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Hard sealing butterfly valve: the sealing pair is made of metal material or harder on both sides of the material is called "hard seal." This seal sealing performance is poor, but high temperature, abrasion resistance, good mechanical properties such as: Steel + steel; steel + graphite; steel + alloy steel; (here the steel may also be cast iron, cast steel, alloy steel may also be surfacing, spraying alloy.) Soft seal: One side of the metal material, the other side is a flexible non-metallic material is called "soft seal." This seal has good sealing performance, but not high temperature, easy to wear, poor mechanical properties such as: steel + Rubber; steel + PTFE [wiki]; etc .. The advantage of soft sealing is good sealing performance, the disadvantage is easy to aging, wear and short life. Hard Seal Long service life, but the seal is relatively poor than the soft seal.