ABSTRACT
Every fluid that passes through a plastic piping system will do one of two things: nothing, or have a deteriorating effect. In other words plastics are either affected by a chemical or they aren’t. When there is reaction, there are visible signs of incompatibility, such as swelling or stress cracking. Swelling is due to the dissolving or solvating of the chemical into the polymer. Stress cracking results from a reaction of the polymer with the chemical. Small cracks appear in areas of concentrated stress, usually in the joints. In either case, polymer degradation can take place very quickly or over a long period of time, depending upon the degree of compatibility or external energy source available to drive the reaction.If there is no reaction, then things will stay as they are until something disturbs or attacks the basic structure of the polymers. This could be induced by exposure to ultraviolet (UV) light, mechanical shock, thermal stress, or a similar reaction. However, for the vast majority of chemicals, a properly selected plastic is nonreactive for the life of the installation.Before a system can be designed, the materials of construction must be identified with clarity. There are usually only one or two best choices of plastic pipe that are suitable for a given application. Teflon resin (PFA or FIFE) pipe is certainly compatible with just about any fluid, but at what cost? The goal should be to determine the best material. This requires a thorough understanding of the forces and stresses exerted on a pipeline, the behavior of fluids, the reactions and compatibility of materials, and a basic familiarity of various methods of installation. Besides installing a system that will perform as designed, it is also the engineer’s responsibility to install a system that is the most cost-efficient. To achieve this, selection of products can only be made after performing a complete analysis of life cycle and installed cost along with product performance. Once the selection process is complete, refer to Appendix 1 for a chemical resistance table for common thermoplastic piping materials. In addition, Section III of this chapter lists common chemicals and cross-references to formal chemical names. 44
One approach to material selection has been to simply install a new pipe line the way it has always been done in the past. If 316 stainless steel worked well for system A then it should work as well or better for system B, provided system B has similar operating conditions. This “can’t go wrong” line of thinking all too often results in overdesign of systems at best, or poor design and system failure at worst. Fortunately this line of thinking is changing, due to increased awareness of materials and basic economics.The more common method of pipe selection is to narrow down a list of possible candidates based on compatibility and cost. As with any selection process, what is actually followed is a process of elimination. A formalized method of pipe selection by process of elimination is presented here. The method shown is accomplished in two steps. The first step is establishing the list of all suitable plastic piping materials based purely on a compatibility viewpoint. The second step examines this list of compatible materials from a product availability and installed cost perspective to determine the best selection(s). This elimination exercise systematically determines the ideal material(s) for a given application.The first step is to determine all possible piping materials based on an examination of the process operating conditions. The procedure listed here can be used in its present form or can be varied to suit a particular situation. The list contains what may seem rather obvious questions, but many times important conditions and considerations are overlooked. The following questions should be answered: 1. What is the fluid being transported, including the presence of any suspended solids? If possible, determine information such as the pH and percent concentration of the fluid if it is used at less than full strength. The presence of reactive by-products of the fluid (an example may be a radical hydroxyl group formed by the degradation of some strong oxidizers) and other chemicals found in trace amounts must not be overlooked.2. What are the maximum and minimum temperatures of the fluid? Use worst-case process conditions to establish temperature extremes.3. What are the maximum and minimum temperature of the ambient conditions plus that of any equipment in close proximity to the piping? Do not neglect to include the thermal gain of dark-colored pipe exposed to direct sunlight. This can run 20-30°F (10-15°C) above ambient.4. What are the mixing possibilities of two or more fluids? Exothermic or endothermic reaction must be considered with the minimum and maximum temperature. Exothermic temperature gain can be 200°F (95°C) or more.5. What are the maximum and minimum operating pressures of the
system? As with temperature, the worst-case process extremes should be used. Often, 50% pressure added to operating pressure will take in to account water hammer and shock.6. What are the types and location of valves in the system? This should be known to determine if rapidly closing valves will result in water hammer or surges. Purity considerations will also influence valve selection.7. What is the support system, or is it buried, in trench, etc.? Knowing this is necessary to determine the economics of one plastic pipe compared with another based on number and type of supports needed. Some plastics are better suited for buried installations due to their lower notch sensitivity or impact strength.8. How is it connected to tanks, pumps, and possible vibration points? Certain plastics are better able to absorb mechanical vibration and shock than others.9. Is it a drainage application or pressure system? Some plastics are not available molded in drainage patterns, so fabricated fittings may be all that are available.10. Is the fluid being transported flammable? Most building or fire codes restrict the transportation of flammable fluids in low-melt-point materials such as plastics or even copper and some other metal pipes. Clearly, the most important consideration in selecting a plastic pipe is identifying the fluid or medium being transported. If the transported medium is not compatible with the plastic then elimination is easy. However, some installations may require only short-term exposure, or economics dictate that a more expensive material is simply not justified and it is more economical to simple replace a material periodically rather than opt for more expensive piping. This may occur with valves, since valves can easily be replaced and a completely suitable metal valve may be many times the cost of a conditionally resistant plastic valve. However, this is more often the exception rather than the rule.
