Design Considerations

Section Contents:

Product Characteristics
Technical Data
Installation
Cost
Maintenance Requirements

Product characteristics. The choice of piping system materials should be based on overall long-term value – including long-term costs, reliability, versatility, environmental impact and impact on drinking water safety and public health. PVC piping systems deliver value because of their high quality, corrosion-resistant, durable and environmentally sound characteristics.

Durability. PVC pipe is engineered and tested tough. It provides proven resistance to costly leakage. It offers specifiers and building owners and managers unmatched corrosion resistance and reliability. PVC pipe is not electrically conductive and is unaffected by excessively hard or soft water, changes in pH and the chemical constituents found in both domestic and industrial wastewater. In the household, PVC piping resists attack by cleaners and other household chemicals. Because PVC withstands conditions that other pipe materials cannot, it frequently is selected in place of, or to replace, other pipe materials.

Reliability. PVC pipe resists scale buildup, pitting and tuberculation. Its internal surface remains smooth and deposit-free with corrosive or hard water. As a result, PVC pipe resists bacteria buildup and maintains high flow efficiency which reduces pumping requirements and virtually eliminates the need for disassembly or cleaning for the life of the system. PVC pipe is also flexible. It can bend without breaking. Both pipe and joint assemblies withstand pressure surges and shock. PVC pipe is resistant to impact, general wear and abrasion, providing specifiers and, ultimately, building owners and managers with reliable service and less costly maintenance.

Water quality. Increasing public attention to water quality and health are additional factors favoring PVC use in piping applications. PVC pipe delivers water as clean and pure as it receives. It imparts no taste or odor to the water it transports. And PVC pipe helps to maintain uniform water temperature due to its low thermal conductivity. PVC pipe has been tested against the ANSI/NSF 61 standard and other health effect and drinking water standards for more than 35 years and has consistently performed better than the standards require. Meanwhile, in recent years, competitive materials have come under scrutiny for possible health effects from leaching heavy metals into drinking water.

Fire performance. While some construction materials ignite easily and burn rapidly, PVC does not. PVC piping products meet or exceed all applicable building code requirements for installation in fire-resistive construction. The hazards to persons or property in a fire from PVC pipe are no greater than the hazards posed by other piping materials. That’s because PVC and all other types of piping are typically installed within building walls and are not directly exposed to fires when they occur.
Technical data.

Drain, waste and vent systems. PVC pipe can be used in residential, commercial and industrial drain, waste and vent systems. Pipe and fittings are joined by solvent cementing and are available in sizes 1.25" through 12" in diameter. Here are some of the things that specifiers in particular need to keep in mind.

Grades. PVC pipe identified as Schedule 40 DWV pipe, or informally as Schedule 40 pipe, is used for both above-grade and below-grade installations. A special 3.25-inch outside diameter pipe, which has a thinner wall than Schedule 40 DWV, is referred to informally as Schedule 30 pipe. This pipe is intended to fit within a 2 x 4 inch wide stud wall assembly.

Limitations. While PVC pipe is resistant to chemicals, specific analysis should be performed before the pipe is specified for any special waste system. Plastics are affected by ultraviolet radiation. Pigments are added to the PVC to make pipe and fittings resistant to degradation. PVC can be exposed to sunlight during construction, but prolonged exposure to the sun is not advised. PVC pipe and fittings can withstand normal temperatures encountered in a sanitary and storm drainage system. The recommended maximum temperature for continuous drainage applications is 140 F.

Laying lengths. PVC pipe is available in 10-foot and 20-foot lengths. Laying lengths for common fittings are identified in Table 1 and Table 2.

Expansion and contraction. PVC pipe has a higher expansion and contraction rate than metallic pipe. The rate translates into an expansion of 3.36 inches for every 100 feet with a temperature change of 100 F. Where PVC pipe is subjected to severe temperature fluctuations, provisions must be made for expansion and contraction. This can be accomplished by use of expansion joints or offset piping arrangements.
 
Hangers and supports. Horizontal PVC pipe must be supported at maximum 4-foot intervals. Contractors should support vertical piping at every normal floor level. At midpoints, they should provide intermediate guide supports and install hangers and supports to allow for thermal expansion and contraction.

Hot and cold water distribution systems. CPVC pipe can be used in residential, commercial and industrial hot and cold water distribution systems. Pipe and fittings are joined by solvent cementing. A full complement of fittings and valves is available for potable water applications. Pipe and fittings are available in CTS (Copper Tube Size) ½ through 2 inches and in IPS (Iron Pipe Size) ¼ through 12 inches. Connections between CPVC pipe and other piping materials are accomplished by use of adapter fittings. Adapter fittings connect by threaded joints, compression connections, flanges or mechanical connections.

