The selection of gaskets and bolts for oil&gas,petrochemical industries

Gaskets and bolts selection.

Pipe Flange are widely used in pressure vessels and pipelines,including weld neck flange,slip on flange,and blind flange, etc. Flange joints greatly improve the installation efficiency of equipment and pipelines, facilitate maintenance, and promote standardized and modular construction. A flange connection consists of a pair of flanges, gaskets, bolts and other components. The selection of gaskets and bolts directly affects the sealing performance of the flange joint. There have been many accidents of flange joint leakage caused by improper use of gaskets or bolts.

The selection of gaskets and bolts is mainly based on: flange type, sealing surface type, temperature, pressure, and medium characteristics. Moreover, bolt strength, gasket type, and flange type should be matched with each other. According to current domestic and international standards, gaskets are mainly divided into three categories according to their materials: non-metallic gaskets, semi-metallic gaskets, and metallic gaskets. Bolts are also divided into three categories according to their strength: low-strength, medium-strength, and high-strength bolts. There are many types of gaskets and bolts, and complete systems are available in chemical industry standards (HG), petrochemical industry standards (SH), national standards (GB), and American standard ASME. This article introduces the selection of gaskets and bolts for flange joints in harsh working conditions (ultra-low temperature, high temperature, strong corrosion, high pressure) in the petrochemical industry, to provide reference for the safe design of related working conditions.

1. Stud Bolt Selection

The common bolt materials for ASME B16.5 and ASME B16.47 flanges are shown in Table 1.

According to ASME B16.5, bolts with allowable stress greater than or equal to A193 B7 material belong to high-strength bolts. High-strength bolts can be used for all types of flange joints; medium-strength bolts need to prove that they can press the gasket to ensure the sealing of the flange joint; low-strength bolts can only be used with non-metallic gaskets in low-pressure working conditions, in hopes of maintaining the sealing of the flange joint under low preload. The chemical industry standard HG stipulates that bolts with tensile strength greater than 800MPa belong to high-strength bolts, and those with normal temperature yield strength less than or equal to 240MPa belong to low-strength bolts. The application temperature range of bolts is also affected by differences in bolt materials and processing techniques, as specified in Table 1.

Table 2 Stud Bolt for Chemical HG and Petrochemical SH Flanges

It should be noted particularly that: ① The applicable temperature of A453 Gr.660 bolts in chemical industry standards is -29～525℃, while in ASME standards it is -198～538℃; ② A453 Gr.660 is a heat-resistant stainless steel with a coefficient of thermal expansion similar to that of austenitic stainless steel. In ASME B31.3 2012 and previous versions, A453 Gr.660 bolts were only suitable for -30～538℃ working conditions, but in ASME B31.3 2016 and later versions, these bolts are specified to be applicable to -198～538℃. A domestic valve manufacturer has applied this material bolt to LNG valves, and it is recommended to confirm the working condition applicability with the supplier before procurement.

2. Gasket Selection

Common gaskets for flanges mainly include non-metallic gaskets, semi-metallic gaskets, metallic gaskets, etc., as shown in Table 3.

The selection of gasket materials should consider medium corrosiveness, toxicity, flammability, pressure rating, temperature, flange rigidity, bolt strength, etc. Gaskets should have good resilience, good corrosion resistance, and appropriate deformation capacity. There are many factors to consider in gasket selection, which will not be introduced here and will be analyzed according to specific working conditions.

3. Ultra-Low Temperature Working Conditions

The ultra-low temperature working conditions referred to in this article are those with design temperatures between -101℃ and -196℃, involving ethylene, liquid nitrogen, liquefied natural gas, etc., most of which are flammable and explosive media with high leakage risks.

First, in terms of flange types, the above ultra-low temperature medium pipe flanges mostly use welding neck flanges, and the flange sealing surfaces generally adopt raised face connections, with ring joint surfaces used less frequently. If the ring joint surface is used for the flange sealing surface under ultra-low temperature working conditions, the gasket type can only be stainless steel oval ring gasket or octagonal ring gasket. Metal ring gaskets require higher preload, and according to Table 4 “Matching Table of Common Bolts and Gaskets for Flanges [3]”, high-strength bolts need to be selected. However, under ultra-low temperature working conditions, high-strength bolts cannot be selected, and only medium-strength bolts such as A320 B8M Class 2 after strain hardening treatment can be used, resulting in certain leakage risks in flange joints, and bolt strength verification may be required when necessary.

