Vacuum flange (NW KF CF ISO)

    The vacuum flange connection is a non-permanent sealed connection, which is characterized by maintaining the sealing performance after multiple disassembly and assembly, and it can replace the aging or damaged sealing device. The commonly used flanges in vacuum systems include rubber ring sealing flanges and metal gasket sealing flanges.

    1 KF (NW) flange

    Small diameter pipelines use rubber rings to seal vacuum flanges, called KF (or NW) flanges, and are widely used in low, medium and high vacuum systems. Its characteristics are that it can be used repeatedly, easy to operate, and economical. However, KF flanges have two major problems: one is the limited heat resistance when the equipment is baked, and the other is the air permeability of hydrogen and helium.

    When using KF flanges, a central bracket with an O-shaped rubber ring is installed between the two KF flanges, and the outer side can be easily installed and disassembled quickly through the clamp. When using fluorine rubber ring, the baking temperature can generally reach 150℃.

    2 ISO flanges are O-shaped rubber ring sealed vacuum flanges with a wide range of diameters.

    3 The inside of the CF flange has a raised sharp knife edge, which is deformed by squeezing an oxygen-free copper gasket (or other soft metal) to achieve the purpose of sealing the vacuum. It is a sealing method of all metal materials and is widely used in ultra-high In a vacuum environment. CF flanges are also often represented by ICF flanges. The number after CF is the inner diameter of the standard pipe, and the number after ICF is the outer diameter of the flange.

    CF flanges are divided into two types: fixed type and loose sleeve type. The fixed flange is an integrated structure. The loose-fit flange is composed of the blade part of the extruded copper washer and the fixed part by bolts. Through holes or threaded holes can be opened on the flange. Flanges with through holes should be fixed by bolts and nuts, and flanges with threaded holes can be fixed directly with bolts. The flange material is mainly SUS304 or SUS304L, because SUS304 emits less gas, processing and welding are easier.

    The feature of CF flange is that it does not distinguish between male and female. It is made of metal, including oxygen-free copper gasket, so it can withstand high temperature (above 250°C) baking. The disadvantage is that the price is high, and the oxygen-free copper washers cannot be reused many times.

    The installation requirements for CF flanges are relatively high. Because oxygen-free copper washers deform after being squeezed, it is very important to tighten the bolts with equal force. If the bolts are not evenly applied, the flanges at the diagonal position will be lifted up, causing air leakage. The torque to tighten the bolts is approximately as follows: M4 bolts 20kgf*cm, M6 bolts 70kgf*cm, M8 bolts 100kgf*cm.

    There are a few points to note when using CF flanges: the bolts must be coated with MoS2 to prevent the bolts and nuts from seizing and jamming, the outer circumference of the flanges must be aligned, and the flange faces must be kept a little apart to make room for the bolts to be tightened after baking. .

    In the CF flange, the knife edge must be cut into the metal sealing ring to achieve vacuum sealing. After long-term use, the knife edge will gradually become dull. In addition, the knife edge is easily injured after being hit by a hard object. One of the remedies at this time is to use IPD metal seals. IPD metal sealing ring does not rely on the knife edge of the flange, but relies on the inclined surface of the outer periphery of the knife edge to seal.

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