In the natural gas sector, flanges serve as critical connection and sealing components, playing a vital role across the entire industry chain—from natural gas extraction and gathering to transportation, processing, storage, and end-use applications. They must specifically address the industry’s core characteristics: "high pressure, flammability, explosiveness, extensive long-distance pipelines, and, in some cases, corrosive media." As a result, their application scenarios and technical requirements are tightly focused on "leak-proof performance, high-pressure resistance, and adaptability to complex operating conditions," which can be detailed further through the following key stages:

1. Natural Gas Extraction and Mine-Gathering & Transportation Stage: Withstanding High Pressure at Wellheads and Handling Complex Mine Conditions

The upstream natural gas extraction process—specifically at wellheads and mine-field gathering systems—is the first critical application area for flange technology, with the core requirement being Resisting high-pressure gas flow impacts at the wellhead, while enduring corrosion from mining environments and the effects of outdoor conditions. , to prevent natural gas leaks from posing safety risks.

• Wellhead assembly (gas production tree) connection The gas gathering tree, serving as the core control unit at the wellhead, relies on high-pressure flanges to connect its valve body, four-way connections, casing head, and tubing head. Common flange types include butt-welded or integral flanges. These flanges must withstand the high pressures of 10–70 MPa at the wellhead—though pressure levels can vary significantly across different gas fields, with some high-pressure fields exceeding 50 MPa. Additionally, they need to resist corrosive media such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂), which may be present in the natural gas. In "acidic gas fields," flange materials should be made from sulfur-resistant carbon steels like 35CrMo or 20CrMo, or from corrosion-resistant alloys such as Incoloy 825, ensuring compliance with the NACE MR0175 standard to prevent sulfide stress corrosion cracking. For conventional gas fields, flanges can typically be fabricated from high-quality carbon steel grades like 20# or low-alloy structural steels, balancing strength requirements with cost considerations.
• Connection of Mine Field Gathering Pipelines and Equipment : The gathering pipelines (mostly ranging from DN50 to DN300) connecting the wellhead to the gas gathering station at the mine site must be connected in sections using flanges. These flanges also serve as interfaces for installing essential equipment such as filters (which remove dust and condensate from natural gas), flow meters (used to measure production rates), and pressure transmitters. Given that most mining sites are located in remote, outdoor environments, the flange designs need to withstand extreme temperature fluctuations—ranging from -30°C to 40°C—as well as harsh conditions like frequent windblown sand, rain, and snow erosion. Therefore, the sealing structures must balance both "resistance to extreme cold and heat" and "dust- and water-proof" performance. In colder regions, flexible graphite gaskets (capable of withstanding temperatures from -200°C to 650°C) are typically used, while nitrile rubber gaskets are suitable for moderate-temperature areas. Additionally, flange sealing surfaces often feature raised-face or male-and-female configurations to minimize the ingress of windblown debris into the sealing gaps, ensuring long-term reliability under challenging operational conditions.

II. Long-Distance Natural Gas Pipeline Segment: Designed to Handle High-Pressure Transport Over Extensive Distances and Diverse Operational Conditions Along the Route

Natural gas long-distance pipelines—such as national-level projects like the West-to-East Gas Pipeline and the Sichuan-to-East Gas Transmission—are the backbone of cross-regional transportation, and flange applications must meet Long-distance high-pressure transmission, complex geographical conditions along the route (such as permafrost, mountainous regions, and water bodies), and the need for regular maintenance. , and must also align with the "resilience" of the pipeline system.

