A flange joint is only as good as its gasket. The flanges and bolts may be perfect, but the wrong gasket will leak — or blow out — the first time pressure or temperature spikes. Knowing the main flange gasket types helps you supply complete flange kits and answer the technical questions buyers ask before they order.
The three gasket families
Gaskets fall into three broad groups:
- Non-metallic (soft) gaskets — sheet materials for low to moderate service.
- Semi-metallic gaskets — spiral wound, kammprofile and jacketed types for most process duties.
- Metallic gaskets — ring type joints (RTJ) for high pressure.
The dimensional standards are ASME B16.21 for non-metallic flat gaskets and ASME B16.20 for metallic and semi-metallic gaskets used with ASME B16.5 and B16.47 flanges.
Non-metallic (soft) gaskets
Soft gaskets are cut from sheet and used mostly with raised-face and flat-face flanges in Class 150 and 300.
- CNAF (compressed non-asbestos fibre): general utilities, water, air, low-pressure steam.
- PTFE: excellent chemical resistance; used in acids, alkalis, food and pharma.
- Flexible graphite: high temperature and good sealing; often with a metal insert for strength.
- Rubber (EPDM, NBR, neoprene, Viton): water, oil and low-pressure lines; common with flat-face flanges.
Soft gaskets are cheap and easy to cut, but they have pressure and temperature limits and may creep under load.
Spiral wound gaskets
The spiral wound gasket is the workhorse of refineries, chemical plants and power stations. It is made by winding a V-shaped metal strip and a soft filler together in a spiral.
A typical spiral wound gasket has:
- Winding strip: usually SS 304 or 316L; alloys like Monel, Inconel or titanium for special service.
- Filler: flexible graphite (most common) or PTFE.
- Outer (centering) ring: usually carbon steel; centres the gasket inside the bolts and acts as a compression stop.
- Inner ring: protects the winding from the fluid and prevents inward buckling. Inner rings are required by ASME B16.20 for PTFE-filled gaskets and for many larger sizes and higher classes, and many end users specify them for all spiral wound gaskets.
ASME B16.20 colour codes
Spiral wound gaskets carry a colour code on the outer ring so material can be identified in the field:
| Winding material | Colour on ring |
|---|---|
| 304 SS | Yellow |
| 316L SS | Green |
| 321 SS | Turquoise |
| 347 SS | Blue |
| Monel 400 | Orange |
| Inconel 600 | Gold |
The filler is shown by a stripe: grey for flexible graphite and white for PTFE.
Ring type joint (RTJ) gaskets
RTJ gaskets are solid metal rings that sit in machined grooves on RTJ-faced flanges. As bolts are tightened, the ring deforms slightly into the groove to make a metal-to-metal seal.
- Type R: oval or octagonal cross-section; the most common, used with ASME B16.5 flanges.
- Type RX: pressure-energised version for higher pressures.
- Type BX: for API 6A flanges in very high-pressure wellhead service.
RTJ gaskets must be softer than the flange groove so that the ring deforms, not the flange. Typical materials and maximum hardness are soft iron (about 90 HB), low carbon steel (about 120 HB) and 316 stainless (about 160 HB). RTJ is common at Class 600 and above, and in wellhead, high-pressure gas and hydrocracker lines.
Other semi-metallic gaskets
- Kammprofile (grooved metal): a serrated metal core with soft facing layers; excellent for heat exchangers.
- Double-jacketed: soft filler enclosed in a metal jacket; used on heat exchanger and large-diameter flanges.
How to choose the right gasket
Ask the buyer or check the specification for:
- Flange facing: RF, FF or RTJ.
- Pressure class and temperature: soft gaskets for low duty, spiral wound for most process duty, RTJ for high pressure.
- Fluid: acids and chlorides affect winding and filler choice.
- Size and standard: ASME B16.5 or B16.47 Series A/B, or EN 1514 for EN 1092 flanges.
- Bolting: the bolts must deliver enough load to seat the gasket. See stud bolt grades explained.
For the flange side of the joint, read flange types explained and our guide to flange pressure class ratings.
Selling gaskets as part of a flange kit
Many overseas buyers prefer one supplier for flanges, studs, nuts and gaskets together. Quoting a complete joint kit:
- increases order value,
- reduces the buyer's coordination work,
- avoids mismatched components on site.
List each component separately with material, standard and quantity. QuoteMitra makes it easy to build these multi-line quotations with weights and terms.
Frequently asked questions
What is the most common flange gasket?
For raised-face process flanges, the spiral wound gasket with 316L winding and graphite filler is the most common choice worldwide.
Can a gasket be reused?
No. Spiral wound and RTJ gaskets are designed to deform when compressed. Always fit a new gasket when a joint is opened.
What does "SS316/GRAPH/CS" mean?
It describes a spiral wound gasket with 316 stainless winding, graphite filler and carbon steel outer ring. An inner ring would be added as "/SS316 IR".
Which gasket is used for flat-face flanges?
Full-face soft gaskets (with bolt holes), such as CNAF or rubber, are used with flat-face flanges, especially on cast iron or plastic equipment.
Key takeaways
- Soft gaskets suit low-pressure utilities; spiral wound suits most process service; RTJ suits high pressure.
- ASME B16.20 colour codes identify winding and filler material.
- RTJ rings must be softer than the flange groove.
- Quote complete joint kits — flanges, bolting and gaskets — to win larger orders.
Suppliers who understand the whole joint, not just the flange, become the first call when a project starts.
