The functions of soft seal valve and the functions of hard seal valve can supple with each other. Soft seal valve has better sealing than hard seal valve. At present, the sealing of hard seal valve can meet the requirements. Soft seal valve has good sealing except for its bad ageing resistance, abrasion resistance and short service life while hard seal valve has long service life.

One of side of sealing pair is made of metal and the other side is made of flexible non-metallic materials. For that, the valve is called as soft seal valve. Soft seal valve has good sealing while poor high temperature, abrasion resistance and mechanical properties.
 
Soft seal means that one of sealing pair is made of materials having low hardness. In general, soft seal seat is made of non-metallic materials having certain intensity, hardness and high temperature resistance to ensure zero leakage. Nevertheless, service life and adaption to temperature is worse than hard seal valve. Although some manufacturers asserted that their hard seal valve could realize zero leakage, the sealing of hard seal valve is still worse than soft seal valve. However, corrosion resistance of soft seal valve to some corrosive medium cannot meet the technological requirements.
 

Both two sides of sealing pair are made of metal materials or other materials having high hardness. So the valve is called as hard seal valve. Hard seal valve have poor sealing but high temperature resistance, abrasion resistance and mechanical properties are better than soft seal valve, such as steel + steel, steel + copper, steel + graphite and steel + alloy steel. The steel can be cast iron, cast steel, alloy steel, bead welding alloy or spraying alloy.