Is Choosing HDPE Over Ductile to Achieve a Tighter Radius a Good Idea?
An installer may consider HDPE for Horizontal Directional Drill projects due to past experience or the potential for a tighter radius. HDPE does bend and provides a tighter radius than ductile iron, but there are drawbacks to using HDPE instead of ductile iron.
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What if the trench is 40 feet deep? Can HDPE handle that? Ductile iron pipe can. Using HDPE would require a significant increase in wall thickness due to HDPE lacking hoop strength compared to the stronger DI pipe. An increase in the DI pipe's wall thickness would also be required compared to a shallower 6-foot bury. However, the increase in thickness would be insignificant compared to HDPE, which would dramatically affect water flow.
The increased wall thickness would require a larger pipe diameter to achieve the same flow rate. This would then increase the cost of valves and fittings. Such a decision can significantly impact a project.
Both types of pipe are designed as flexible conduits. HDPE's weakness in bedding conditions is more critical than DI pipe. Proper bedding is needed to control HDPE pipe deflection. Due to DI pipes' strength, native trench conditions according to ANSI/AWWA C150/A21.50 are adequate for most applications.
HDPE is also subject to internal deflection caused by a vacuum. A catastrophic failure on the downside stream of a pipeline may create a vacuum that could collapse the HDPE pipe. The HDPE will not return to its original shape. Your 12-inch water line could become an 8-inch and be unable to perform as intended. Air release valves are often installed to prevent such a failure. Would you rather trust an air release valve or sleep well knowing you installed DI pipe?
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, various soil types, and different pressure levels?
The most common materials for manufacturing 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 that provide service to homes, offices, buildings, and businesses vary in size from as small as half an inch to 6 inches. Pipe wall thickness, a key characteristic for determining structural strength and pressure rating, is usually expressed as a ratio of wall thickness to pipe diameter. The question remains: what type of pipe material and size is best for each system?
FORCE MAINS AND WATER SUPPLY PIPES
Water distribution systems consist of either force mains or gravity sewers. Force mains rely on pressure heads induced by water pumps to generate flow, while gravity sewers rely on gravity to allow 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 pressure head from the elevation difference between the user and the community’s elevated water storage tank. Though gravity feed is used, it is not an example of gravity flow since pumps initially elevate the water into the tank. The pressure is measured in feet of head, determined by the elevation difference between the water level in the storage tank and the user's spigot. The available driving head is further reduced by in-line losses to friction, flow velocity, and minor head losses imposed by fixtures and appurtenances. The resultant head pressure within the pipe must be contained by the pipe wall itself without rupturing and by all joints and fixtures connecting pipeline segments.
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Pipes can be damaged by factors beyond internal pressure, such as water hammer. This occurs when water flow under pressure is suddenly stopped by closing a valve, or when water flow changes direction, as in a pipe bend. A strong enough water hammer can cause a pipe to break or explode. Water hammer can be minimized by ensuring pipe flow velocities are less than 5 feet per second (fps) or by installing air traps, stand pipes, air release valves, vacuum relief valves, and water hammer arresters. Reinforcing pipe bends with concrete thrust blocks or mechanical joint restraints can also mitigate the impact of water hammer. These measures prevent pipe bends from dislocating or breaking.
The potential for pipe breakage is primarily determined by the characteristics of the pipe materials and their response to applied internal and external forces. Certain pipe materials may be too brittle, chemically unsafe, or only suitable for large diameter pipes.
GRAVITY FLOW SEWERS
Gravity sewers are primary for pipelines in public spaces, carrying stormwater and sewage to treatment facilities or natural water bodies. Flows are driven by gravity along pipes installed with a sloping gradient. These networks consist of many branch pipelines feeding into a central sewer main that carries the bulk of the flow.
Sewers are designed to carry flows in an "open-channel" condition until the flow depth reaches the pipe's diameter. Sewer pipes typically have larger diameters than force mains or water supply pipelines carrying similar flows since force mains have additional pressure-driven energy. Sewers need a minimum designed flow velocity to keep them self-cleaning and prevent sediment accumulation, typically 2 to 2.5 fps.
Installation of sewer pipes often requires significant excavation depths to maintain smooth flow grades, adding to construction costs. Their depth and size make them less susceptible to vehicle impact and vibration loads but more vulnerable to earth movements causing misalignment and damage. Urban environments with existing buried utilities complicate installation, adding to costs and maintenance difficulties.
METAL PIPES
Cast Iron Pipe was the primary metal pipe for urban water main construction until the 1970s. Cast iron is still found in older urban water distribution systems. It was easy to manufacture and install but is brittle and prone to cracking. Displacement from earth movements and impacts from heavy traffic shorten its expected lifetime. Freezing temperatures and expanding ice further damage cast iron water mains.
Ductile Iron Pipe replaced cast iron pipe and is more flexible, stronger, and less brittle. It handles impacts and vibrations better and is less susceptible to freezing damage. However, both iron pipes are prone to corrosion, weakening joint connections and thinning pipe walls. To guard against corrosion, ductile iron pipes are often lined with a cement mortar coating, isolating the metal walls from the water. Its resistance to pressure and structural strength makes it ideal for water force mains.
Steel Pipe is costlier than ductile iron, but it is inherently resistant to rust and corrosion, and it is lighter and stronger. Joints are welded, ensuring overall pipeline strength. However, steel pipe is susceptible to temperature-induced strains due to its higher thermal expansion coefficient. Engineers must consider this when designing steel pipelines to prevent buckling. Despite this, its greater strength allows for larger diameter pipes and higher flow rates.
Copper Pipe connects water mains to households and businesses, continuing into the house's plumbing and fixtures. Type K copper piping, which has a thicker wall and higher pressure rating, is used for these connections. Copper is soft, easy to manipulate, and resistant to freezing. Copper lines can be thawed or kept from freezing with mild electrical currents through the conductive pipe.
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