Do you know what the markings on the spiral wound gasket represent

author: www.qishine.com

2026-03-06

In industrial sectors such as petrochemicals, power generation, and pharmaceuticals, "leaks, spills, drips, and emissions" at flange connections are major enemies of safety and environmental protection. The metallic spiral wound gasket is a critical sealing component, and its markings serve as its essential "identification card" for ensuring fail-safe operation. Correctly reading and interpreting these markings is a necessary skill for every engineer, procurement specialist, and installer. This article systematically decodes the seven key elements of spiral wound gasket markings to help you select the correct type and ensure safe application.

The names of all the parts of the spiral wound gasket and the meanings of the markings

1. Company Logo: The Starting Point of Quality and ResponsibilityThe trademark or company name on a Spiral Wound Gasket is far from just a simple graphic. It is a symbol of quality commitment and the source for traceability and accountability. The logo of a reputable manufacturer signifies that its products adhere to strict management systems from raw materials and production processes to quality control. When selecting, prioritize brands with a long-standing, good reputation in the industry—this is the first line of defense against risk. XIAMEN QISHINE has been in this industry for 14 years.

2. Gasket Size
The "Gasket Size" in the marking refers to the gasket's nominal diameter.
Definition: Nominal Diameter (DN) is a numerical designation related to the internal diameter of a pipe or flange, e.g., DN150, DN300. It represents a standardized series of sizes, not an exact measured inner diameter.

Purpose: It directly determines the gasket's inner diameter, ensuring the gasket can be correctly centered and fitted within the flange bolt circle.
Key Point: It must exactly match the nominal diameter of the mating flange. A DN150 gasket must never be used on a DN125 or DN200 flange.

Spiral wound Gasket Size Specification

3. Gasket Pressure
The gasket pressure marking indicates its nominal pressure rating.
Definition: Nominal Pressure (PN, or Class) is a standardized pressure rating for pressure vessels, piping, and flanges, e.g., PN16, PN40, Class 150, Class 600. It represents the ANSI B16.20 Spiral Wound Gasket's designed comprehensive load-bearing capacity for specific medium pressures and temperatures.

Importance: The pressure rating determines the gasket's structural design (e.g., number of winding layers, density of the "V"-shaped winding strip) and thickness. Higher-pressure rated gaskets are thicker and more dense.

Core Principle: The gasket's pressure rating must be equal to or higher than the design pressure rating of the flange and piping system. Using a lower-pressure rated gasket for a higher-pressure application is absolutely prohibited as it poses a significant safety hazard.

Spiral Wound Gasket Include Winding, Inner Ring and Outer Ring.

4. Inner Ring Material: The Internal Anchor
Located inside the winding, the inner ring's main functions are centering (for easier installation alignment), filling the central area, and protecting the winding strips from direct media erosion or inward buckling instability under cryogenic/vacuum conditions.

Do you know what the color of the outer ring represents?

Common Inner Ring Materials and Typical Applications:

Material Grade	Characteristics	Typical Application Environment
304	General-purpose austenitic stainless steel, economical, resistant to general corrosion.	Water, steam, air, oil products, non-strongly corrosive chemical media. Wide temperature range (-200°C ~ +700°C).
316/316L	Contains molybdenum, significantly better pitting and crevice corrosion resistance than 304.	Seawater, chemical media (containing halide ions like chloride), phosphoric acid, acetic acid, etc. Mainstream choice in the chemical industry.
321	Contains titanium, excellent resistance to intergranular corrosion.	High-temperature environments (up to 800°C), e.g., hot oil systems, flue gas ducts.
Carbon Steel	Low cost, poor corrosion resistance.	Non-corrosive oil, gas, water systems, often used with external anti-corrosion coatings.
Special Alloys	e.g., Monel, Hastelloy, Titanium (Ti).	Extreme corrosion environments: concentrated sulfuric acid, hydrochloric acid, strong alkalis, high-temperature acetic acid, etc.

5. Filler Material: The Sealing Agent
The filler strip is alternately wound and gripped by the metal strips; it is key to forming the seal. Its softness compensates for micro-irregularities on the flange faces, achieving initial sealing.

