Sulfur Trioxide Dry O-Ring Compatibility: Which Compounds Hold Up

Dry sulfur trioxide (SO₃) is an aggressive acid anhydride used in sulfuric acid manufacture and specialty sulfonation service. Seals on dryer outlets, converter ducts, absorption towers, process valves, and transfer flanges see a strong oxidizing gas that attacks many everyday elastomers. Dry SO₃ is a hard ask for common oil-service stocks. Start with a fluorocarbon family that the chart rates excellent for this row.

This guide maps the handbook Chemical Compatibility Chart row for Sulfur Trioxide Dry to practical compound choices. Wet SO₃, oleum, and sulfuric acid rows sit nearby on the chart when your SDS wording differs.

Where it shows up

• Sulfuric acid contact plants: dryer, converter, and absorption-tower flanges and ducts moving dry SO₃ gas
• Oleum and SO₃ transfer: valves, pump skids, and loading connections on concentrated sulfur trioxide streams
• Sulfonation and specialty chemical service: reactors and feed lines where dry SO₃ is the reagent
• Instrument and sample taps: gauges and analyzer fittings on SO₃ process piping
• Maintenance and retrofit: cover seals and manway gaskets on legacy acid-plant equipment

Service is usually dry gas in steel or stainless equipment, sometimes with heat from the converter train. Chart ratings assume a generic commercial compound near room temperature. Moisture, heat, and co-fluids change outcomes. Keep the seal dry when the SDS calls for dry SO₃.

How dry sulfur trioxide stresses an O-Ring

Dry SO₃ is a strong oxidizing acid anhydride. Many hydrocarbon elastomers swell, harden, or crack under continuous contact. On this chart row, FKM, FFKM, and PTFE rate excellent (1). EPDM, silicone, fluorosilicone, and butyl land in the good (2) band for many static uses when the grade and temperature fit. Nitrile, HNBR, and neoprene rate not recommended (4). Polyurethane and Aflas® show insufficient data (X) here. Heat and any moisture that forms oleum or sulfuric acid raise the stakes further. See Chemical Compatibility and Failure Analysis.

Handbook chart ratings for sulfur trioxide dry

Same rating badges and layout as the interactive chart. Search Sulfur Trioxide Dry on the Chemical Compatibility Chart to pin the live row.

Best family on the chart row: Viton® / FKM (V). FFKM and PTFE also rate excellent (1). EPDM, silicone, fluorosilicone, and butyl rate good (2). Nitrile, HNBR, and neoprene are not recommended (4). Polyurethane and Aflas® are insufficient data (X) on this row.

Recommended O-Ring materials

First choice: Viton® / FKM

FKM (Viton®) is rated excellent (1) and is the chart’s preferred letter for dry sulfur trioxide. It is the everyday fluorocarbon start for SO₃ process valves, dryer connections, and static cover seals when temperature and pressure stay inside commercial FKM windows. Use 70 Shore A for most static flanges; step hardness or add a back-up ring when pressure and clearance gap climb.

Also usually excellent on this row

• FFKM 1Excellent Use for mixed aggressive acid streams, higher heat, or when plant standards call for perfluoroelastomer.
• PTFE 1Excellent Chemically inert but rigid. Consider PTFE-encapsulated when you need rebound with a PTFE skin.

Often workable (good)

• EPDM 2Good May suit milder static duty; prefer FKM for continuous dry SO₃ when the chart is your guide.
• Silicone (VMQ) 2Good Possible for some static low-pressure uses; watch tear, abrasion, and plant standards.
• Fluorosilicone (FVMQ) 2Good Often workable for limited static contact; confirm grade immersion data for continuous dry SO₃.
• Butyl (IIR) 2Good May fit some static covers; FKM remains the preferred everyday start on this row.

Materials to use with caution or avoid

• Nitrile (Buna-N) 4Not recommended Petroleum workhorse in mineral oils; avoid for continuous dry SO₃ contact on this chart row.
• HNBR 4Not recommended Same weakness as standard nitrile for dry sulfur trioxide here.
• Neoprene (CR) 4Not recommended Avoid in continuous dry SO₃ duty on this row.
• Polyurethane XNo data No chart guidance for this fluid. Prefer a 1-rated family for critical SO₃ seals.
• Aflas® (FEPM) XNo data Insufficient data on this row. Verify grade immersion results before specifying.

Physical properties to match (after chemistry)

A chart 1 means the polymer family usually survives the fluid. Finish the spec with the gland and process needs:

• Hardness: 70 Shore A for most static covers and flanges; 90 Shore A or a back-up ring when pressure and gap climb (Shore A Hardness).
• Temperature: handbook chemical ratings are near room temperature. Converter and dryer lines run warmer and age seals faster. Confirm continuous temperature against the Temperature Guide.
• Dry vs wet: moisture turns SO₃ toward oleum or sulfuric acid. Match the exact fluid name on the SDS and on the chart.
• Static vs dynamic: face and cover seals are mostly static; shafts and rods need abrasion and extrusion control beyond a chemical letter (groove design).
• Size: lock ID × CS (or the AS568 dash) on the Master O-Ring Size Chart before arguing compound.

Practical selection checklist

1. Confirm dry SO₃ vs oleum vs sulfuric acid from the SDS, and whether moisture can enter the seal cavity.
2. Note temperature and whether the seal sees gas, condensate, or both.
3. Start with FKM at 70 Shore A for general static duty unless a plant standard already calls for FFKM or PTFE-encapsulated.
4. Avoid nitrile, HNBR, and neoprene for continuous dry SO₃ on this chart row. Treat polyurethane and Aflas® as unrated until you have grade data.
5. Pin Sulfur Trioxide Dry on the Chemical Compatibility Chart. For critical service, contact The O-Ring Store with fluid grade, temperature, hardness, and duty cycle.

Quick answers

Which O-Ring for dry sulfur trioxide?
Start with FKM (Viton®) at 70 Shore A for general static duty. FFKM and PTFE also rate excellent. EPDM, silicone, fluorosilicone, and butyl are usually good for milder static service. Prefer FKM when dry SO₃ is continuous and aggressive.

Why FKM for dry SO₃?
Dry sulfur trioxide is an oxidizing acid anhydride. On the Sulfur Trioxide Dry chart row, FKM rates excellent (1) and is the preferred letter. Match the fluid name and keep moisture out of the seal cavity when the process is specified as dry.

Is dry SO₃ the same as sulfuric acid or oleum on the chart?
Related acid rows exist on the chart. Use the exact fluid name that matches your SDS and operating condition.

Chart ratings are guidance for generic commercial compounds near room temperature. Concentration, temperature, moisture, and co-fluids change outcomes. Evaluate specific parts before use in critical applications.