Plumb your plant air system with plastic pipe
By S. N. Robichaud
Ease of installation and greater economy certainly are big reasons to consider thermoplastic pipe for plumbing your compressed air system.
For years, metal piping materials -- primarily copper and black iron -- have been the overwhelming favorite for plumbing compressed-air systems. Recent advances in materials technology, however, have made thermoplastic pipe a safe and economical alternative to traditional materials.
A big advantage of metal pipe, tubing, and fittings is that installers are familiar with them and the techniques for joining them. Black iron is inexpensive, but installation is time-consuming and labor-intensive. Moreover, threaded joints often serve as a source of leakage. This leads to higher operating costs because compressors must operate overtime to compensate for the leakage. Although connections between copper pipe and fittings are less prone to leakage, copper components are more expensive, and installation, again, is labor intensive -- especially when large diameters are involved.
But these aren't the only drawbacks to metal piping systems. Interior corrosion can cause scaling and pitting on inside surfaces. As the corrosion products combine with moisture and other contaminants, they accumulate on the inner surfaces of the pipe and fittings, increasing their roughness. As the ID becomes rougher, pressure drop though the system increases. Again, this ends up costing money by reducing efficiency of the compressed air system. Perhaps more importantly, particles can dislodge and clog or damage end-of-line equipment.
The good and bad of PVC
Because of these drawbacks, compressed air system users have been seeking alternatives to traditional metal pipe and tubing. Over the past five to ten years, industrial plastics have been developed that present an attractive alternative to metal piping.
PVC piping is relatively inexpensive, easy to install, lightweight, and corrosion resistant. However, PVC has one major drawback. It is brittle. An inadvertent impact could cause the piping to shatter, endangering surrounding personnel. Most PVC pipe manufacturers warn against using PVC for compressed air service due to potential liability from such failures. The Plastic Piping Institute, in their Recommendation B, states that plastic piping used for compressed air transport in above-ground systems should be protected in shatter-proof encasements, unless otherwise recommended by the manufacturer. In many states, the Occupational Health and Safety Administration (OSHA) has stepped in and regulated against using brittle plastics such as PVC in these applications, and additional states are following suit.
The strictest standard in the country has been issued by California's OSHA. It includes five tests, as well as a requirement for comprehensive marking of the pipe and fittings. These tests include long-term hydrostatic, short-term burst, and three specialized impact tests -- all to ensure the safety and ductility of the system. The impact tests include striking frozen, pressurized pipe with both blunt and sharp strikers, using various forces, and striking a frozen pipe with a hemispherical striker, using various forces.
Manufacturers are required to present the results of these tests for review upon request. When specifying a thermoplastic system, for safety's sake it is important that your supplier meets Cal-OSHA regulations, regardless of the state in which the system will be installed.
An attractive alternative to PVC
New thermoplastic piping systems -- using high-density polyethylene (HDPE), for example -- overcome the brittleness problems associated with PVC. They efficiently and reliably deliver compressed air with lower material and installation costs and longer service life than with metal systems. They offer a margin of safety missing from PVC.
These new thermoplastics are safe because they expand at the point of failure, tearing open rather than fragmenting dangerously. They do not accumulate scale on their ID, nor does pitting or corrosion occur, and they are unaffected by synthetic and mineral oils used in compressors.
The internal surface of thermoplastic piping typically has a roughness factor, C, of about 150 to 165. Metal piping systems, on the other hand, start out with an interior surface roughness factor of about 120. This factor is inversely proportional to friction head losses: as C increases, system pressure drop over a given length at a given flow decreases. This means that when installed, the ID of thermoplastic pipe and fittings is smoother, so lower pressure drop occurs than with metal piping components. Moreover, because it is less prone to accumulating particulate contamination, and corrosion does not occur, the ID of thermoplastic piping systems remains cleaner and smoother.
The substantially rougher surface of metal piping allows contaminants to collect in the millions of tiny pits and crevices on the ID of the pipe. In addition, moisture and other contaminants can react with the metal itself and produce corrosion products that also accumulate. Over time, these contaminants and corrosion particles continue to collect and build up to form scale. As the scale builds, it roughens the ID of the pipe and fittings, which increases pressure drops. Ultimately, the higher pressure drop increases the demand on the system's compressors, which increases operating cost. Moreover, pipe scale particles can dislodge and damage equipment when carried downstream. Because thermoplastics do not promote the formation of pipe scale -- even when exposed continuously to condensation -- these problems do not exist with thermoplastic piping systems.
Thermoplastic systems also offer low cost and quick installation. Heat-fusion welding makes pipe joining quick, easy, and extremely reliable because there are no threads to leak. Unlike PVC, no glues or cements are used, so there is no cure time. Testing can be conducted immediately after installation. In contrast, some glues may require as long as 24 hours to fully cure before full system pressure can be applied. In addition, fusion welding can be performed in any environmental condition using simple, lightweight tools without prior experience. These tools are available for low-cost rental or purchase.
A cost comparison shows that black iron is less expensive than standard thermoplastics. However, in tool set up and installation comparisons the plastic system takes only half the tool set-up time and one third the time to weld each joint. Moreover, in many pipe sizes the weight of black iron is 10 times that of thermoplastic pipe, making iron components more difficult to maneuver, support, and install, especially in larger sizes.
Thermoplastics are even more cost effective when compared to copper. The price of thermoplastic pipe is less than copper, although the cost of the fittings is a little more. Overall, the total systems appear similar, but, again, the real savings appear in the installation time and maintenance of the system. Soldering of copper pipe is much more time consuming than the socket fusion method used with thermoplastics.
When done properly, soldering a joint can take from five to seven minutes by a well-trained and experienced plumber or mechanical contractor. If done improperly, the joints can leak, especially in large-diameter systems. On the other hand, unlike soldering, socket fusion can be performed in less than two minutes by anyone with minimal training, and with less chance for error. The result is a strong, leak-free joint every time.
Considerations for thermoplastics
An important consideration when designing a thermoplastic compressed air system and selecting the appropriate thermoplastic material is thermal expansion. Thermoplastics expand and contract from thermal changes more than metals do. This must be taken into account during system layout by allowing for expansion at corners or by building in expansion loops and offsets.
Another important consideration is pressure rating. Be sure to select a material and construction with an adequate margin of safety for the pressures to which your system will be exposed. In the case of a safety failure or a temperature rise, a system should still perform at the operating pressure.
Scott N. Robichaud, is assistant vice-president of engineering, Asahi/America, Inc., Malden, Mass.