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Flange Gasket Marking and Selection Guide

[Abstract]:Accurate marking identification lays the foundation for precise matching of flange gaskets, while scientific selection serves as the core guarantee for reliable sealing and leakage prevention.

In industrial applications such as pipeline transportation, pressure vessels and chemical equipment, the tightness of flange connections directly affects production safety and operational efficiency. As core sealing components, flange gaskets are a leading cause of leakage due to incorrect marking identification and improper selection. Industry data shows approximately 60% of flange leakage failures result from mismatched or improperly selected gaskets. Therefore, understanding standard gasket markings and mastering scientific selection methods is essential for industrial professionals. This article comprehensively explains flange gasket identification, classification, selection logic and practical application guidelines.

Gasket markings serve as formal product specifications governed by unified domestic and international standards, including GB/T 9126, ASME B16.21 and EN 1514-1. A complete marking consists of six core parameters to ensure accuracy in procurement, installation and replacement. Common standards include GB/T 9126 for non-metallic flat gaskets and GB/T 4622 for spiral wound gaskets, while ASME B16.21 is widely adopted internationally. For example, “GB/T 4622.2-2017” specifies spiral wound gaskets complying with Chinese national standards.

Standard type codes are used for quick identification: Non-metallic Flat Gasket (NF), Spiral Graphite Gasket (SPG), Ring Type Gasket (RG), and Expanded PTFE Gasket (PTFE). Material markings distinguish base and reinforcing materials. Non-metallic gaskets are marked with base materials such as Flexible Graphite (FG) and Nitrile Rubber (NBR). Spiral wound gaskets adopt “metal strip + filler” marking, e.g., “304 + FG”. Metal ring gaskets are labeled with alloy grades including 316L stainless steel and Inconel 625.

Dimensional specifications cover Nominal Diameter (DN) and flange face types, including Raised Face (RF), Flat Face (FF), and Male & Female Face (MFM), e.g., “DN100 RF” for DN100 raised face flanges. Pressure ratings are defined by PN or Class standards, which are not directly equivalent (e.g., PN1.6 ≈ Class150). Structural indicators apply to special gaskets: spiral wound gaskets are marked with Inner Ring (I), Outer Ring (O), or Inner & Outer Ring (IO), while metal ring gaskets include R, RX and BX cross-section types.

Full marking example: GB/T 4622.2-2017 SPG IO 304+FG DN50 PN1.6 RF. This refers to a standard spiral wound gasket with inner and outer rings, 304 stainless steel strip and flexible graphite filler, suitable for DN50, PN1.6 raised face flange applications. Accurate marking interpretation eliminates matching errors.

Gasket selection must be based on temperature resistance, pressure tolerance and media compatibility. Non-metallic flat gaskets feature simple structure and cost efficiency for low-pressure and ambient-temperature conditions. NBR gaskets offer excellent oil and water resistance for -20℃ to 120℃ and PN≤1.6MPa water and oil media. EPDM gaskets resist acid, alkali and steam, applicable at -50℃ to 150℃ and PN≤2.5MPa. Flexible graphite gaskets withstand extreme temperatures from -200℃ to 650℃ with strong chemical resistance, ideal for high-temperature steam and flue gas systems under PN≤6.4MPa. PTFE gaskets deliver superior chemical inertness against most acids and alkalis for chemical pipelines within -200℃ to 260℃, with slight limitations in rigidity and cold flow performance.

Spiral wound gaskets combine metallic rigidity and non-metallic elasticity, serving as universal solutions for medium and low-pressure conditions. 304+FG gaskets operate steadily at -200℃ to 650℃ under PN0.6–16.0MPa for steam, oil and general chemical media. 316L+PTFE types adapt to highly corrosive environments within -200℃ to 260℃. Metal ring gaskets rely on metallic elastic deformation for sealing, suited for high-temperature and high-pressure scenarios. Type R fits PN1.6–10.0MPa and ≤600℃; Type BX serves ultra-high pressure of PN16.0–42.0MPa and temperatures up to 800℃ with superalloy materials, requiring high-precision flange surfaces. Composite gaskets with non-metallic cores and metal reinforcements balance corrosion resistance and structural stability for medium-to-high pressure corrosive conditions, such as metal-reinforced PTFE gaskets working at PN≤10.0MPa and -200℃ to 260℃.

Scientific selection follows a three-dimensional matching principle: working conditions, medium characteristics and flange type. For temperature and pressure matching: select PTFE or graphite gaskets for sub-zero environments below -50℃ to avoid rubber embrittlement; adopt flexible graphite or superalloy gaskets for temperatures above 400℃ to prevent thermal softening. For pressure grading: non-metallic flat gaskets for PN≤1.6MPa, reinforced spiral wound gaskets for PN2.5–10.0MPa, and metal ring gaskets for PN≥16.0MPa. Elastic gasket types are recommended for fluctuating pressure conditions such as pump outlets.

Media compatibility is critical: PTFE, fluororubber or 316L+PTFE gaskets for acid, alkali and salt corrosion; NBR or carbon steel+graphite gaskets for oil media; flexible graphite gaskets for high-temperature steam and flue gas; wear-resistant metal ring or outer-ring spiral gaskets for particle-containing slurry and dust media. Flange matching rules: non-metallic flat gaskets for PN≤2.5MPa slip-on flanges; reinforced spiral and metal ring gaskets for PN≥4.0MPa butt-weld flanges. RF flanges match spiral and composite gaskets; FF flanges adapt to flat non-metallic gaskets; MFM and TG flanges fit precision spiral and metal ring gaskets.

Four common on-site selection errors must be avoided. First, ignoring temperature-pressure coupling: the actual pressure resistance of PN1.6 rubber gaskets drops below PN1.0 at 150℃, requiring derating for high-temperature service. Second, cost-oriented mismatching: ordinary rubber gaskets fail rapidly in corrosive media. Third, structural mismatching: flat gaskets displace on RF flanges and deform excessively on TG flanges. Fourth, over-specification: high-pressure metal gaskets cause poor fitting and leakage under low-pressure conditions.

Typical selection cases: Chemical hydrochloric acid pipeline (DN100, PN1.6, 25℃, strong corrosion) — GB/T 9126-2017 PTFE Flat Gasket DN100 PN1.6 FF; High-temperature steam pipeline (DN200, PN4.0, 400℃) — GB/T 4622.2-2017 SPG IO 304+FG DN200 PN4.0 RF; Oil & gas high-pressure pipeline (DN150, Class600, 350℃) — ASME B16.21 RG BX 316L DN150 Class600.

With industrial upgrading, gasket technology is evolving toward intelligence and long service life. Advanced gaskets adopt RFID chips for full lifecycle traceability of production batches and inspection records. Asbestos-free structures have completely replaced traditional asbestos gaskets, while high-temperature alloys and nano-reinforced PTFE are emerging for extreme working conditions. Mastering gasket marking decoding and standardized selection effectively minimizes leakage risks and stabilizes industrial connection systems, constituting core professional competency for industrial practitioners.

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Flange Gasket Marking and Selection Guide

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