Until recently, process cooling applications carrying cooling liquids have been dominated by three primary materials: polyvinyl chloride (PVC), type 304 or type 316 stainless steel, and carbon steel.1 Each of these materials offers specific advantages with regard to cost, corrosion resistance, strength or reduced long-term maintenance requirements; however, none of these materials possesses all of these benefits simultaneously.
There is another material that exhibits all of these characteristics, but it has gone relatively unnoticed within the process cooling industry. High-density polyethylene (HDPE) has a low installed material and labor cost, does not corrode, has excellent mechanical properties and requires little maintenance. So why hasn’t HDPE caught on as a piping material?
Piping BackgroundTo understand the relatively slow progression of HDPE in process cooling applications, we first need to consider the reasons for the popularity of other piping materials. For many years, PVC has been considered an inexpensive piping option for some process cooling applications. One of the top three thermoplastics produced globally (the other two are polyethylene and polypropylene),2PVC offers attractive benefits such as low cost, midrange chemical resistance and ease of installation (using solvent cement). However, it also has some limitations - including an industry accepted minimum temperature rating of 32°F (0°C). 3Even when produced with additives, PVC piping systems are extremely notch-sensitive and often are considered structurally compromised when operating within the average 0°F to 60°F (-18 to 16°C) temperature range for process cooling applications. Regardless of the known failure modes of PVC, many system designers and original equipment manufacturers have implemented PVC piping systems at temperatures approaching 32°F and lower in an effort to reduce installed system costs. Not surprisingly, such installations have resulted in some system failures.
For this reason, many engineers who specialize in process cooling application have opted to avoid thermoplastics in favor of stainless and carbon steel. Within the process cooling community, stainless steel has long been considered the best-performing solution, albeit a rather costly one. Much of the upfront cost is due to the cost of materials (pipes, fittings and valves) and the somewhat specialized installation labor required to weld a metal piping system. There is also an increased infrastructure cost due to the extensive structure required to support the system in place. And while the robust nature of stainless steel can offer some long-term cost advantages, it should be noted that stainless steel is not corrosion-proof but rather corrosion resistant. Adding exterior corrosion-resistant coatings and thermal insulation can increase the cost of stainless steel piping systems, and long-term interior pitting by erosion is still possible. Such pitting can cause poor system performance and, in some cases, premature piping system failure.
Carbon steel generally is considered to be a robust and less expensive alternative to stainless steel within the process cooling industry. However, carbon steel has short- and long-term corrosion issues that must be taken into account along with the potential need for an insulating jacket to reduce the possibility of condensation and ice buildup. In most cases, using carbon steel for a process cooling application requires the addition of corrosion inhibitors, which can increase operation costs and have an adverse environmental impact should a leak occur. And even with these corrosion inhibitors, some form of internal and external corrosion inevitably will occur within the piping system over time, possibly causing internal pitting and incrustation of the metal. As with stainless steel, these factors will contribute to poor system performance, increased wear on secondary systems (pumps, heat exchangers) and an increased potential for equipment failure and excessive long-term maintenance costs.
HDPEAlthough commonly used in the utility service sector to convey bulk water and natural gas, HDPE has long been bypassed as an industrial piping solution for process cooling applications in North America. When it has been used in industrial plants, HDPE usually has played a secondary role providing makeup water service or waste drain plumbing within a given industry, many times taking a backseat to PVC, stainless steel and carbon steel for the actual plant process piping systems (process cooling, chemical and water distribution).
Yet HDPE presents a good all-around solution for mid- to low-temperature applications. To contemplate HDPE as a modern industrial piping system for process cooling, we must consider the overall mechanical strength characteristics, welding technologies and short- and long-term cost considerations associated with HDPE.