• Segmentation of Long-Distance Pipelines and Equipment Connection : Long-distance pipelines primarily rely on welded connections for their main lines. However, in sections where pipelines cross or pass under obstacles—such as rivers or railways—and at equipment connection points within stations and facilities (e.g., compressor stations or distribution stations), flanges are still extensively used. For instance, at compressor stations, the connections between compressor inlets/outlets and pipeline systems, as well as at distribution stations where pressure regulating valve assemblies interface with pipelines, all require high-pressure butt-welded flanges (typically rated PN10 to PN40, corresponding to pipeline design pressures of 6–10 MPa). These flanges are made from materials that match those of the pipelines themselves—most commonly forged steel flanges compatible with X80 or X70 linepipe steels, such as ASTM A694 F80—to ensure that the flange and pipeline maintain consistent strength and toughness, thereby preventing stress concentrations caused by material mismatches.
• Pipeline Emergency Repair and Maintenance Assistance In the event of a leak or failure in long-distance pipelines, the repair can be swiftly carried out using a "flange + short pipe" assembly as the emergency repair section. Typically, the flanges used for repairs are either loose-type or slip-on flanges, offering the advantage of eliminating the need for precise alignment with the pipeline axis during installation. These flanges can accommodate minor pipe misalignments, significantly reducing the time required for emergency repairs—while conventional welding repairs may take several hours, flange-based repairs can be completed in just 1 to 2 hours. This approach minimizes disruptions to natural gas transmission and lowers associated losses. Additionally, ancillary components along long-distance pipelines, such as cathodic protection test stakes and drain valves, are also connected to the pipeline via small-sized flanges (DN25–DN50), making it easier to perform regular inspections and maintenance tasks.

III. Natural Gas Processing and Purification Stage: Addressing Media Corrosion and Ensuring Process Sealing Requirements

After natural gas is extracted, it must undergo processing—such as dehydration, acid removal, and hydrocarbon separation—to meet pipeline transmission or domestic-use standards. During this stage, flanges are required. Corrosive media and high/low-temperature process conditions in the tolerance treatment process , while ensuring the treatment system achieves "zero leakage" (preventing acidic media from spilling out and corroding equipment, or natural gas leaks that lead to waste).

• Connection of the dehydration and deacidification unit In dehydration units (such as those using triethylene glycol), the connections between the absorption tower, regeneration tower, and heat exchangers—as well as in acid removal systems (e.g., the MDEA process for acid removal), where the absorption tower is linked to the flash drum—must utilize corrosion-resistant flanges. Given that the processes involve triethylene glycol (which can reach temperatures exceeding 200°C during high-temperature regeneration) and MDEA solutions (an alkaline medium prone to corroding carbon steel over prolonged exposure), the flange materials should be either 304 stainless steel (for typical corrosion conditions) or 316L stainless steel (for more aggressive environments, such as media containing high concentrations of Cl⁻ ions). Additionally, the sealing surfaces are often designed as tongue-and-groove or male-female types, paired with PTFE gaskets that offer excellent resistance to acids, alkalis, and extreme temperatures up to 260°C. This ensures that corrosive media cannot leak through the sealing interface while also preventing the gasket from swelling or being eroded by the process fluids.
• Connecting the Dehydrogenation and Liquefaction Units : When natural gas requires hydrocarbon removal (to produce LNG and LPG), the flanges used in the liquefaction unit—where temperatures can drop as low as -162°C—must specifically meet "cryogenic resistance" requirements. For instance, flanges connecting LNG storage tank feed lines or cryogenic heat exchangers should be made from materials with exceptional low-temperature toughness, such as 06Ni9DR low-temperature steel, paired with cryogenically rated gaskets like modified PTFE gaskets capable of withstanding temperatures as low as -200°C. Additionally, flange welds must undergo low-temperature impact testing to prevent brittle fractures under freezing conditions. Meanwhile, in hydrocarbon removal units, connections for fractionation towers and condensers—which handle light hydrocarbons like propane and butane (both highly flammable and explosive)—require flange seals that combine "metal-wound gaskets with raised-face or grooved-face sealing." The springback properties of the metal-wound gasket help compensate for flange deformations caused by temperature fluctuations, ensuring a leak-proof seal even in extreme cold environments.

4. Natural Gas Storage and End-Use Applications: Aligning Storage Pressures with Domestic Safety Requirements

In natural gas storage facilities—such as underground gas storage sites and LNG tanks—and at end-use points—including city gate stations, industrial customers, and residential gas users—the application of flanges must strike a balance between functionality, safety, and efficiency. Storage pressure stability, endpoint security protection, and civilian convenience , to prevent gas supply safety issues caused by flange problems.

