How do cryogenic valves work?

Transportation of Cryogenic Media

Cryogen leakage is both dangerous and expensive, reflecting the high costs involved in creating cryogenic gases. The components of any valve will contract and expand at different rates due to their varying material compositions and exposure times to cryogens.

Due to constant heat gains from the environment, valve and piping insulation are essential when dealing with cryogens. Gas processing involves managing the physical properties of gases such as LNG, nitrogen, oxygen, argon, and helium. These liquids are unstable at atmospheric pressure and can rapidly transform into the gas phase if left unchecked.

Cryogenic valves can operate effectively at extremely low temperatures and are crucial in fields like the petrochemical and aerospace industries, where there is a need to handle fluids at very low temperatures. Although the definition of cryogenic temperature varies by industry, these valves typically operate between -40°F (-40°C) and below -320°F (-196°C).

Uses of Cryogenic Valves

Cryogenic valves are primarily found in industrial facilities to facilitate processes conducted at cryogenic temperatures. Below are some common applications:

Liquid Nitrogen Gas (LNG) Terminals and Plants

In the oil and gas industry, cryogenic valves control liquified gases like liquid nitrogen, methane, and helium. These gases are cooled to cryogenic temperatures for safe, non-pressurized storage and transport. Larger volumes can be stored or transported at lower pressure levels. Before cooling these gases into liquid, it is crucial to remove condensate, moisture, CO2, and H2S to prevent downstream corrosion issues.

Cryogenic Processing

The field of cryogenics saw significant advancement from World War II, leading to its commercialization by Ed Busch. By cryogenically tempering metal tools rather than using heat treatment, he extended their lifespan two to four times. Today, you can find cryogenic valves and equipment in steel production, food freezing, and biotech applications like vaccines.

Aerospace Applications

In aerospace, cryogenic fuels have gained widespread acceptance. Liquid hydrogen and liquid oxygen are often used as propellants for space shuttles, either alone or with jet fuel, necessitating the use of cryogenic valves.

Types of Cryogenic Valves

There are various types of cryogenic valves, each suitable for different applications. Selecting the right type is essential because cryogenic fluids are highly sensitive, and any leakage can result in costly and harmful consequences.

Cryogenic Butterfly Valves

Triple-offset butterfly valves are ideal for cryogenic service due to their non-friction, metal-to-metal seal that provides bubble-tight shut-off and ensures the long-term integrity of the sealing mechanism. These valves offer quick opening and closing actions and are ideal for remote operation.

Cryogenic Ball Valve

The most common valves for liquid gas applications are double-seal ball valves. They offer excellent flow characteristics and provide a tight seal. These valves incorporate a vapor space below the gland to allow gasification, maintaining the gland packing near ambient temperature, which limits thermal conductivity.

Cryogenic Globe Valves

These valves have a spherical shape with a disc that rotates 90° to the plane of its body seat, providing effective long-term sealing. However, they may not resist erosion and are not recommended for systems with high flow rates. Globe valves should ideally have a full Stellite valve trim to mitigate wear and tear, as cryogenic fluids tend to lose their lubricity.

Cryogenic Gate Valves

Cryogenic gate valves feature a wedge-shaped gate that opens and closes in line with the mating body seat, minimizing pressure drops when fully open. While they are desirable for larger valve sizes, they are more challenging and expensive to actuate compared to butterfly valves and are not ideal for remote operation.

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Standards

While some companies have specific requirements for cryogenic valve service, most requirements stem from established engineering standards such as MSS SP-134, BS, ASME 16.34, and ISO.

MSS SP-134

MSS SP-134 is the dominant code in North America for cryogenic valves. It covers material requirements, design, dimensions, fabrication, pressure testing, and non-destructive examination of cryogenic valves with body/bonnet extensions.

• Materials: Materials in contact with cryogenic fluids should exhibit suitable mechanical properties as outlined in ASME standards.
• Design: The body/bonnet extension should isolate the stem packing and valve mechanism from cryogenic fluid temperature effects.
• Fabrication: All valve welds should align with ASME standards and be performed by qualified welders.
• Production Pressure Testing: Valves should be cleaned, degreased, and tested according to ASME specifications before use.

BS Standards

The British Standard specifies the design, manufacture, and testing of valves for cryogenic service. It includes guidelines for maintaining stem packing temperatures and pressure relief provisions to manage thermal expansion.

• Extended bonnets and precise stem orientation to ensure valve functionality.
• Pressure relief to manage thermal expansion and liquid evaporation.
• Prohibition of flat-seated discs for globe valves in favor of conical shapes.

ASME 16.34

ASME 16.34 provides detailed specifications on valve components, including testing requirements, material selection, and pressure guidelines.

• Shell tests and leakage detection devices.
• Comprehensive material selection requirements.
• Description of nominal pipe size and marking specifications.

ISO Standards

ISO standards outline design, manufacture, and testing requirements for cryogenic valves. These specifications cover a wide temperature range, ensuring safe operation.

• Materials: Both metallic and non-metallic materials must meet ISO specifications for the rated minimum temperature.
• Design: Valves must perform safely within their rated temperature and pressure ranges, following ISO and ASME norms for minimum wall thickness.

Cryogenic Valves Testing

Cryogenic valves undergo various tests to ensure functionality and safety, including shell strength tests and leakage detection. These tests adhere to multiple standards, guaranteeing the reliability of the valves in extreme conditions.

• Shell Strength Test: Normally hydrostatic or pneumatic, performed at 1.5 times the maximum working pressure.
• Shell and Seat Leak Test: Pneumatic tests pressurize to 1.1 times, and hydrostatic tests to 1.3 times the maximum working pressure.
• Cryogenic Prototype and Production Test: Conducted following BS procedures to validate performance.