Relieve the pressure

The Pressure Equipment Directive (PED), which came into force on 29 November 1999, is to become mandatory throughout the European Union on 29 May 2002.

The Pressure Equipment Directive (97/23/EC) is closely aligned to the equivalent UK Regulations (SI 1999/2001) so British manufacturers should find no difficulty in complying, which is just as well, as the penalty in the UK for failure to comply can be a fine of up to £5000 or even a prison sentence.

Many companies are already using CE marking to gain a competitive advantage and increase their sales into mainland Europe. The CE Mark confirms that a manufactured product complies with one of the directives, which are designed to ensure the free movement of goods throughout the EU.

The Pressure Equipment Directive covers equipment operating at 0.5barg or higher, such as vessels, valves, piping, safety accessories (including relief valves, bursting discs) and pressure accessories (including control and isolation valves, pressure indicators and switches). Flanges, nozzles, couplings and other elements attached to pressurised items are also included.

Companies producing packaged equipment must also comply — even if they buy in CE marked sub-assemblies — by following the steps of the directive and CE marking the completed assembly. The good news is that the directive provides a number of alternative routes to compliance, and each manufacturer can choose the route which best matches their design, production and quality control process.

A stepwise process for under-standing and applying the PED has been developed by Stockton & Billingham College, lead partners in the ADAPT ACE Project (Appli-cation of CE marking). Under the auspices of the European Social Fund (ESF), the project was set up to support small to medium enterprises (SMEs) in the UK.

What is a typical pressure equipment assembly? A heatless regenerative desiccant compressed air or gas dryer (photographed) provides a good case study. It is assumed that the manufacturer already operates a full quality assurance system to ISO 9001.

A twin vessel dryer comprises two packed beds of alumina desiccant beads, connected by a complex valved manifold, in which one bed dries the process gas stream, while the other is regenerated by a heated backflow of dry gas. Periodically, the two vessels are switched so that the first bed of desiccant is regenerated while the other dries the gas. Depending on the size and duty of the vessels, the changeover cycle can be sequenced every few minutes using time-controlled changeover valves, or can be automatically controlled from a humidity sensor in the dried gas stream.

The complete assembly comprises the two packed-bed vessels, each with a pressure relief valve, drain valve, pressure indicator, differential pressure switch, inlet and outlet isolation valves and solenoid actuated 3-way changeover valves, plus pipework. Altogether, a complex piece of equipment.

The complete assembly is within the scope of the PED, and each of the equipment items is considered in its own right. For simplicity only one of the dryer vessels will be categorised and a changeover valve, to see how the directive applies to individual items and then to the complete assembly. Every item of the assembly would need to be assessed in this way before a conclusion is reached. Though in this case the vessels fall into the most stringent category, Category IV, so this is applied to the whole assembly.

Simple but effective

Taking, as an example, the dryer manufactured by Pressure Products, two vertical pressure vessels each house a packed bed of alumina desiccant beads. The simplicity of the design obviates the need to heat the bed for regeneration by purging with dry air to expel the adsorbed moisture from the bed. On this particular dryer, the changeover cycle is controlled by a dewpoint measurement device on each dryer vessel, which delays the regen-eration cycle until the desiccant bed requires it. At the end of its useful life, the desiccant can even be removed from the vessel and re-placed with new material while the equipment is still in-situ.

The dryer vessel has to be categorised as follows:

Type of pressure equipment: Vessel-flooded volume of 3064 litres (one of two dryer vessels), operating at 7.0 barg.

The vessel is not covered by any exclusions from the directive.

Volume (V) and Pressure (PS) must be used as the assessment criteria.

The fluid group: Air - Group 2 Gas (i.e. not Group 1 dangerous gases).

Conformity assessment table: Chart 2: Vessels for Group 2 gasesPressure Vessel: Volume (V) 3064 litres (stated by manufacturer)

Maximum pressure (PS) 7.0 barg.

PS.V 21448 bar litres

Category IV (applies when PS.V above 3000 bar litres)

Purpose designed poppet type switching valves are used in this application, in which a Teflon O-ring seals against a replaceable stainless steel seat. This permits easy replacement of the contact parts when they become worn by the fine abrasive dust displaced from the desiccant beds during the regeneration process. Finally the whole assembly must be assessed to determine the category to which it is to be CE marked, though it is not necessary to CE mark individual items of equipment within a packaged unit. The process is shown in the panel opposite for the change-over valve and the complete unit.

It is not yet mandatory to supply CE marked products within the PED, so in theory it is not compulsory to follow the process. In practice though, some purchasers are already insisting upon CE marking, and in many other cases the presence of a CE Mark gives a considerable market advantage. Any product or assembly which will be supplied or first com-missioned after 29 May 2002 must meet the requirements of the PED and be CE marked as necessary.

Route to compliance

The major steps for compliance are: identifying, confirming and classifying the fluid and equipment.

A 3-way changeover valve: Type of pressure equipment: Pressure Accessory (pneumatically-actuated changeover valve)

Not covered by any exclusions

Nominal size (DN) and pressure (PS) are the assessment criteria for these items

Fluid group: Group 2 Gas

Conformity assessment table: Chart 7: Piping for Group 2 gases

Maximum pressure (PS) 7.0 barg.

Nominal size (DN) 150

PS.DN 1050

Category I (applies when PS.DN is greater than 1000 and below 3500)

Complete Air Dryer Assembly

Type of pressure equipment: Assembly (complete package)

Maximum pressure (PS) 7.0 barg.Not covered by any exclusions

Category of each component as assessment criteria

Fluid group: Group 2 Gas

Integration Assessment to Category IV (the highest category of any item other than safety/ relief valves is applied to the assembly)

Assessment of the protection of the assembly against exceeding permissible operating limits as referred to in the Essential Safety Requirements of the PED.