[Home](/)/[Resources](/resources)/PED Deep Dive EUPEDDeep dive # PED Category Classification Deep Dive: Pressure, Volume, and Fluid Group Logic The single most consequential decision in a PED project happens before a single test is run: which category does your equipment land in under Annex II? Get it wrong and you either over-engineer a compliance file for equipment that only needed self-declaration, or you ship equipment that a market surveillance authority will flag for lacking Notified Body involvement it legally required. This deep dive walks through exactly how the Annex II matrices work, why fluid group is the variable that moves everything, and where PED classification most commonly goes wrong. Copy Link[Share on WhatsApp](https://wa.me/?text=https%3A%2F%2Fkrono-labs.com%2Fguides%2Fpressure-equipment-directive-ped-deep-dive) At a glance Governing directive 2014/68/EU (PED) Classification basis PS × V or PS × DN, by fluid group Categories I through IV, plus Art. 4(3) SEP NB required from Category II upward Neighbouring regimes SPVD, TPED On this page * [1\. Category is calculated, not chosen](#category-is-calculated) * [2\. Fluid Group 1 vs Group 2: the variable that moves everything](#fluid-groups) * [3\. Reading the Annex II matrices by equipment type](#matrices) * [4\. Article 4(3): sound engineering practice and the SEP trap](#sep) * [5\. Module selection once you know the category](#module-selection) * [6\. Assemblies: when the whole system outranks its parts](#assemblies) * [7\. PED vs SPVD vs TPED: three pressure directives, one confusion](#neighbouring-regimes) * [8\. Common classification pitfalls](#pitfalls) ## 1\. Category is calculated, not chosen As [the base PED guide](/guides/pressure-equipment-directive-ped) explains, category under PED 2014/68/EU is not a design choice — it is the output of a calculation defined in Annex II, driven by maximum allowable pressure (PS), volume (V) or nominal diameter (DN), and fluid group. Manufacturers accustomed to other CE directives, where you often select the conformity route that fits your budget and timeline, sometimes assume PED works the same way. It does not. Two engineers with the same vessel geometry and the same pressure rating can land in different categories purely because one specified a Group 1 fluid and the other a Group 2 fluid. This matters commercially as much as technically. Category determines whether a Notified Body is mandatory, which directly affects both the cost and the lead time to market. A manufacturer who under-classifies a product — deliberately or by oversight — does not get a cheaper compliance path; they get a non-compliant product with a Declaration of Conformity that does not match what Annex II actually required. The rest of this deep dive works through the mechanics: how fluid group changes the calculation, how to read the matrices for different equipment types, the Article 4(3) exemption that some equipment falls under instead of a numbered category, and how category maps to the module you are actually allowed to use. ## 2\. Fluid Group 1 vs Group 2: the variable that moves everything Annex II runs on two fluid groups. Group 1 covers fluids classified as dangerous under the relevant hazard criteria — flammable, toxic, oxidising, or explosive substances, along with gases and superheated liquids that behave unpredictably under pressure loss. Group 2 covers everything else: water, steam, compressed air, nitrogen, and other fluids that do not carry those specific hazard classifications. The practical effect is that the same physical vessel, at the same PS and V, reaches a materially higher category when specified for a Group 1 fluid. A vessel that would comfortably sit in Category I or II holding compressed nitrogen can require Category III treatment — full Notified Body involvement including design examination — holding a flammable process gas at the same pressure and volume. This is intentional: the directive is weighting classification toward the consequence of failure, not just the mechanical stress involved. For manufacturers building product lines that get filled with different fluids depending on the customer application — a common pattern in industrial gas equipment — this means fluid group has to be decided per variant, at the design stage, before category and conformity route can even be discussed. Treating fluid group as a labelling afterthought is one of the more expensive mistakes in PED classification. ## 3\. Reading the Annex II matrices by equipment type Annex II is not a single matrix — it is a family of matrices, each keyed to a different equipment type and axis combination. Applying the wrong matrix to your equipment type is a distinct failure mode from misjudging fluid group, and it shows up most often when a manufacturer copies a classification approach from a different product family. Equipment type Classification axis Note Vessels (Group 1 fluid) PS × V (bar·litre) The steepest curve in Annex II. A vessel holding a flammable or toxic gas reaches Category III or IV at pressure/volume combinations that would be Category I or II for a Group 2 fluid. Vessels (Group 2 fluid) PS × V (bar·litre) Water, steam, compressed air, and nitrogen sit on the more forgiving curve — the same physical vessel can stay in Category I or II well past the point a Group 1 fluid would push it into NB territory. Piping (Group 1 fluid) PS × DN (bar·mm) Uses nominal diameter (DN) rather than volume. Gas pipework carrying hazardous fluids escalates quickly with diameter, which is why plant designers size Group 1 piping runs deliberately. Piping (Group 2 fluid) PS × DN (bar·mm) Steam and process-water piping follow a separate, less punishing DN curve — most utility piping in a facility sits comfortably in Category I or II. Fired or otherwise heated equipment (steam/hot water > 110°C) V (litres), fixed thresholds Boilers and similar equipment use volume-based thresholds independent of the general PS×V curve, reflecting the added risk of an internal heat source. Safety