Last updated: April 2026
The filler is the part of a spiral wound gasket that actually carries the seal. The metal winding provides recovery, the centring ring provides location and a compression stop, but it's the soft filler — graphite, PTFE, mica, or ceramic — that flows into the flange surface and stops the leak. Picking the wrong filler is the single most common cause of premature gasket failure on a correctly-rated flange.
This guide covers the four fillers used in commercial spiral wound gaskets, the duty envelope of each, and the failure modes you're avoiding when you specify them correctly.
Quick-Pick Matrix
| If your service is... | Default filler | Why |
|---|---|---|
| Steam, hydrocarbons, refinery, < 450°C oxidising | Flexible graphite | Best recovery; tolerant of thermal cycling |
| Acids, alkalis, solvents, high-purity (food / pharma) | PTFE | Universal chemical inertness; FDA-compliant |
| Cryogenic to -200°C | Flexible graphite (with inner ring) | Graphite stays flexible; PTFE risks brittle failure on thermal shock |
| Flue gas, exhaust, regenerator (> 500°C, oxidising) | Mica | Graphite would oxidise; mica is fire-safe |
| Reformer, hot exhaust (> 800°C) | Ceramic | Only filler with continuous service in this range |
Flexible Graphite
The default filler for ~80% of spiral wound applications. Made from exfoliated natural graphite calendered into thin sheet, then wound between the V-section metal strip. Best balance of sealability, recovery, temperature tolerance, and cost.
Where it fits
- Steam systems — saturated and superheated, all classes.
- Hydrocarbons — crude, naphtha, gasoil, residue, fuel gas.
- Refinery hot service — fractionation, hydroprocessing reactor effluent.
- Cryogenic — LNG service down to -200°C; graphite stays flexible where soft polymers go brittle.
Where it doesn't
- Strong oxidisers — concentrated nitric acid, chlorine, oleum will attack graphite.
- Above ~450°C in oxidising service — graphite slowly oxidises (loses mass) when air is present. Inert atmospheres extend the limit to ~650°C.
- High-purity duties — graphite particulate contamination is unacceptable in pharma, semiconductor, or food service. Use PTFE.
Grades to specify
Standard sealing-grade flexible graphite is ≥98% carbon. For high-temperature or aggressive service, specify oxidation-inhibited grades (with passivating additives) or nuclear-grade (≥99.5% C, low chlorides) for stress-corrosion-sensitive stainless flanges.
PTFE (Polytetrafluoroethylene)
The chemical specialist. Virtually inert across the entire pH range, FDA-compliant, and contamination-free — but limited to ~260°C continuous service.
Where it fits
- Aggressive chemicals — strong acids (HCl, H₂SO₄, HNO₃), alkalis, solvents.
- High-purity service — pharmaceutical reactors, food & beverage, semiconductor.
- Mixed-service plants — when one gasket spec has to cover many duties without contamination risk.
Where it doesn't
- Above 260°C — PTFE creeps badly above its glass transition; the seal relaxes.
- Hot oleum / fluorine compounds — the few media that attack PTFE.
- Dynamic / vibration duties — graphite has better recovery for cyclic loads.
Mica
A high-temperature, fire-safe filler made from layered phlogopite or muscovite mica plates, bonded with vermiculite or silicate. Used where graphite would oxidise and PTFE would melt.
Where it fits
- Flue-gas and exhaust ducts, stack flanges, regenerator headers.
- Fire-safe service per API 6FB / ISO 10497 — keeps a seal during a fire excursion.
- Continuous service to 800°C (peak 1000°C).
Where it doesn't
- Wet service — bonded mica can swell and lose seal in liquid water.
- Aggressive acids — silicate binders are attacked.
Ceramic
Specialist filler for the very hot end — reformer outlet flanges, ethylene cracker furnace tubes, exhaust manifolds. Continuous to 1000°C, peak 1200°C. Lower compressibility than graphite or PTFE; needs higher seating stress and tighter flange-finish control.
Common Mis-Specifications
- PTFE for hot steam. Above 200°C PTFE creeps; the bolt load relaxes within weeks.
- Graphite for concentrated nitric acid or chlorine. Oxidisers degrade the filler; switch to PTFE.
- Standard graphite for hot oxidising flue gas. Specify oxidation-inhibited or step up to mica.
- PTFE for cryogenic. Below ~-100°C PTFE goes brittle; graphite is the right choice.
- Graphite on stress-corrosion-sensitive austenitic stainless near a heat source. Standard graphite can leach chlorides; specify nuclear-grade or low-chloride filler.
Frequently Asked Questions
Flexible graphite is the default filler for around 80% of spiral wound applications. It handles steam, hydrocarbons, and most refinery service from cryogenic temperatures up to 450°C in air or 650°C in inert atmospheres, with the best recovery and thermal-cycling tolerance of any filler.
Use PTFE when the media will attack graphite (concentrated nitric acid, chlorine, strong oxidisers), when graphite contamination is unacceptable (food, pharma, semiconductor), or when one gasket spec has to cover many chemical duties without compatibility risk. The trade-off is the temperature limit: PTFE creeps badly above 260°C.
Mica is the right filler when graphite would oxidise (continuous service above 450°C in oxidising atmospheres) and PTFE would melt. Typical applications are flue-gas ducts, regenerator headers, exhaust manifolds, and fire-safe flanges where the gasket must keep a seal during a fire excursion to 800°C continuous and 1000°C peak.
Yes — graphite is compatible with austenitic stainless steel for the vast majority of duties. The exception is stress-corrosion-sensitive service: hot stainless flanges in chloride-containing environments. In those cases specify nuclear-grade or low-chloride graphite (≤50 ppm Cl) to avoid leaching corrosive ions onto the flange face.
No — the flange class (ASME 150, 300, 600, 900, 1500, 2500) and the centring/inner ring construction determine the cold working pressure. The filler determines the temperature envelope. Both have to be right: a Class 600 spiral wound gasket with PTFE filler is rated to 1480 psig but only to 260°C — push it to 300°C and the PTFE will creep and the seal will fail.
Related
- What is a spiral wound gasket?
- Bolt-up to ASME PCC-1
- Spiral wound vs RTJ
- Spiral wound gaskets — full range
Sources
- ASME B16.20 — Metallic Gaskets for Pipe Flanges, Annex A (filler selection)
- API 601 — Metallic Gaskets for Raised-Face Pipe Flanges
- FSA-G-604 (Fluid Sealing Association) — Spiral Wound Gasket Technical Handbook