High-Performance Butterfly Valve — Double Offset, CF8M SS316 Body and Disc, TFM1600 Seat, API 609 ANSI 150

The high-performance double offset butterfly valve is technically different from all conventional catalog butterfly valves in one fundamental aspect: thanks to its double offset geometry, the disc immediately separates from the seat at the start of opening and does not touch the seat again until the last moment of closing — eliminating the continuous sliding between disc and seat that is the main cause of wear in conventional center-lined butterfly valves. The result is a valve with significantly longer seat life, lower operating torque in the intermediate position, and more precise sealing in the closed position.

The CF8M (SS316) body and disc, TFM1600 (high-density modified PTFE) seat, and 17-4 PH (precipitation-hardened stainless steel) stem constitute the highest-performing material combination in the Cematic butterfly valve family. Designed according to API 609 — the most demanding standard for industrial butterfly valves — for chemical, petrochemical, Oil & Gas, and process fluid services where standard butterfly valves are insufficient. 2" to 10" diameters in stock; up to 18" available by special order.

Double Offset — why it is different from conventional butterfly valves

In a conventional center-lined butterfly valve (zero offset), the axis of rotation passes exactly through the center of the disc. When opening and closing, the disc edge continuously slides over the entire perimeter of the seat during the full 90° of travel. This continuous sliding:

• Generates friction and wear with each opening and closing cycle
• Requires higher operating torque throughout the travel
• Wears the seat uniformly around the entire perimeter over time

In a double offset butterfly valve, the axis of rotation is offset in two directions relative to the center of the disc:

• First offset (axial): The axis is offset laterally relative to the plane of the seat — when rotating, the disc does not describe a perfect circle but an elliptical path that causes it to lift off the seat immediately at the start of opening.
• Second offset (radial): The axis is also offset perpendicular to the pipeline axis — this reinforces the seat-lifting effect and improves force distribution during closing.

The practical result: the disc only touches the seat at the final instant of closing — not during the travel. Wear is concentrated at that single moment of final contact, instead of being distributed along the 90° of travel as in a conventional butterfly valve. The seat's service life increases proportionally.

The TFM1600 seat — high-density modified PTFE

TFM1600 (also called modified PTFE or high-density PTFE) is a fluoropolymer superior to conventional PTFE in the most critical properties for high-performance valve seats:

• Higher density and lower permeability: TFM1600 has a denser molecular structure than conventional PTFE — lower permeability to gases and fluids, which improves seat sealing, especially in low differential pressure gas service where standard PTFE can experience micro-leakage.
• Lower cold flow (creep) tendency: Conventional PTFE tends to permanently deform under load at temperature — a phenomenon called cold flow or creep. TFM1600 has significantly lower creep tendency, maintaining sealing force for longer and over more operating cycles.
• Higher resistance to fine abrasion: With metal backing in double offset designs, TFM1600 better resists abrasion from fine suspended particles than a soft elastomer (EPDM, Viton) — which wears out more quickly due to particle abrasion.
• Practically universal chemical resistance: TFM1600 inherits the chemical compatibility of PTFE — resistant to acids, alkalis, solvents, hydrocarbons, and virtually any industrial fluid at temperatures up to 180 °C.
• Replaceable seat: The TFM1600 seat can be replaced in the field when worn — without replacing the entire valve.

The 17-4 PH stem — the highest mechanical strength in its class

The 17-4 PH stainless steel stem (Grade UNS S17400, also known as 630) is a precipitation-hardened stainless steel — a type of stainless steel that combines the mechanical strength of tool steel with the corrosion resistance of stainless steel. Its properties:

• Tensile strength: 1,000–1,100 MPa in H900 condition — three times higher than standard SS316 (515 MPa)
• Corrosion resistance: Similar to SS304 in non-chlorinated environments — sufficient for the process fluids where this valve is specified
• Fatigue resistance: The higher mechanical strength of 17-4 PH provides greater resistance to fatigue from repeated cycles — critical in high-performance valves with frequent opening and closing cycles
• Why not SS316 or SS420: In high-performance double offset valves, the stem is subjected to greater torsional and bending stresses than in a conventional center-lined butterfly valve. 17-4 PH provides the necessary mechanical safety margin for these stresses without increasing the stem diameter — maintaining the valve's compact design.

