Are you still hitting the welded joints of pressure vessels with a hammer during hydrostatic testing? If yes, then you’re due for a refresher on the pressure testing requirements of ASME Section VIII Division 1 since this requirement was for pressure vessels back in the mid 1940’s. This article will help you by highlighting the main requirements of, and differences between, the hydrostatic test for new pressure vessels fabricated according to ASME Section VIII, Division 1 and the hydrostatic leak test for new piping systems made under ASME B31.3. Since it is the most popular way to test a pressure vessel or piping system, the focus of this article will be on the hydrostatic test, article UG-99 of Section VIII Division 1 (2013 Edition), and the hydrostatic leak test, paragraphs 345 & 345.4 of B31.3 (2012 Edition). Please refer to these Codes and their interpretations for the complete requirements. Moreover, verification with the local jurisdiction is essential to ensure that their requirements are not stricter than those of the Codes.
Pressure testing during the fabrication process
In Section VIII Division 1, the hydrostatic test is considered the final step in the fabrication of a pressure vessel before the application of the U-code symbol stamp. The test has to be done after completion of the vessel, after all pre-test examinations have been performed and, when required, after post-weld heat treatment (PWHT). Note that a preliminary hydrostatic test prior to the PWHT is allowed to prevent an additional PWHT cycle when repairs are required following an unsuccessful hydrostatic test. For pressure vessels that cannot be safely filled with liquid or adequately dried when used in a service that cannot tolerate the test liquid, the alternative to a hydrostatic test is a pneumatic test as described in UG-100. The code requires special care for this type of test considering the amount of energy stored in compressed gas. The other alternative for vessels or vessel parts that cannot be accurately calculated is a proof test that follows UG-101.
Vessels can be tested either in the vertical or horizontal position, independent of the in-service position. When vessels are made of multiple parts, welded or bolted, the parts can be hydrostatically tested independently and U-Part stamped, but they must be tested assembled before the application of the U-Code symbol stamp for the whole unit.
As in Section VIII Div. 1, ASME B31.3 requires that all new piping systems, defined by the code as “interconnected piping subject to the same set or sets of design conditions,” be subject to a hydrostatic leak test before initial operation. This has to be done after any heat treatment and code required examinations, excluding owner-specified examination.