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Irrigation Piping Materials: Past & Present

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The conclusion of World War II marked the introduction of new technologies to private industry that were developed for the war effort. One that would certainly revolutionize the residential/commercial marker of the irrigation industry was thermoplastic pipe.

Born in the 1940s, plastic irrigation pipe really hit stride in the `60s, replacing galvanized steel, copper water tube, asbestos-cement and cast-iron pipe, all used in early residential/commercial irrigation systems. Because of its excellent corrosive resistance, light weight, high strength-to-weight ratio, durability and resiliency, more than 100,000 miles of PVC pipe is installed each year in North America.

Although some of these early materials are not as mainstream in today's residential/commercial irrigation market, contractors are periodically confronted with their presence on an older site. It is helpful to understand the properties and performance of each.

Steel Pipe

Galvanized steel pipe is almost impossible to distinguish from wrought-iron pipe. Nevertheless, wherever you find steel pipe, you'll find corrosion and chemical deposits that can accumulate and ultimately restrict flow. As the pipe ages, the condition worsens, dramatically affecting the performance and coverage of an irrigation system.

Designers estimate that in 15 years of operation, the flow capacity in steel pipe will be diminished by as much as 50 percent. In addition, rust and scale flaking can clog irrigation nozzles, and this condition too worsens with time. As a result, maintenance problems can recur if tapping into steel pipe at the supply source, and should be considered in the design and installation of sprinkler systems.

Cast-Iron Pipe

Historically, cast-iron pipe has been used for city water mains, although asbestos-cement pipe is giving it a good run. Although cast-iron piping rusts, it does not affect the performance of the pipe, rather it reduces its flow capacity. Designers estimate that in 18 years of operation, the flow capacity of cast iron pipe is diminished by as much as 50 percent. Here, too, there is a concern with rust clogging sprinkler orifices, particularly after any kind of maintenance or repair is performed.

Installation techniques for cast-iron pipe are critical to its proper specification, as the installation depth determines the recommended maximum pressure that should be applied. Cast-iron pipe should be fully supported by the trench bottom because it is extremely brittle, and backfill should be free of rocks and debris.

Asbestos-Cement Pipe

Asbestos-cement pipe is light weight and strong, and is completely free of rust and corrosion. Besides the two obvious materials, asbestos-cement pipe is also comprised of silica. Being nonmetallic, it will not conduct electricity and resists electrolysis and galvanic action.

As with cast-iron pipe, installation of asbestos-cement pipe is important because of its extreme brittle character. The trench and the backfill material must be meticulously uniform and free of debris, particularly rocks, to avoid pipe damage. Concrete thrust blocks are required at all fittings, angles and stubs, so that internal water pressures can't pry fittings from the pipe.

Copper Water Tubing

Copper tubing is made for an abundance of purposes, but for the irrigation industry, copper water tubing is the appropriate product. Many plumbing codes require the use of copper in irrigation piping systems, and some designers still prefer copper water tubing for risers in shrub areas, as it won't rust and quickly tarnishes, thus blending with the plant material.

There are three types of copper water tube -- K,L and M. Type K is commonly used for 2-inch or less water services, like the piping connecting the city water main and yard meters, because it has the thickest wall. Type M, the thinnest-walled copper water tubing, is generally used in turf irrigation systems. All three are manufactured in rigid, straight lengths, but types K and L are also available in soft-annealed.

Although the working pressure of copper water tubing can handle most water pressures encountered in residential/commercial irrigation systems, the working pressures of normal soldered fitting joints are well below that of the tube and should be addressed at the design and installation levels.

Thermoplastic Pipe and Standards

Plastic pipe tends to be the product of choice in the residential/commercial irrigation industry for several reasons. It is relatively inexpensive, strong and durable, light weight and easy to work with, and is resistant to corrosion, rust or rot. Possibly the only advantage that metal pipe seems to have over plastic pipe is its strength, although for practical purposes, much of this strength is wasted by the lesser demands of most residential/commercial irrigation systems.