Conventional vs. Double Offset Butterfly Valve — when to specify each

Construction Materials

Technical Specifications

• Type: Double offset wafer butterfly valve (high performance butterfly valve)
• Design standard: API 609
• Face-to-face: API 609
• Actuator mounting: ISO 5211
• Flange: ANSI B16.5 Class 150
• Inspection and testing: API 598
• Body material: CF8M (SS316)
• Disc: CF8M (SS316)
• Seat: TFM1600 (modified PTFE) — replaceable
• Stem: 17-4 PH (UNS S17400)
• Diameters in stock: 2" (DN50), 2½" (DN65), 3" (DN80), 4" (DN100), 5" (DN125), 6" (DN150), 8" (DN200), 10" (DN250)
• Diameters by special order: 12" (DN300), 14" (DN350), 16" (DN400), 18" (DN450)
• Nominal pressure: ANSI 150 lb
• Maximum temperature: ≤180 °C
• Compatibility: Chemicals and corrosive fluids under moderate conditions

Dimension table with ISO 5211 mounting

Fluid compatibility — CF8M + TFM1600

• ✅ Diluted acids — H₂SO₄ up to 30%, HCl up to 20%, HNO₃ up to 25%
• ✅ Alkalis — NaOH up to 40%, KOH up to 30%
• ✅ Liquid and gaseous hydrocarbons — gasoline, diesel, LPG, natural gas
• ✅ Organic solvents at moderate concentration
• ✅ Seawater and saline solutions up to moderate temperatures
• ✅ Process steam up to 180 °C
• ✅ Petrochemical process fluids and process gases
• ✅ Natural gas with H₂S and wet CO₂
• ⚠️ Concentrated acids (H₂SO₄ >50%, HF): CF8M body may be insufficient — consult us
• ❌ HF, concentrated ClO₂: use PFA anti-corrosion valve

Automation — ISO 5211, reduced operating torque

The double offset significantly reduces the operating torque in the intermediate position (10°–80°) compared to a conventional butterfly valve — the disc is separated from the seat during almost the entire travel. The maximum torque is concentrated in the last degrees of closing. This feature allows for:

• Lower torque actuators in intermediate diameters: For 4" to 8", the operating torque of the double offset valve is lower than that of the equivalent conventional butterfly valve — verify with Cematic the correct torque for each diameter/pressure combination
• Higher control precision: Constant and low torque during travel improves positioner response and positioning accuracy in modulating control
• Pneumatic actuator single or double acting: See butterfly valves with pneumatic actuator →
• Electric actuator ON/OFF or modulating: See butterfly valves with electric actuator →

Main Applications

• Chemical industry — corrosive fluids with high cycle frequency: The double offset is the first choice when the butterfly valve operates more than 10,000 cycles per year in corrosive fluids. In inorganic chemistry, organic synthesis, and reagent handling, conventional butterfly valves require more frequent seat replacement — the double offset significantly reduces this maintenance frequency.
• Petrochemical and refineries — hydrocarbon service at temperature: Piping classes in refineries and petrochemical plants frequently specify API 609 for butterfly valves in hydrocarbon service. TFM1600 resists hydrocarbons and their derivatives without the swelling that EPDM or NBR would experience.
• Oil & Gas — natural gas with H₂S and CO₂: Wet natural gas with dissolved H₂S and CO₂ is incompatible with standard elastomers in the long term. TFM1600 is not attacked by these components, and the CF8M body resists typical process gas in production and transport facilities.
• Power generation — steam and high-temperature fluids: Process steam up to 180 °C, heating fluids, and process fluids in power plants where conventional butterfly valves with EPDM are no longer viable due to temperature. TFM1600 maintains its sealing properties up to 180 °C without the progressive degradation of EPDM above 120 °C.
• Solar thermal and geothermal energy: Concentrated solar power (CSP) installations with thermal oil at 150–300 °C and geothermal plants with corrosive process fluids at high temperatures. CF8M + TFM1600 covers the operating range of these systems in standard thermal fluid distribution diameters.
• Pulp and paper — process chemicals: Bleaching lines with diluted ClO₂, NaOH, diluted H₂SO₄, and other chemicals in the kraft process where conventional butterfly valves require frequent seat replacements. The double offset in CF8M + TFM1600 significantly extends the maintenance interval in these services.

For the standard iron butterfly valve with EPDM/NBR/Viton seat, see iron wafer butterfly valve with SS316 disc →

TFM1600 Datasheet

RPTFE Datasheet