Flexible Wedge Disk:
The wedge type disk can also be obtained in a flexible design, developed primarily to overcome sticking on high temperature services subject to great temperature changes. In one form the single piece construction of the disk is maintained but the two seating faces are separate from each other except for a short connecting axle or spud at the centre of the disk. In this way each disk face is permitted a certain degree of independent movement which, besides eliminating the possibility of sticking in the closed position, facilitates tightness on both the upstream and downstream sides of the disk over a wide range of pressures and temperatures.
Soft sealing is a recent development in which plastic or synthetic rubber rings are inserted into the faces of the disk or body seat rings to provide soft sealing backed up by contact of the metal to metal faces. Soft seals provide improved sealing characteristics over a very useful range of temperatures for a great variety of fluids. The resilience of the seal gives good recovery from deformation by accumulated 'dirt' and solid particles in the service fluid. Another asset is the reduction in effort required to operate the valve. For block and bleed control, sometimes known as double blocking, the use of soft seals can secure the requirement for positive tight closure on both sides of the disk with the aid of only one valve. This may avoid the possible use of two conventional valves to obtain the same results. Another development in soft sealing offering positive tight shut-off with high resilience is the wedge gate valve in which a facing of synthetic rubber is bonded to the disk casting. Both upstream and downstream faces and also the lower periphery of the disk are covered with the rubber. Sealing round the upper two-thirds of the disk is axial by wedge action while round the lower onethird of the disk the seal is achieved by radial contact between the rubber covered lower part of the disk and the base of the body.
There are three basic designs of stem arrangements Inside screw (rising stem), Inside screw (non-rising stem) & Outside screw (rising stem) The first mentioned is relatively the least expensive and is most commonly found on the smaller sizes of valves. A useful feature is that the position of the stem serves to indicate the position of the disk. Because the stem threads are inside the valve body and so open to
attack by the service fluid, inside screw valves are not usually used for fluids having corrosive or erosive properties or for high temperature services where consequent expansion and contraction may cause binding of the threads. In the case of the inside screw, non-rising stem design, the stem does not move axially but merely rotates. This arrangement is particularly useful where headroom is limited. Also, the elimination of the up and down movement of the stem reduces the amount of wear on the gland packing. On the outside screw, rising stem valve, the stem threads are situated outside the valve body and so are not subjected to possible effects of the pipeline fluid. The stem threads are accessible for lubrication and the position of the stem provides an indication of the amount of valve opening. Adequate headroom is required for the rising stem, for which some form of protection should be arranged to guard against possible damage.