The European Pressure Equipment standardisation system: state of the art

Following to the remarks made by many different sources, I was recently involved in a comparison among the different EN standards dealing with Pressure Equipment in order to identify possible differences. Among the topics in which the main harmonised standards (EN 12952  – Water  Tube Boilers,  EN 12953 –  Shell  Boilers,  EN 13445 – Unfired Pressure Vessels, EN 13480 – Piping) show remarkable differences, I have selected the method for high temperature design (in the so called “creep range”, where materials start behaving like liquids, increasing their strain under constant stress) and the hydrostatic test pressure (the value of the test pressure is prescribed, or at least recommended, in the Pressure Equipment Directive, however the prescriptions are not completely clear and may give raise to different interpretations). Without going too much into details about the reasons of the differences, I will only mention the most significant results of these comparisons.

Example 1 – Thicknesses (mm) of a cylindrical shell having an I.D. = 1000 mm, made of Low Alloy  steel  2,5Cr-1Mo at 100 bar and 500°C

Example 2 – Thicknesses (mm) of a cylindrical shell having an I.D. = 1000 mm made of Austenitic Stainless Steel ASME SA 240 304 at 100 bar and 600°C (1)

From  the  two  examples  presented  above,  it  is  evident  that  the  thickness  of  the  same cylindrical shell operating in the creep range (at 100000 or at 200000 hours) is not the same when it is part of a water tube boiler, of a pressure vessel or of a piping system. In example 1, with a service life of 100000 hours (11 years), the boiler standard is the one which gives the minimum thickness (49 mm), while the maximum thickness (59 mm) is obtained with the piping standard. In example 2, for the same service life, the minimum thickness (69 mm) is for a pressure vessel (provided it is monitored in service!), all other standards give 84 mm. Well, let’s hope that cylindrical shells for high temperature applications are clever enough to understand to which kind of pressure equipment they are belonging, and possibly to make a reasonable forecast about the designer’s ideas about future monitoring in service: in this way they will be able to develop the necessary strength characteristics! I personally must confess that I am not clever enough to understand why a piece of piping working at 100 bar and 500°C may be thinner when its lifetime is 200000 hours (22 years) than in the case of a shorter lifetime (100000 hours = 11 years). If this were true, it could be extremely dangerous to interrupt after 11 years the life of a pipe originally designed for a lifetime of 22 years!

Dealing with the hydrostatic test pressure, I have considered 3 different examples:

Example 1: cylindrical shell, 1 m inside diameter, 40 mm thickness, joint efficiency 100%, corrosion allowance 1 mm, material fine grained carbon steel P355 NH EN 10028.3, design pressure 100 bar, design temperature 350°C.

Example 2: cylindrical shell, 1 m inside diameter, 20 mm thickness, joint efficiency 100%, no corrosion allowance, material austenitic stainless steel 1.4571 EN 10028.7, design pressure 50 bar, design temperature 200°C .

Example 3: cylindrical shell, 1 m inside diameter, 8 mm thickness, joint efficiency 85%, corrosion allowance 1 mm, material carbon steel P355 GH EN 10028.2, design pressure 10 bar, design temperature 200°C . The shell is closed by an elliptical end, 5 mm thick, with no corrosion allowance, material austenitic stainless steel 1.4571 EN 10028.7.

For all the three cases I have calculated the hydrostatic test pressure according to the prescriptions of the harmonised standards for pressure equipment mentioned above (note that a harmonised   standard,   by   definition,   should   be   a   standard   giving   the   so   called “presumption of conformity” with the reference directive, in our case the Pressure Equipment Directive). It is surprising to see the amount of disagreement about the interpretation of the same PED requirement among the different EN standards, prepared of course by different CEN Technical Committees. By the way, in the comparison I have also included the values of the test pressures which should be reasonably obtained if we try to give the same interpretation of the PED with the use of a different (non harmonised) standard (ASME Section VIII division 1).

At the end I must say that the situation of EN standards for Pressure Equipment is an excellent mirror of the actual political situation of the European Union: everyone is trying to bring forward his own ideas, without looking too much at other people’s ideas, and possibly ignoring the final goal of the work. But please, do not blame too much European standardisers: differently from European politicians, they are not paid at all (moreover, they have to pay some contribution to their relevant standard bodies in order to have the great honour to work for CEN). However, if you look at the mess European politicians are now doing with the Euro, you will have to recognize that possible problems concerning the stability of pressure equipment are certainly negligible if compared to problems concerning the future financial stability of Europe.

Fernando Lidonnici