At a glance
- Two questions select the table: the phase at the maximum allowable temperature TS, and group 1 or 2 under the CLP classification (Art. 13(1)).
- Vapour pressure at TS more than 0.5 bar above atmospheric? Then the liquid counts as a gas (Art. 4(1)(a)(i)). The TS you assign is a classification decision.
- The safety data sheet of the fluid or the mixture is the source — not the main constituent, and not your gut feeling.
Why this question weighs heaviest
Annex II contains nine tables. Which one applies follows from three things: the equipment type, the phase of the fluid and the group the fluid falls into. A vessel with a group 1 gas is assessed in table 1; the same vessel with a group 2 liquid in table 4. The limits in those tables sit orders of magnitude apart.
Put differently: a mistake in the fluid decision is not a rounding error. It moves your equipment to a different chart, and with it often two categories.
Question 1: gas or liquid
The directive does not draw the line on how the fluid looks at room temperature, but on behaviour at the maximum allowable temperature TS. The gas side under Article 4(1)(a)(i) covers:
- gases, liquefied gases and gases dissolved under pressure;
- vapours;
- and also liquids whose vapour pressure at the maximum allowable temperature is more than 0.5 bar above normal atmospheric pressure (1,013 mbar).
If that vapour pressure stays at or below 0.5 bar above atmospheric, the liquid side of Article 4(1)(a)(ii) applies.
Question 2: group 1 or group 2
Article 13(1) divides fluids into two groups. Group 1 consists of dangerous fluids as classified under the CLP Regulation (EC) No 1272/2008: among others explosives, flammable gases, oxidising gases, flammable liquids, self-reactive and pyrophoric substances, oxidisers, organic peroxides and substances with acute toxicity. Fluids whose maximum allowable temperature is above their flashpoint also fall into group 1. Group 2 is the remainder: everything not in group 1.
The decision therefore does not follow from your feeling about the fluid but from the hazard classes on the safety data sheet. That sheet is your source; open it and read sections 2 and 9.
| Fluid | Phase at usual TS | Group | Table for a vessel |
|---|---|---|---|
| Compressed air | gas | 2 | 2 |
| Nitrogen | gas | 2 | 2 |
| Natural gas | gas | 1 (flammable) | 1 |
| Oxygen | gas | 1 (oxidising) | 1 |
| Acetylene | gas, additionally unstable | 1 | 1 |
| Water ≤ 110 °C | liquid | 2 | 4 |
| Steam or superheated water > 110 °C | gas | 2 | 5 for fired equipment |
| Thermal oil above its flashpoint | liquid | 1 | 3 |
This table is a reading aid, not a classification. The group of a concrete fluid or mixture follows from its safety data sheet and from the temperature you assign as TS.
Four places where it goes wrong
Mixtures judged on the main constituent
A mixture is classified as a mixture, not as its largest component. A few per cent of an acutely toxic component can put the whole into group 1. The safety data sheet of the mixture decides, not that of the raw material.
TS set too generously
A manufacturer who takes TS wide for margin can lift the fluid over its flashpoint or over the 0.5 bar vapour pressure — and thereby switch tables himself. Margin in TS is not free.
Today's fluid instead of tomorrow's
Fill the vessel with a different fluid later and the classification changes. That is not theoretical: buffer vessels and storage tanks often get a different content during their life. Record in the instructions for which fluid and which TS the equipment was assessed.
The gas blanket above a liquid
An expansion vessel, hydrophore or sprinkler tank almost always has a gas cushion above the liquid: air or nitrogen under pressure. That cushion drives the classification, and gas lands in a much stricter table than liquid. One overlooked checkbox, and the same vessel jumps from Article 4(3) to category IV.
Frequently asked questions
What is the difference between group 1 and group 2 in the PED?
Group 1 consists of dangerous fluids as classified under the CLP Regulation (EC) No 1272/2008 — explosive, flammable, oxidising, acutely toxic, among others — and of fluids whose maximum allowable temperature is above their flashpoint. Group 2 is everything not in group 1. The group decides which Annex II table applies.
Is water group 1 or group 2?
Water is group 2. The follow-up question is the phase: at a maximum allowable temperature where the vapour pressure stays no more than 0.5 bar above atmospheric, the liquid side applies; above that, and for steam or superheated water above 110 °C, the gas side.
Why does the vapour pressure count at 0.5 bar?
Article 4(1)(a)(i) counts liquids whose vapour pressure at the maximum allowable temperature is more than 0.5 bar above normal atmospheric pressure of 1,013 mbar on the gas side. The reason is physical: such a liquid can flash-evaporate on failure and then behaves like a gas.
How do I classify a mixture?
On the mixture itself, through the safety data sheet of that mixture, and not on the main constituent. A small fraction of an acutely toxic or flammable component can put the whole into group 1.
Does the category change if I later fill the vessel with a different fluid?
Yes. The classification belongs to the fluid and the temperature the equipment was assessed for. Use it for a different fluid and the assessment no longer covers it. Record the assessed fluid and TS in the instructions for use.
Not sure about the table?
The free tool selects the table from equipment type, phase and group, and shows which rule was decisive.
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