Fitness-For-Services : Engineering analyses for the evaluation of "crack-like" indications
Piping and pressure vessels in the US, and often worldwide, are designed and manufactured in compliance with one of the American Society of Mechanical Engineers (ASME) Piping Codes or some section of the ASME's Boiler and Pressure Vessel Code. These are design and construction codes and have significant restrictions on what types of indications are allowed to remain in the component after inspection. Anything that is planar or "crack-like" must be repaired before the component is accepted.
Once the component is placed in service the original design code may or may not be applicable depending on the regulations in the State where the component is located. In "Code States," the regulatory branch governing pressured equipment often refers to the National Boiler Inspection Code (NBIC) for guidance. For piping and pressure vessels designed to an ASME Code refers to the "Code of Record" for guidance. Basically, the NBIC refers back to the original design code and mandates that the original design code be followed for repair or alteration guidance. If planar "crack-like" indications are found during an inspection this reference stream would mandate that these indications have to be repaired.
However, repair introduces the potential for additional indications to be introduced into the component and the possibility of damage done to the component during the repair process. An alternative to this "repair on discovery" approach is to perform a Fitness-For-Service (FFS) evaluation. This evaluation utilizes engineering analyses (heat transfer analyses, stress analyses, and fracture mechanics analyses) to evaluate the "crack-like" indications to determine if leaving them in the component will compromise its safe operation. There are several levels of evaluation that may be performed. The first level, discussed here, addresses how the safety factor in the component may be determined if the "crack-like" indication is not removed. Other approaches address how the "crack-like" indication will behave during future operations; for example, will it grow in size and, if so, at what rate?
The first step in performing one of these evaluations is to gather the information concerning the design and operation of the component. The operational aspect is very important because the design basis may include additional safety margin above the actual operating conditions. Once that information is obtained then there are two approaches used to evaluate the remaining safety margin. The first would be to use the guidance in API 579/ASME FFS-1 regarding the Failure Assessment Diagram (FAD) to evaluate the specific indications found. For example, Figure 1 shows a typical FAD developed for a circumferentially oriented "crack-like" indication located on the inner surface of a pressure vessel (piping is treated in a similar fashion).
Figure 1: Vessel FAD
The dark blue curved line is the FAD and establishes the flaw size that would result in a through wall failure of the vessel. However, looking at the diagram it is not really clear what the factor of safety is. In fact, the diagram is fairly confusing. Some additional information may help. The size of the "crack-like" indication is shown in the heading of Figure 1. The indication is 10 inches long and has a depth (through wall) of 0.158 inches. Since the wall thickness, also listed in the header, is 0.375 inches. The indication extends through over 40% of the wall thickness. The internal pressure used in this analysis is 50 psig. To evaluate the factor of safety, you find the intersection of the red (Elastic Model) with the FAD. Then you drop down the light blue line to its intersection with the green line. Reading the value of that intersection off the right (Vessel Pressure) axis you find that the failure pressure is about 325 psig. Since the analysis pressure was 50 psig the factor of safety on failure is over 6. However, this is a fairly inefficient means of determining whether the component is safe to run at those conditions. In addition, one of these diagrams has to be developed for each indication found.
An alternative approach, which can be done before the scheduled inspections, is to iteratively solve the problem for various indication sizes and factors of safety. In addition, the same sort of analysis can be done for an indication of the same length but extending through the wall. The result can be used to evaluate whether the failure mode is a leak or a burst. An example of this approach is shown in Figure 2. This is the same vessel for which Figure 1 was developed.
Figure 2: Tolerable Flaw Diagram
Figure 2 is much easier to understand. The crack lengths and depths are shown on the horizontal and vertical axes. There are individual curves for each factor of safety evaluated and the leak-before-break region is clearly seen. If this analysis is done before the inspection occurs then during the inspection the actual indications found can be plotted and an immediate resolution may be had. For example, Figure 3 shows Figure 2 with indications found during an inspection. Clearly, there are a large number of indications that might be safely left in the vessel while others should be repaired immediately.
Figure 3: Tolerable Flaw Diagram with Indications Shown
The question of whether or not the indications may be safely allowed to remain in the vessel depends on the operating conditions. If the vessel is subjected to variations in pressure or temperature or if the vessel is operating in the creep regime then the issue of propagation due to fatigue and/or creep comes in to play. I will address these issues in a follow-on blog.
If you need any further information regarding the approach discussed here please contact me at firstname.lastname@example.org
Mike Cronin is the Director of Engineering of Intertek’s Asset Integrity Management Group. He has over 40 years of experience in the fields of applied mechanics and fracture mechanics. Mike has worked on the Space Station, US Navy aircraft carriers and surface ships, aircraft engine failure, steam turbine rotor failures, high energy piping failures and fitness-for-service.