The Scope and Application of Pressure Testing Valves

To understand the term ‘allowable leakage rate’ and demonstrates its empirical nature, it is beneficial to presents a brief and timely analysis of the international standard ISO 5208 for valve pressure testing. For this purpose, it is also important to analyze the refinement provided by the API 6D valve specification to the ‘allowable leakage rate’ values of ISO 5208. This in turn highlights the importance of a comprehensive assessment of the valve’s life cycle to achieve the best performance beyond the testing stage.

By M. Sc. José Luis Trejo Rosales – Vacoisa 

With the advancement of valve technology, pressure test procedures and criteria have transformed by following the evolution of the standardized tests that measure a valves’ sealing capacity. The oil and gas industry’s primary valve standard is ISO 5208: Industrial Valves – Pressure Testing of Metallic Valves. The purpose of this standard is to establish procedures and acceptance criteria for the verification of valve quality in terms of seat tightness or sealing capability under standard test conditions.

The main difference between the international standard ISO 5208, and other current normative documents in the oil and gas industry for valve pressure testing, lies in the fact that this ISO document introduces the term ‘allowable leakage rate’. This term, or concept, is applied during the testing period and as such constitutes a key stage within the valve life cycle.

‘Allowable Leakage’

When analyzing the ISO 5208 standard, it is relevant to note the levels or ‘leakage’ rates that it establishes, which range from a ‘fine leakage’ level through various levels of ‘gross leakage,’ called ‘allowable test leakage rates.’ In this regard, please note that the standard document regulates a level of ‘standardized leakage.’ For quick reference, see table 4 of ISO 5208:2015 or Annex A of this paper.

It is also important to note that the ‘standardized leakage’ level in ISO 5208’s ‘allowable leakage rates’ are not supported by any informative or normative annex that includes at least a single data correlation about the pressure controlling part’s materials, which are intrinsically involved in the valve sealing capability. The ‘allowable leakage’ values set by ISO are empirical, as evidenced in ISO 5208:2015.

The term ‘allowable leakage rate’ is extremely critical. Despite the fact that a new valve’s materials have a full resistance, are mitigated, and have not been exposed to operating conditions, they still exhibit ‘allowable leakage’. It is therefore key that ISO 5208 users are aware that, regardless of the redundancy of the standardized tests to which the valves have been submitted (FAT – Factory Acceptance Test, OSAT – On-Site Acceptance Test, among other tests required by customer), the tested valves are not exempt from ‘permissible leakage’, even at the testing stage; this is considered acceptable for valve quality purposes.

Customized Rates

Fortunately for valve users in the oil and gas industry, the ‘allowable leakage rates’ of ISO 5208 are adapted and adjusted to the particular requirements of product standards or specifications. An example of this is the API 6D specification for valves in pipeline and piping systems, which complements and enhances the ISO 5208 ‘allowable leakage rates’ by appropriately limiting it in terms of valve seal technology.

Indeed, the API 6D Valve specification competently circumscribes what can be expected from valve sealing capability for oil and gas applications. Furthermore, it does so in terms of seal technologies. According to API 6D, the tests and acceptance criteria of ISO 5208 are specified exclusively for two types of valve seats: ‘soft-seated’ and ‘metal-seated’ technology. Note that the standardized terminology for seat seals at this point is specific and refers exclusively to ‘soft-seated’ and ‘metal-seated’. Thus, it leaves no room for mixed terms such as ‘Primary Metal – Secondary Soft seat (PMSS)’.

To be more specific, the term ‘soft-seated’ refers to seats with polymeric inserts (PTFE, PEEK, among others). Consequently, it is appropriate to describe them as seats with polymeric seals; it is thanks to the resilient properties of polymer materials that maximum sealing capacity can be achieved. Based on API 6D logic, a valve with a soft-seated technology corresponds to a Permissible Leakage Rate A, a non-visible leakage rate for the duration of the test according to ISO 5208.

The term ‘metal-seated, alternatively is inherently related to the use of metal seats without polymer inserts. They are metallic seals, which, following API 6D philosophy, correspond a Permissible Leakage Rate D from ‘ISO 5208, associated with a level of ‘leakage’ or internal pass from the test stage.

The ‘allowable leakage’ levels associated to a soft-seated and metal-seated technologies, respectively, are documented in Annex B. Annex B shows the ‘maximum allowable leakage rate’ for hydrostatic and pneumatic testing. At this point in the analysis, it is critical to note that the ‘allowable leakage’ levels, when measured and improved by API 6D (Rate A for soft-seated and Rate D for metal-seated) provide to the valve users a safe test criterion in terms of reliability. This is because each new edition of ISO 5208 relaxes the allowable leakage rates, accessing in every new edition to allowable leakage levels grosser than previous versions of that document.

ANNEX A: Sealing Capability In Valve Closing Elements

ANNEX B – Sealing Capability In Valve Closing Elements

Valves Optimal Performance

Finally, to achieve the valves’ best performance, it is essential to promote selection of them, in an integral way, analyzing each of the stages involved in valves life cycle, not limited exclusively to pressure testing stage. The above issue is relevant, because as it has been shown, no matter how much users double, triple, or even quadruple the amount of testing (with the associated impact on costs and delivery times that represents), valves will show allowable leakage, even if it is visible or not visible.

The right valve selection is a process that must consider all stages of the life cycle: engineering, manufacturing, testing, and operation, among other key factors. This process goes through the analysis of material compatibility in real operational scenarios. The above are critical aspects that are beyond the regulatory scope. This strategic activity is mainly the responsibility of professional engineering entities, which are not limited to transcribing technical standards to justify valve selection and specification. Valves have an impact on the safety of processes, infrastructure, human resources, and the environment.

Final Thoughts

From a technological perspective, it is acknowledged that the ISO 5208 valve pressure test constitutes the most demanding non-destructive test that valves must pass to ensure manufacturing quality, and also have to recognize that there is an implicit systemic limitation in the ‘allowable leakage rates’ of the international standard ISO 5208, which is usually disregarded. This limitation can be corrected and substantially improved by assessing the valve’s full life cycle. This improvement has a positive impact on the reliability of the valves beyond the testing stage.

ABOUT THE AUTHOR

José Luis Trejo Rosales is a professional in the hydrocarbons sector, speaker at technology dissemination forums, and member of the Association of Petroleum Engineers of Mexico. He is a Mechanical Engineer from the Higher School of Mechanical and Electrical Engineering, and Master in Materials Science from the Higher School of Physics and Mathematics of the National Polytechnic Institute of Mexico. He has collaborated for the Mexican Petroleum Institute assuming the position of pipe specifications specialist in industrial and offshore installations. José is an expert in the total life cycle of engineering and process valves, including specification, manufacturing, distribution and service. As a valve professional, he has represented European manufacturers, and has been a valve specification manager at Cameron.