As seen in figure 1, HDPE is a highly ductile material, which allows it to absorb adverse structural loads, such as surge pressure impact from opening and closing of control valves or unforeseen seismic loading. Under such conditions, HDPE retains its structural integrity - unlike PVC, which is highly strain-sensitive and has a tendency to develop stress cracking under increased load conditions, possibility resulting in system failure. HDPE has the highest impact resistance of the most common thermoplastics and retains this structural integrity through its entire temperature range. It has low-temperature impact resistance similar to that of ABS (approximately 5 ft.-lb./in. of notch at -40°F/C as determined through Izod impact strength testing), and it also retains 50 percent of its 73°F (23°C) strength at its upper-temperature limit of 140°F (60°C). In comparison, PVC loses 80 percent of its 73°F strength at 140°F.4
Unlike metallic piping systems, HDPE does not scale, pit or corrode. Aided by poor thermal conductivity, HDPE requires little or no insulation in medium- and low-temperature applications, which further reduces the installed cost and long-term maintenance of this piping option. HDPE also benefits from excellent UV resistance, which allows outdoor use in direct sunlight without the need for external coatings. With its excellent chemical resistance properties, HDPE can transport strong acids, caustics and other highly corrosive chemicals. HDPE pipe possesses an ultra-smooth interior wall that increases hydraulic efficiency and will not degrade over its operational design life of 25 to 50 years continuous service, as with other thermoplastics such as PVC and metallic piping systems.5This benefit translates into better performance, less stress on secondary equipment like pumps and heat exchangers, and a significant reduction in long-term costs and maintenance compared to alternative materials of construction.
It should be noted that when temperature fluctuates, the same physiological properties that make HDPE such a robust piping system also promote greater expansion/contraction than materials such as stainless and carbon steel. However, when this effect is properly considered during the engineering phase, it can be resolved easily through the use of expansion loops or offset bends and natural changes of direction in the piping system.
HDPE also makes used of advanced fusion welding technologies. Heat fusion as a piping system assembly method creates a joint that is as strong as or stronger than the pipe itself.
Cost ConsiderationsShort-term cost benefits include material costs, installation labor costs and mechanical infrastructure costs. In general, HDPE pipe and fittings cost slightly less than carbon steel and significantly less than type 304 and 316 stainless steel. However, the price relationship between schedule 80 PVC and HDPE is not quite so clear cut. Schedule 80 PVC has a slight material component price advantage up through 6" IPS, however, HDPE has a slight cost advantage from 8" IPS to 10" IPS and an above 10" IPS.
Labor, by far, is the most difficult cost to quantify, due in part to standard labor rates as outlined by the Mechanical Contractors Association of America’s (MCAA) Labor Estimating Handbook and the perceived costs by contractors. However, using the MCAA labor rates, HDPE labor per joint (heat fusion) is less expensive than both welded carbon and stainless steel.
Infrastructure is the last consideration of the short-term cost components. While HDPE does requires more support per linear foot than metallic piping systems, the significant weight reduction of a thermoplastic piping system potentially translates into a reduction in the total mechanical piping load and ultimately a reduction in the sizing of the required support structure.
The Bottom LineApplying a safe, reliable and cost effective piping solution to a process cooling application should not require a compromise between safety and security and cost control. Nor should it require choosing corrosion resistance over reduced service life. HDPE provides all the benefits - high strength, safety, reliability, cost effectiveness and corrosion resistance - in a robust plastic piping system. As these advantages become more widely understood, this alternative piping material is almost certain to gain new ground in process cooling.
References1. Puckorius, Paul, “Cooling Water Treatment vs. Cooling Systems, Part 4,”Process Cooling, July 2002 (www.process-cooling.com/Articles/Water_Works/ff8d8dc32a5b7010VgnVCM100000f932a8c0____).
2. ACC Resin Production Sales Stats, Nov. 2009, Released Jan. 12, 2010 (www.americanchemistry.com/s_acc/sec_news_article.asp?CID=206&DID=10599).
3. Georg Fischer LLC, Plastic Piping Systems - Product Guide, #1250 (8/09).
4. PVC Pipe – Design and Installation,AWWA Manual M-23, American Water Works Association, Denver, 2004.
5.Handbook of Polyethylene Pipe, First Edition (2006) – Plastic Pipe Institute, Washington, D.C.