Chapter 1

Introduction

Within the European Union the coming into force of common national laws in the pressure equipment field, all based on the very same legal act of the European Parliament and the European Council, the so-called Pressure Equipment Directive (PED) [1] created a serious need for corresponding complementary standards harmo- nized at a European level, adopted by the European Committee for Standardization (CEN) at the request of the European Commission.

The PED requires certain types of pressure equipment brought onto the European Market to comply with the so-called essential safety requirements (ESR), in order to ensure the required safety of pressure equipment. Compliance with the requirements of a relevant harmonized standard provides for a product the presumption of con- formity with the ESR that the standard addresses. The harmonized standards need not be used, they are only one means of demonstrating compliance with the ESR of the PED, but they are the only means that provide directly the presumption of con- formity with the ESR.

This need for a harmonized standard created a unique chance and challenge: The chance for a new approach to Design by Analysis (DBA), using all the knowledge in engineering mechanics - theoretical as well as practical - and all the experience with numerical methods and with commercially available hard- and software, used in simulations of the behaviour of structures under various actions.

Work on this new approach, called Direct Route in Design by Analysis (DBA- DR), started in 1992, the first sketch of a draft dates October 1992. The draft went through (informal) enquiries repeatedly and formed the basis of an EU-research project, which rendered proposals for changes and a handbook [3] with numerous examples, input listings, etc.

This new approach, DBA-DR, is now laid down in a normative annex, Annex B of Part 3: Design, of the harmonized standard EN 13445: Unfired Pressure Vessels [2]. The relevant parts of this standard were approved on 23 May 2002: Part 1: General, Part 2: Materials, Part 3: Design, Part 4: Fabrication, Part 5: Inspection and Testing.

Since then, this new approach has been used in numerous industrial applications and research projects; numerous papers deal with this approach and are dedicated to it directly [5-21 ].

2 Pressure Vessel Design: The Direct Route

Industrial applications and investigated examples have shown that this Direct Route is a major step forward in DBA. This new approach is sound, gives the de- signer (and the user) not only the presumption of conformity to the ESR of the PED, but also, at the same time, much insight into the behaviour of components and the safety margins against failure modes.

Furthermore, the DBA-DR has shown to be of great help in the determination of safety-critical points and of critical actions. Therefore, this new approach can lead, and has already led, to design improvements and to improved in-service in- spection periods and dedicated in-service inspection procedures.

Applications have also pointed out one basic problem: The growing gap between analysis software capabilities on one side and the expertise of the users on the other.

Some of the software tools are so easy to use that little thinking is required to obtain fantastically looking, colourful pictures of stress distributions, and many users tend to believe their results are correct because they look so good and con- vincing. Wrong results look usually as good as correct ones.

The Direct Route has made DBA easier to use in the design process, more straightforward and logical in the design decisions, but technical knowledge of en- gineering principles and careful analysis of results is still a prerequisite of good workmanship. It is still the analyst who has to decide on the model, the geometry, and the boundary conditions. It is practically always necessary to use part models, and the decision on the boundaries and the boundary conditions is a very critical one, requiring thought, and, possibly, additional investigations.

A good DBA still requires from the analyst

�9 good workmanship with regard to the tools used, �9 knowledge of the basic engineering principles and the phenomena involved, �9 fantasy and creativity with regard to the selection of the models used, �9 fair knowledge of the legal requirements pertaining to design, �9 fair knowledge of manufacturing and testing procedures, and especially �9 extreme carefulness in each step, from the design specification to the design

report.