Thermoplastic Valves

In many pressure piping systems the most important components are the valves. Valves deserve more critical attention for several reasons. Valves can directly influence the product or process. They contain moving components that can fatigue, bind up, expand, swell, wear, or break. Valves can be exposed to sudden pressure surges or changes in direction of fluid flow. The very fact that they are handled leads them to be subjected to rough handling. Valves usually contain several close-tolerance parts and may include elastomers that may react differ­ently to the transported media. The presence of solids or suspended particles can adversely affect valve performance. Therefore, the selection of thermoplastic valves must be given careful consideration.Thermoplastic valves are not very different from metal valves in then-function and operation. The obvious difference is the materials of construction. Thermoplastics are limited to lower operating temperatures and pressures. Quite often the thermoplastic valve will have a much lower pressure rating than the piping system itself. For example, 1 in. (32 mm) PVC pipe will be pressure rated at 630 psi (43 bar), while most PVC valves of the same size are rated at 150 psi (10 bar) or a maximum of 232 psi (16 bar) for some ball valves. This is sufficient, however, to safely handle most industrial applications.Since thermoplastics do not undergo corrosion, close-tolerance components remain close-tolerance. Thermoplastic valves are often used in an otherwise all-metal system due to their long service life and generally lower cost compared to stainless and other exotic metals.In addition to corrosion resistance, the manufacturing advantage of being able to injection mold a completed product offers considerable cost savings over assembly of machined metal components. In addition, the postmolding fabrica­tion methods and field fabrication possibilities offer significant installation ad­vantages. This versatility comes at the expense of size. Thermoplastics are generally one-third to one-half larger than a comparable metal valve. However, they are proportionally lighter.This chapter addresses the most popular thermoplastic valve designs and 212

materials of construction and offers samples of most designs. It is intended to provide a general overview and examples of what thermoplastic valves are available. The samples shown by no means represent all the different styles, sizes, and designs by different manufacturers. Where possible valve actuation has been presented.