Design and Installation Principles of Low-Temperature Valves

In current large and medium-sized ethylene plants, as well as in coal-to-methanol purification units, deep cryogenic separation is commonly employed for components such as hydrogen and methane. To meet these requirements, low-temperature valves are widely used. Understanding the design requirements and special structures of low-temperature valves is crucial to avoid issues during selection and installation.

Low-temperature valves, especially ultra-low-temperature valves, operate at extremely low temperatures. When designing such valves, special requirements must be considered in addition to general valve design principles.

Design Requirements for Low-Temperature Valves:

1. The valve should not become a significant heat source for the low-temperature system. Excessive heat influx can lead to rapid internal fluid evaporation, abnormal pressure build-up, and potential hazards.
2. Low-temperature media should not adversely affect handwheel operation and packing sealing performance.
3. Components directly in contact with low-temperature media should have explosion-proof and fire-resistant structures.
4. Components operating at low temperatures cannot be lubricated conventionally, necessitating structural measures to prevent friction damage.

Selection of Materials for Low-Temperature Valves:

Main Body Materials:

• Selection factors include the lowest operating temperature, mechanical properties required at low temperatures, wear resistance, corrosion resistance, and weldability.

Valve Body, Bonnet, Seat, and Disc (Gate):

• Selection principles vary based on the operating temperature. Austenitic stainless steel is preferred for temperatures below -100°C, while ferritic stainless steel is suitable for temperatures above -100°C.

Valve Stem and Fasteners:

• Nickel, chromium-molybdenum alloy steels are recommended for temperatures above -100°C, while austenitic stainless acid-resistant steel is suitable for temperatures below -100°C. Surface treatments such as chrome plating or nitriding are essential to enhance hardness and prevent thread galling.

Selection of Gaskets and Packing Materials:

• Gasket and packing materials must maintain reliable sealing and resilience at low temperatures. Various materials such as asbestos rubber sheets, PTFE, and graphite with metal reinforcement are commonly used.

Special Structures of Low-Temperature Valves:

Low-temperature valves include gate valves, globe valves, ball valves, butterfly valves, check valves, etc., with structures similar to general valves. Key considerations include:

• Valve Body: Must withstand temperature fluctuations without deformation.
• Bonnet: Utilizes a long-neck design to protect the packing chamber and accommodate insulation.
• Valve Disc: Utilizes flexible or open-type discs to ensure reliable sealing.
• Valve Stem: Requires surface treatments to prevent galling and ensure proper sealing.
• Packing Chamber: Positioned away from low-temperature zones to maintain sealing effectiveness.

Preventing Abnormal Pressure Build-Up:

• Incorporating pressure relief holes or safety valves to release excess pressure.
• Proper labeling of relief hole directions on valve bodies.
• Installation of drain valves to remove residual fluids and prevent abnormal pressure build-up.

Installation Requirements for Low-Temperature Valves:

• Valves handling liquid media should have upward valve stems to prevent abnormal pressure build-up.
• Valves with pressure relief holes should have clearly marked directions, visible even when insulated.