The 80/20 Rule Isn’t Just for Dangerous Goods

Ask a logistics professional why a tank shouldn’t be offered for transport between 20% and 80% full, and the answer comes fast: surge. Ask why a tank of vegetable oil or fruit juice concentrate is exempt from that same discipline, and the usual answer is a shrug — “that’s a dangerous goods rule.” It isn’t. It’s a physics rule that happens to be written down in a dangerous goods regulations.

It’s Physics, Not Chemistry

The degree-of-filling requirement in the IMDG Code exists to stop a partially filled tank from generating unacceptable hydraulic force as the liquid surges inside it. That force comes from mass, momentum, and motion — a ship rolling in a seaway, a tanker braking hard on a highway — not from the liquid’s UN number or packing group. A road tanker of drinking water filled at 50% fill surges exactly as violently as a tank of the same volume and density carrying a Class 3 liquid. The tank shell, baffles, and mountings don’t know or care what’s inside.

What IMO’s CSS Code Says

This is exactly why the Code of Safe Practice for Cargo Stowage and Securing addresses tank ullage in its own right. In essence, it warns against offering a portable tank for shipment in a fill condition that can produce dangerous hydraulic surge forces — and it says this while discussing stowage and securing generally, not as a dangerous-goods-specific clause. The CSS Code governs the stability and securing of everything loaded aboard ship. Surge is a stability hazard first; it only becomes a “dangerous goods” topic because DG tanks are the ones most rigorously regulated.

Same Discipline for Non-Regulated Liquids

In practice, anyone loading non-regulated liquids into an ISO tank should respect two distinct principles borrowed from the dangerous goods world, not one. First, the 20–80% rule: if the tank has no internal baffles, don’t ship it partially filled in that middle band, where sloshing can compromise stability on the road or at sea. Second, the TP1 expansion methodology: calculate the maximum safe fill limit using the liquid’s density at 15°C and 50°C, so there’s enough ullage left when the cargo heats up to the 50°C reference temperature — non-regulated liquids expand in hot climates exactly like regulated ones do.

Non-regulated liquids don’t carry the elevated vapour pressure or packing-group triggers that call for TP2 or other overlay provisions, so TP1’s general formula is the right one to apply for that thermal check. No UN number required for either principle — the tank, and the sea or the road, respond to the same physics either way.

Watch Surge Happen

It’s easier to believe once you’ve seen it. This CFD physics simulation of a partially filled tanker under braking shows the surge wave forming and slamming the tank head — and how internal baffles break that wave up. No chemistry involved; it’s the same behaviour whether the tank holds diesel, drinking water, or milk.

Simulation by SIMULIA XFlow (Dassault Systèmes), via The Awesomer.

Whether or not the cargo ever appears on a dangerous goods manifest, the tank doesn’t know the difference. Neither does surge.