At a glance
- The category follows from numbers and the fluid — never from quality, and never from negotiation (Art. 13, Annex II).
- The expensive steps: from category II a notified body is involved and welding procedures and welders must be approved; from III a recognised third-party organisation for NDT personnel and the material appraisal come on top.
- Exactly on a boundary line? The lower category applies (Annex II point 4). But calculate with exact values, not rounded ones.
What a category actually is
A PED category says nothing about the quality of your equipment. It says how much energy is released if the equipment fails, and therefore how much evidence the legislator wants to see before it may be placed on the market. An excellently built vessel can be category IV; a mediocre one category I. The category follows exclusively from numbers and fluid properties.
The classification is set by Article 13 of Directive 2014/68/EU: pressure equipment is classified by ascending level of hazard in accordance with Annex II. That annex contains nine tables. Which table applies depends on three things, and those three together decide the entire outcome.
The three ingredients of the classification
1. What kind of equipment it is
A pressure vessel, industrial piping, or fired or otherwise heated equipment with a risk of overheating. That last group has its own table (table 5) and therefore ends up in a high category sooner than a vessel of the same size. Safety accessories and pressure accessories follow their own rules again.
2. Which fluid it contains
Two questions: is it a gas or a liquid at the maximum allowable temperature, and does the fluid fall into group 1 or group 2. Group 1 is the dangerous group, classified via the CLP Regulation (EC) No 1272/2008; group 2 is everything not in group 1. A liquid whose vapour pressure at the maximum allowable temperature is more than 0.5 bar above normal atmospheric pressure (1,013 mbar) is treated as a gas for classification — that follows from Article 4(1)(a)(i).
3. How big it is
For vessels: the maximum allowable pressure PS, the volume V and the product PS·V. For piping: PS, the nominal size DN and the product PS·DN. Those numbers set where you land in the chart of your table.
Below all thresholds of Article 4(1)? Then the equipment falls under Article 4(3) and there is no category. That is not an exemption from everything, but it is a very different regime.
What really changes per category
Not the category itself costs money, but what is attached to it. First the picture, then the detail.
| Category | Permitted modules (Art. 14) | Notified body | Welding and NDT (Annex I, 3.1) | Materials (Annex I, 4.2) |
|---|---|---|---|---|
| Art. 4(3) (SEP) | none — there is no conformity assessment | no | sound engineering practice of a Member State | no requirement from the PED |
| I | A | no | no approval requirement from point 3.1 | manufacturer substantiates |
| II | A2, D1 or E1 | yes | point 3.1.2: operating procedures ánd personnel for permanent joints approved | manufacturer substantiates |
| III | B (design type) + D B (design type) + F B (production type) + E B (production type) + C2 H | yes | point 3.1.2 and 3.1.3: NDT personnel approved by a recognised third-party organisation | particular material appraisal (PMA) assessed by the notified body |
| IV | B (production type) + D B (production type) + F G H1 | yes | same as category III | same as category III |
The step that costs the most sits between categories I and II: a notified body comes into the picture and your welding procedures and welders must be approved. The second step sits between II and III: there an approval requirement for NDT personnel arises for the first time — and that approval can only be given by a recognised third-party organisation (point 3.1.3) — and a particular material appraisal is assessed by the notified body.
Aside: "NoBo" and "notified body" are the same thing — NoBo is simply the abbreviation. Who holds which role exactly (and what the recognised third-party organisation and the user inspectorate do) is set out on the conformity assessment modules page.
The boundary lines: just below is not a safe place
Annex II point 4 states that the demarcation lines in the tables indicate the upper limit of each category. If your point lies exactly on a line, the lower category applies. That is favourable, but it is also exactly why designing just below a line is risky.
An example. A vessel with water at 110 °C or below, PS 10 bar and V 500 litres comes to PS·V = 5,000 bar·L. In table 4 that stays below the thresholds of Article 4(1)(a)(ii) and the equipment falls under Article 4(3). Raise PS to 21 bar at the same volume, or keep 12.6 bar at 800 litres, and suddenly you face category I with an EU declaration of conformity and a CE marking — table 4 requires both PS above 10 bar and PS·V above 10,000 bar·L for that.
The practical lesson: do not only run your design point, run the margin around it. A customer who later asks for a higher PS, or a design change of a few hundred litres, can silently push you a category up. The full report therefore states, per parameter, how much room remains to the next category.
Four cases that regularly go wrong
Several chambers in one item of equipment
For pressure equipment with several chambers, the highest category applicable to the individual chambers applies. Where a chamber contains several fluids, the fluid that requires the highest category decides (Art. 13(2)). A heat exchanger with steam on one side and cooling water on the other is not classified on the average.
Safety accessories
Safety accessories fall into category IV under Annex II point 2. There is one exception: accessories manufactured for specific equipment may be classified in the category of that equipment. A safety valve placed on the market on its own is therefore category IV.
Pressure accessories
Pressure accessories are classified under Annex II point 3 on their own PS, and on their volume V or nominal size DN. Where both apply, the higher of the two outcomes decides.
Assemblies
An assembly is more than the sum of its parts. Article 4(2) sets its own regime for assemblies generating steam or superheated water above 110 °C, and for assemblies placed on the market as a whole. The assembly gets its own assessment; the individual parts do not finish the job.
Frequently asked questions
What does SEP mean in the PED?
SEP stands for Sound Engineering Practice. It is not a category but the regime of Article 4(3): equipment that stays below all thresholds of Article 4(1) is designed and manufactured in accordance with the sound engineering practice of a Member State, is accompanied by adequate instructions for use, and does not bear the CE marking under this directive.
Which category is the heaviest?
Category IV. Only the modules B (production type) + D, B (production type) + F, G and H1 are permitted there, and a notified body is involved in each of them. Category IV is also the default category for safety accessories placed on the market on their own.
Can I choose a higher category myself?
Not the category itself — that follows compulsorily from Annex II. But under Article 14(3) you may apply a conformity assessment procedure of a higher category. That is done, for example, when one quality system has to cover an entire scope of supply.
What happens if my equipment sits exactly on a boundary line?
Then the lower category applies. Annex II point 4 states that the demarcation lines indicate the upper limit of each category. Do calculate with exact values and not with rounded numbers; rounding PS or V can flip the outcome.
Does the category also determine in-service inspection?
Not directly. The CE marking covers placing on the market; use is national law. In the Netherlands, for example, the pressure equipment decree designates which categories per Annex II table are subject to inspection before entry into service and periodic re-inspection — see the page on the Netherlands.
Which category is your equipment in?
The free tool gives the table, the category and the permitted modules, with the reasoning at every step. No account, no e-mail address.
Run the free check or see the full report