UN31A and the metal IBC

The family of metal codes

Metal IBCs are coded by what they hold and the metal they are made of (49 CFR 178.705). The first two digits are the same as for every IBC: 11 for solids loaded or discharged by gravity, 21 for solids under a pressure above 10 kPa (1.45 psig), 31 for liquids. The letter is A for steel, B for aluminium and N for other metals. So a stainless IBC for a liquid is 31A, and so is a carbon steel one; the code doesn't say which steel. The body material and its minimum thickness appear in the plate's second block.

Which liquids need one

The main reason to choose metal is fire. Plastic IBCs holding combustible or flammable liquids can melt or burn fairly rapidly and release their whole contents, while metal IBCs fitted with a pressure-relief vent can withstand fire for at least 30 minutes (Wikipedia, IBC). Fire guidance follows from that: plastic and composite IBCs should never hold Class I flammable liquids, and metal IBCs are suitable for those as well as Class II and III combustibles (IBCTanks.com on NFPA guidance). Your local fire code sets the limits on quantity and location.

The other reasons are service life and cleaning. A sanitary product moved between process steps, or a container refilled for years, is often better in stainless with a manway you can clean and inspect through. See stainless or carbon steel.

How the plate differs from a caged tote’s

The first block reads the same way: UN symbol, code, packing group letter, date, country, maker, stacking load and gross mass. The second block on a metal IBC must be on a corrosion-resistant metal plate and adds the body material and its minimum thickness in mm, the maximum loading and discharge pressure where one applies, and the maker's serial number (49 CFR 178.703(b)(2)). The serial number matters because a steel IBC is a long-lived asset. Its test records, which the owner must keep under 49 CFR 180.352(g), follow that number for years.

Testing a steel IBC over its life

The periodic rules are the same as for a caged tote: a leakproofness test and an external inspection every 2.5 years, and an internal inspection at least every five years. The internal inspection on a metal IBC also confirms the wall still meets its minimum thickness; one that doesn't is removed from hazmat service (49 CFR 180.352(b)(3)). Unlike the bottle of a composite tote, a steel body can be repaired and returned to service after it has been retested and marked. That is part of why steel IBCs are bought to come back.

Fittings on a steel IBC

Steel IBCs carry more fittings than a caged tote, and each is chosen for the product. A manway on top gives access for cleaning and inspection. A pressure and vacuum relief vent protects the shell when product warms or is pumped out. A bottom outlet, often a 2 in or 3 in ball or butterfly valve, drains the sloped or coned base. Many have a grounding lug for bonding during transfers of flammable liquids, which the static rules call for when the flash point is 60 °C or lower (49 CFR 173.35(k)).

Ask which gaskets are fitted at the manway and the valve, and whether they suit your product. A stainless shell is wasted if the gasket swells in the solvent.

Corrosion allowance

Metal IBCs for products that corrode the metal must be built with extra wall thickness, a corrosion allowance, unless the product is known not to attack it (49 CFR 173.35(c)). That is one reason to send the SDS: the maker checks the product against the steel and the wall before the design is approved for it.

Short answers

Is UN31A stainless or carbon steel?

Either. A means steel of any type or surface treatment. The plate’s second block names the body material and its minimum thickness.

Can a UN31A IBC carry a Packing Group I liquid?

No. No IBC of any build may be filled with a Packing Group I liquid under 49 CFR 173.35(j).

What does 31B mean?

A rigid aluminium IBC for liquids. 31N is the code for other metals.

Can a steel IBC carry Packing Group I liquids?

No. No IBC may be used for a Packing Group I liquid under US rules, whatever its material.

Is stainless or carbon steel better for solvents?

Carbon steel suits many solvents and costs less; stainless suits corrosive or water-based products and repeated cleaning.

Does a UN31A plate show specific gravity?

Metal IBCs show maximum permissible gross mass rather than a specific gravity number. Divide that by the volume to see what density fits.