VALVE STEM PACKING LEAKAGE LEVEL

Leakage in industrial valves is generally divided into two categories: internal leakage and external leakage. Internal leakage refers to the passage of media through the sealing surfaces after the valve has been closed, affecting the shut-off function of the system. External leakage refers to the escape of media into the external environment through joints such as valve stem packings and flange gaskets, and is the primary source of fugitive emissions.

An important distinction must be clarified in valve leakage detection. Conventional evaluation of external leakage is based on visible observation under naked-eye conditions — that is, whether there is any medium exudation, dripping, or bubble formation at the stem or flange connections under visual inspection. This is the standard typically used to determine "zero leakage" during hydrostatic or pneumatic testing.

Fugitive leakage, on the other hand, refers to trace, invisible leakage. Such leakage cannot be detected by the naked eye and is only discovered using high-sensitivity instruments. It occurs mainly in the dynamic sealing area of the stem packing and the static sealing area of gaskets. Because the gasket joint in the valve body is a static seal, it is relatively easy to meet low-leakage standards. However, the valve stem packing is a dynamic seal — the stem continuously rubs against the packing during movement, which can easily lead to particle carry-over and fluctuations in sealing force. Therefore, the packing area is the core difficulty in controlling fugitive emissions from valves.

In response to the stringent control of volatile organic compound (VOC) fugitive emissions, a relatively comprehensive international standard system has been established.

EPA Method 21 is a standard detection method established by the U.S. Environmental Protection Agency (EPA) for detecting VOC leaks from valves, pumps, flanges, and other equipment and piping components using portable instruments. It is important to note that EPA Method 21 is not an emission limit standard, but a detection method standard. It specifies the technical requirements for detection instruments, probe scanning procedures, calibration procedures, and data recording methods. It is widely used in Leak Detection and Repair (LDAR) programs and also serves as the detection method basis for valve type-testing standards such as API 624 and API 641. The U.S. Clean Air Act (CAA), based on EPA Method 21 test results combined with the type of process media, specifies corresponding leak limits and repair deadlines, with specific requirements depending on the type of media the valve services.

The German TA-Luft (Technical Instructions on Air Quality Control) and VDI 2440 guidelines together form important technical regulations for fugitive emission control in the European Union. According to TA-Luft and VDI 2440, the leak rate for flange connections under a test pressure of 1 bar shall not exceed 10⁻⁴ mbar·L/(s·m). Furthermore, the limits are refined based on temperature: 10⁻⁴ mbar·L/(s·m) for temperatures below 250°C, and 10⁻² mbar·L/(s·m) for temperatures above 250°C. VDI 2200 specifies the selection, calculation, design, installation, and testing procedures for bolted flange connections. Testing typically employs a helium mass spectrometer with a test medium of 97% helium, and the leak rate is read directly from the leak detector — which differs from the U.S. system that uses methane as the medium and concentration limits (ppmv) as the indicator.

API 622 is a test standard for the fugitive emission performance of packing material itself. Before applying for API 624 or API 641 certification, an API 622 packing certificate must first be obtained. Its test conditions cover comprehensive factors such as temperature, pressure, thermal cycling, and mechanical cycling, with a packing leak rate not exceeding 100 ppm under the specified test conditions.

API 624 is a fugitive emission type-testing standard for rising stem valves (e.g., gate valves, globe valves) equipped with graphite packing. This standard specifies the use of methane with a purity of not less than 97% as the test medium. During the test, the valve must undergo 310 mechanical cycles and 3 thermal cycles (high temperature point 260°C ± 14°C), while maintaining a test pressure of 41.4 barg (600 psig) at the high temperature point.

API 641 is a fugitive emission type-testing standard for quarter-turn valves (e.g., ball valves, plug valves). It also uses methane with a purity of not less than 97% as the medium, and the valve under test must complete 610 mechanical cycles and 3 thermal cycles.

The acceptance criteria for both API 624 and API 641 are the same: the leak value shall not exceed 100 ppmv throughout the entire test, and no adjustment to the packing system is permitted during the test. This 100 ppmv limit provides a clear definition of the allowable leak rate from the dynamic stem packing seal under design conditions — serving as a reference value for engineering design.

ISO 15848 is an internationally applicable industrial valve standard for the measurement, testing, and qualification procedures for fugitive emissions. It consists of two parts: ISO 15848-1 specifies type testing and classification systems for valves, and ISO 15848-2 specifies production acceptance testing for valves.

ISO 15848-1 classifies stem seals into three leakage classes based on the measured leak rate (mg/(s·m)): Class A ≤ 10⁻⁶ mg/(s·m), Class B ≤ 10⁻⁴ mg/(s·m), and Class C ≤ 10⁻² mg/(s·m). Typically, bellows-sealed valves or certain specially designed quarter-turn valves can achieve Class A; stems with PTFE or elastomeric rubber seals can achieve Class B; while flexible graphite packed stems generally correspond to Class C.

Since the leak rate unit of ISO 15848 (mg/(s·m)) and the concentration unit of API standards (ppmv) have different physical meanings, there is no simple conversion relationship between them. ISO 15848 characterizes leakage by the mass of medium leaking per second per unit length of stem, while API standards characterize it by the volumetric concentration of leaking gas in air — reflecting different evaluation dimensions.

API 624 and API 641 have incorporated the detection method principles of EPA Method 21 into their type-testing frameworks. However, the limit requirements of API 624 and API 641 are primarily based on methane testing. Their applicability to small-molecule gases such as hydrogen and helium must be separately evaluated — because small-molecule gases are more permeable and may result in higher actual leak rates than methane under the same sealing conditions.