• Storage facility connection The wellheads for injection and production at the underground gas storage facility, along with the surface gathering and transmission pipelines connecting to the storage reservoir, must be equipped with high-pressure flanges (pressure rating PN25–PN40, suitable for the 10–20 MPa operating pressure of the gas storage facility). The material selected should be sulfur-resistant low-alloy steel (e.g., 20CrMo) to prevent corrosion from potentially acidic media present in the storage reservoir. For the LNG storage tanks (atmospheric-pressure cryogenic tanks), the top inlet, bottom outlet, and safety valve connections must utilize cryogenic-grade flanges made from 06Ni9DR material. Flange sealing surfaces should be either flat or raised-face types, paired with cryogenically rated asbestos-rubber gaskets. Additionally, all welds between the flanges and the tank walls must undergo helium mass spectrometry leak testing to ensure absolute integrity—preventing any leakage of LNG (which is highly volatile and can easily form explosive mixtures when exposed to air).
• End-user connection In city gate stations, the connections between natural gas pressure regulating valve assemblies, metering devices, and pipelines must use medium- to low-pressure flanges (pressure ratings PN1.6–PN10, suitable for urban network pressures ranging from 0.4 to 4.0 MPa). These flanges are typically made of Q235B carbon steel or 304 stainless steel—though corrosion resistance is prioritized in residential applications. The sealing surfaces feature raised-face designs, paired with nitrile rubber gaskets that offer excellent cost-effectiveness and consistently reliable sealing performance, making it easy to disassemble and maintain pressure regulators and flow meters later on. For industrial users—such as steel plants and chemical facilities—where gas boilers and burners are connected to pipelines under high-temperature conditions (with boiler outlet pipes exceeding 200°C), the flanges must be crafted from heat-resistant carbon steels like 20G or low-alloy heat-resistant steels such as 15CrMoG. These are complemented by flexible graphite gaskets, which prevent gasket degradation and failure even at elevated temperatures. While residential gas pipelines predominantly rely on threaded connections, small-sized flanges (DN25–DN50) made of brass or 304 stainless steel are still essential for connecting valves in neighborhood pressure regulation cabinets and before building-level gas meters, ensuring both safety and corrosion resistance in household settings.

V. Core Technical Requirements for Flanges in the Natural Gas Sector

The "flammable, high-pressure, and partially corrosive medium" characteristics of the natural gas industry dictate that flange applications must meet stringent standards far exceeding those for general industrial use. The core technical requirements can be summarized into three key points:

1. Precise matching of material and pressure rating : Must comply with industry-specific standards for the natural gas sector, such as API 6A (wellhead flanges), API 5L (pipeline steel flanges), and GB/T 9113 (steel pipe flanges). Pressure ratings should be carefully selected based on actual operating conditions—choosing "neither excessive nor insufficient"—with PN63-PN100 recommended for high-pressure wellhead applications, PN10-PN40 suitable for long-distance pipelines, and PN1.6-PN10 ideal for end-user systems. Additionally, material selection must align with the nature of the medium: opt for sulfur-resistant steel if the medium contains acidic components, choose low-temperature steel for cryogenic applications, or use stainless steel when necessary, ensuring proper material matching to prevent safety risks caused by "material mismatch."
2. Sealing performance with zero-leakage guidance : When natural gas leaks reach concentrations of 5%–15%, they can form an explosive limit. Therefore, flange seals must achieve "zero leakage"—in high-pressure applications, use metal ring gaskets such as octagonal or elliptical gaskets, suitable for pressures above PN40; for medium- and low-pressure scenarios, opt for metal-wound or flexible graphite gaskets; and in cryogenic environments, rely on cryogenically rated gaskets. Additionally, the machining precision of flange sealing surfaces must meet Ra1.6 to Ra3.2 standards, minimizing surface defects that could lead to leaks.
3. Structural adaptation to operational requirements : Integral flanges for high-pressure wellheads (high strength, with no weld-related weak points); butt-welded flanges for long-distance pipeline stations and facilities (excellent welding performance, ensuring tight connections with pipelines); slip-on flanges for emergency repair scenarios (offering flexible installation); and cryogenic flanges designed for low-temperature storage applications (outstanding toughness and resistance to low-temperature impact). Meanwhile, flange bolts must be made of high-strength materials—such as Grade 8.8 or Grade 10.9—and tightened according to specified torque values to prevent bolt loosening, which could lead to sealing failure.

In summary, the application of flanges in the natural gas industry essentially involves selecting an optimal "material-structure-sealing" solution tailored to the specific pressure, temperature, and medium characteristics of each stage across the entire value chain. Their performance directly determines the safety, stability, and cost-effectiveness of natural gas transportation, making them a critical component that ensures the efficient operation of the natural gas industry chain.