As engineers in fluid control systems, our work is centered on managing risk and ensuring operational integrity. A recurring and critical topic in this field is valve leakage. While the specification "zero leak" is common, it is a technical misnomer. From a mechanical engineering perspective, a true hermetic seal with zero molecular passage in an industrial valve is not feasible due to factors like microscopic surface imperfections, material porosity, and degradation from operational cycles, temperature, and pressure.
Therefore, our objective as technical professionals is to apply a clear and quantifiable framework to define acceptable performance. This is the function of valve leakage detection standards. They provide the precise, universally understood language necessary for valve specification and testing. This guide provides a technical overview based on my experience in valve design, testing, and application, focusing on the functional differences between the core API and ISO standards and how to apply them to specific operational contexts.
A precise discussion of valve leakage requires differentiating between its two primary forms. They have different origins within the valve and carry distinct operational consequences.
This is any leakage that leaks from the high-pressure side of the valve body's pressure boundary to the low-pressure side. The most common points of emission are the stem packing and body-to-bonnet gaskets. Fugitive emissions are a primary concern for safety and environmental compliance. The release of flammable, toxic, or regulated media poses direct risks to personnel and can result in significant regulatory penalties under frameworks like the U.S. Clean Air Act.[1] Consequently, controlling fugitive emissions is a critical design and maintenance priority, especially in the hydrocarbon and chemical processing industries.
This refers to leakage across the valve's obturator (the closure element) when the valve is in the closed position. The process medium remains contained within the pipeline. However, the impact of internal leakage on process control and efficiency is significant. It can result in product loss, energy waste from pressure loss, and cross-contamination between process streams. Most importantly, it can compromise the positive isolation required for safe maintenance and shutdown procedures.
The selection of a testing standard is a critical engineering decision. The American Petroleum Institute (API)[2] and the International Organization for Standardization (ISO)[3] offer the dominant standards, and they reflect different testing philosophies.
Leakage Type | API | ISO |
Fugitive Emissions | API 624 / 641 (Pass/Fail Packing Test) | ISO 15848 (Graded Performance Classification) |
Seat Leakage | API 598 (Industry Baseline Production Test) | ISO 5208 (Graded Rates for Precise Specification) |
When a low-emissions valve is required, the specification typically involves either ISO 15848 or API 624/641. These standards are not directly interchangeable. For all fugitive-emission test protocols—including ISO 15848 and API 624/641—the valve’s pressure boundary (body and bonnet joints) must remain leak-free during testing. The distinction between standards lies in what part of the sealing system is evaluated for performance.
ISO 15848-1 is a type test that evaluates the external sealing performance of the entire valve assembly. It provides a graded classification rather than a simple pass/fail result, based on:
Tightness Class: A quantitative measurement of leakage rates using helium or methane.
Endurance Class: A qualification based on the valve's performance over a defined number of mechanical and thermal cycles.
Temperature Class: Defines the temperature range for which the valve's performance is certified. A valve certified to ISO 15848 carries a detailed performance descriptor. This allows an engineer to specify a valve with a high degree of precision for its intended service life and conditions, making it a common choice for critical applications and in regions with stringent environmental regulations, such as the EU.
Developed to meet the needs of the North American petroleum industry, API 624 (for rising stem valves) and API 641 (for quarter-turn valves) are type tests focused specifically on the performance of the stem packing. The test protocol is highly prescriptive: the packing is subjected to a set number of mechanical and thermal cycles using methane as the test medium. The single acceptance criterion is a measured leakage of less than 100 ppmv, with no adjustments permitted during the test. An API certification provides a clear assurance that the valve's packing system has passed a standardized and demanding protocol widely accepted in North American refineries and chemical facilities.
The production valve leak test for seating integrity is typically conducted according to API 598 or ISO 5208.
API 598 is the most widely used standard for routine production testing of valves. Its acceptance criteria are direct: for resilient (soft) seated valves, no visible leakage is permitted during the test. For metal-seated valves, a small leakage rate, defined in drops per minute (liquid) or bubbles per minute (gas) based on the valve's nominal size, is allowed. It is a functional and universally accepted standard for general industrial applications.
ISO 5208 provides a more granular system of defined leakage rates. Rate A specifies "no visually detectable leakage" and is functionally equivalent to the API 598 requirement for soft-seated valves. The standard then provides a series of subsequent rates (e.g., AA, B, C, etc.) that correspond to specific, calculated maximum leakage rates based on the valve's diameter (DN). This system is extremely useful when a specific, quantifiable level of seat tightness is a critical design parameter, such as in hazardous gas service where something less than "bubble-tight" is unacceptable.
The correct standard depends entirely on the application's location, industry, and criticality.
North American Refinery: Fugitive emissions specifications will almost certainly require API 624 or API 641 certification. Standard seat leakage requirements will be met by testing to API 598.
European Process Plant / Renewable Energy Project: The regulatory environment often requires the detailed performance data provided by ISO 15848 for fugitive emissions. Seat testing may reference ISO 5208 or EN 12266-1.
Critical Gas Isolation: For any application involving hazardous gas or requiring absolute shutoff for safety, seat leakage should be specified to ISO 5208 Rate A. This removes any ambiguity regarding performance expectations.
General Industrial Service (e.g., Water, Utilities): For standard applications, production testing of every valve to API 598 provides the necessary quality assurance for reliable service.
A test certificate is a validation of performance against a specific standard. True operational reliability, however, comes from an engineering-level understanding of that standard—its methodology, its limits, and its applicability to a given process.
At Neway Valve, our engineering philosophy is grounded in this principle. Our investment in comprehensive in-house testing facilities for fire-safe, cryogenic, and fugitive emission evaluations is driven by the need to generate precise performance data. This data informs our designs and allows us to provide sound technical guidance to our clients. The objective is always to ensure the specified solution is appropriate for the application's lifecycle demands.
The selection of valve leakage standards involves nuanced technical trade-offs. If you are evaluating these criteria for a project, I encourage you to consult with our technical team to discuss the specific requirements of your application.
[1] https://www.epa.gov/laws-regulations/summary-clean-air-act
[2] https://www.api.org/-/media/apiwebsite/products-and-services/2025_intnl-usage_report_web-final.pdf
[3] https://www.iso.org/standard/61441.html
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