Conventional types of gate valves are generally ideal for services that require infrequent operation and where the disk is kept fully open or fully closed. They are not intended for modulating or throttling purposes. The velocity of flow against a partly opened disk may cause vibration and chattering with possible damage to the seating surfaces and throttling can subject the disk to the erosive effects of wire drawing.
The principal vanauon in the design of gate valves is in the type of sealing element employed and reasonably the valves are often described in terms of the sealing element, e.g. solid wedge, parallel slide, parallel double disk, and split wedge. There are many other types but the designs in most common use are the wedge gate valve and parallel slide valve.
Wedge Gate Valve:
The fluid controlling element in this design of gate valve is a rigid one-piece, wedge-shaped, disk containing no loose parts. The disk is accurately guided in the valve body to ensure that contact between the disk seat faces and body seat faces is limited to only a small portion of the closing and opening movements. Because of the wedge action, tight sealing can be achieved without assistance from the fluid pressure.
Parallel Slide Gate Valve:
The design feature of this valve is a parallel and flexible sealing element containing two half-disks which slide between two corresponding parallel seat faces in the body. A spring is usually contained between the two halves of the disk. This spring has three functions: to provide a flexible closure member, to hold the disk halves in sliding contact with the body seats under zero or near zero pressure so that a wiping action is obtained, and to dampen down any tendency to vibrate of the disk parts. Closure sealing is achieved by the thrust of the line pressure forcing the disk against the seating face on the downstream side of the body and the resultant contact pressure secures fluid tightness. The closure member of a parallel slide valve has a high degree of flexibility, which is particularly useful on services subject to appreciable variations of temperature. Expansions or contractions of working parts can be accommodated without affecting the operation of the valve.
Parallel Double Disk Gate Valve:
This valve has two parallel disk halves which are forced outwards against the body seat faces by means of a spreader or wedge when contact is made with a stop in the bottom of the valve body. The first opening movement releases the disk halves and continued operation raises them clear of the body seat openings.
Split Wedge Gate Valve:
Similar to the parallel double disk valve but the disk is wedge-shaped. Again a spreader device is used to force the disk halves against matching tapered seats in the valve body and the first opening movement releases the disk halves from contact with the body seats.