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The Pressure Equipment Directive 2014/68/EU defines safety requirements for pressure equipment and assemblies depending on their level of hazard, as well as conformity assessment procedures that are applied during the design, the manufacturing, and the final assessment to check the conformity with the safety requirements. For this purpose, pressure equipment and assemblies covered by the PED are classified into categories, from which the applicable conformity assessment procedures are derived.
The PED is relevant for process plants, particularly for the sizing and procurement of vessels and pipes. The classification according to the PED is taken into account during the sizing of the components already, as a higher classification according to the PED means higher procurement costs due to the required more elaborate conformity assessment procedures.
The PED applies to pressure equipment and assemblies with a maximum permissible pressure (PS) of more than 0.5bar above the atmospheric pressure of 1.013bar(a). The term pressure equipment means pipes, vessels, and pressure accessories. A functional unit consisting of several pieces of pressure equipment is called an assembly.
Exclusions from the scope of the PED that are to be considered are listed in article 1 section 2. These include components that are covered by other norms and directive, such as motors, pumps, compressors, and turbines.
Based on a pattern defined in the PED, pressure equipment and assemblies are divided into four categories which are denoted with the roman numerals I, II, III, and IV. Depending on the category, the conformity with the safety requirements defined in the directive must be verified using predefined procedures. A special case are pressure equipment and assemblies which are within the scope of the PED but do not have to fulfill the safety requirements and do not have to be subjected to a conformity assessment procedure. With reference to article 4 section 3, the design, manufacturing, and final assessment of pressure equipment of this type has to be done in accordance with "sound engineering practice".
The safety requirements for pressure equipment are defined in annex I of the PED. Among other things, these include specifications for the design of the pressure equipment, the manufacturing, the final assessment, and the materials.
As part of the design of pressure equipment, the pressure resistance must be demonstrated either by calculation methods or by experimental design methods considering reasonably foreseeable process conditions. For this, the impact of wear, corrosion and other chemical influences, as well as fatigue must be taken into account.
Pressure equipment is to be fitted with safety accessories, if there is a possibility that the permissible pressure limits are exceeded. These accessories are to be designed such that the maximum permissible pressure (PS) is not permanently exceeded. However, a short term exceedance of the PS of not more than 10% is permitted.
The final assessment of pressure equipment shall include a pressure resistance test, which is usually done in the form of a hydrostatic pressure test. The test pressure must be chosen such that the load on the pressure equipment is at least equal to the higher one of the two values resulting from the following conditions.
The conformity assessment tables defined in annex II of the PED are used for the classification of pressure equipment into one of the four categories of the PED depending on several factors. A difference is made between pipes and vessels. The process media is classified as either dangerous or not dangerous, and as either being in a gaseous or liquid state of matter. Liquids with a vapour pressure at the maximum possible temperature of more than 0.5bar above the atmospheric pressure of 1.013bar(a) are treated as gaseous. From these factors eight different groups of pressure equipment result, each with its own conformity assessment table. An exception within this classification is fired or otherwise heated pressure equipment with the risk of overheating, intended for the generation of steam or superheated water. A separate, additional conformity assessment table exists for equipment of this type. Including this exception, there are a total of nine conformity assessment tables.
In the conformity assessment tables the x‑axis represents the maximum permissible pressure (PS) and the y‑axis either the nominal diameter, in the case of pipes, or the volume, in the case of vessels. The area formed by each table is divided by an individual pattern into the four categories of the PED. To determine the category applicable for a given component, the corresponding table is identified and then the maximum permissible pressure (PS) and the volume or nominal diameter, respectively, are plotted on the corresponding axis.
The following graphic shows the steps to determine the conformity assessment table applicable for a given pressure equipment. When determining the category, the exceptions defined in article II for selected conformity assessment tables have to be taken into account. Also, safety accessories such as safety valves and rupture discs are classified into category IV.
In annex III of the PED, the available conformity assessment procedures are defined for each of the four categories in the form of the following single modules and combinations of two modules. According to the category into which a pressure equipment or an assembly is classified, the manufacturer has to choose one of the available conformity assessment procedures.
We consider the requirements of the Pressure Equipment Directive when determining the capacity and process conditions of the plant components in order to avoid classifications into higher categories and to reduce the procurement costs.
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