Worldwide, countless fluid carrying systems in operation today employ highly engineered plastics to provide the essential elements of any fluid system, be it residential, retail or commercial. These elements include fittings, seals, piping and tubing—all of which today can be engineered from plastics due to considerable advances in materials technology and manufacturing processes.
Stainless steel, brass and copper components are increasingly giving way to advanced plastic resins which are lightweight, less expensive, easier to manufacture and install and free from corrosion buildup and harmful lead content. The Graduate’s Benjamin Braddock received sound advice when he was told that “there’s a great future in plastics.”
Material Technology Advances
The key component to any fluid carrying system is the fittings. Although there are many variations on fittings, both plastic and metallic, the basic function remains the same: to enable the successful transfer of fluid or gas from one point in a system to another point without leakage. Fittings have come a long way since the days of wooden joints and castings; over the years, designs and materials have advanced considerably to provide improved performance, reliability, ease of installation and safety.
The advent of engineered plastics has brought about a myriad of changes to fittings, which make manufacturing much more efficient and use by installers that much simpler. The introduction of plastics to the manufacture of fittings has greatly enhanced the water treatment and fluid systems industry. Design freedom allows OEMs to incorporate specific configurations directly into their components. Plastic fittings can be made to meet exact operating parameters, tube sizes and common space restrictions. Large multiple-impression molds enable fitting manufacturers to produce consistent products in high volume. Today’s plastic fittings are simple, reliable and very versatile in their functions.
Most importantly for fluid systems, connections made using plastic fittings will be absolutely leak-proof when installed properly.
Plastics Versus Metals
Materials commonly used for water purification and fluid handling systems include stainless steel, copper and brass. Stainless steel meets most requirements but is expensive, heavy to handle and limited to threaded or barbed connections. Copper is less expensive, except when raw material prices jump, as they did just a few years ago. Copper, too, is heavy and has to be soldered. Copper is also vulnerable to harsh or aggressive water. With the implementation of lead-free laws in California and Vermont, use of brass for wetted surfaces in plumbing and fluid systems has become problematic, and new lead-free or low-lead products are needed as replacements. Plastic fittings, of course, do not have lead issues. We can expect to see other states pass similar lead-free laws in the years ahead.
Plastics are also less expensive than metals, a good deal lighter and therefore easier to handle. A simple push-to-connect capability allows plastics to be installed in tight, confined spaces and under counters.
Acetal and polypropylene are staple plastic resins used commonly for fluid systems: acetal is known for its high-tensile strength and predictable performance; polypropylene because it is lightweight and has greater chemical resistance than acetal. Plastics are also available in a range of colors, which plays a helpful role in simple visual identification of hot and cold water lines.
The evolution of plastics has allowed significant changes in the methods used to connect tubing, pipe or hose to a fitting. Many household water purification systems are now manufactured largely with molded plastic, which includes the interconnecting tubing. In fact, plastic fittings, valves and tubing are commonly used today to connect hot and cold water sources to faucets and fixtures in homes.
Making the Connection
Since their introduction, push-in fittings have rendered realignment issues a thing of the past. This is not to say that the push-in fitting was accepted without resistance. At first, installers were rightly skeptical that something so simple in design and made of plastic would work reliably. However, as installers used push-in fittings and discovered how easy they were to work with and how reliable they were, wide acceptance and use developed.
Until the advent of plastic components for use in fluid systems more than a decade ago, most connections were limited to threaded ends, soldered joints or barbs.
Threaded fittings, also known as compression fittings, can cause plastic tubing to deform when compressing the end against a ferrule, and this made alignment difficult after removal for service. Barbed end fittings have also been commonly used with a variety of hose types; they require a relatively soft material for insertion, and, in most cases, some kind of clamp. Although adequate for certain types of systems, they may create cost and design problems for water purification systems as they require larger inside tube diameter, forcing use of correspondingly larger outside diameter (OD).
While push-in fittings require complex engineering design and precise, high-quality manufacturing, they are actually quite simple to understand and certainly to use.
They are unique in their design because they depend on just two components to work: a flexible “collet” with integral plastic or stainless steel teeth to serve as a gripper and an O-ring. The teeth in the collet are molded into flexible legs. These legs are closed together when empty and expand outward when a tube (copper or plastic) is inserted past the O-ring to an internal stop. The collet teeth grab the tube OD and hold it firmly in place. The O-ring is usually a food-grade rubber compound (EPDM). It is seated inside the fitting body and forms a firm seal on the tube OD. The fitting body can be in any number of configurations made of plastic such as tees, elbows, union and threaded connectors.
As with any product, proper installation is critical to performance and reliability. Some push-in fittings are limited for use with either plastic or copper tubing. Tubing must be cut square and free of score marks, burrs and sharp edges. More versatile designs are used with linear low-density polyethylene tubing without requiring any change to the fitting itself.
Push-in fittings can also be mated with male and female threads. Standard elbows, tees and connectors can be made to fit threaded female parts by selecting what is known as a stem adapter, which has a tube stem on one end and threading on the other.
Significantly for ease of installation and time considerations, no tools are required with push-in fittings—the only tool required is a tube cutter or plumber’s pipe cutter for copper. This makes installation easy, and makes a big difference in time spent on installation. Disconnection for service is just as fast—simply push the collet square against the fitting to free the tube.
The future of plastic fittings is limited only by the development of new resins and the ability of molding machines to process them. The “old school” reliance on metal has given way to a broad acceptance of plastic push-in fittings, which are simple to use, highly reliable over the operational life of a fluid handling system and, when installed correctly, absolutely leak-proof.