Those words have been used to start thousands and thousands of Engineered Composite Repair System™ (ECR System) repair designs in the last 20 years, and unfortunately, many designs have used just that as their basis. Why is this a problem? In many cases it may not be, but it certainly cannot be made as a blanket statement. As with any engineering design, there are many variables that come into play, but as a general statement, the design equations for this type of defect in the ASME PCC-2 Article 4.1, and in the ISO 24817, are not considered reliable under a certain defect diameter size. Effectively, regardless of whether you truly have a “pinhole,” a minimum hole (defect) diameter should be used to ensure a reliable result is obtained from the design. ISO 24817 specifically states that a minimum defect size of 15mm (0.6in) must be used in the equations where the ASME PCC-2 does not currently provide this guidance. This article attempts to clarify this misunderstanding in the industry as well as have a discussion on other defect types of concern for all Type B (through-wall) repair designs.
German writer and statesman, Johann Wolfgang von Goethe, once wrote, “Knowing is not enough; we must apply. Willing is not enough; we must do.” The problem with this is that sometimes the knowledge is not widely known, and thus cannot yet be applied. Part of the mission of NRI’s technical department, NRI University, is to educate the industry on the technical requirements and processes for successfully integrating composite repairs into their maintenance and repair programs. Without a firm understanding of the needs and design principles, it is difficult for a composite user to know the difference in design needs and what is required for a successful outcome. Based on my experience and general discussions with the industry, Type B design requirements and limitations fall firmly into that category. So, let’s address the “knowledge” portion here, that way we can move to “apply.”
To understand the subtle differences and design techniques, we will review two primary topics: 1) defect geometries and 2) defect minimum size limitations.
The standards identify three primary defect geometries for Type B design cases: 1) circular or near-circular defect, 2) a circumferential slot defect, and 3) an axial slot defect. Potential samples of each may be seen in the figures here.
As one can see, the defect types have significantly different considerations with regard to geometries, and with the difference this then makes in which design equation should be used, it becomes critical to the success of the repair that the information is correct, verified, and valid. A change in defect size or type can have bigger ramifications on the design needs than many users realize who have not spent time analyzing the requirements from the equations or any previous composite repair designs. Perhaps you start with only a pinhole, but if you then seal that leak (which is required before installing an ECR System™) in a manner that changes the defect size or geometry, such as by putting a small band clamp on, then the design must reflect that change with the geometry and size of that leak-stop device. In addition, the size of the defect may also potentially change if you have an area that is not fully prepared or requires additional filling around the defect.
Defect Size Limitations:
In considering the limitations of defect sizes, the bigger concern lies in the minimum sizes used, rather than the maximum sizes. Although it may seem that counterintuitive that the concern is having a defect that is too small, the way in which the equations determine the repair thickness simply break down when the defect size does not have a large enough size associated. In the graph below, a typical design using a constant repair thickness and adjusting hole size, the resulting pressure capacity may be viewed and one can see that as the defect diameter goes down, the calculated pressure capacity exponentially grows to infinity. Obviously, this does not represent the real world.
As was stated previously, the ISO 24817 standard gives guidance on what the minimum defect size should be, and in this case, it is represented by the yellow box in the chart, which indicates the equations should be used only for (circular) defect sizes of at least a 15mm (0.6in) diameter. The ASME PCC-2 does not provide this guidance, although the design equations are the same as the ISO, and this leads to a perceived “loophole” of using a very small circular diameter within the design equations, which would lead to the requirement of the composite repair being unrealistically thin but meeting a very high design pressure rating. Again, as with all repair design, there are a variety of variables and the actual minimum diameter where the equations become unreliable can shift, the resulting exponential curve to infinity is the same and shows the breakdown of the equations’ reliability.
Similar data and graphs may be obtained with the other defect geometries and size limitations as well. They look slightly different, but the result is the same and shows a range of equation reliability that does and should limit the use of those equations for repair design. So, when performing a design or reviewing a proposed composite repair design, be cautious of the limitations associated and ensure that not only will it satisfy the requirements to be compliant, but that it also works practically in the real world.
While we have not delved too deeply into the granular details of the design equations themselves, it is my hope that this short article will help to start the dialogue on this misunderstood and misinterpreted topic of Type B composite repair designs. As previously stated, it is NRI’s goal to ensure the industry has the knowledge it needs to properly and successfully use ECR Systems for the repair of their piping and pipeline networks. Our engineers are happy to continue the conversation in detail and answer any questions that may have come to mind in reading this article, along with any other questions you may have. But now that you are armed with this knowledge, the second part of the earlier quote becomes the next step, and we must be “willing to apply” and “willing to do.” As leadership guru, John Maxwell has said, “It’s not knowledge that will make you successful; it’s what you do with that knowledge, it’s how you apply that knowledge.”
What can you do today to apply this knowledge and remove any doubt in successfully using Engineered Composite Repair Systems™?
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