An Introduction to Valve Low-Leakage Testing

In the industrial field, valves are key components for controlling fluid flow, and their sealing performance is crucial. Low-leakage valves are preferred in many high-demand scenarios due to their excellent sealing performance. Low-leakage valves refer to valves with an extremely small actual leakage, where conventional hydrostatic or pneumatic sealing tests cannot accurately determine leakage, requiring more advanced methods and instruments to detect minute external leaks. Such minimal leakage to the external environment is called low leakage. With the continuous development of industrial production, the demand for valve sealing performance is increasing, and the importance of low-leakage testing is becoming more prominent.

Currently, the commonly used international standards for low-leakage valve detection mainly include three: the U.S. Environmental Protection Agency EPA Method 21 Volatile Organic Compound Leak Detection, ISO 15848 Industrial Valves: Measurement, Testing and Qualification of Low-Leakage Performance, and SHELL MESC SPE 77/312 Industrial Valves: Low-Leakage Measurement, Classification System, Qualification Procedure, and Type Approval and Product Testing for On-Off and Control Valves.

EPA Method 21 only specifies the detection method without classifying leakage levels and is considered a local regulatory standard with a relatively narrow application range. In contrast, ISO 15848 and SHELL MESC SPE 77/312 evaluate valve performance based on three aspects: tightness class, durability class, and temperature class. Tightness class mainly targets leakage at the stem and body seals, divided into grades A, B, and C. ISO 15848 specifies that leakage at the body seal should be ≤50 cm³/m³, while both standards calculate stem leakage based on the stem diameter. Among them, ISO 15848 grade A represents the highest sealing level, while grades B and C correspond to SHELL MESC SPE 77/312. Generally, the sealing level of low-leakage valves is below grade B, and bellows-sealed valves, which use metal bellows at the stem sealing position, can achieve a sealing level below grade A.

To obtain certificates in compliance with these standards, valve manufacturers must complete and pass prototype testing, usually conducted by established third-party testing institutions. Manufacturers need to select from different testing procedures, acceptance criteria, and test variables, such as medium, temperature, number of cycles, and thermal cycles. There are various methods to obtain leakage rate data, such as diaphragm testing and bagging testing. Valve manufacturers can test for specific tightness grades, durability grades (mechanical and thermal cycles), and test temperatures. However, a disadvantage exists: it is not a fully unified procedure for evaluating emission performance, making it difficult for end-users to compare the overall performance of one manufacturer with another. Different manufacturers may choose different performance classifications based on their products, so end-users must evaluate test results according to specific needs or applications.

Low-leakage valves mainly concern external leakage, and the following points require emphasis:

Valve castings must meet ammonia test requirements. Due to the small molecular weight of ammonia, it easily penetrates, and casting quality is a key factor affecting sealing performance. High-quality castings can effectively prevent medium from leaking from the interior to the external environment, ensuring overall valve sealing.

The seal between the stem and bonnet often uses a gasket, which is a static seal. This type of seal is relatively easy to meet leakage requirements because the stem and bonnet remain relatively stationary under normal operation, and the contact between sealing surfaces is stable. By properly selecting gasket materials and installation procedures, leakage risk can be effectively reduced.

Stem sealing usually adopts a graphite packing structure. Because the stem seal is dynamic, the movement of the stem can carry graphite particles from the packing, causing leakage. Therefore, special low-leakage packing is required, and clearances between the packing and the stem, the packing and packing box, and the gland with the stem and packing box must be strictly controlled. Additionally, the surface roughness of the stem and packing box should be controlled. These measures are crucial for ensuring sealing performance at the stem.

Valve leakage can be internal or external. Internal leakage refers to medium leaking from the inlet side to the outlet side through the sealing surface, while external leakage refers to medium leaking from the valve to the external environment. Both can have serious consequences:

Safety Risks: Valves transporting flammable, explosive, toxic, or harmful media may cause fire, explosion, or poisoning accidents, severely threatening personnel safety. For example, in chemical plants, if a valve carrying flammable gas leaks and encounters an open flame or static spark, it may trigger a fire or explosion, causing significant casualties and property loss.

