How to Select Materials for Butterfly Valves？

Principles of Material Selection: Compatibility is More Important Than Special

a.Valve Body Material: Not “The More Expensive, The Better”

• Cast Iron vs. Ductile Iron: Many assume ductile iron outperforms cast iron for all conditions. 

• However, in sulfur-bearing media (e.g., natural gas), graphite in ductile iron reacts with sulfur to form sulfides, causing hydrogen embrittlement — making gray cast iron more corrosion-resistant in such cases. Cast iron with nickel content ≥13% is recommended, as nickel inhibits sulfur corrosion.

• Stainless Steel “Model Traps”: 304 stainless steel butterfly valves work for neutral media but suffer pitting corrosion from chloride ions in chloride-bearing media (e.g., seawater). 316 stainless steel (with molybdenum) is preferred, but above 250°C, molybdenum precipitates and corrosion resistance drops. Hastelloy C276 should be used instead — a detail often overlooked in high-temperature applications.
• Plastic Body “Temperature Threshold”: PVC butterfly valves are low-cost and suitable for normal temperature and low pressure. However, above 60°C, PVC releases plasticizers, reducing body strength by over 50% and risking rupture under pressure fluctuations. Hot water systems must use CPVC (chlorinated polyvinyl chloride), with a maximum temperature resistance of 95°C.

b.Valve Seat Material: The “Last Line of Defense” for Sealing

• NBR “Oil Resistance Misconception”: Nitrile rubber (NBR) has good oil resistance but swells and fails in aromatic oils (e.g., gasoline). Fluororubber (FKM) should be used instead. A hidden risk: NBR hardens and loses elasticity below -10°C, causing cold-shrink leakage in outdoor pipelines in northern winters.

• PTFE “Filling Secrets”: Pure PTFE has poor wear resistance. In practice, it is filled with fiberglass (improved wear resistance), carbon fiber (enhanced thermal conductivity), or bronze powder (reduced friction coefficient). However, filling ratios matter: fiberglass content over 25% makes PTFE brittle and prone to cracking. Many low-cost butterfly valves use excessive filler to cut costs, shortening seal life.
• Metal Seat “Matching Principle”: Metal seats are common in high-pressure butterfly valves (PN ≥ 40). Hardness grading between the seat and disc is required: if the disc is 316 stainless steel (≈180 HB), the seat should be Hastelloy (≈230 HB). Similar hardness causes scoring and leakage due to mutual friction. This “hardness difference” rule is rarely stated in manuals but is a common field failure cause.

c.Valve Stem Material: The “Unnoticed Load Bearer”

• Chrome Plating “Thickness Rule”: Standard stems are chrome-plated for rust prevention. Plating thickness should be 0.02–0.05 mm. Excessive thickness (over 0.1 mm) reduces adhesion and causes flaking; insufficient thickness fails to form a complete protective film. In humid environments, pinholes (diameter < 0.01 mm) in the plating create galvanic cells, accelerating corrosion — a common cause of “unexplained rusting” stems.
• Duplex Steel “Application Boundary”: 2205 duplex steel stems offer strong corrosion resistance but experience metallurgical changes and reduced toughness above 300°C. Inconel 625 alloy should be used instead. Many high-temperature application users overlook this trait, leading to stem fractures after long service.