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Comparison between American and European Pressure Vessel Rules

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1 Comparison between American and European Pressure Vessel Rules
Fernando Lidonnici Convenor of WG’C’/CEN TC54 Sant’Ambrogio Servizi Industriali SRL - Milano

USA: the ASME Code (Section VIII for Pressure Vessels) is the safety standard generally accepted, but there are differences in the different States (in some of them it is imposed by law, in other it is required for insurance purposes only). Europe: the PED (Process Equipment Directive) is a law and therefore it is compulsory in all the European countries. It requires compliance with the Essential Safety Requirements (ESRs) only, not with a detailed set of rules. The use of a “Harmonized Standard” (EN for Pressure Vessels) assures “Presumption of Conformity” with the PED, but other standards are also acceptable, provided they take into account the ESRs.

ASME: requires strict compliance with the section of the ASME code concerning a given pressure component (there is a specific section for each type of component). The ASME stamp (different for each section) assures full compliance with every detail contained in that section. The Manufacturer is responsible to take into account the requirements and loading conditions specified by the Purchaser.. PED: the Manufacturer (not the Purchaser) is fully responsible for the safety of the equipment in all the possible conditions: normal operating, exceptional, testing, transport, erection, maintenance, etc. The CE mark means that only the ESRs have been complied with, not a specific code or standard. Design and fabrication must be made on the basis of a specific Risk Analysis, different for each component, under the responsibility of the Manufacturer.

Nominal design stresses for Steels in Section VIII division 1 are limited by a very high safety coefficient on tensile strength (3,5). In the creep range the characteristics of the welding material are supposed to be the same as the base metal. DBA (=Design by Analysis) and Fatigue Analysis only possible with Section VIII division 2 (special vessels). DBE (Design by Experiment) only possible where calculation formulae are not provided in the Code. Section VIII division 1 covers all materials (Steels, Cast Iron, Al alloys, Cu alloys, Ni alloys, Titanium, etc.)

5 DESIGN TO PED + EN Safety coefficients are only recommended, however deviations from recommended values must be supported by additional prescriptions giving the same level of safety. The harmonized Pressure Vessel Standard considers the recommended value of 2,4 on tensile strength; this value may be lowered to 1,875 (amendment already approved) if additional prescriptions are taken. The actual characteristics of weld metal must be taken into account, particularly in the creep range. DBA is always possible, fatigue analysis is mandatory for all vessels subject to cyclic loadings. DBE possible only for small components (in EN it will be covered in an amendment already approved). EN is limited to steels, Spheroidal Cast Iron and Aluminium alloys only (amendment already approved for Aluminium and design of Steels in the Creep Range)

Only SA (or ASTM) materials may be used (plus a few EN materials considered in the Code). Allowable design stresses based on hot tensile / creep properties are directly provided in the Code. The material Manufacturer is not required to give a guarantee on such characteristics. Only a few SA / ASTM material specifications give impact properties at room or low temperatures.

There are three possibilities: Use of EN harmonized material standards. These standards usually contain hot tensile characteristics and impact test values which are therefore automatically guaranteed by the material Fabricator. Use of materials for which an EAM procedure has been carried out (only possible for a few materials not yet contained in EN standards). Use a PMA (Particular Material Appraisal) = original specification plus supplementary requirements to be guaranteed by the material Fabricator.

8 JOINT EFFICIENCIES (of main welded joints)
ASME: from 0,7 (no NDT) to 1 (full NDT). Joint efficiency 1 required for Lethal Service, large thicknesses, materials with particular welding difficulties. It is possible to use 0,7 also for large vessels. PED + EN 13445: also from 0,7 (no NDT) to 1 (full NDT). EN limits the use of testing group 4 (joint efficiency 0,7) to carbon steels (temperatures +10 / +200 °C) and austenitic stainless steels (temperatures -50 / +300°C) only, with pressure smaller than 20 bar and pressure x volume smaller than (above 100°C) or bar x litre, provided the content is not dangerous (Group 2 Fluids only).

ASME VIII division 1: the lower of (formula to be applied to all pressure parts of a specific vessel) PED (Recommended): the higher of (formula to be applied to all the pressure parts of a specific vessel), however not less than Use of lower hydrostatic test pressures is permitted if alternative measures assuring the same safety level are taken. EN : the hydrostatic test pressure may be limited to the maximum value compatible with the actual thicknesses of the main pressure parts determined by design conditions. For Vessels of Testing Group 4 a higher test pressure is provided. Note: PS = Design Pressure, = ratio between nominal design stress at 20°C and nominal design stress at design temperature.

ASME: Authorization given to the Manufacturer to affix an ASME Stamp (specific for the particular kind of pressure component concerned) after a survey carried out by ASME and the National Board of Pressure Vessel Inspectors. Design, fabrication and inspection activities carried out under the surveillance of an Authorized Inspection Agency. Particular precautions are to be taken for vessels in “Lethal Service”. PED: Specific certification procedures (modules) have been provided in order to cover 4 different hazard categories of vessels. Some modules are based on QC (Quality Control) procedures, other on QA (Quality Assurance) procedures (lower degree: self certification, higher degree: either full QC (step by step checking), or full QA (Qualification of the Manufacturer). For Vessels the hazard category depends on the content (dangerous / / non dangerous) and on its physical state (liquid or gas / vapour). Each module provides a different involvement of a Notified Body, specifically authorized by one of the European countries. Note: also assemblies made of several pressure component are subject to a certification procedure.

11 PED: Conformity assessment table for dangerous gases

12 PED: Conformity assessment table for non dangerous gases

13 PED: Conformity assessment table for dangerous liquids

14 PED: Conformity assessment table for non dangerous liquids

15 PED: Conformity assessment modules for the various hazard categories

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