Valves are essential equipment for the safe operation, maintenance, upgrading and development of gas transmission, distribution and storage systems. Improper selection or poor quality of gas valves can lead to leakage, shutdown and other accidents. Once an accident occurs, it may at least disrupt normal social life and production, and at worst cause heavy losses to national and public life and property. Therefore, the selection of gas valves must be approached with great care.

Current Application Status of Gas Valves

With the development of the gas industry, the demand for gas-specific valves is growing steadily, and valve manufacturers continue to introduce new products to meet the needs of market competition. At present, buried gas valves in China are mainly divided by structure into gate valves, ball valves and butterfly valves. Actuation methods mainly include manual, gear (worm) operation, electric, pneumatic and gas-hydraulic combined operation. Installation options are either with a valve chamber or direct burial. According to relevant data, the most widely used valves in urban coal-gas transmission and distribution systems are manually operated gate valves. Taking Tianjin as an example, three districts — Hebei, Hongqiao and Beichen — have more than 120 kilometers of medium-pressure pipelines and over 300 valves, of which more than 80% are gate valves, followed by butterfly valves and ball valves. Before the gas source conversion project, these pipelines carried manufactured (coal) gas, which contains considerable impurities — in particular "gas tar gel" formed by a mixture of tar, aromatic hydrocarbons and dust — which frequently affected valve sealing or even "seized" the valve stem. Therefore, valve selection focused on valves whose structural features could solve this problem, ensuring flexible operation and zero leakage. After years of practice and data analysis, gate valves (including parallel double-disc gate valves and flexible-seal single-disc gate valves) proved suitable. However, with the completion of the natural-gas-replacing-coal-gas project, the nature of the gas has changed. Natural gas is cleaner and drier than manufactured gas, but contains sandy dust and is delivered at higher pressure; under high pressure, the sandy dust creates strong erosion of the valve cavity, and natural gas also contains highly corrosive hydrogen sulfide. How to select valves for natural gas in urban buried pipelines is therefore a new challenge facing the industry.

Natural gas is flammable, explosive and highly corrosive, so natural gas valves installed in underground pipe networks should meet the following requirements:

Corrosion-resistant materials: natural gas transported in pipelines contains large amounts of hydrogen sulfide before desulfurization (a toxic and extremely corrosive gas that reacts with iron to form iron sulfide, which flakes off and corrodes mechanical equipment). Even after desulfurization, residual hydrogen sulfide remains. Therefore, sulfur-resistant, corrosion-resistant materials must be selected for pipeline valves.

Rational structure: buried gas valves should be full-bore in design to reduce flow resistance and allow passage of pipeline cleaning pigs or pipeline detectors, while saving operating costs; structural height should be minimized to reduce installation costs; the top of the valve should be equipped with a fully enclosed open/close position indicator so operators can clearly see the valve status at any time and avoid misoperation.

Excellent sealing: leakage requirements for natural gas valves are extremely strict. The standard CJ3055-95 "Testing and Inspection of Urban Gas Valves" stipulates that soft-sealed valves must exhibit no detectable internal leakage at 1.1 times the nominal rated service pressure, and hard-sealed valves may allow internal leakage of less than 0.3·DN mm³/s at 1.1 times the nominal rated service pressure. External leakage is absolutely not permitted. Buried and critical valves generally adopt a fully welded body structure. To ensure the sealing performance of pipeline valves, the sealing pair must offer excellent corrosion resistance, wear resistance, self-lubrication and elasticity.

Easy operation: the vast majority of underground pipeline valves are operated manually, so the valve should have low operating torque and a limited number of turns over the full stroke, allowing rapid gas source isolation in the event of an accident.

Simple maintenance: valve components should adopt low-maintenance or maintenance-free designs to minimize inspection and servicing work, reducing situations where valve maintenance requires road closures and disrupts traffic.

At present, gas valves used in China's natural gas industry fall into three main structural categories: gate valves, ball valves and butterfly valves. These valve types are analyzed and compared below from three perspectives:

With the continuous development of China's gas industry and science and technology, more new technologies and materials will be applied to the manufacture of gas valves. Accordingly, the selection criteria for gas valves should also be continuously improved to meet the needs of gas users.