Giving an Assembly a PED category – A help or a hindrance?

If you manufacture CE marked assemblies that fall within the scope of the Pressure Equipment Directive, you’ll be familiar with the concept that, with the exception of those that fall within Article 4(3) (i.e. Sound Engineering Practice), they have a hazard category. But what if those assemblies don’t really require a category and that attributing them one is more of a hindrance than a help?

Categories classify pressure equipment according to a level of hazard, and they have two main purposes. One is to determine which Conformity Assessment Procedure (i.e. Module(s)) can be applied, and the other is to affect the obligations in a handful of essential safety requirements. The categories for pressure equipment are ascertained by referring to Conformity Assessment Tables in the Directive. However, you won’t find such a table for assemblies. So how are they categorised?

Well, you won’t find anywhere in the Directive a requirement to ascertain the category of an assembly. And I think that’s for a good reason; you don’t need one to assess its conformity. Article 14 paragraph 2.6(b) of the Directive tells you that the assessment of the integration of the various parts of the assembly is determined by the highest category applicable to the equipment concerned other than the safety accessories. So, in other words, all you need to know to choose a Conformity Assessment Procedure is the highest category applicable to the equipment in the assembly, ignoring the safety accessories. The assembly itself doesn’t need a category for you to do this.

Is this just semantics? I don’t think it is. The problem with attributing a category to an assembly is that it implies that there’s one corresponding level of hazard that envelops the assembly, when this is not the case. It creates the wrong mindset. A more accurate way to view it is that the various items of pressure equipment in the assembly each have their own level of hazard, as indicated by their respective categories.

But how about those essential safety requirements that are affected by the category? Doesn’t an assembly need to have a category for these?

There are just five essential safety requirements which are affected by categories: ESR 3.1.2, 3.1.3, 3.2.2, 4.2(b) and 4.3.

  • ESR 3.1.2 relates to permanent joining, ESR 3.1.3 to NDT personnel. However, the category of permanent joints between the items of pressure equipment in an assembly are determined individually, not by categorising the assembly, so an assembly’s category is not relevant.
  • ESR 4.2(b) and 4.3 relate to materials. But materials, with the exception of those for permanent joining, are not used in the manufacture of assemblies; they are used in the manufacture of pressure equipment. ESR 4.2(b) is not appliabale to joining materials, and the obligations in ESR 4.3 that are affected by a category are not applicable to joining materials either. So, again, an assembly’s category is not relevant.
  • The fifth of these essential safety requirements is ESR 3.2.2, and it allows for category I series-produced assemblies to be pressure tested on a statistical basis. I’ll address this point a little later.

So, putting aside ESR 3.2.2, why do we give assemblies categories? Possibly because PED guideline A-39 says ‘the Directive clearly defines a category for each assembly in Article 14 paragraph 2.6 (b)’. But does Article 14 paragraph 2.6 (b) really say that? It makes no reference to an assembly having a category, referring only the highest category of equipment within the assembly. I think that Article 14 paragraph 2.6 (b) tells the manufacturer how to assess, not categorise, its assembly.

But in which case, why does the guideline say assemblies have categories? The issue it’s addressing is a weakness in the drafting of one of the exemptions from the scope of the Directive.  Article 1(2)(f) exempts pressure equipment from the Directive if it’s covered by one or more of a number of listed Directives and, importantly, is classified no higher than category I. The problem is that without being assigned categories, assemblies would not benefit from that exemption.

So you can understand why it might at first sight seem helpful to say that assemblies have categories. And it is to an extent. But I think it’s like swallowing a spider to catch a fly. It resolves the issue of the exemption in paragraph 2 (f) and also allows for the pressure testing on a statistical basis which I referred to earlier, but it leaves most assembly manufacturers wondering what the significance is of the hazard category that’s seemingly enveloped their entire assembly. It gives the impression that the assembling of the parts has inexplicably introduced a higher level of hazard in some parts of the assembly than they otherwise would have had.

I think it would be simpler to just interpret Article 1 paragraph 2 (f), and also the option to pressure test series-produced assemblies on a statistical basis, by considering the highest category applicable to the equipment in the assembly ignoring the safety accessories, thereby making the need to categorise assemblies redundant. And by not assigning an assembly a category, the manufacturer is in a better position to clearly see the different levels of hazard that its assembly’s constituent parts truly present.

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