GATE VALVES of various types have been in use for many years. In general, Gate Valves are so called "hard seal" valves in which the gate valve is sealed in a metal to metal or hard seal. Such hard seal valves have numerous drawbacks in use. First, in order to make a seal the seal surfaces must match perfectly. This requires very precise machining and fitting of the valve parts which is expensive. Secondly, the seal faces are readily damaged and the seal leaks. Thirdly, the seal surfaces are likely to stick because of corrosion, gum formation or any of a variety of reasons, resulting in the valve becoming inoperative or broken by attempts to release it. More recently, "soft seal" gate valves have been proposed. These valves have elastomer inserts at the sealing surfaces to make and complete the seal under pressure. These valves solve the problems discussed above in connection with hard seal gates valves, but have problems of their own. The known soft seal valves most often use double matching piece gate construction where the gate pieces are mechanically fastened around the seal member. The double piece construction is expensive to form. Moreover, it has been found that fluid leakage results around the fasteners holding the pieces. Leakage also results when the pieces are slightly misaligned due to machining and assembly errors. It is also possible for the fasteners to fail when the gate having misaligned pieces is urged into engagement with the valve body in the fluid passage. Other soft seal valves propose using a single piece gate with the seal molded onto the gate body. Bellows Seal ValvesThe single piece gate construction requires special equipment and elaborate processes to produce and as a result is expensive to manufacture.

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