Selection of Cryogenic Valves for Liquefied Natural Gas

Abstract

Liquefied natural gas (LNG) plays an increasingly critical role in national energy production. Cryogenic valves used in LNG systems are essential components of these systems. This article outlines the design and selection process for LNG cryogenic valves, focusing on material selection and specific processing requirements. It defines the criteria for selecting LNG cryogenic valve types, analyzes their design features and structural integrity, and details the key inspection and testing requirements.

Overview

Natural gas, regarded as a clean, efficient energy source, plays a vital role in optimizing energy consumption, improving air quality, and reducing greenhouse gas emissions. The growth of China’s natural gas and LNG industries has significantly contributed to the nation’s economic and social progress. In 2010, China imported 9.34 million tons of LNG, marking a 75% increase over the previous year. The construction of LNG receiving terminals, liquefaction plants, satellite stations, and other infrastructure projects is progressing steadily. The expansion of the LNG industry is a key factor in the growing demand for natural gas. Cryogenic valves are critical components in LNG systems, and their design, selection, and manufacturing must meet stringent standards. These valves must comply with stricter safety and reliability requirements than those used in liquid oxygen and nitrogen systems. Cryogenic valves for LNG applications must comply with stringent requirements for reference standards, material selection, structural design, and testing procedures. This article focuses on the selection and design of cryogenic valves for LNG applications, providing a detailed analysis of their performance under operational conditions.

Reference Standards

The design, manufacturing, and inspection standards for cryogenic valves in LNG systems are outlined in Table 1 – Main Standards for Cryogenic Valves in LNG Applications. The most widely adopted design standard is British Standard BS6364, internationally recognized. The low-temperature testing requirements in this standard are cited by various industry and corporate standards. Shell (SHELL) has established its own design and inspection standards for cryogenic valves. The Chinese standard GB/T24925, which replaces JB/T7749, also addresses cryogenic valves and stipulates requirements for cryogenic treatment prior to valve finishing.

Material Selection and Processing Requirements

LNG cryogenic valves are essential for controlling and regulating flow within LNG systems. Valve selection must account for general design principles as well as factors such as safety, reliable sealing, and performance under cryogenic conditions, especially when handling flammable or explosive media.

1. Selection of Primary Materials

From a materials science perspective, materials with face-centered cubic (FCC) lattices, such as austenitic stainless steel, copper alloys, and aluminum alloys, do not exhibit low-temperature brittleness. Although aluminum does not become brittle at low temperatures, its low hardness makes it unsuitable for sealing due to poor wear and abrasion resistance. Copper and copper alloys, with their lower strength, are also limited in cryogenic valves. Carbon (C) and chromium (Cr) alloy steels lose impact strength below -20°C, restricting their use to temperatures of -30°C and -50°C, respectively. Nickel steel with a nitrogen content of 3.3% can be used at temperatures as low as -100°C, while nickel steel with 9% nitrogen content is suitable for use at temperatures down to -192°C. Austenitic stainless steel, nickel alloys, Monel alloy, Hastelloy, titanium, aluminum alloys, and bronze are suitable for use at extremely low temperatures, as low as -273°C. Considering both material reliability and cost-effectiveness, austenitic stainless steel is an ideal choice for cryogenic applications due to its high toughness and excellent structural stability, and superior wear resistance at low temperatures. For valves that require welding, low-carbon austenitic stainless steel with excellent weldability is preferred.

Table 1 Main standards for cryogenic valves for LNG

2. Selection of Sealing Materials

Fluoroplastics and rubbers commonly used in sealing become brittle, undergo cold flow, or exhibit dimensional expansion when exposed to LNG at low temperatures. Therefore, these materials are unsuitable for use in gaskets or packing in LNG valves. Asbestos is not used due to its carcinogenic properties and propensity for causing leakage. For LNG valve designs, the valve seat typically incorporates a soft sealing structure made of modified polytrifluorochloroethylene (PCTFE), which compensates for metal deformation under cryogenic conditions. Graphite is employed for flange ends and packing boxes owing to its superior sealing properties at low temperatures.

