenzene 80 %. It is not a stow for these cargoes today.

SP 386 lists what the offeror has to weigh: tank capacity and geometry, insulation, loading temperature, voyage duration, ambient conditions, the properties of the stabilizer and nearby heat sources. The full text and the shipper’s confirmation are covered in Polymerizing substances, special provision 386 and shipper’s confirmation.

What to ask before accepting the booking

• Date the tank was filled, and the inhibitor type and level at filling.
• How many days the stabilization is good for at 45 °C, counted from filling, with terminal dwell and transhipment included. The Flaminia tanks stood ten days on the quay before loading.
• Whether temperature control is required, and the control and emergency temperatures if it is.
• Planned stow position relative to heated fuel tanks and heated cargo.