The ASME Code -- The Winds of Change
Publication Date: 2012-May-07
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The ASME Code - The Winds of Change
The Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers has been accepted throughout the world as setting the safety standard for pressure equipment. However, just as we must adapt to meet the demands of our ever changing world, the ASME is in the midst of a number of very significant changes that will benefit the industry which it serves. This article will summarize some of the significant initiatives that ASME has adopted or is considering for adoption for the Boiler and Pressure Vessel Code.
Change in Safety Factor for Allowable Stress Determination
Probably the most significant initiative is related to the allowable stress basis used for determining the mechanical strength (thickness) of pressure vessels and heat exchangers. There are a number of criteria used in establishing the Code allowable stress that depend on the material and the temperature at which the equipment is to be used. At temperatures below the range where creep and creep rupture govern the selection of allowable stresses, the maximum allowable stress for Section VI11 Division 1 is the smallest of the following:
1/4 of the specified minimum tensile strength at room temperature; 1/4 of the tensile strength at temperature;
2/3 of the specified minimum yield strength at room temperature 2/3 of the yield strength at temperature.
For many of our applications (primarily carbon and low alloy steels below about 650 OF), the allowable stress basis is governed by the safety factor on the tensile strength; e.g. tensile strength divided by 4. The factor of safety of four on tensile stress for the allowable stress basis is approximately fifty years old. Obviously, a lot has changed in the steel making technology, fabrication, inspection, etc. in the past fifty years that deserve consideration when evaluating an appropriate allowable stress basis for today's industry. Perhaps more important, there are European codes that currently use less conservative basis for allowable stress that results in some equipment being considerably thinner (less expensive) than a comparable ASME Code vessel. (The proposed European Unified Pressure Vessel Code may use a safety factor on tensile strength as low as 2.3.)
ASME has recognized the need to reconsider the basis for allowable stress determination, and will issue a special Addenda to be released sometime this summer. This addenda will revise the safety factor from 4 to 3.5 on tensile strength for Div. 1 applications. For carbon and low alloy steel equipment operating below the...