Given the wide variety of pipe materials available, how do engineers and contractors select the right one for their diverse project applications? What are the best materials for different systems, for various soil types, and for different pressure levels?
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The most common materials for the manufacture of water main pipes and fittings are metal (cast iron, ductile iron, steel, and copper), clay and concrete pipe (vitrified clay, reinforced concrete, and asbestos cement), and plastics (PVCs, HDPE, and fiberglass). The most common pipe diameter for water mains is 6 to 16 inches, with 8, 10, and 12 inches also being used. Branch lines providing service to individual homes, offices, buildings, and businesses vary in size from as small as half an inch in diameter up to 6 inches. Pipe wall thickness (a chief defining characteristic for determining a pipe&#;s structural strength and pressure rating) is measured differently by different types of materials, but is usually expressed as a ratio of the wall thickness to the pipe&#;s diameter. The question remains, what type of pipe material and size (or combination of several pipes in a distribution system) is best for which system? And what are those systems?
FORCE MAINS AND WATER SUPPLY PIPES
Waster distribution systems consist of either force mains or gravity sewers. The first rely on applied pressure heads induced by water pumps to generate flow in the pipes. The second rely on gravity (and the fact that the water runs downhill) to allow for water flows. Force mains tend to be smaller in diameter since the applied pressure can cause high-flow velocities even in small diameter pipes.
Water supply force mains usually get their applied pressure head directly from the elevation difference between the user and the community&#;s elevated water storage tank. Though this utilizes gravity feed, it is not an example of gravity flow since pumps were used to put the water into the elevated tank in the first place. The pressure is measured in feet of head by the elevation difference between the water level in the elevated storage tank and the spigot at the user&#;s household. With water&#;s density of 62.43 pcf, one foot of water head is equivalent to 0.43 psi. The available driving head is further reduced by in-line losses to friction (based on the roughness or smoothness of the pipe&#;s interior wall), flow velocity (based on the pipe&#;s interior diameter), and minor head losses imposed by fixtures and appurtenances (pipe bends, tees, valves, meters, flanges, etc.). The resultant head pressure within the pipe has to be contained by the pipe wall itself without rupturing or cracking and by all of the joints and fixtures connecting the pipe line segments.
Pipes can be damaged by other factors besides internal pressure. One such potential impact is water hammer. This is the shock that occurs when water flow under pressure is suddenly stopped by closing a valve, or when water flow abruptly changes direction as in a pipe bend. A strong enough water hammer can cause a pipe to break or even explode. Water hammer can be minimized by ensuring pipe flow velocities are less than 5 feet per second (fps) or by the installation of air traps, stand pipes, air release valves, vacuum relief valves, and water hammer arresters. The impact of water hammer at pipe bends can be minimized by reinforcing them with concrete thrust blocks or mechanical joint restraints (such as metal rings attached to the pipe and dead-bolted into an adjacent fixed structure). The dead weight of the blocks or tensile strength of the restraining rings will prevent the pipe bend from becoming dislocated or even broken.
The potential for pipe breakage in any pipeline is primarily a function of the material characteristics of the pipe materials and how they respond to applied internal and external forces. Certain pipe materials will be too brittle. Others will be chemically unsafe to use in water supply applications. Other pipe materials can only be effectively used as large diameter pipes.
GRAVITY FLOW SEWERS
Gravity sewers are the other primary use of pipelines in public spaces. Gravity sewers are networks of underground pipelines that carry stormwater to discharge at natural bodies of water and transmit sewage to a wastewater treatment facility (though both may use intermediate pump stations to overcome flat topography and loss of flow gradient). In both cases, flows are driven by gravity and elevational differences along the length of the pipes that have been installed with a sloping gradient. These pipe networks consist of many branch pipelines that feed into a central sewer main that carries the bulk of the accumulated flows to its final destination.
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Sewers are sized and designed to carry flows in essentially an &#;open-channel&#; flow condition, at least until the depth of flow in the pipe increases to the pipe&#;s diameter. The diameter of the sewer pipe typically exceeds the diameter of a force main or a water supply pipeline carrying the same flows since the force mains have additional energy supplied by the applied pressure. However, sewers do need a minimum designed flow velocity to ensure that it remains self-cleaning and prevents the accumulation of sediment and debris that could clog the pipe (typically 2 to 2.5 fps).
Because of the need to maintain smooth flow grades even in variable terrain, the excavation depths needed to install a sewer pipe in its trench can be significant. Given the potentially large quantities of flow that sewers must carry, their diameters must be proportionally large. Having to install them in urban environments with their potential for traffic disruption and presence of existing buried utilities adds to the difficulty of building a sewer network. Together, these factors can add up to significant construction and installation costs. Their depth and size make them less susceptible to loads from vehicle impacts and vibrations. But they are more vulnerable to damage from earth movements which misalign the pipes, causing cracks and dislocated joints. And difficulty of access can make operations and maintenance more difficult.
METAL PIPES
Cast Iron Pipe was the original metal pipe used for most urban water main construction throughout the 20th century until the s. Cast iron can still be found in the older sections of urban water distribution systems. It was relatively easy to manufacture and install. However, it is very brittle, making it prone to cracking and structural breakage. Since all urban water mains are subject to displacement from earth movements and impact loads from heavy truck traffic, the expected lifetime of a cast iron pipe is relatively short. Each applies a bending moment to the pipe length, which can cause it to crack and rupture. Additional damage occurs to cast iron water mains as a result of freezing temperatures and expanding ice within the water mains.
Ductile Iron Pipe was designed to replace cast iron pipe and has largely done so. Ductile iron pipe is more flexible, stronger, and less brittle than cast iron. Therefore, it can handle shocks from impacts and vibrations better and is less susceptible to failure from freezing conditions. However, both types of iron pipe are susceptible to corrosion over time which can weaken joint connections and effectively thin a pipe wall. To guard against corrosion, the interior walls of ductile iron pipe are often lined with a coating of applied cement mortar. This isolates the metal pipe walls from the water it is carrying. Its resistance to pressure and structural strength make it an ideal choice for water force mains.
Steel Pipe is more costly than ductile iron pipe; it is also inherently resistant to rust and corrosion and is lighter and stronger. Joints can be made by welding pipe ends together, ensuring overall pipeline strength. One problem it has is a susceptibility to temperature-induced strains. With a higher coefficient of thermal expansion, steel pipe increases more with hotter temperatures and contracts more with colder temperatures. Contractors and engineers have to take this into account when designing and installing a steel pipeline network to prevent potential buckling of the pipe lengths. However, its greater strength allows for the manufacture of larger diameter pipes capable of carrying greater flow rates.
Copper Pipe is used to make the final run from the water main to the households and businesses receiving the water. This use of copper continues on into the house with all of the household&#;s plumbing pipes and fixtures. Specifically, Type K copper piping is used for connection lines from the water main. This has a thicker pipe wall thickness and a higher pressure rating than other commercially available copper pipe (Type L and Type M). Copper is relatively soft, easy to manipulate, and forms into pipes and fixtures of variable sizes and shapes. This makes for ease of installation, simplicity of joint welding connection, and resistance to freezing. Copper lines can be thawed or prevented from freezing in the first place by the application of mild electrical current through the conductive copper pipe.
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