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# PED Article 9 Pressure Categories and Fluid Groups Explained

The Pressure Equipment Directive 2014/68/EU assigns pressure equipment to conformity assessment categories I through IV — or below the threshold as SEP (Sound Engineering Practice) — based on the fluid it contains, its maximum allowable pressure, and its volume or nominal size. Getting this categorisation wrong means either over-engineering your compliance approach or, more dangerously, under-specifying the notified body involvement that the Directive requires.

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At a glance

Fluid groups

Group 1 (hazardous) / Group 2 (all others)

Category range

SEP, I, II, III, IV

SEP threshold

Below lowest Annex II table line — no CE mark

Directive reference

2014/68/EU, Article 9 & Annex II

## PED categorisation framework — key concepts

### Fluid group classification under Article 13

PED 2014/68/EU Article 13 divides fluids into two groups. Group 1 covers hazardous fluids: substances and mixtures defined as explosive (H200–H205), flammable (H220–H228), oxidising (H270–H272), toxic (H300, H310, H330), or highly toxic — essentially CLP-regulated hazardous substances. Compressed air, nitrogen, water, and steam are Group 2 (all fluids not belonging to Group 1). The fluid group classification is the first mandatory determination because it selects which Annex II table applies. A vessel containing propane (H220 — extremely flammable gas) is Group 1; an identical vessel containing compressed nitrogen is Group 2, and the category outcome will be different even at the same PS and V.

### Determining variables: PS, V, and DN

Three parameters drive the Annex II table look-up. PS (maximum allowable pressure) in bar gauge — the highest pressure the equipment is designed to withstand in normal operation. V (volume) in litres for pressure vessels — the internal volume. DN (nominal size) in millimetres for piping — the nominal pipe diameter per EN ISO 6708. For accessories (valves, safety devices), DN is used. For assemblies of multiple items, Article 4(2) requires assessment of the assembly as a whole, applying the most onerous category from the Annex II assessment of each constituent item. Steam has its own tables regardless of whether it could be classified as a Group 2 gas.

### Reading the Annex II tables — which table applies

There are nine tables in Annex II, and selecting the correct one is the core skill. Tables 1 and 2 cover pressure vessels containing gases: Table 1 for Group 1 gases, Table 2 for Group 2 gases (including steam — see Note 3 at the bottom of the Annex). Tables 3 and 4 cover pressure vessels containing liquids: Table 3 for Group 1 liquids, Table 4 for Group 2 liquids. Tables 5 and 6 cover piping: Table 5 for Group 1 gases and liquids (separate zones within the table), Table 6 for Group 2 gases and liquids and steam. Tables 7, 8, and 9 cover pressure accessories, safety accessories, and assemblies. Within each table, find the PS value on one axis and V or DN on the other — the intersection zone gives the conformity assessment category.

### Category I through IV and SEP — what each means

SEP (Sound Engineering Practice) applies below the lowest threshold line in each table. Equipment in the SEP zone is not subject to PED conformity assessment procedures and must not bear the CE mark — it must be designed to sound engineering practice but requires no DoC. Category I is the lightest formal PED category: Module A (internal production control — manufacturer self-assessment with no notified body involvement) or A2. Category II adds options D1 and E1 (quality assurance modules). Category III requires Module B (EC type examination) plus D, F, E, or H (various production control modules), or G (unit verification). Category IV is the most stringent: Module B+D, B+F, or G — the notified body is involved in both type examination and ongoing production surveillance.

### Fire tube boilers, steam generators, and special rules

Steam pressure vessels and boilers have specific provisions. Steam generators and fire-tube boilers are assessed per Table 2 (Group 2 gas — steam is treated as a non-hazardous fluid per Note 3 to Annex II) but with Article 9(3) specifying that pressure vessels designed to generate steam or superheated water at temperatures above 110°C are always minimum Category I. Fire-tube boilers with volume V greater than 2 litres and PS greater than 0.5 bar are always at least Category III regardless of the table intersection. This 'floor' provision prevents low-pressure but large-volume steam generators from falling into SEP.

### Pressure accessories: valves and safety devices

Pressure accessories — including shut-off valves, check valves, pressure reducing valves, and safety valves — are assessed using Tables 6 (general accessories), not the vessel or piping tables. For safety accessories (pressure relief valves, burst discs, limiting devices), Table 9 in Annex II applies. The key parameter for accessories is DN (nominal size), combined with PS. Safety accessories designed to protect equipment that would otherwise be in Category III or IV are automatically assigned Category IV — a frequently overlooked provision that forces many safety valve manufacturers into the most stringent conformity route.

## PED conformity assessment process — step by step

01

Identify the contained fluid. Obtain the Safety Data Sheet (SDS) for each fluid and review Section 2 (hazard identification) and Section 9 (physical/chemical properties). Check if any CLP hazard statements in the H200–H228 or H270–H272 or H300–H330 range apply — if yes, the fluid is Group 1.

