Valve Selection for Flammable and Explosive Media

In process systems that handle flammable and explosive media, valves are critical control components. The selection of valves directly affects the safety, reliability, and efficiency of the system. To ensure safe and stable operation, valve selection should consider various factors. These factors include valve type, structure, material, sealing performance, connection method, and operational characteristics. This article will explain the key factors that need to be considered when selecting valves for such environments.

Shutoff Reliability

Shutoff reliability is the most important consideration when selecting valves, especially in systems that handle flammable and explosive media. Even a small leak can cause a serious safety incident. Therefore, valves must provide efficient and reliable shutoff functionality. The most common shutoff valves, ranked by their shutoff reliability from high to low, are as follows.

Gate Valves: These valves have high shutoff reliability and provide excellent sealing. They are suitable for applications requiring complete closure.

Globe Valves: These valves offer good shutoff performance and are commonly used for precise flow regulation.

Ball Valves: These valves have reliable shutoff and are suitable for quick opening and closing.

Plug Valves: These valves have average shutoff performance but meet requirements in certain special applications.

Butterfly Valves: These valves have lower shutoff reliability. They are suitable for flow control but are not recommended for high-reliability applications.

In practice, valves with higher shutoff reliability, such as gate or globe valves, should be chosen to ensure that no media leaks.

Pressure Drop and Energy Efficiency

Unless the process system has specific requirements, valves with low pressure drop should be selected. This helps minimize energy loss. Pressure drop across a valve is directly related to its ability to handle fluid. Choosing valves with low pressure drop improves system energy efficiency. Common shutoff valves, ranked by pressure drop from low to high, are as follows.

Ball Valves: These valves have the lowest pressure drop and are suitable for applications requiring low flow resistance.

Plug Valves: These valves have relatively low fluid resistance and are ideal for quick adjustments.

Gate Valves: These valves have higher flow resistance but provide better sealing.

Globe Valves: These valves experience higher pressure drop and are used for precise flow regulation.

Butterfly Valves: These valves have the highest pressure drop and are suitable for simpler flow control.

When there is a conflict between shutoff integrity and pressure drop, shutoff integrity should take priority. This is especially true in high-security process systems.

Special Media Requirements

For special media, such as liquefied petroleum gas (LPG) or liquid hydrocarbons, valve cavity sealing is important. To prevent the formation of closed cavities under high pressure, valves that are less likely to form sealed cavities should be selected. Globe or butterfly valves are preferred in these cases. If a valve with a sealed cavity structure is used, it should be equipped with a vent hole to release excessive internal pressure. This prevents safety hazards. Unless the process has specific requirements, standard-diameter valves should be chosen. This ensures enough flow capacity and avoids reduced-diameter valves.

Connection Method and Installation Requirements

The connection method of the valve directly affects the system's sealing and installation convenience. In most cases, welded connections should be preferred. This is especially important when the valve pressure rating exceeds Class 600 LB or PN 64. Welded connections provide better sealing and structural stability. If threaded connections are used, additional sealing welding should be applied to ensure tightness.

Specific Valve Design and Selection

Valves are designed differently depending on the operating pressure, flow requirements, and medium characteristics. Here are design recommendations for common valve types.

Gate Valves: For DN50 and larger sizes, wedge-shaped elastic valve discs should be used for better sealing.

Globe Valves: For flow regulation, tapered or ball-shaped valve discs are recommended, as they perform better than flat valve discs.

Butterfly Valves: A three-eccentric or high-performance double-eccentric design is recommended. These designs improve sealing and extend the lifespan. Middle-line type butterfly valves should be avoided.

Check Valves: Wafer-style check valves are not recommended due to their poor sealing performance, which can cause leakage.

For valves with DN1000 and above, such as gate valves, ball valves, and swing check valves, a venting and draining system should be installed. This system facilitates maintenance and prevents freezing.

Safety Requirements

In systems handling flammable and explosive media, valve safety is very important. Non-metallic sealed valves should have fire-safe and anti-static designs. These valves should pass fire safety tests to ensure they are safe in fire-prone environments. Additionally, valves should be designed to prevent both internal and external leakage during operation. They must meet the leakage class requirements of ISO 5208 and the low-leakage standards of SPE 77/312.

Material Selection

Pressure-bearing components of valves should be made from high-quality metal materials. The quality of these materials should be strictly controlled. This includes checking for surface defects, internal defects, non-metallic inclusions, and grain size. Material selection is especially important in special environments, such as hydrogen environments. For valves with pressure ratings exceeding CLASS 600 or PN 64, material quality must be controlled strictly. Brittle materials, such as cast iron, should not be used because they can break under high pressure.

Sealing Elements and Surface Treatment

Valves should use high-strength materials for bolted valve cover bolts. The gaskets should have high gasket coefficients (m) and specific pressure (y) to ensure sealing at connection points. The sealing surfaces of the valve disc, seat, and upper seals should undergo hardfacing with carbide welding. This process improves wear resistance and minimizes leakage. The valve stem surface should undergo appropriate surface hardening. This treatment extends the service life of the valve and reduces leakage caused by wear.

Conclusion

In systems handling flammable and explosive media, selecting the right valve is crucial for ensuring system safety and reliability. By considering factors such as shutoff performance, pressure drop, special media requirements, connection methods, safety features, material selection, and sealing elements, operators can ensure stable system operation. This helps prevent safety incidents caused by valve failure. Valve selection should be carefully based on specific process requirements, media characteristics, and operating environments. This ensures safe, efficient, and reliable valve operation.