Pipe Materials: Understanding Polyvinyl Chloride Standards

Pipe Materials: Understanding Polyvinyl Chloride Standards

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Polyvinyl chloride is the most common pipe material used in onsite wastewater treatment systems. PVC pipe manufacturers must follow accepted standards for dimensions and material strength. Onsite system installers must have some understanding of these standards in order to ensure that the correct PVC product is selected for the job. 

Nominal diametersAll pipes have a nominal diameter. This value is typically the nearest 1/4 or 1/2 inch value of the actual internal diameter. These nominal diameters are “name” only. By the standards listed below, each pipe size and material type must be manufactured with an internal and external diameter that meets the accepted tolerances. 

Iron pipe standard Many PVC pipe products conform to IPS external diameter standards.  This allows PVC (with IPS dimensions) to be fitted to standard steel pipe. PVC pipe that conforms to ASTM D1785 and D2241 has IPS external diameters.

Material codesPVC compounds range in strength and chemical resistance. In order to specify PVC pipe for a given task, material codes were developed to provide standardization. These codes provide information about the impact strength, chemical resistance and hydrostatic design stress of various PVC compounds. For wastewater systems, most PVC pipe has a material code of PVC 1120 or PVC 1220. The material code must be known in order to compare the pressure rating of various combinations of external diameters and wall thicknesses.

ASTM InternationalASTM International provides widely recognized standards for material performance and uniform dimensions. These standards are critical to the success of piping systems; manufacturers must follow these standards in order to ensure that the designers and installers can depend on pipe materials to withstand internal and external forces and depend on the standardization of pipe dimensions.

ASTM D1785 — Schedule 40, 80 and 120 This standard established the dimensions of Schedule 40, 80 and 120 PVC pipe. Pipe that has the “Schedule” designation has the same external diameter and wall thickness independent of pipe material. In other words, 2-inch-diameter Schedule 40 PVC has the same external diameter and wall thickness as 2-inch-diameter Schedule 40 steel pipe.    

The higher the schedule number, the thicker the wall and the more pressure it can withstand. While Schedule 40 has a thinner wall than Schedule 80, for the same nominal diameter, the external diameter is the same for Schedule 40 and Schedule 80. Because the wall thickness increases, the internal diameter of Schedule 80 decreases. Thus, Schedule 80 can withstand more pressure but will transmit less fluid because of the reduced cross-sectional area. For example: A 1-inch Schedule 40 pipe has an outside diameter of 1.315 inches and an inside diameter of 1.029 inches. Whereas a 1-inch Schedule 80 pipe has an outside diameter of 1.315 inches and an inside diameter of 0.936 inches.

D2241 — Pressure-rated, standard dimension ratio PVC pipe — This standard provides specification for the wall thicknesses and external diameter of various PVC materials that are needed to meet established pressure ratings. Standard Dimension Ratio is a method of rating pressure piping. The SDR is the ratio of pipe diameter to wall thickness and can be expressed as:

For example: An SDR 21 means that the outside diameter (D) of the pipe is 21 times the thickness of the wall.

Relatively speaking, high SDR ratio pipe has a thinner wall, and a low SDR ratio has a thicker wall. Thus, high SDR pipe has a lower pressure rating, and low SDR pipe has a higher pressure rating. Some SDR pipe is pressure rated by “class.” All pipes with the same class number have the same pressure ratings. A class/pressure rating is associated with SDR pipe with a specific material designation.

SDR and schedule comparisonBoth SDR (D2241) and scheduled (D1785) pipe have outer diameters that are based on IPS, but their inner diameters differ. SDR pipes of the same outer diameter have larger inner diameters/smaller wall thicknesses than similar sized Schedule 40 pipe. 

Additional plastic pipe standards — Most PVC pipe is either approved for potable water or for use as a drain, waste and/or vent pipe. NSF International makes this determination. Most PVC pipe is marked NSF-pw (potable water) or NSF-DWV (drain, waste or vent). This demarcation is especially important for PVC fittings. It is assumed that NSF-pw fittings are under pressure and, therefore, have more cement surface-area. Likewise, the NSF-DWV pipe is generally used in gravity flow or low-pressure flow situations, and these fittings have less surface area for bonding. 

Pipe marking and identification — All PVC has external markings along its length. The standardization for the markings has the manufacturer’s name or trademark, the standard to which it conforms (for example ASTM), the nominal pipe size, and the material designation code. That information is followed by the pressure rating if it is meant for pressure or DWV if for drainage. Next, the markings indicate standard dimension ratio or schedule number. Lastly, a marking indicates whether the material is approved for potable water use. 

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President-Elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to kim.peterson@colepublishing.com.

This article is part of a series on piping materials:


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