Economic Loss: Medium leakage leads to raw material waste and may require additional energy to maintain system pressure, increasing production costs. In petroleum refining, if valves in crude oil pipelines leak, it not only wastes crude oil but may also require extra energy to maintain normal production, increasing operational costs.

Environmental Pressure: Valve leakage in chemical or petrochemical industries can contaminate soil, water, or air, violating environmental regulations and facing fines or production shutdowns. With stricter environmental requirements, leakage causing pollution may lead to heavy fines or forced shutdowns, resulting in reputational and financial loss.

System Stability: In precision control systems such as pharmaceuticals or semiconductor production, valve leakage can disrupt medium purity or flow stability, affecting product quality. For example, in high-purity gas delivery systems for semiconductor manufacturing, valve leakage may reduce gas purity, impacting semiconductor quality and causing significant economic loss.

Therefore, performing low-leakage valve tests to detect sealing performance in advance is a key preventive measure and an important part of industry compliance.

Different countries and industries have established clear standards for low-leakage valve testing to ensure accurate and consistent results. Widely used standards include:

API 598 (American Petroleum Institute Standard): Commonly used in oil and gas, specifying valve pressure tests, including body and seat tests (core of low-leakage testing), with detailed requirements for test medium (water, air, or nitrogen), test pressure, holding time, and leakage judgment.

ISO 5208 (International Standard): Applicable to industrial gas valves, similar to API 598, emphasizing zero visible leakage or quantifiable allowable leakage. Standards vary based on valve type (gate, ball, globe) and operating conditions (high pressure, high temperature) to accurately assess sealing performance in practical applications.

GB/T 13927 (Chinese National Standard): Equivalent to ISO 5208 and mandatory in the domestic valve industry. It specifies test pressure range (usually 1.1× design pressure or 1.0× rated pressure), hold time (10–30 minutes), and detection methods (bubble test, pressure decay). Compliance ensures valve quality meets industry requirements and enhances market recognition.

Bubble Test: The valve is filled with compressed air or nitrogen (0.6–1.0 MPa), and the sealing areas (seat, packing) are submerged in water. Bubble formation indicates leakage, with bubble count used to assess leak size (≤1 bubble/min is acceptable). Suitable for low-pressure valves or high-precision valves (ball, butterfly), particularly for detecting minute leaks. Key points: ensure internal cleanliness, immerse seals fully, and prevent bubbles sticking to surfaces.

Pressure Decay Test: Fill the valve with test medium (water or gas) and maintain a set pressure. Pressure drop is monitored during hold time. If the drop exceeds the allowable range (e.g., ≤0.05 MPa/30 min), the valve is considered leaking. Suitable for high-pressure, large-diameter valves (gate, globe), especially where bubble tests are impractical. Key points: remove internal air, maintain stable ambient temperature.

Ammonia Leak Detection: Ammonia gas is used as a tracer, filling the valve, with a mass spectrometer detecting external leaks. Extremely sensitive, capable of detecting <10^-9 Pa·m³/s. Suitable for high-precision valves in nuclear, aerospace, semiconductor, or toxic/radioactive medium applications. Tests must be conducted in sealed chambers with clean, dry valve surfaces.

Sealing Surface Condition: Scratches, wear, contamination, or deformation can cause leakage. Surfaces should be inspected and, if necessary, resurfaced to ensure accurate results.

Test Medium Selection: Gas is sensitive to detect micro-leaks but affected by temperature and humidity; liquids are stable but less precise. Select medium based on valve type and standards.

Test Pressure and Hold Time: Low pressure may miss leaks; high pressure may damage seals. Hold time must be sufficient but not excessive. Follow standard parameters strictly.

Environmental Factors: Temperature fluctuations cause medium expansion/contraction; high humidity may form condensation that interferes with detection. Conduct tests in dry, stable environments (20–25°C).

Low-leakage valve testing plays a critical role in industrial production. By strictly following international and domestic standards, using appropriate testing methods, and considering factors affecting accuracy, valve sealing performance can be effectively ensured, safeguarding industrial safety, stability, and environmental protection. Low-leakage valves and rigorous testing are indispensable in industries including oil and gas, chemical, pharmaceutical, and semiconductor. With continuous technological advancement, methods and standards for low-leakage testing are expected to improve further, providing stronger support for sustainable industrial development.