A resilient-seated butterfly valve has only a handful of components, but every one of them works in a different environment. The body carries pipeline pressure, the disc faces flow velocity and sealing friction, the seat does the sealing, the stem transmits actuator torque, and the bushings quietly keep everything aligned. Get any one of the five wrong — a seat that swells in oil, a stem that seizes in chlorinated water, a coating that blisters in hot water — and the valve fails long before its design life.
This guide breaks down the materials used in soft-seated butterfly valves one component at a time, with selection tables, application combinations and the standards that validate them. It covers the material options Laux Valve uses across its wafer, lug, double flange, U-section, AWWA C504 and double eccentric series.
The body is the pressure-retaining envelope of the valve. For most soft-seated butterfly valves in water and industrial piping, ductile iron is the default choice; cast iron, carbon steel and stainless steel are selected when pressure, temperature or corrosion push beyond it.
| Material | Common Grades | Strengths | Limits | Typical Use |
|---|---|---|---|---|
| Cast Iron | GG25 / HT250 | Low cost, easy machining | Brittle, poor shock & thermal resistance | Low-pressure water lines, budget projects |
| Ductile Iron | GGG40 / GGG50, ASTM A536 65-45-12 | High strength and toughness, PN10–PN25 | Needs epoxy coating for corrosion | Default body for wafer, lug, double flange, U-section and AWWA C504 valves |
| Carbon Steel | WCB (ASTM A216) | Higher temperature and pressure, weldable | Requires coating, poor raw corrosion resistance | Steam, high-temperature water, power plants |
| Stainless Steel | CF8 (304), CF8M (316), CF3M (316L) | Excellent corrosion resistance, hygienic | 3–5× the cost of ductile iron | Food, chemicals, coastal plants, double eccentric high-performance valves |
| Aluminium Bronze | C95400 | Outstanding seawater resistance | Galvanic care needed at flanges | Marine, desalination, offshore |
At Laux Valve, standard water-service bodies are ductile iron GGG40/GGG50 coated inside and out with fusion-bonded epoxy, rated PN10/PN16 (ANSI 150) and built to EN 593 or AWWA C504. The AWWA C504 rubber-seated series uses this combination for municipal waterworks; the double eccentric series moves to CF8M stainless steel for aggressive service.
The disc sits directly in the flow stream, so it sees the full media chemistry plus the constant friction of the seat. Three base materials cover nearly all cases, and the coating on the disc often matters more than the base metal.
| Coating | Typical Thickness | Best For | Notes |
|---|---|---|---|
| Nickel Plating | 10–25 µm | Clean water, HVAC, general industrial | Hard, smooth sealing surface |
| Epoxy Coating | 250–300 µm | Potable water, wastewater | NSF/ANSI 61 and WRAS approved grades available |
| Nylon 11 (Rilsan) | 250–350 µm | Drinking water, abrasive media | Excellent abrasion resistance, food contact approved |
| Halar (ECTFE) | 300–500 µm | Aggressive acids and solvents | Bridges the gap to full PTFE lining |
Rule of thumb: if the seat is rubber, a nickel-plated or epoxy-coated ductile iron disc is usually enough. If the media attacks the coating, step up to solid CF8M or a PTFE-lined design.
The seat is what makes a butterfly valve "soft-seated." The rubber choice is decided almost entirely by media chemistry and temperature.
| Seat | Temp. Range | Best For | Avoid |
|---|---|---|---|
| EPDM | -30 to 120 °C | Hot/cold water, dilute acids and alkalis, ozone and UV | Oils, fuels, hydrocarbons |
| NBR | -20 to 90 °C | Oils, fuels, fats, oily wastewater, compressed air | Ozone, strong oxidizers |
| FKM (Viton) | -20 to 180 °C | Strong acids, chemicals, high temperature | Steam, ketones, amines |
| Silicone | -50 to 180 °C | Food, pharmaceutical, hot air | Abrasive media, oils |
| PTFE / RPTFE | -40 to 180 °C | Near-universal chemical resistance | Rigid seat — needs eccentric (double offset) design |
EPDM is the default for water and wastewater; NBR wins whenever oil is present; FKM is the choice for chemical dosing lines; and PTFE/RPTFE takes over for aggressive media on PTFE lined butterfly valves and double offset valves. For a deeper comparison of the two most common water-service rubbers, see our guide EPDM vs NBR Butterfly Valve Seats.
Also check how the seat is attached: bonded (vulcanized) seats give the tightest shutoff and simplest maintenance-free design; replaceable/back seats let you swap the rubber without changing the valve body on large sizes.
The stem transmits actuator torque to the disc and is the highest-stressed moving part. It must stay straight under load, resist the same media as the disc, and — critically — be retained safely inside the body.
| Stem | Grade | Best For |
|---|---|---|
| SS420 | AISI 420 | Standard water service, EPDM/NBR seated valves |
| SS431 | AISI 431 | Higher strength with moderate corrosion resistance |
| SS316 | AISI 316 | Corrosive media, food, coastal |
| 17-4PH | ASTM A564 630 | High torque and high cycle duty — double offset valves |
| Duplex 2205 | A890 4A / S31803 | Seawater, chlorides, desalination |
Every Laux soft-seated valve uses a blowout-proof stem design: the stem has a retaining shoulder that locks under the body, so line pressure can never eject it. The top flange is machined to ISO 5211 (F05/F07/F10) for direct mounting of manual gearboxes, electric and pneumatic actuators.
Stem bushings do three quiet jobs: they keep the stem concentric with the seat, they cut operating torque, and they electrically isolate the stem from the body to slow galvanic corrosion.
If a valve feels stiff after a few years in service, the bushing is usually the first suspect — which is why we recommend replaceable bushing designs on DN300 and above.
| Application | Body | Disc | Seat | Stem | Notes |
|---|---|---|---|---|---|
| Potable water | GGG40/50 epoxy | DI nickel or Nylon 11 | EPDM | SS420 | NSF/ANSI 61, WRAS certified wetted parts |
| Wastewater | GGG40/50 epoxy | DI nickel | NBR / EPDM | SS420/431 | NBR if oil or fat traces present |
| HVAC chilled/hot water | GGG40/50 epoxy | DI nickel | EPDM | SS420 | U-section valves for flangeless installation |
| Seawater / desalination | Aluminium bronze or CF8M | Aluminium bronze | EPDM | Duplex 2205 | Watch galvanic pairs at flanges |
| Chemical processing | PTFE lined | CF8M | PTFE / FKM | SS316 / 17-4PH | Verify compatibility chart per media |
| Food & beverage | CF8 / CF8M | SS316 polished | Silicone / white EPDM | SS316 | FDA and 3-A sealing materials |
| Double offset HP | CF8M | CF8M | RPTFE | 17-4PH | Zero leakage to EN 12266-1 Rate A |
Always ask for a material test certificate (EN 10204 3.1) covering the body, disc, stem and seat traceability on any project valve.
Send these seven items and a supplier can lock the right material combination immediately:
Laux Valve manufactures soft-seated butterfly valves in ductile iron and stainless steel, with EPDM, NBR, FKM and PTFE seats, nickel/epoxy/Nylon discs and blowout-proof stems — all tested to EN 12266-1 Rate A. Download the product catalogs and material datasheets, or contact our engineers with your media and pressure class for a material recommendation within 24 hours.