accessories and pressure accessories Inherits the category of the equipment they protect A safety valve or pressure switch does not get classified on its own merits — it takes the category of the vessel or system it is protecting, which is a common source of confusion when accessories are sourced from a different supplier. Safety accessories and pressure accessories are the row worth remembering: they do not get classified independently against Annex II — they inherit the category of the equipment they protect, which is easy to miss when accessories are procured from a separate supplier with its own compliance paperwork. ## 4\. Article 4(3): sound engineering practice and the SEP trap Equipment that falls below the Annex II category thresholds entirely does not simply escape PED scope with no obligations at all — it falls under Article 4(3), commonly referred to as sound engineering practice, or SEP. SEP equipment cannot bear the CE mark for PED purposes and does not go through a numbered conformity module, but it still has to be designed and manufactured to meet PED's essential safety objectives in Annex I, and the manufacturer still has to be able to demonstrate that in the event of a market surveillance inquiry. This is the trap: SEP is sometimes treated as "no PED obligations at all," when in reality it is a lighter-weight compliance obligation, not an absent one. Small accumulator vessels, low-pressure cylinders, and simple fittings below the thresholds typically fall here — and manufacturers are still expected to maintain documentation showing the equipment was engineered to recognised good practice, even without a Declaration of Conformity or a CE mark for PED specifically. A related and separate carve-out exists in the Simple Pressure Vessels Directive (SPVD) for a narrow class of welded vessels holding air or nitrogen — see Part 7 below for how that overlaps with, but does not replace, the general PED categories. ## 5\. Module selection once you know the category Once category is established, the directive restricts which conformity assessment modules you are permitted to use — this is not an open menu. Each category maps to a specific set of allowed modules, and the options narrow as risk increases. ### Category I Module A onlyNotified Body: No Internal production control. The manufacturer designs, tests against the relevant harmonised standard, and signs the Declaration of Conformity without third-party sign-off. ### Category II Module A2, D1, or E1Notified Body: Yes, for surveillance or verification A Notified Body carries out unannounced production checks (A2) or audits either the quality system for production (D1) or the finished product (E1) — the manufacturer still self-declares design conformity. ### Category III Module B+C2, B+D, B+E, B+F, or GNotified Body: Yes, mandatory EU type-examination (Module B) is now paired with a production-side module, or the manufacturer runs unit verification (Module G) for one-off equipment. This is where a Notified Body first reviews the design itself, not just production. ### Category IV Module B+D, B+F, G, or H1Notified Body: Yes, the strictest routes only The same design-plus-production pairing as Category III, but restricted to the highest-scrutiny module combinations — H1 (full quality assurance with design examination) is the ceiling of PED conformity assessment. One-off or very low-volume equipment has an additional lever worth knowing about: Module G, unit verification, lets a Notified Body assess a single item of equipment directly rather than the manufacturer's design and production system as a whole. It is available at Category III and IV and is frequently the right fit for bespoke reactors or custom pressure vessels that will never be produced as a series. ## 6\. Assemblies: when the whole system outranks its parts PED does not only cover individual vessels and piping runs — it also covers assemblies, meaning several pieces of pressure equipment assembled by a manufacturer to constitute an integrated, functional whole. A packaged boiler skid, a compressed-gas storage-and-distribution unit, or a process module combining several vessels and interconnecting piping are all classic assemblies. The classification question for an assembly is not simply "what is the highest category of any individual component." The assembly is assessed as a whole, considering interactions between components — how the piping, vessels, and safety accessories function together — which can push the assembly's overall category above what any single item would reach in isolation. In practice, the highest-category individual component tends to set a floor, with the interaction risk sometimes pushing the assembly higher still. This is a frequent source of underestimated compliance scope: a manufacturer sources several Category I or II components, integrates them into a skid, and assumes the finished assembly inherits the lower individual ratings — when in fact the assembly itself needs its own classification exercise under Annex II. ## 7\. PED vs SPVD vs TPED: three pressure directives, one confusion PED is the general-purpose regime for stationary pressure equipment, but it is not the only EU pressure directive, and manufacturers regularly need to determine which one — or which combination — actually governs their product. Regime Scope Marking PED 2014/68/EU Stationary pressure equipment above 0.5 bar: vessels, piping, safety accessories, pressure accessories, and assemblies. CE mark, plus the Notified Body number where one was involved. SPVD (Simple Pressure Vessels Directive) A narrower carve-out for simple welded vessels holding air or nitrogen — a lighter-weight regime than full PED for a specific, low-complexity equipment type. CE mark, under the Simple Pressure Vessels rules rather than the general PED matrices. TPED 2010/35/EU Pressure equipment built for the transport of dangerous goods — cylinders, tubes, drums, cylinder bundles, and tanks that move between locations rather than sitting fixed in a plant. The Pi (π) mark