Low-Temperature Flange Sealing Solutions for Cryogenic Chemical Pipelines

Abstract

On LNG carriers, boil-off gas (BOG) can be reliquefied into LNG using lower-temperature cooling media such as nitrogen and helium. However, achieving a reliable seal for cryogenic gases remains a significant technical challenge. Based on the LNG reliquefaction project, this paper analyzes the key factors influencing flange sealing performance and explores gasket selection and sealing method optimization under cryogenic conditions. The results demonstrate that the inclusion of low-temperature compensation disc springs, combined with optimized bolt preload, significantly reduces flange leakage rates. This study aims to provide a technical reference for low-temperature sealing applications in marine environments.

With the rising use of cryogenic gases, the demand for reliable sealing technologies has increased significantly. Among the various sealing methods, bolted flange connections consisting of flanges, gaskets, bolts and nuts remain one of the most cost-effective and widely adopted solutions in fluid systems. Cryogenic sealing is especially critical in maritime applications such as LNG carriers. For example, the reliquefaction process involving a nitrogen–helium mixture places stricter demands on flange integrity. Effective sealing is achieved by compressing the gasket to eliminate microscopic gaps between sealing surfaces. Studies show that differential thermal contraction of materials at cryogenic temperatures is a primary cause of leakage. Although related studies have been conducted, comprehensive technical solutions are still lacking. This paper proposes a multi-faceted improvement strategy, from gasket material selection to structural optimization, with an emphasis on the application of low-temperature compensation disc springs to promote the advancement of cryogenic flange sealing technology.

1. Basic Principle of Flange Sealing

Figure 1 illustrates the basic structure of a flange connection.

Figure 1 Basic structure of a flange connection

A flange connection is composed of three primary components: bolts, flanges, and a gasket. These components function together to ensure a reliable seal, with the gasket serving as the most crucial element. Its primary function is to fill the gap between the flange sealing surfaces, eliminate surface irregularities, and create a sealing barrier through compression deformation. When a preload is applied to the bolts, the gasket is compressed, resulting in elastic deformation. This deformation enables the gasket to conform more precisely to the flange surface, filling microscopic gaps and forming a tight, reliable seal. However, when the pipeline is pressurized, internal axial forces tend to separate the flanges, stretching the bolts and reducing the compressive load on the gasket.

If the compressive load falls below a critical threshold, leakage may occur. The residual load on the gasket, known as the effective tightening force, must consistently exceed the pipeline’s operating pressure to ensure sealing integrity. Once the system reaches its working pressure, the gasket’s elastic recovery properties become essential for maintaining the seal. Its ability to maintain contact pressure compensates for minor surface irregularities and helps ensure ongoing sealing performance. Even under fluctuating operating conditions, such as changes in temperature and pressure, the gasket’s elasticity allows it to recover from slight deformations and continue providing a stable, leak-free seal.

2. Performance Indicators of Flange Seals

Sealing Effect: The primary function of flange seals is to maintain the integrity of pipelines or containers, making the sealing effect a critical indicator of their performance. Parameters such as leakage, air tightness, and water tightness are commonly used to evaluate the sealing effect.

Temperature Resistance: Temperature resistance is vital for flange seals under varying working conditions. Their ability to function effectively in extreme environments, whether in high or low temperatures, along with the temperature range they can withstand, are key indicators for assessing flange seals.

Pressure Resistance: Flange seals are subjected to different working pressures across various engineering projects, and their ability to perform consistently over time is vital for the success of a project. In general, the pressure resistance of flange seals must meet the specific pressure requirements of the working conditions.

Wear Resistance: Friction and wear can significantly impact the performance of flange seals, making wear resistance a critical performance indicator. It is typically evaluated using exchange testing methods conducted under contact pressure.

Chemical Resistance: Flange seals are exposed to a wide range of media in various industrial environments, making chemical resistance a crucial performance indicator. Factors such as pH value, acid or alkali concentration, and other properties of the medium are commonly used to evaluate their chemical resistance.

3. Factors Affecting Flange Sealing

Flange sealing is primarily achieved by bolts applying a preload that compresses the gasket. Failure typically manifests as leakage of the medium at the gasket, with the two most common types being permeation leakage and interface leakage.

Permeation leakage refers to the seepage of the medium through microscopic pores within the gasket material and is more commonly observed in non-metallic gaskets. These gaskets are typically made from materials such as wood fibers, minerals, carbon fiber, or graphite, all of which have relatively loose internal structures with fine pores. When a pressure differential exists between the inside and outside of the system, the medium can gradually pass through these pores, resulting in leakage.

Interface leakage occurs at the contact surface between the flange and the gasket and is the most common type of flange failure. It can result from several factors, including inadequate gasket performance, insufficient bolt preload, pipeline vibration, gasket aging, flange surface wear, and other mechanical or environmental influences. Over extended periods of operation, the likelihood of interface leakage increases, making it a critical concern in long-term sealing performance.

