PRESSURE VESSELS IN CYCLIC SERVICE
|The ASME Boiler and Pressure Vessel
Code was first published in 1915. The purpose of this Code was to drastically
reduce the number of casualties associated with boiler and pressure vessel
explosions. In 1915, about 1,000 people were killed each year even though
the number of boilers in use was small. Today, the number of boilers and
pressure vessels in use is tens of thousands times as great as it was in
1915, and the contained volumes and pressures are much greater. Yet, since
about 1965, no one has been killed by accidents caused by inadequate rules
in the ASME Code.
The sections of the ASME Code that cover boilers and pressure vessels are Section I and Section VIII, respectively. Section I, and Section VIII, Division 1 of the Code do not adequately address cyclic service. This was not a problem for many years, but today, many industrial processes involve very high numbers of cycles. Section VIII, Divisions 2 and 3 have rules for cyclic service, but Section I and VIII, Division 1 do not. Consequently, some boilers and pressure vessels are improperly designed for cyclic service. An example of boilers that may fit this category is Heat Recovery Steam Generators (HRSG). An example of pressure vessels that may be improperly designed for cyclic service is Pressure Swing Absorbers (PSA). In order for the fatigue data of Section VIII, Division 2 to be properly used for Section I and Section VIII, Division 1 vessels, it is necessary to adjust the stress intensification factors for assumed flaws, or to limit the weld details and perform nondestructive examinations appropriate to find and correct the potential flaws. This will increase costs, and is often neglected.
When boilers and pressure vessels are in cyclic service, the ordinary details for welded construction may not be appropriate because of built-in “notches.” Another problem is that the welds may not be properly examined. Sections I and VIII, Division 1 do not always require radiography, and even when they do, the radiography should be supplemented by ultrasonic examination, before, and periodically during operation.
An associated issue is whether or not “full penetration” welds are required, and whether or not the penetration is required to be verified, and by what means. Failure to achieve “full penetration” creates a notch which can cause the vessel to crack and fail in service.
The issues of whether or not vessels are in cyclic service, how these
vessels were designed and fabricated, and which ASME Code rules are used,
and how they are used, are very important considerations when evaluating
vessel failures or potential failures. All of these issues are within
our expertise and experience. We have helped our clients successfully
resolve conflicts associated with these issues.