Creep and fatigue are phenomena that, through poor design or installation techniques, will damage a plastic irrigation piping system. Creep occurs when water pressure in the pipe exceeds the pressure rating. This stretching of the pipe thins the walls and, over time, can cause failure. Continual pressure surges, or water hammer, can create fatigue in pipes and fittings, which too can lead to piping system failure.

Pressure ratings, determined by the hydrostatic design stress, size and wall thickness of the pipe, represent the estimated maximum pressure that water can exert in the pipe with a "high degree of certainty" that failure will not occur. The pressure rating of plastic pipe is not the safe working pressure to which the pipe can be subjected over time.

Thermoplastic pipe for irrigation purposes is available in three product lines -- SDR-PR, PVC and PE pipe. Nomenclature visible at regular intervals along pipe sections identify specifications needed to select the appropriate material at the distributor or in the field. The piping is coded as follows: letters on the pipe identify the kind of material used, and the four digits that follow indicate the type and grade of material (the first two digits), and the hydrostatic design stress in units of 100 psi (the last two digits).

Because of the range of pressure ratings for schedule wall plastic pipe, Standard Dimension Ratio - Pressure Rated (SDR-PR) pipe was developed and has become the mainstay of the residential/commercial irrigation industry. Each series of wall thickness is based on a "standard dimension ratio" -- the ratio of pipe diameter to wall thickness.

The variety in types and grades of plastic pipe make industry standards significant. These standards serve to identify each kind of plastic pipe; dimensions; workmanship; sustained pressure; burst pressure; and more.

Poly Vinyl Chloride Pipe

Without question, poly vinyl chloride (PVC) is the most durable thermoplastic pipe made, due to its strength and its resistance to chemicals. Residential/commercial irrigation systems now often are exclusively PVC, however in some regions of the U.S., designers and contractors use asbestos-cement for larger pressure mainlines, and polyethylene (PE) for sub-mains and laterals.

Generally, when PVC is used for irrigation system piping, SDR-PR pipe is preferred, because of the uniform pressure ratings for all pipe sizes and because pressure ratings for SDR-PR are higher than those for schedule pipe in the larger sizes. There are standards for schedule 40, 80 and 120 PVC pipe.

Poly Ethylene Pipe

Poly ethylene pipe is a lower strength piping material than PVC and therefore is more prone to creep and fatigue over time. Consequently, PE has lower pressure ratings. Insert type fittings that aren't solvent welded create more friction in the system and can require more labor to install.

Special care should be exercised during installation.

Because it is so flexible, PE pipe tends to slither in the trench, which can affect sprinkler placement and alignment during installation. This flexibility also makes it more difficult to use drain valves to prevent freeze damage. An air compressor is usually required for winterization.

Poly ethylene is available in low-density (Type I), medium-density (Type II) and high-density (types III and IV) classifications. Types I and II are the most flexible and available in longer coils, making the pipe easier to lay using fewer fittings in long runs.

Poly ethylene pipe is made in two styles in both schedule-wall and SDR-PR wall pipe, SDR-PR being almost the exclusive product for residential/commercial irrigation systems. With some PE pipe, the inside diameter can be the constant, control dimension because of the use of internal insert fittings. The outside diameter of the wall increases in thickness to handle greater pressures. The second style uses the outside diameter for all sizes and the inside thickness varies.

Preferences in the use of PE and other pipe material are often found regionally and even locally. Rarely will you see anything but PVC systems in the Southwest, while you might see more PE pipe in the Rocky Mountain region and the East. Regardless, with the variety and abundance of piping materials available to the contractor, more efficient, durable residential/commercial irrigation systems are attainable.

The revolution in irrigation system piping material that has occurred over the last 50 years has essentially created the irrigation contractor and the residential/commercial irrigation market. The hydraulics of water transportation has become more precise, and properly designed irrigation systems perform better and last longer. Selecting the right material for your system's framework ensures a lasting and effective system.

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