Table 4 Matching Table of Common Bolts and Gaskets for Flanges [3]

Flanges for low-pressure working conditions such as CL150 and CL300 can also choose non-metallic gaskets, and their applicable temperatures and pressures are shown in Table 5. However, due to the low resilience and relatively poor sealing performance of non-metallic flat gaskets, non-metallic gaskets are not recommended for flange joints under ultra-low temperature working conditions.

Table 5 Applicable Temperatures and Pressures of Gaskets for Ultra-Low Temperature Working Conditions

Metal wound gaskets are made by spirally winding steel strips and filler strips on a metal central axis. They have good elasticity and resilience, and the number and density of windings can be adjusted according to working pressure. They have good sealing performance within a certain temperature and pressure range, and the sealing specific pressure can be appropriately adjusted through the structure of the wound part, which can maintain sealing performance under low stress. At present, metal wound gaskets are mostly used for pipe flanges of ultra-low temperature media such as liquefied natural gas and ethylene, with rich application experience. The main risk points of wound gaskets are: if there is no inner ring or outer ring, the welded points of the metal strips in the wound part are prone to divergence under high preload, and this may even occur when both inner and outer rings exist. At present, with the improvement of suppliers’ research and manufacturing capabilities, the reliability of gaskets has been greatly improved by adopting solid structures for the inner and outer rings of wound gaskets and multiple welding of metal strips.

4. High Temperature Working Conditions

The high temperature working conditions referred to in this article are those with design temperatures above 650℃ (above 450℃ for oxidizing media), involving refining, chemical and other industries. In addition to flammable and explosive characteristics, the media also pose risks such as high-temperature burns. Under such high-temperature working conditions, welding neck flanges are generally selected, and the 可选 gasket types include metal ring gaskets and specially designed wound gaskets, etc. The 可选 bolt materials include non-strain-hardened stainless steel or high-temperature high-alloy steel (such as UNS N07718, etc.).

Metal ring gaskets include octagonal ring gaskets and oval ring gaskets. Under bolt preload, plastic deformation occurs due to the gasket hardness being lower than the flange surface, thereby forming a seal. High-temperature working conditions may cause irreversible damage to the gasket and flange joint, reducing flange durability. The flange may need to be replaced after maintenance, which is generally not economical.

According to specifications such as HG/T20631, the applicable range of graphite wound gaskets is -200～650℃, and the temperature should not exceed 450℃ when used for oxidizing media. Under the long-term action of high-temperature media and oxidizing media, carbon molecules in graphite will be oxidized into carbon dioxide and volatilize in the form of gas, leading to damage to the graphite material and thus a decline or even failure of sealing performance. Both experiments [4] and engineering application cases show that graphite oxidation is the main cause of graphite gasket failure.

In recent years, vermiculite, as an alternative to graphite, has been increasingly used in engineering projects. Vermiculite is expanded at high temperature to make its volume larger, forming an intermediate structure similar to expanded graphite, and then chemically expanded to make vermiculite flakes expand like graphite, forming flakes with high flexibility and low porosity [5]. A typical vermiculite wound gasket is Thermiculite, which can maintain gasket integrity and low damage rate below 982℃, ensuring good sealing performance, and has been applied in some fields of the petrochemical industry, such as flange joints of high-temperature reactors, high-temperature steam, high-temperature oil and gas, high-temperature ethylene pipelines, etc. The application of Thermiculite gaskets reduces the number of shutdowns, increases operating time, reduces leakage risks, and improves operating efficiency.

5. Strong Corrosion Working Conditions

The strong corrosion working conditions referred to in this article include media such as hydrochloric acid, sulfuric acid, chlorine gas, and phosgene of various concentrations. The corrosion risk is high, and the harm to personnel and the environment is great. The selection of gaskets and bolts should first be based on the type of pipeline flange. For sulfuric acid, according to different concentrations and temperatures, the 可选 flange materials include carbon steel, stainless steel (austenitic steel, high-alloy austenitic steel, etc.), Hastelloy, and steel lining, etc.; for hydrochloric acid, according to different concentrations and temperatures, the 可选 flange materials include high-alloy austenitic steel, Hastelloy, and steel lining, etc. For steel flanges, the 可选 gaskets include wound gaskets, toothed gaskets, and non-metallic gaskets, and the metal strip and plate materials are selected with reference to the flange materials. For flanges below CL300, non-metallic flat gaskets such as filled modified polytetrafluoroethylene gaskets and expanded polytetrafluoroethylene gaskets can be selected according to temperature and medium corrosiveness. Bolt strength and materials can be selected according to Table 4.