At low temperatures, the material’s thermal expansion coefficient decreases and molecular movement slows, leading to volume shrinkage. This shrinkage can cause the flange diameter to contract and deform, reducing the clamping force on the gasket and increasing the risk of leakage. Additionally, materials tend to become more brittle in low-temperature environments, which may cause the flange to crack under excessive stress. Therefore, the design of low-temperature flanges must carefully account for fluctuations in temperature and pressure to ensure long-term reliability and sealing integrity.

Figure 2 Interface Leakage and Permeation Leakage

3.1 Impact of Working Conditions

Working conditions include pressure, temperature, and the physical and chemical properties of the medium. In practical engineering applications, pressure or the medium alone has a limited effect on sealing performance. However, when temperature variations are involved, the interaction among these factors can significantly increase the risk of leakage. At different temperatures, the physical properties and permeability of the medium can change. Specifically, under low-temperature conditions, molecular movement slows, which may reduce the leakage rate. However, the sealing material becomes more susceptible to performance degradation.

Temperature fluctuations can cause creep and stress relaxation in flanges, bolts, and gaskets, leading to a reduction in sealing force. Non-metallic gaskets, in particular, are more susceptible to accelerated aging and reduced elasticity due to temperature changes, which can compromise their sealing performance. In addition, uneven temperature distribution can cause inconsistent thermal expansion among components, further increasing the risk of leakage. Therefore, it is essential to maintain a uniform temperature distribution within the sealing system and to avoid sudden temperature changes whenever possible. Since working conditions are determined by the production process and cannot be fundamentally changed, the most effective approach is to optimize the sealing structure and carefully select the appropriate gasket materials.

3.2 Influence of the Flange Sealing Surface

The structural shape and surface characteristics of the flange sealing surface play a significant role in sealing performance. Common types of sealing surfaces include flat, raised face, and tongue-and-groove. The flat sealing surface features a simple structure and is easy to manufacture, making it suitable for anti-corrosion linings. However, its large contact area makes it difficult to compress uniformly and more prone to extrusion.

The raised face sealing surface improves contact pressure distribution by incorporating a boss or groove, which helps achieve more uniform stress and enhances sealing reliability. The tongue-and-groove type effectively limits seal deformation and extrusion, offers strong support and protection, and provides excellent resistance to media corrosion, making it ideal for high-sealing-requirement applications.

In addition, the flatness and perpendicularity of the flange sealing surface have a direct impact on the quality of the contact seal. Irregularities in the surface or misalignment with the flange centerline can lead to uneven distribution of sealing force, resulting in overstress in some areas and insufficient contact in others, which can ultimately cause leakage.

3.3 Influence of the Gasket

The gasket’s compression and rebound properties, material composition, and structural design all significantly impact the sealing performance of the flange. Under normal operating conditions, the tensile force applied by the bolts balances the internal axial force of the medium, compressing the gasket and forming an effective seal.

This compressed state ensures firm contact between the gasket and the flange sealing surface, effectively preventing medium leakage. However, axial forces from within the medium may cause slight deformation of the bolts and sealing surfaces, reducing the initial compressive load. If the gasket has sufficient resilience, it can recover its original shape as the external force diminishes or disappears, thereby generating renewed compressive force and reestablishing a reliable seal.

3.4 Influence of Bolt Preload

Properly increasing bolt preload can significantly enhance the sealing performance of the entire flange assembly. However, excessive preload may damage the gasket and undermine the sealing effect. The preload level should be appropriately selected based on the type of gasket to ensure optimal sealing reliability. Additionally, due to friction and other variables, it is often difficult to achieve precise preload control using manual methods or a torque wrench in real-world conditions.

3.5 Influence of Preload Tools, Methods, and Frequency

Various methods are available for tightening bolts to achieve effective flange sealing. Traditional approaches include manual tightening, the use of torque wrenches, and nut angle control. With technological advancements, more precise methods, such as strain gauge monitoring and ultrasonic control have emerged, greatly enhancing preload accuracy and consistency. During the tightening process, bolts interact with one another, and bolts tightened later can alter the preload of earlier ones, potentially causing loosening. Studies comparing clockwise and diagonal tightening sequences have shown that clockwise tightening helps improve preload uniformity, although diagonal tightening is still more commonly used in engineering practice. Furthermore, the number of preload applications also affects the final sealing outcome. A single round of tightening often results in uneven preload distribution. In industrial settings, bolts are typically tightened in three stages, with some procedures including a secondary tightening after 24 hours to ensure even and lasting preload.

4. Selection and improvement methods of low-temperature flange gasket materials

There are various types of flange gaskets. According to the properties of their materials, they can be roughly divided into three categories: metal gaskets, metal-non-metal combination gaskets and non-metal gaskets.