Filler Materials:

Gray: Flexible Graphite

White: PTFE

Light Green: Ceramic

Pink: Mica Graphite

Common Filler Materials and Typical Applications:

Material	Properties	Typical Application Environment
Flexible Graphite	Most widely used. Extremely wide temperature range (-200°C ~ +1600°C in inert atmosphere), good resilience, thermally and electrically conductive.	High-temperature/high-pressure steam, hot oil, hydrogen, hydrocarbons, and most chemical media. Avoid strong oxidizing acids (fuming sulfuric acid, nitric acid).
PTFE	"King of Plastics," excellent chemical inertness, resistant to almost all chemicals, high cleanliness.	Pharmaceutical, food, strong acid/alkali applications requiring high cleanliness and strong corrosion resistance. Temperature limit approx. -200°C ~ +260°C.
Ceramic Fiber	High-temperature resistance (up to 1200°C+), fireproof, insulating.	High-temperature flue gas, exhaust ducts, boiler outlets, and other high-temperature but not high-pressure applications.
Non-Asbestos Fiber	Environmentally friendly, balanced performance, relatively low cost.	General industrial media like water, oil, solvents under medium temperature and pressure, replacing traditional asbestos.

6. Outer Ring Material: External Reinforcement and Positioning
Encasing the outer part of the winding, the outer ring's main functions are reinforcing the winding to prevent unraveling, aiding in centering during installation, and improving the gasket's overall compression resistance and blow-out resistance.

Material Selection Principle: Typically the same as or similar to the inner ring material to ensure coordinated performance with the flange and inner ring during temperature and pressure changes. Common materials are the same range as for the inner ring (304, 316, carbon steel, etc.). In non-demanding applications, to save costs, the outer ring material grade may be slightly lower than the inner ring, but its strength must be sufficient.

7. Standard: The "Law" for Manufacturing and Acceptance
The standard number in the marking is the sole authoritative basis for the gasket's design, manufacturing, and acceptance, ensuring product interchangeability and reliability.

Qishine is ideal choice for EPC companies and project contractors.

Website: www.qishine.com
Email: qishine@qishine.com
Tel: 0592-5225595

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Common Standard Systems:
1. Chinese National Standard (GB/T):
GB/T 4622.2-2022 "Spiral Wound Gaskets – Technical Requirements": Basic product standard.
GB/T 9129-2022, etc., series standards for pipe flange gaskets.

2. Chinese Chemical Industry Standard (HG/T) – Primary Choice for Domestic Design:
HG/T 20610-2022: European System (for PN series flanges, e.g., HG/T 20592).
HG/T 20631-2022: American System (for Class series flanges, e.g., HG/T 20615).
These are the standards most frequently specified on drawings for domestic petrochemical and chemical projects and must be strictly matched.

3. American Standard (ASME):
ASME B16.20: Internationally common (especially in American projects), used with ASME B16.5 flanges.

4. International Standard (ISO):
ISO 21298, etc., common in international EPC projects.

Golden Rule: The gasket standard must correspond exactly to the standard of the connecting flanges. Checking the flange markings or drawing requirements is the definitive method for selecting the gasket standard.

Practical Interpretation: A Complete Marking Example

Suppose you see a gasket marked as:
Qishine 4in 300# 304/FG/CS ASME B16.20

You can immediately interpret:
1. Manufacturer: The manufacturer is Qishine, responsible for its quality.
2. Size & Pressure: For use with a flange having a nominal diameter of 4 inches and a nominal pressure rating of 300#.
3. Material Structure: "304/FG/CS" represents: Inner ring is 304 stainless steel, filler is Flexible Graphite, outer ring is Carbon Steel.
4. Governing Standard: ASME B16.20.

This means you need to install it on a flange also marked as 4in, 300#, with a matching flange face type (e.g., Raised Face RF) conforming to the ASME B16.5 standard.

Conclusion
Correctly reading, interpreting, and applying the markings on metallic spiral wound gaskets is a fundamental skill for ensuring the safe, stable, and long-term operation of industrial facilities. It connects the links of design, procurement, installation, and maintenance. Remember these seven key elements and perform a final verification before each installation. Let every marking become a reliable footnote for the safe operation of your plant. For special service conditions, always consult professional technical personnel or the manufacturer for final confirmation.

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