March 9, 2012
The type of inspection needed for piping systems depends on which code or standard is invoked for the project. Understanding the order inspection requirements can make or break a contractor. This article should shed some light on the various pitfalls that can be avoided by understanding the relevant testing requirements.
Although the era of modern tube and pipe manufacturing is less than 100 years old, it’s difficult to imagine manufactured goods without these products. Some of the applications for tube and pipe are fairly simple, such as towel bars and handrails, whereas others are very sophisticated, such as pressure tubing for power boilers and oil country tubular goods (OCTG).
Just as the applications vary, so do the standards to which the tube or pipe must be manufactured and tested. Like the tubes themselves, the methods for testing can be complex. For example, testing tube or pipe for a high-pressure application is not as clear-cut as it would seem. Any of five standards could apply; it depends on the application. Commonly used pressure standards are:
A thorough familiarity with the testing standards is necessary before bidding on a project to build a piping system. A copy of the code should be available on-site because it forms a part of the contract, and it should be consulted to obtain all the welding and testing requirements.
Many contractors fly blind, unaware of the code or specification they should comply with during construction or repair. Furthermore, many lack knowledge about the portions of the project that require testing. This can present a serious problem after the welding is complete because many contractors take on piping orders and do not realize that radiography is involved. The additional cost can take a substantial bite out of the profit.
Many methods are available for testing welds—magnetic particle test (MT), liquid penetrant test (PT), radiography test (RT), and the ultrasonic test (UT). Knowing how each works, and the appropriate applications for each, is critical for successful testing.
MT and PT are used for detecting surface flaws. MT relies on a magnetic field and steel particles; a discontinuity in the weld allows the magnetic flux to leak, attracting the particles. PT uses a liquid penetrant that is applied to the weld, then removed; a developer draws any remaining penetrant out of any cracks or crevices. RT and UT are volumetric tests. RT uses X-rays to create a 3-D image of the weld, whereas UT uses sound waves to find discontinuities in the weld.
MT and PT. Surface cleaning is necessary to prepare the welds for either MT or PT. Conditioning, or grinding, also is used to smooth out these areas for surface inspection. Every little ripple or crevice between weld passes can create a flaw indication, each of which must be evaluated.
Fillet-, socket-, and branch-connection weld configurations normally are evaluated by MT or PT. These don’t lend themselves to radiography. When radiography is used on these configurations, the images usually are difficult to interpret.
UT. This test sends sound waves through the material; when they hit a flaw (reflector) they bounce back to the transducer probe and are displayed on an oscilloscope monitor. The height and depth of the flaw then can be evaluated against standard calibration blocks for acceptance or rejection of the weld.
An advantage of UT is that the area to be inspected does not have to be roped off to reduce radiation exposure to personnel, as is required in RT. However, UT does have some drawbacks. The surface condition, workpiece configuration, weld contour, and weld shape can affect the test results, and marking and documenting indications is very time-consuming and difficult. Inspection technician training and experience are vital. Also, the test results are temporary. After interpreting the image on the monitor, the technician moves to the next weld, and the image disappears from the oscilloscope.
RT. Unlike the temporary nature of UT, RT produces a film or image that can be reviewed by anyone at a later time. Radiography under ASME B31.3 is usually applied as follows:
It actually gets more complicated than this. ASME B31.3 has several subcategories, some of which are more stringent than others. Applying a more stringent service category than necessary could result in welds being rejected as nonconforming when in fact they may be perfectly acceptable. This could lead to needless rework of welds, which comes with significant cost—time and materials to grind out the weld, rework of the weld itself, and an additional RT on the new weld.
On the other hand, applying normal fluid service criteria to piping that is made for cyclic severe service could result in accepting piping welds that are unacceptable for the application, resulting in an in-service failure. The results in this case could be disastrous.
Another consideration is that performing a test is one process; evaluating a weld to determine if it is acceptable is a different process altogether.
For example, it isn’t uncommon for a contractor to require a radiographic inspection by requesting “Radiograph per ASME Section V, Article 2, or per ASTM E94.” These documents specify only the method, materials, quality requirements, exposure controls, and documentation required for performing radiography; they do not list or describe any acceptance requirements. Acceptance criteria list all the types of voids or flaws that might be found and the limits or extent that they are permitted. For instance, it specifies the amount of porosity that may be permitted, along with the sizes and spacing between pores that must be measured and evaluated.
The appropriate code or standard that invoked the radiography should be stated as well. It should have the acceptance criteria that will be used. Testing methods are normally separated from acceptance criteria because they can be used for making the RT under many different codes, whereas acceptance criteria are very specific to each code or product.
A close review of the project’s documentation will help alleviate many of these problems. An upfront review, before accepting the order, is the first step in establishing a thorough inspection plan to reveal the full cost of the project, knowing which tests must be performed, and preventing rework on welds that fail.
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