By Veso Sobot, P. E n g .
Plastics play a vital role in providing dependable piping service. In applications such as landscape irrigation systems, drain, waste, and vent (DWV) systems, and sewage transport, polymer-based products can offer myriad advantages.
Modern materials, such as polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), and high-density polyethylene (HDPE), are increasingly being specified for large-diameter, buried pipelines installed by water and wastewater utilities, as well as for smaller diameter DWV applications and cold-water delivery systems. This article examines not only the advantages and design considerations of these plastic products, but also the ways in which using plastic piping can help curb energy consumption.
The effect of leaks on energy
Leaking and broken water and wastewater infrastructure causes more than 8.3 trillion L (2.2 trillion gal) of water to be lost annually in the United States. In many distribution systems, the amount of water lost or unaccounted for can be between 20 and 50 percent.1 Beyond water loss, pipe damage, and the resulting billions of dollars in repair and replacement costs, there can also be public health issues should contaminants enter the system when pressure is reduced.2
Corrosion of pipes made of traditional materials can cause numerous water quality problems. Not only does corrosion reduce the pipe’s hydraulic carrying capacity, but the resulting deposits can also help harbor nuisance and pathogenic microorganisms. Leaching of metals can impart a metallic taste to the water and stain the plumbing, with some pipe failures prompting extended ‘Boil water’ notices.
As if these impacts were not enough, broken and decaying pipe infrastructure exacts yet another significant cost in terms of energy. Pumping water represents as much as seven percent of the nation’s total electricity consumption and accounts for 70 to 90 percent of municipal water utilities’ operating costs.3
The pumping process is energy-intensive for several reasons. Water has a density of almost 1000 kg/m3 (62.4 pcf), requiring significant work to transport. Additionally, friction from passing water through conduits results in energy loss—the higher the flow velocity, the larger the losses. This is especially important when treated water is moved over great distances at high velocities. As the population increases, aggregate demand for water is expected to rise, along with the energy needed to provide it.
Leaks increase a piping system’s energy consumption by imposing extra demands—water must be continuously pumped from the source to the leak location. To have sufficient pressure at the point of demand, upstream pressures must be increased to compensate for the pressure lost from leaks. Moreover, the water escaping from a buried water main can erode the surrounding soil and possibly damage nearby infrastructure.
Deteriorated pipes are not very hydraulically efficient, so more energy is needed to force water through their rough, interior surface, as compared to newer, smoother piping. The combined effect of poor hydraulics and leakage in older piping is a doubling or tripling of the energy needed to operate the system when compared to the same system made of new pipe. In one example, an un-rehabilitated pipe network incurred daily energy costs for pumping of $3380, while the rehabilitated version of the same system had an energy cost of $1245.4 (Savings may vary.)
Since a large portion of the energy used to pump water and drive it through the distribution systems is likely to come from fossil fuel combustion, each unit of energy consumed also entails a certain amount of greenhouse gas (GHG) emissions. In other words, leaks arguably and indirectly contribute to greenhouse gases by increasing the energy demands from the nation’s water systems.
Advancements in plastic piping
One way to help make the nation’s piping systems more energy-efficient would be to minimize the number of leaks in pipe networks. Replacing and rehabilitating current water distribution systems made of traditional materials with plastic piping products could help reduce leakage and, in turn, improve energy efficiency. The installation of high-density polyethylene (HDPE), polyvinyl chloride (PVC), and chlorinated polyvinyl chloride (CPVC) piping products is generally much easier than that of traditional piping materials. Additionally, some below-ground installations of plastic piping can be done non-invasively, further minimizing construction time and energy use.5 At the same time, technological advances have made plastic piping products more economical. These polymer-based materials are not subject to corrosion, which helps them maintain their flow capability over time and be less prone to leaks.
The durability and reliability of plastic piping, and its ability to meet stringent water quality and fire performance standards, have made it an established alternative to more traditional piping products.
Plastic piping products come in a variety of lengths, diameters, wall thicknesses, and pressure classes, along with a full complement of standard fittings, valves, and couplings. They are almost always compatible with other pipe materials and can be specified for system upgrades.
Indoors, plastic piping can offer specifiers and building owners protection against costly leakage and breaks caused by corrosion. In public utilities applications, the plastic piping’s durability is reflected in its low break rates when compared to alternatives. For example, a National Research Council (NRC) of Canada study found polyvinyl chloride water distribution pipe experienced on average 0.5 breaks per 100 km (62 mi) annually, compared to 32.6 breaks for cast iron and 7.9 breaks for ductile iron.6
Plastic piping’s smooth surface is neither electrically conductive nor affected by extremely hard or soft water, pH changes, or chemical constituents of wastewater. Plastic piping resists attack by cleaners and other household chemicals, and can withstand pressure surges, shock impact, general wear, and abrasion. Plastic piping can deliver water as clean and pure as it receives, imparts no taste or odor, helps maintain uniform water temperature, preserves its high flow efficiency, and can cost less to maintain. As it is lightweight, plastic pipe costs less to ship, and generally can be fabricated, cut, and installed more quickly than alternatives.