Commonly Used Flanges and Sealing Gaskets:

1. Non-Metal Gaskets
   • PTFE Gaskets
   • PTFE Coated Gaskets
   • Rubber Gaskets
   • Asbestos Rubber Gaskets
   • Fiber Rubber Gaskets
2. Metal-Non-Metal Composite Gaskets
   • Metal Wound Gaskets
   • Metal Coated Gaskets
   • Metal Corrugated Composite Gaskets
   • Metal Flat Gaskets
   • Metal Ring Gaskets
   • Metal Toothed Gaskets
3. Metal Gaskets
   • RX Type Gaskets
   • BX Type Gaskets
4. Special Ring Gaskets
   • Octagonal Ring Gaskets
   • Oval Ring Gaskets

4.1.1 Low-Temperature Resistance

Currently, commonly used low-temperature gasket materials include PTFE, silicone, and fluororubber. Among these, polytetrafluoroethylene (PTFE) offers excellent low-temperature resistance and can withstand temperatures as low as -200°C. In contrast, fluororubber and silicone have relatively weaker low-temperature resistance, generally withstanding temperatures around -60°C. Table 1 presents the performance of some low-temperature-resistant gaskets.

Table 1 The performance of some low-temperature-resistant gaskets

4.1.2 Sealing Reliability

When selecting low-temperature gasket materials, it is essential to consider not only their low-temperature resistance but also their sealing reliability. Generally, materials with a low friction coefficient, minimal compression deformation, and good recovery properties ensure better sealing performance. For example, polytetrafluoroethylene (PTFE) offers excellent sealing capabilities and is commonly used for low-temperature gaskets.

4.1.3 Corrosion Resistance

Corrosion can be a concern when using low-temperature gaskets, as different media can affect their performance. Therefore, it is important to select appropriate gasket materials based on the properties of the media. For instance, PTFE or fluororubber is often chosen when dealing with acidic or alkaline media, as both materials exhibit excellent corrosion resistance.

4.2 Sealing Improvement Methods

In addition to selecting the appropriate gasket materials for the core components, other methods can also be employed to achieve the desired sealing effect. These include adjusting the flange connection method, optimizing the number of bolts, using sealants, incorporating compensating disc springs, and more.

4.2.1 Adjusting the Flange Connection Method

Improving the flange connection method can enhance the sealing performance of low-temperature flanges. This can be achieved by increasing the sealing size of the connection structure and optimizing the flange design. Additionally, strengthening the flange connection or adding a sealing gap between double flanges can further improve sealing effectiveness.

4.2.2 Increasing the Number of Bolts or Changing the Bolt Arrangement Method

Increasing the number of bolts or modifying the bolt arrangement can enhance the tightness of the connection on the sealing surface of low-temperature flanges. It is important to ensure that the bolts are evenly distributed and that the load is uniformly distributed across the flange. One widely studied method is the use of super bolt preloaders, which replace traditional nuts and bolts. By installing a circle of pre-studs around the main studs, the force can be more effectively transmitted to the connecting parts.

4.2.3 Strengthening Cleaning and Maintenance of Sealing Parts

Dirt and impurities in the sealing parts can negatively affect the sealing performance of low-temperature flanges. Therefore, regular cleaning and maintenance are essential. This includes keeping the connection parts clean, inspecting the wear of the flanges and gaskets, and replacing gaskets when necessary. These practices can significantly improve the sealing performance of low-temperature flanges.

4.2.4 Low-Temperature Sealants

Among the low-temperature sealants that have been widely studied, products such as DW series glue, NHJ-44 nylon-modified epoxy glue, and HC-02 tetrahydrofuran polyether epoxy glue are commonly used. These sealants are primarily tested for low-temperature performance, including use at liquid nitrogen temperatures. Typically, high-temperature curing or post-treatment can improve the curing properties of the sealant. However, these sealants require a longer curing time at room temperature, which limits their use in engineering applications.

4.2.5 Low-Temperature Compensation Disc Spring

The sealing performance can be improved by adding a compensation disc spring between the nut and the flange. The mechanical energy during compression is converted into the elastic potential energy of the disc spring. Over time, as the bolt-flange joint undergoes changes under various working conditions, which may lead to insufficient bolt preload, the disc spring releases its stored elastic potential energy. This process generates force by rebounding, compensating for gasket and bolt deformation, and maintaining the preload on the bolt. Tests on a flange seal filled with 2 MPa helium showed no noticeable leakage after being cooled with low-temperature liquid nitrogen, as detected by a helium detector.

5. Conclusion

In the context of reliquefaction ships, LNG readily vaporizes at ambient temperature, and if the boil-off gas (BOG) generated during transportation is not promptly managed, it can result in significant economic losses. Re-condensing BOG into LNG using a lower-temperature cooling medium is an effective solution. Since transporting cryogenic fluids requires pipelines, ensuring reliable flange sealing becomes crucial. Based on the LNG reliquefaction project, the study found that using colder nitrogen and helium gases for BOG cooling, combined with cryogenic compensating disc springs to counteract bolt deformation at low temperatures, effectively reduces leakage risk. Moreover, dividing the bolt pre-tightening process into three stages, and applying one-third of the total force at each stage can greatly enhance sealing performance and further minimize cryogenic gas leakage.