3. Low-Temperature Treatment

LNG is stored at a temperature of -163°C. As steel cools, it rapidly transitions from the austenitic state, and below the M-point, martensitic transformation occurs. At this point, the diffusion of iron and carbon atoms is restricted, and the transformation is limited to the reorganization of the iron lattice. This diffusionless phase transformation causes a dimensional change in valve components. Studies have shown that the unit volume of martensitic steel is 1.7% larger than that of austenite. The resulting volume expansion and shear forces can rupture the lattice, leading to partial bulging of the workpiece surface, which can severely impact valve sealing and operational performance. To counteract the volume changes caused by martensitic transformation, low-temperature treatment is required prior to final valve processing.

4. Cleaning of Valves

Valve pressure tests, including shell strength and internal leakage tests, typically use water as the testing medium. In LNG systems, water or impurities in the valve cavity can condense into solids at low temperatures. The valve's opening and closing motion can scratch the sealing surface or valve stem, leading to internal or external leakage. Therefore, cleaning the valve internals is crucial to ensuring optimal valve performance. It is more effective to clean the valve after the water pressure test, or alternatively, to use air pressure testing in place of water pressure testing.

Type Selection

The most commonly used valve types in LNG systems are ball valves, butterfly valves, globe valves, and check valves. Gate valves are prone to jamming under cryogenic conditions, and some European design standards have discontinued their use in LNG systems. An integrated valve structure is preferred to minimize leakage points and enhance sealing efficiency. Top-mounted valve designs must include an inspection port for ease of maintenance. To further minimize leakage, ball valves typically incorporate a soft sealing structure.

Design and Structural Features

Extended Stem with Raised Stuffing Box

Valve packing is typically made from non-metallic materials such as graphite. Due to differences in thermal expansion between metals and non-metals at low temperatures, leakage may occur. The extended stem design creates a transitional cavity between the fluid medium and the packing, enabling the valve stem to operate within a controlled temperature range. This design ensures the packing remains within the normal operating temperature range of the stuffing box, thereby guaranteeing reliable valve operation. Reference values for valve stem extension are provided in standards such as MSS SP 134 and BS 6364, with SHELL’s design standards offering more detailed specifications.

Middle Cavity Pressure Relief Structure

LNG occupies 625 times the volume of natural gas under standard temperature and pressure (STP) conditions. Under normal operating conditions, LNG is stored in the valve cavity of LNG valves, such as ball and gate valves. During intermittent pipeline operations or maintenance, LNG in the middle cavity can vaporize, creating overpressure within the valve, potentially jeopardizing valve safety. To mitigate this, LNG valves must include a self-pressure relief structure in the middle cavity to automatically release excess pressure when it exceeds safe thresholds.

Reliable Sealing Structure

Controlling external leakage is crucial for LNG, which is highly flammable and explosive. Leakage typically occurs at the valve body/gland flange and upper stem packing. Lip seals at these points effectively control external leakage. External leakage is governed by Germany’s TA-LUFT certification, while the SHELL standard and ISO 15848 outline guidelines for inspections.

Fireproof and Anti-static Design

Fireproof and anti-static properties are critical factors when selecting LNG valves. Fire protection requirements are specified in API 607 and API 6FA. Anti-static design requirements must be verified through relevant testing, and valve samples are selected for inspection during the verification process.

Testing and Inspection Features

The testing and inspection of cryogenic valves include both room temperature and low-temperature tests.

Low-Temperature Test: This test is conducted in accordance with BS 6364, using liquid nitrogen at -196°C as the cryogenic medium and helium as the pressurizing medium. It is primarily used to detect internal leakage at low temperatures.

External Leakage Inspection: External leakage detection is typically performed using helium mass spectrometry after the low-temperature test has been completed and the valve has returned to room temperature. This method evaluates the effect of temperature on valve sealing. The inspection follows ISO 15848-2.

Middle Cavity Pressure Relief Test: To verify the reliability of the valve’s self-pressure relief structure, this test is performed as part of the valve inspection, as outlined in Appendix B of API 6D.

Conclusion

Cryogenic valves used in the LNG industry include ball valves, butterfly valves, globe valves, check valves, and others. This guide provides an overview of the structure, performance, and technical design standards for cryogenic LNG valves. It also highlights the specific requirements for material selection, valve type, and structural design that distinguish LNG valves from standard valves. Furthermore, the guide outlines the specific inspection requirements for cryogenic valves. It serves as a reference for designers in the selection of LNG cryogenic valves.