In the early days of the GATE VALVES, the gate was parallel-sided, and was pushed into place in the main pipe line, to shut off the flow by relying on the pressure on the upstream side. The pressure of the fluid on the upstream side was relied on to create the necessary force to seal off the valve. Elaborate methods were developed in subsequent years so that the gate was pushed tight against both faces by mechanical means, making the action of the valve independent of the pressure in the pipe line. This type of construction had the disadvantage of a large number of delicate parts which were subject to attack by the fluid in the pipe line.

In the 1860's, a man named Chapman, while chopping wood one day, looked at the shape of the axe head and invented the solid wedge type gate valve as we know it today. Later, as the tendency to combine the advantages of both the solid wedge and double disk gate valve was tried, a design was evolved known as the split wedge. This type of construction has the advantage, although more expensive than the solid wedge, of being able to adapt itself to small amounts of distortion occasioned by pipe line strain. It also offers the peculiar advantage for those special installations where it is desired to seal the space between the seats with steam or inert gases.

Lately, a new variation has come on the market, the so-called "flexible wedge", wherein the wedge is in one piece but cut out between the two seats in such a way as to provide a small degree of flexibility. It offers the added advantage of relieving the valve spindle from excess stress when it is necessary to close the gate valve when subjected to extremely high temperature. In this connection it might be interesting to note that the atomic energy field again has dipped back into the 1920's for a valve construction with parallel seats, and relying on the pressure in the pipe line to keep the valve tight. This has been possible because, although the pressures encountered with pressurized water reactors are high, the temperature is less than 600