Cryogenic Valves in LNG Applications

Overview

Liquefied Natural Gas (LNG) is natural gas cooled to -163°C. This turns the gas into a liquid. The liquid form is easier to store and transport. It saves space and reduces costs. LNG is highly flammable and explosive. So, valves used in LNG facilities must be very reliable and safe.

Valves are a small part of the total investment in LNG stations. But they account for over 50% of maintenance costs. Many LNG accidents are due to valve failures. These can cause leaks, shutdowns, and even explosions.

LNG Valves

Valve Materials

LNG valves operate at very low temperatures, around -163°C. At this temperature, metals can become brittle and unsafe. To prevent this, valves are made from special stainless steels like 304, 304L, 316, and 316L.

• 304 and 304L: These are strong and resist corrosion.
• 316 and 316L: These offer the best stability at low temperatures.

These materials are chosen for their strength and toughness. The valves undergo special treatment to ensure they remain strong and leak-proof.

Special Requirements

Design and Process

Preventing leaks is crucial. Valve bodies should be made in one piece. They should use welded connections instead of flanges. This design reduces potential leak points.

• Materials: Low-carbon stainless steels like 304L or 316L are preferred for welding parts.
• Testing: Components must pass rigorous tests, like ultrasonic or radiographic tests.
• Cryogenic Treatment: Parts are treated at -196°C to ensure they perform well at low temperatures.

Extended Bonnet

Cryogenic valves like gate, globe, ball, and butterfly valves need extended bonnets. This design helps with insulation and prevents freezing.

• The bonnet is made from the same material as the valve body.
• It is welded to the valve body and packing box.
• Heat treatment is applied after welding to eliminate stresses.

Overpressure Protection

When valves are closed, trapped LNG can vaporize due to ambient heat. This causes a rapid pressure rise. Overpressure can lead to leaks or valve failure. Valves use internal or external relief systems to prevent this.

• Internal Relief: Uses holes or elastic seats to balance pressure.
• External Relief: Uses a valve on the valve body to vent excess pressure.

Common Types of Cryogenic Valves

Gate Valves

Standard cryogenic gate valves are unidirectional. They have a 3mm hole to balance pressure. For bidirectional sealing, external relief valves are used instead.

Ball Valves

Cryogenic ball valves are typically top-entry type.

• Soft-Seal Ball Valves: Cost-effective for small-diameter, low-pressure uses.
• Hard-Seal Ball Valves: Suitable for larger, high-pressure uses due to their abrasion resistance.

For LNG, soft-seal seats made of PCTFE are used for temperatures down to -196°C. Hard-seal ball valves use metal-to-metal sealing for larger sizes and higher pressures.

Butterfly Valves

Cryogenic butterfly valves need bidirectional sealing. They are usually butt-welded.

• Connection: Stronger and more reliable than flanges, making maintenance easier.
• Structure: Double or triple eccentric structures are preferred. They minimize friction, reduce wear, and extend life.

Globe Valves

Globe valves use different sealing surfaces to handle temperature changes.

• Preferred Seals: Conical or spherical surfaces are better for cryogenic uses.
• Marking: Bidirectional sealing globe valves are marked with the flow direction.

Check Valves

Cryogenic check valves can be swing, dual-plate, or axial types. Swing and dual-plate valves are more reliable. Lug and wafer types are not recommended due to leakage risks.

Valve Sealing

LNG's small size and high permeability require high sealing performance. Valves must be designed with anti-static, fire, and explosion-proof measures.

Stem Sealing

Valve leaks can be internal or external. External leaks are more dangerous due to LNG's flammability. Stem seals use materials like PTFE, impregnated PTFE asbestos rope, or flexible graphite. These seals combine soft and hard elements with elastic load devices to ensure long-term performance.

• Metal Bellows Seals: These are used for zero leakage and high reliability, meeting TA-Luft standards.

Body Flange Sealing

Ideal cryogenic gaskets are soft at room temperature but rebound well at low temperatures. Spiral wound gaskets made of stainless steel and flexible graphite are commonly used. These gaskets maintain performance even at low temperatures.

Fasteners

Fasteners must be tough at low temperatures. They are typically made of austenitic stainless steel. They undergo strain-hardening and are lubricated with molybdenum disulfide. This ensures uniform load distribution and prevents leaks. Torque wrenches should be used to ensure proper tension and avoid overloading.

Anti-Static Measures

Cryogenic valves must prevent static electricity buildup. This can cause sparks and fires. Static conduction devices should be installed between valve components. This safely dissipates static electricity. Metal-sealed valves do not require these devices but must undergo resistance testing.

Cryogenic Testing

LNG valves undergo conventional strength and sealing tests. They also undergo specific low-temperature tests. These ensure performance. Tests include:

• Packing and Flange Gasket Sealing Test: No visible leakage at -196°C for 15 minutes under nominal pressure.
• Seat Sealing Test: Soft seals show no visible leakage. Hard seals have a leakage rate of less than 0.1xDN (ml/s).
• Operational Test: Smooth operation without sticking at -196°C for five cycles.

Conclusion

LNG is a clean and efficient energy source. It is important in both energy consumption and industry. Valves used in LNG service operate at -163°C. They require special materials, designs, sealing methods, and maintenance. Cryogenic valves must pass performance tests to be used in extreme conditions.

FAQs

1. What materials are used for LNG valves? Austenitic stainless steels like 304, 304L, 316, and 316L are used due to their performance at low temperatures.
2. Why is an extended bonnet necessary for cryogenic valves? An extended bonnet keeps the packing above 0°C, preventing it from freezing and ensuring smooth operation.
3. What is the purpose of pressure relief in cryogenic valves? Pressure relief prevents overpressure, which can lead to leaks or valve failure.
4. What type of seals are used for stem sealing in cryogenic valves? Stem seals use a combination of soft and hard seals with elastic load devices for long-term performance.
5. How are cryogenic valves tested? They undergo strength, sealing, and operational tests, including low-temperature tests to ensure reliability.