instead of CE, reflecting TPED's alignment with ADR/RID transport agreements rather than the general product-safety framework. The distinguishing question is almost always: is the equipment stationary or transportable, and if it is a simple welded vessel, does it fall within SPVD's narrower scope instead of the general PED matrices? Getting this wrong means building a technical file, and potentially engaging a Notified Body, against the wrong legal instrument entirely — the file has to be rebuilt against the correct one, since a PED Declaration of Conformity does not substitute for a TPED Pi-marking process or vice versa. ## 8\. Common classification pitfalls ### Classifying by the vessel's nameplate pressure instead of maximum allowable pressure (PS) PS is a specific defined term in PED — the maximum pressure for which the equipment is designed, as specified by the manufacturer — not an operating pressure, a test pressure, or a supplier's marketing spec. Using the wrong pressure figure shifts the entire Annex II calculation. ### Treating fluid group as fixed for a product line across all markets A product built to handle multiple fluid types across different customer applications needs its category recalculated for the most hazardous fluid group it is actually specified for in each configuration — not just the fluid group of the first market it launched in. ### Assuming SEP equipment needs no documentation at all As covered in Part 4, Article 4(3) equipment still has to meet Annex I's essential safety objectives and be capable of demonstrating that — SEP is a lighter compliance burden, not a zero one. ### Classifying an assembly by its highest-rated single component only As covered in Part 6, assembly classification has to account for interactions between components, not just the component list's ceiling. ### Confusing PED, SPVD, and TPED because the products look similar A gas cylinder built for storage at a fixed industrial site and one built to be transported between locations can look nearly identical, but they sit under different directives entirely — see Part 7. ## Frequently asked questions ### Can a manufacturer choose to apply a higher PED category than the calculation requires? Yes, and it is sometimes done deliberately — for example when a product line will be reused across several fluid groups and the manufacturer wants one technical file and one Notified Body relationship rather than re-classifying every variant. What is not permitted is applying a lower category than Annex II calculates, even if the manufacturer believes the equipment is safer than the matrix suggests. ### Does the fluid group depend on what the customer actually puts in the vessel, or what it is designed for? Classification is based on the fluids the equipment is designed and intended to contain, as specified by the manufacturer, not on how a given customer ultimately uses it. A vessel specified for Group 2 fluid but later filled with a Group 1 fluid by an end user is a misuse issue, not a reason to reclassify the product at the design stage — though manufacturers should document intended fluids clearly precisely because this gets challenged. ### What happens if my equipment sits just below the Category I threshold? It falls under Article 4(3) as sound engineering practice — see Part 4 below. It does not get CE marked under PED at all, but the manufacturer still owes the essential safety objectives in Annex I and typically documents this in a simplified technical file rather than a full PED conformity file. ### Do assemblies get their own category, or do they inherit categories from their components? Assemblies get evaluated as a whole and can reach a higher category than any single component would reach on its own, because the assembly's overall risk includes interactions between components — the highest-category individual item does not automatically set the assembly's category, but it is usually the floor. ### Is a Notified Body required for every Category II item, or only some? Every Category II item requires some form of third-party involvement — the choice among Module A2, D1, or E1 changes what the Notified Body actually reviews (surveilled production, audited quality system, or verified finished product) but does not remove the requirement for one to be involved at all. ### How does PED interact with the Machinery Regulation when a pressure vessel is built into a machine? Pressure equipment that is an integral part of a machine and performs a specific function within it — a hydraulic cylinder inside a press, for instance — is generally addressed under the Machinery Regulation as part of the machine's overall conformity assessment, rather than separately under PED. PED applies to the pressure component in its own right only when it is placed on the market as a standalone item. ### Can the same physical vessel design carry two different PED categories depending on which market variant is sold? Yes, and manufacturers with multi-fluid product lines run into this constantly. The same welded vessel shell sold as a Group 2 (compressed air) variant and a Group 1 (industrial gas) variant can land in different categories under Annex II purely because of the fluid group, which means two separate technical files and potentially two separate conformity routes for what is mechanically the same part. **Disclaimer:** This page is an educational resource only and does not constitute legal or engineering advice. PED category classification depends on the precise pressure, volume, diameter, and fluid group specifications of your equipment — always verify against the current text of Directive 2014/68/EU Annex II and consult a qualified pressure equipment engineer or Notified Body before finalising a classification. 🇪🇺 CE Marking roadmap for your product Every standard, document, and test that applies — free, no account required. See your free roadmap[ Want an expert to take your product through 🇪🇺 CE Marking compliance for you? 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