GEORGE GALANES, NBIC MAIN COMMITTEE CHAIR, AND LUIS PONCE, MANAGER OF TECHNICAL SERVICES
Among the BULLETIN archives, two older articles on pressure testing are still relevant and worth reading.
In the 2006 fall edition, George Galanes, one of the authors of this article, discussed material degradation, which can occur from prolonged exposure of inservice pressure-retaining components to elevated temperature service. His article focuses on the dangers of performing repeated hydrostatic tests above the normal working pressure of an inservice pressure-retaining item (PRI) with degraded material toughness and the risks of brittle fracture while testing at room temperature. The second topic is that the perceived benefits of hydrostatic testing can’t be used to determine the future condition of the component and only provide a snapshot in time of the current condition.
On the other hand, Robert Schueler’s article in the 2008 summer edition emphasized the importance of the equipment selected for the pressure test, including the piping, fittings, blank flanges, and fasteners. These items must be sufficiently rated for the test pressure and temperature.
It’s important to note that risks associated with pressure testing during the fabrication of new pressure-retaining components are far different than risks associated with pressure-testing inservice components, as described earlier. When conducted, the pressure test is the last step in the fabrication or inservice component repair process before stamping and signing the applicable Manufacturer’s Data Report or Form “R” report to ensure no leaks from workmanship. Those involved in pressure testing must treat it with the same importance as the other elements of the quality program. I wonder how many inspectors can recall an incident during a pressure test or a near miss from which they were fortunate to walk away. The Occupational Safety and Health Administration website (osha.gov) has several reports of personnel who have been hospitalized or died due to a pressure test gone wrong.
When conducted safely, the pressure test should not fail, so why have there been incidents in the past? One possibility is that the pressure test is typically the last step before stamping and certification, and certificate holders may feel rushed to complete a project. Another is that the pressure test sometimes occurs after normal working hours or over the weekend, which can be less-than-optimal times for testing personnel and the inspector.
The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME BPVC) and the National Board Inspection Code (NBIC) have quality system requirements for some aspects of pressure testing, but not all of them. For example, the NBIC requires the quality system “shall describe” the process used to ensure all required examinations and tests have been successfully performed and accepted by the inspector. ASME Section VIII Division 1 has similar wording in part, “The Quality System shall describe the … examinations, sufficiently to permit the Inspector … to determine at what stages specific inspections are to be performed.”
The codes place the responsibility on the certificate holder to approve and the inspector to accept the process to be used in production for the testing of equipment. This is where Schueler’s article becomes important and may be used by the certificate holder to create a procedure to ensure pressure testing is conducted safely. The code requirements in the XX-99 and XX-100 paragraphs provide the rules for minimum and maximum pressures and temperatures, and even holding times as in the NBIC, but neither code addresses how to select pressure testing material, e.g., piping, fittings, flanges, fasteners, proper bolting sequence, and torquing values.
Let’s focus on some code of construction requirements, which include the minimum test pressure and the maximum or upper limit of the test pressure, and, of course, the test temperature limitations. All design calculations typically include the minimum test pressure, but a fair question is how many designs address the upper limit of the test pressure. ASME Section I PG-99.1 requires that no part of the boiler shall be subjected to a general membrane stress greater than 90% of yield strength (where the yield strength is determined using the 0.2% offset method), and the code places responsibility on the designer to determine the value. Inspectors and Review Team Leaders should ask designers and engineers to demonstrate the method to calculate the 90% of yield strength (0.2% offset).
ASME Section IV HG-510 is more basic, stating the hydrostatic test shall not be exceeded by more than 10 psi, and ASME Section VIII Division 1 UG-99 (c) allows the inspector to reject the vessel if the pressure test exceeds the calculated test pressure to the point of visible permanent distortion. ASME Sections VIII Division 2 and Division 3 are much more comprehensive, providing the necessary guidance to designers and engineers for determining the upper limit of the test pressure. Inspectors and Review Team Leaders should verify this upper limit is included in the Manufacturer’s Design Report.
Remember – Regarding the calculated test pressure, RCI-1 requires the inspectors in part to "verify design calculations... are complete and meet the requirements of the Code and the Certificate Holder's quality program."
The upper limit of the test pressure is critical because it could affect future pressure tests as Galanes calls attention to in his article. It is important to re-emphasize that increased internal pressure required for hydrostatic testing can place undue risk of failure for aged inservice components with no benefit. There is no need to verify the design for a repaired component, and lower pressures for pressure tests function to check for leaks and the integrity of the repair.
As a side note for any hydrostatic or pressure testing being performed on aged equipment, the risk of damage to surrounding equipment that safely supports the component inservice must be checked to ensure the structural integrity is present to support the static weight of water fully.
Lastly, NBIC Part 3, 4.4.1 c) Initial Service Leak Test can be misapplied when not used properly because this type of test may be conducted when permitted by the original code of construction. This term is not defined in the NBIC, but ASME B31.1, 137.7.1 contains an example. There, we find that an initial service leak test permits starting system pumps, compressors, or other equipment where an opportunity exists to examine for leakage before full-scale operation.
None of the ASME BPVC of construction e.g., Sections I, IV, all the VIIIs, X, or XII, permit this type of pressure test. It is allowed in ASME B31.1 but only for non-boiler external piping. The last sentence in paragraph 137.7.1 reads, “An initial service test is not applicable to boiler external piping.” The question then becomes: Where can this be used? This test can only be used on repairs and alterations outside the scope of the ASME BPVC when the NBIC activity will be stamped with the "R" symbol and the applicable Form R Report will be signed.
Pressure testing should never be taken for granted. Errors have resulted in hospitalization and even death.
This last step in the process should be conducted by following the code rules and the certificate holder’s procedures to ensure the safety of personnel and to ensure the PRI will be safe to operate once installed. To read either article, go to the BULLETIN tab.