These KF-25 vacuum fitting VitonŽ O-ring replacements are designed for ISO-KF standards with a size of NW-25. They are commonly used to build foreline vacuum plumbing and process systems. They are often called quick flange fittings because they are quick to assemble. A circumferential clamp and centering ring form the vacuum seal between the connecting flanges (please see top right figure - click to enlarge). The centering ring contains a rubber elastomer O-ring. The standard sizes for quick flange fittings are KF-10, KF-16, KF-25, KF-40, and KF-50 with flange sizes NW-10, NW-16, NW-25, NW-40, and NW-50, respectively. They are made of VitonŽ. VitonŽ is well known for its excellent resistance to heat (400°F/200°C) and to aggressive fuels and chemicals. VitonŽ is a registered trademark of DuPont Performance Elastomers.