As with any construction material, it is advisable to follow the manufacturers’ specifications when installing a plastic piping system (and to select a location protected from sharp objects, rough handling, and high heat sources). This allows the materials to be used in the most energy-efficient manner, regardless of whether it is a residential project or a utilities application.
Short-term exposure to sunlight during installation is typically not a problem for polyvinyl chloride pipe due to ultraviolet (UV) inhibitors added to the material. polyvinyl chloride piping may also be used in outdoor applications when painted with a light-colored, water-based acrylic or latex paint chemically compatible with the plastic. The manufacturer may also recommend another type of coating or protective device, depending on the application.
Sounds produced by water running through pipes in the walls is a common concern for the design team in residential and commercial projects. The physics of water flowing through pipe should be considered to reduce the effect of noise on building occupants. Acoustical isolation issues should also be investigated and addressed at the earliest stages of project design.
In plastic piping, vibration caused by the flow of water can be managed by appropriate de-coupling to isolate the pipe from contact with structural elements, or by specifying thermal or rubber isolation for sound/vibrational absorption.7 ‘Water hammer’—the concussion of moving water against the sides of a pipe—is another inherent characteristic of supply systems. Its intensity in plastic piping is approximately one third its intensity in traditional materials. Nevertheless, water hammer can be further reduced by designing for a maximum flow rate of less than 1.5 m (5 ft) per second in pipe diameters of 32 mm (1.25 in.) or larger, and less than 2.4 m (8 ft) per second for diameters of less than 25 mm (1 in.).
Other ways to help minimize the problem include following these design and specification practices:
- avoid critical areas of the structure (e.g. those where differential movement is expected or where piping would have close contact with the wall material);
- use a pipe chase or cavity wall of adequate thickness;
- support the pipe properly away from wall material;
- use long-radius fittings to reduce turbulence; and
- wrap piping with sound-deadening material, or pack the wall cavity with insulation material.
Plastic piping has a greater co-efficient of thermal expansion than traditional materials. This means there is movement of 85.3 mm (3.36 in.) for every 30.5 m (100 ft) of pipe per each 56-C (100-F) change in temperature. Most polyvinyl chloride applications are selected for environments with minimal temperature changes, such as in soil or in air-conditioned buildings. Even in the case of considerable temperature fluctuations, most installations involve relatively short plastic pipe segments where dimensional change is not great.
Where necessary, expansion and contraction can be accommodated by piping offsets or expansion joints, by snaking the line or making similar provisions at changes in direction, or by suspending the pipe and avoiding contact with the building structure.
With proper installation, plastic piping can not only provide several advantages to building owners, but can also help curb the nation’s energy consumption.
1 See D. Brailey and A. Jacobs’s “Energy management in the waterworks industry,” Journal of the New England Water Works Association (94:3).
2 See Richard Gillick et al’s “Occurrence of Transient Low and Negative Pressures in Distribution Systems,” Journal of the American Water Works Association (November 2004).
3 For more information, see the National Research Council (NRC) of Canada’s “Detecting Leaks in Water-distribution Pipes,” Construction Technology Update (No. 40, October 2000).
4 See A Brief Report on Pipe Deterioration Focusing on Leaks, Friction, Energy Use, and Greenhouse Gas Emissions, by Andrew F. Colombo and Bryan W. Karney (University of Toronto’s Department of Civil Engineering, June 2003).
5 For more on the subject of plastic piping rehabilitation projects, see “High-Density Polyethylene Solutions for Water Infrastructure Rehabilitation” by Camille George Rubeiz, PE, in the May 2005 issue of Modern Materials. For more on residential retrofit, see also “Pipeline to Successful Renovations” by Janet Arden in the November 2005 issue of Modern Materials.
6 See B. Rajani and S. McDonald’s “Water Main Break Data for Different Pipe Materials for 1992 and 1993” (National Research Council Canada, 1995).
7 To determine acceptable levels of sound and vibration, one should consult the manufacturer’s data or the American Society of Heating, Refrigerating, and Air-conditioning Engineers’ 2003 ASHRAE Handbook, HVAC Applications (Ch. A47: Sound and Vibration Control).