According to the Hazardous Chemicals Catalog, chlorine gas and phosgene belong to highly toxic media, and the selection of gaskets and bolts can refer to extremely hazardous media. If they are mixtures, the design can be carried out according to toxicity identification. Pure chlorine gas and phosgene have low corrosiveness, and the flanges generally use high-quality carbon steel, low-temperature carbon steel, or austenitic stainless steel. The flange type generally adopts welding neck raised face flanges, and the 可选 gaskets include wound gaskets, toothed gaskets, etc. The inner side of the toothed gasket metal plate is in contact with the medium, and the toothed metal plate can be made of corrosion-resistant metal, but it is expensive, customized, and has a long delivery cycle. Specially designed toothed gaskets can be selected, where the non-metallic surface covering layer completely covers all surfaces of the toothed plate, and the metal plate material can be common steel such as austenitic stainless steel. Bolts should be selected as medium or high-strength bolts according to Table 4. If the toxicity level of the chlorine gas and phosgene mixture is low, and the pipeline material uses steel lined with fluoroplastic, glass-lined, etc., and the temperature is below 200℃ and the pressure is low, the gasket type can be non-metallic flat gaskets, and filled modified polytetrafluoroethylene gaskets, expanded polytetrafluoroethylene gaskets, or other suitable gaskets can be selected.

The gasket selection principles for media such as hydrochloric acid, sulfuric acid, chlorine gas, and phosgene are shown in Table 6.

Table 6 Gasket Selection Principles for Strong Corrosion Working Conditions

6. High Pressure Working Conditions

The high pressure working conditions referred to in this article are those with nominal pressure above PN260 or above CL1500. The higher the pressure rating, the greater the leakage risk of the flange joint. Welding neck flanges are generally selected, and high-strength bolts should be selected for high-pressure working conditions. If medium-strength bolts are selected, designers need to analyze and verify the strength of the corresponding working conditions.

In national and industry standards, HG/T 20256-2016 specifies that the gasket for nominal pressure PN260 is a lens gasket; while for CL1500 and above, the gaskets include metal wound gaskets and metal ring gaskets. The lens gasket originates from the design principle of DIN2696 specification and was early used in high-pressure ammonia synthesis and urea units in China. The sealing surface of the lens gasket is arc-shaped, which maintains a high degree of close fit with the flange groove surface. Moreover, to match the flange sealing surface, the hardness of the lens gasket is lower than that of the flange surface to ensure that the lens gasket undergoes plastic deformation under bolt preload. The load makes the gasket fully expand in the flange groove, and the sealing fit between the gasket and the flange surface is closer to achieve effective sealing. Lens gaskets can be designed according to different pressure working conditions and flange materials.

For CL1500 and CL2500 metal wound gaskets, special design and analysis are required for the strength of the metal strip in the wound part, the purity and density of graphite, the winding density, and the structure of the inner and outer rings, etc. The purity of graphite should not be lower than 99%, and the density and number of windings in the high-pressure wound part are determined by its working pressure. Through calculation and experimental verification, increasing the number of windings and winding density, strengthening the welding process of metal strips, and designing solid inner and outer metal rings can ensure that the wound part does not collapse under high-pressure working conditions.

Metal ring gaskets were early used for high-temperature and high-pressure flanges in the petroleum industry and have shown good sealing performance under high-pressure working conditions. Under bolt preload, the ring gasket undergoes plastic deformation to fill the micro-roughness of the flange groove sealing surface. The hardness of the ring gasket is lower than that of the flange, so that the gasket and the flange can fit closely. Some specially designed high-pressure flanges, such as high-pressure self-tightening flanges, also use all-metal gaskets, with applicable pressures reaching 150MPa. At present, with more and more high-pressure working conditions in the chemical new materials and oil and gas industries, as an important part of high-pressure flange joints, metal gaskets will be used more and more widely.

7. Conclusion

Ensuring the strength safety and reliable sealing of flange joints is a necessary condition to ensure the safe and stable operation of petrochemical plants, and selecting appropriate gaskets and bolts is of utmost importance. This article analyzes the selection of gaskets and bolts from the aspects of low temperature, high temperature, high pressure, and strong corrosion working conditions, to help designers understand and use gaskets and bolts.