Grades. CPVC pipe is available in wall thicknesses of either SDR 11 (Copper Tube Size), SDR-PR, Schedule 40 or Schedule 80. CTS pipe has the same outside diameter as copper tubing. CTS is available in sizes ½ through 2 inches. For installations larger than 2 inches, Schedule 40, Schedule 80 and SDR-PR pipe are installed in sizes up to 12 inches. SDR-PR, Schedule 40 and Schedule 80 pipe have outside diameters that correspond to Schedule 40 steel pipe.

Limitations. CPVC pipe and fittings are pressure rated for continuous use at 400 psi at 73.4 F and 100 psi at 180 F. As temperature increases, pressure rating decreases. Under no circumstances should temperature exceed 212 F. Plastics are affected by ultraviolet radiation. Pigments are added to the CPVC to make pipe and fittings resistant to degradation. CPVC can be exposed to sunlight during construction, but prolonged exposure to the sun is not advised unless protected by a water-based latex paint.

Flow characteristics. CPVC pipe does not corrode or accumulate mineral deposits. It should be designed with a water flow rate between 5 and 12 feet per second. (See Table 3.) Friction loss for flow through fittings is based on equivalent lengths of pipe. Equivalent lengths of pipe for CTS fittings ½ through 2 inches are shown in Table 4.

Water hammer. Intensity of water hammer in CPVC pipe is approximately one-third the intensity of copper or steel pipe.

Laying lengths. CPVC pipe is available in 10-foot and 20-foot lengths. Common laying lengths for CPVC SDR 11 (CTS) fittings are identified in Table 5.

Expansion and contraction. CPVC pipe expands and contracts more than metallic pipe. The rate translates into an expansion of 4.08 inches for every 100 feet with a temperature change of 100 F. Where CPVC pipe is installed in long, straight lengths, compensation for expansion and contraction must be provided. This can be accomplished with the use of offset piping arrangements such as loops or bends.

Hangers and supports. CPVC pipe must be horizontally supported at 3-foot intervals for pipe 1 inch and less in diameter and 4-foot intervals for pipe 1¼ inches and larger in diameter. Vertical pipe should be supported at each floor level with mid-story guides. Contractors should install hangers and supports to allow for thermal expansion and contraction.

Water service systems. PVC pipe can be used in residential, commercial and industrial water service systems. Pipe and fittings are joined by gaskets or solvent cementing. A full complement of PVC fittings and valves is available for potable water applications. Pipe and fittings are available in sizes ½ through 24 inches. Connections between PVC pipe and other piping materials are accomplished by the use of adapter fittings. Adapter fittings connect by threaded joints or compression connections.

Grades. PVC pipe is available in a variety of wall thicknesses, including Schedule 40, Schedule 80, Schedule 120, SDR 13.5, SDR 17, SDR 21, SDR 26, SDR 32.5, SDR 41, SDR 64, DR 14, DR 18 or DR 25. Schedule 40, Schedule 80 and Schedule 120 PVC pipes have outside diameters that correspond to Schedule 40 steel pipe. SDR and DR pipe have outside diameters that correspond to the outside diameter of either steel pipe or cast iron pipe.

Limitations. PVC pipe and fittings are pressure rated at 73.4 F. These ratings decrease as temperature increases. Plastics are affected by ultraviolet radiation. Pigments are added to PVC to make pipe and fittings resistant to degradation. Pipe and fittings may be exposed to sunlight during construction, but prolonged exposure is not advised.

Temperature rating. PVC pipe and fittings are pressure rated for various temperatures. Plumbing codes stipulate a minimum pressure rating of 160 psi at 73.4 F. PVC pipe that can meet the plumbing code requirements of 160 psi at 73.4 F have a wall thickness of Schedule 40, Schedule 80, Schedule 120, SDR 13.5, SDR 17, SDR 21, SDR 26 or DR 14.

Flow characteristics. PVC pipe should be designed with a water flow rate of between 5 and 8 feet per second. Friction loss for flow through fittings is based on equivalent lengths of pipe. Equivalent lengths of pipe for fittings ½ through 8 inches are shown in Table 6. Intensity of the water hammer in PVC pipe is approximately one-third the intensity of water hammer in copper or steel pipe. To reduce the impact of the water hammer, velocity of flow should be controlled. Piping should be sized to maintain a velocity of less than 5 feet per second in pipe sizes less than 1 inch and less than 8 feet per second in diameters of 1¼ inches or larger.
 