02

Classify the fluid as Group 1 (hazardous per Article 13) or Group 2 (all others including steam, water, nitrogen, compressed air). Document the classification rationale in the technical file with reference to the specific CLP hazard classes.

03

Determine the equipment type: pressure vessel (closed container for fluid under pressure, with V in litres) or piping (pipe fittings and fittings used to connect pressure equipment into a system, with DN in mm). Pressure accessories and safety accessories use their own tables.

04

Identify the correct Annex II table: Table 1 (Group 1 gas vessel), Table 2 (Group 2 gas or steam vessel), Table 3 (Group 1 liquid vessel), Table 4 (Group 2 liquid vessel), Table 5 (Group 1 piping), Table 6 (Group 2 piping and steam piping), Table 6 (general pressure accessories), Table 9 (safety accessories).

05

Read PS from the equipment design specification (maximum allowable pressure in bar gauge). For vessels, read V (internal volume in litres). For piping and accessories, read DN (nominal size in mm). If PS×V or PS×DN falls below the lowest boundary line in the table, the equipment is SEP.

06

Find the intersection of PS and V (or DN) in the selected table. The zone of the intersection determines the conformity assessment category: SEP (no PED procedure), I, II, III, or IV.

07

If the equipment is SEP: design to good engineering practice (e.g., relevant EN standards for guidance). Do not affix CE mark. Document the SEP determination in the technical file. The equipment need not be accompanied by a Declaration of Conformity.

08

If Category I–IV: select the conformity assessment module appropriate to that category. Category I: Module A or A2. Category II: A2, D1, or E1. Category III: B+D, B+F, B+E, H, or G. Category IV: B+D, B+F, or G. Engage a notified body for Modules B, D, F, G, and H as required by the selected module.

09

Complete the technical file covering: design calculations, materials used (with EN material standards), weld procedures and welder qualification records, pressure testing records, inspection records, and risk assessment. Issue the EU Declaration of Conformity referencing Directive 2014/68/EU and affix the CE mark accompanied by the notified body's identification number (for Category II–IV).

## Frequently asked questions

### What is the difference between Group 1 and Group 2 fluids under the PED?

Group 1 fluids are hazardous — defined by reference to CLP (Classification, Labelling and Packaging) Regulation criteria in PED Article 13. The relevant hazard classes are: explosive, flammable, highly flammable, extremely flammable, oxidising, toxic, and highly toxic. Common Group 1 fluids in industrial equipment include propane, butane, hydrogen, chlorine, and ammonia. Group 2 fluids are all fluids not in Group 1 — this includes compressed air, nitrogen, oxygen (which is oxidising and therefore Group 1, often incorrectly assumed to be Group 2), CO₂, water, and steam. Steam has specific notes in the Annex II tables that treat it as a Group 2 fluid for table selection purposes, but with a minimum category floor for boilers.

### How do I determine which Annex II table to use for my pressure equipment?

Table selection follows two questions: (1) Is the equipment a pressure vessel, piping, pressure accessory, or safety accessory? (2) What is the fluid group and state (gas or liquid)? For pressure vessels: Group 1 gas → Table 1; Group 2 gas or steam → Table 2; Group 1 liquid → Table 3; Group 2 liquid → Table 4. For piping: Group 1 → Table 5; Group 2 or steam → Table 6. For general pressure accessories (valves, fittings): Table 6. For safety accessories (relief valves, bursting disc devices): Table 9. Equipment containing multiple fluids or designed to contain a range of fluids must be assessed for the worst-case fluid and worst-case PS/V combination.

### What does SEP mean and does SEP equipment need CE marking?

SEP stands for Sound Engineering Practice — it is the zone in the Annex II tables below the lowest category threshold. Equipment falling in the SEP zone is explicitly excluded from the PED conformity assessment procedures by Article 4(3) of the Directive. SEP equipment must NOT be CE marked under the PED — the CE mark on pressure equipment signals conformity with the PED, and affixing it to SEP equipment is a non-conformity. SEP equipment must still be designed and manufactured safely — compliance with relevant European standards (EN 286 for simple pressure vessels, or applicable product standards) provides the guidance, but no formal notified body involvement or Declaration of Conformity is required.

### Where do steam pressure vessels fall in the PED category system?

Steam is treated as a Group 2 fluid for the purposes of Annex II table selection (per Note 3 to Annex II), so steam pressure vessels are assessed using Table 2 (Group 2 gas). However, Article 9(3) provides a minimum category floor for equipment that generates steam or superheated water above 110°C — such equipment is always at least Category I even if the Table 2 intersection would place it in SEP. Fire-tube and water-tube boilers with PS above 0.5 bar and volume above 2 litres fall into at least Category III under the special provisions for steam generators in Article 9(3)(b). In practice, most commercial steam generators and hot water boilers are Category III or IV.

**Disclaimer:** Educational resource only. Regulatory requirements change. Consult a qualified compliance specialist before making decisions.

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