Laying lengths. PVC pipe is available in 10-foot and 20-foot lengths.

Trenching and bedding. PVC pipe should be installed underground in accordance with ASTM D 2774. The pipe trench should be as narrow as possible but of sufficient width to install the pipe. PVC pipe must be installed below the recorded frost level. When rock, hard pan, boulders or other material that can damage the PVC pipe are encountered, the trench should be over excavated a minimum of 4 inches for bedding. Bedding on the bottom of the trench may consist of evenly graded, free flowing granular material free from stones or rocks greater than ¾ inch in diameter.

Backfill material less than ½ inch in diameter should be placed in 6-inch layers around the pipe, with each layer tamped in place. Final compaction is recommended to be 85 percent or greater.

Expansion and contraction. PVC pipe expands and contracts more than metallic pipe. The rate translates into an expansion of 3.36 inches for every 100 feet with a temperature change of 100 F. Where small diameter PVC pipe is installed in long, straight lengths with solvent cemented joints, compensation for expansion and contraction must be provided. This can be accomplished by snaking the pipe in the trench. Pipe flexibility allows for compensation of thermal expansion.
 
Installation. Cutting, joining and installing are simpler for PVC pipe than for other materials. For example, a normal length of PVC drain, waste and vent pipe can be carried by one worker, whereas two workers or a machine would be required to move a similar section of heavier metal piping. The benefits of PVC’s light weight are especially evident to the installer working in tight places. Additionally, small diameter PVC pipe is easily and quickly assembled without the need for soldering, a process that requires the use of a torch, a recognized source of fires and worker injuries.

PVC pipe must be cut square with a wheeled tubing cutter or saw designed for that use. Pipe ends must be deburred and wiped clean and dry, free of oil or dirt.

Solvent cementing of PVC or CPVC pipe and fittings is a multi-step process. Contractors should follow the procedure or instructions for application and use of the PVC or CPVC solvent cement found on the container label. The joint is made while the solvent cement is still wet by inserting the pipe into the fitting socket and rotating one-eight to one-fourth turn. Such joining of PVC or CPVC pipe and fittings should be performed in well-ventilated locations. Contact of primer and solvent cement with the skin should be avoided. Eye protection is recommended during solvent cementing.

When threaded adapter fittings are installed, joint tape or pipe dope specifically approved for use with PVC or CPVC pipe should be applied to male threads.

Cost. PVC piping is attractive from a cost standpoint not only because its initial cost can be less than the cost of other materials but also because its light weight means reduced shipping costs as well as faster, safer and easier handling, cutting and installation. The system’s ease of assembly means reduced equipment and labor requirements and reduced injuries and accidents for installation crews. Additionally, since PVC pipe won’t rust, scale, pit or corrode, it lasts longer and requires less maintenance than competitive materials, thus reducing repair and replacement costs and providing a lifetime of dependable service free of costly and damaging water leaks.

While many variables such as local labor rates and wholesale prices will determine the cost of a piping system, a study conducted by IFT Technical Services showed that on average, a CPVC domestic hot and cold water system cost up to 44 percent less to install than a comparable copper system – even when extra detailing required for firestopping the CPVC system was included. The study also found that a PVC drain, waste and vent system cost up to 37 percent less to install than a comparable cast iron system. This study looked at actual applications in residential high-rise, commercial high-rise, a wood-frame townhouse and a strip shopping center in six different U.S. cities.

Typically, PVC pipe for drain, waste and vent applications has an installed cost from $11.45 to $27.00 per linear foot, depending on the diameter of the pipe. This includes materials, equipment and labor. For water service applications, the installed cost ranges from $8.80 to $27.00. For hot and cold water distribution systems, the installed cost of CPVC ranges from $9.60 to $24.00. For comparative purposes, a 2"-diameter PVC pipe for drain, waste and vent applications has an installed cost of $14.40. The same diameter pipe for water service applications has an installed cost of $14.55. For hot and cold water distribution systems, CPVC has an installed cost of $17.75. A 2"-diameter galvanized steel pipe has an installed cost of $14.45 per linear foot; the same diameter copper pipe has an installed cost of $15.95; and the same diameter cast iron pipe has an installed cost of $16.15. (Source: RS Means, Building Construction Cost Data, 57th Annual Edition, 1999).

Maintenance requirements. (Not applicable.)