PEX : Everything You Need to Know About Flexible Plastic Piping

Anyone who has been to an industry tradeshow knows that plumbing contractors, building owners and specifying engineers have more choices than ever when selecting materials and standards for pipes, valves and fittings (PVF) for domestic water system applications. One product, PEX — an acronym for cross-linked polyethylene — has been overtaking copper and CPVC in the residential market for more than a decade, and is now making its way into the commercial sector for domestic water applications.

With the continued expansion of larger-dimension product offerings and increased code approvals and listings for use in commercial plumbing applications, PEX is strengthening its market share and, much like it did in the residential sector, has the potential to become the dominant player in the world of commercial plumbing.

If you’re not familiar with PEX, here is some information that will help guide you when you are looking to specify, design or install this durable piping product for a domestic water application.

 

Overview

PEX pipe is manufactured to be a copper tube size (CTS) pipe with a wall thickness conforming to a standard dimension ratio of 9 (SDR9). PEX is cross-linked to varying degrees and approved by all model plumbing codes for domestic water piping applications, according to ASTM F876 and F877 standards.

 

PEX types

Not all PEX is created equal. There are currently three different methods for manufacturing PEX, known in the industry as PEX-a, PEX-b and PEX-c. These methods generate pipe that is cross-linked to varying degrees and acceptable for potable-water applications, according to ASTM F876 and F877 standards.

 

  • Engel method (PEX-a) – The Engel method, also called PEX-a, is named after German inventor Dr. Thomas Engel. This method cross-links the polyethylene molecules during the extrusion process when polyethylene is in its amorphic state (above the crystalline melting point). This method is also referred to as a “hot” cross-linking process. With the PEX-a method, cross-linking reaches approximately 85 percent, the highest degree of all PEX types. This level of cross-linking creates the greatest flexibility as well as thermal and elastic memory, which means kinks in the pipe can be repaired with the use of a heat gun. In addition, PEX-a pipe can be expanded to accept a high-flow, ASTM F1960, “cold expansion” fitting.

 

  • Silane method (PEX-b) – PEX-b tubing is cross-linked after the extrusion process by placing the tubing in a hot water bath or steam sauna. The degree of cross-linking for PEX-b is typically around 65 to 70 percent. This method is not as evenly cross-linked as the PEX-a method, nor does it have the same degree of thermal memory, which allows kinked tubing to be reshaped with the use of a heat gun.

 

  • E-beam method (PEX-c) – PEX-c uses an electron beam to change the molecular structure of the tubing — that is, cross-link — after the extrusion process. The PEX-c method requires multiple passes under the beam to reach a 70 to 75 percent degree of cross-linking. Side effects of this process are discoloration due to oxidation (from natural white to yellow, unless other pigment is added), and a slightly stiffer product.

 

Hydrostatic temperature and pressure ratings

PEX-a pipe typically meets the following temperature and pressure ratings:

  • 200 degrees F at 80 psi
  • 180 degrees F at 100 psi
  • 73.4 degrees F at 160 psi

In accordance with ASTM F876, the minimum hydrostatic burst pressure for PEX pipe is 480 pounds per square inch (for 1/2-inch pipe) and 475 psi (for 3/4-inch pipe and larger) at 73.4 degrees F. PEX-a manufacturers produce pipe that bursts at pressures of nearly twice that of the ASTM F876 requirements.

 

Thermal conductivity

PEX-a pipe does not sweat like copper, due to its very low coefficient of thermal conductivity of 0.219 Btu/(hr•ft²•°F). Copper has a coefficient of thermal conductivity between 300 and 400 Btu/(hr•ft²•°F), depending on wall thickness (type K, L or M). The thicker walls of PEX-a pipe act as an insulator, offering insulation values of approximately R-0.19. The heat transfer from copper is much greater; PEX-a offers up to 30 percent better insulating value when comparing uninsulated PEX-a with uninsulated copper pipe.

 

Surge pressure and sound intensity

Typical polymers will absorb sound in the range of 10 decibels per cubic meter (dB/cm); metals, on the other hand, absorb sound in the range of 0.1 - 1 dB/cm. For a given change in velocity, the intensity of sound from a copper pipe will be at least eight times higher than that of PEX-a pipe. Furthermore, using PEX-a instead of copper can reduce peak pressures caused by a quick-acting valve by 18 to 40 percent.

 

Freeze resistance

Due to its flexibility and expansion characteristics, PEX pipe is much more freeze resistant when compared with copper and CPVC. Because of its thermal and shape memory, PEX-a can expand up to three times its extruded diameter. As a result, if water freezes in the pipe, the pipe can flex to accommodate the expansion and then shrink back down after the ice thaws.

 

Chlorine resistance

All PEX manufacturers are required to test the resistance of their pipe products to hot, chlorinated water and list the PEX designation on the pipe’s print stream. Some PEX-a manufacturers meet the highest requirement for chlorine resistance at end-use conditions 100 percent of the time at 140 degrees F. Consult with the PEX manufacturer to verify that the latter’s PEX designation meets the intended application of its products.

 

Suspended piping

PEX can be supported by the same CTS pipe hangers or supports used for metallic pipe. The horizontal support spacing for 1-inch and smaller PEX is 32 inches; for 1 1/4-inch and larger PEX, 48 inches. (Note: Some codes only allow for 32-inch horizontal support spacing, regardless of pipe size. Be sure to check local codes for verification.)

For suspended applications requiring less or more restrictive support spacing, products are available that offer continuous support, providing a great solution to lessen the labor and material costs of installation. 

 

Risers

PEX risers require a CTS riser clamp at the base of each floor. The installation should also include a riser clamp at the top of every other story to limit the expansion and contraction to pipe lengths less than 25 feet. This translates to an expansion of about 1 1/2 inches in 25 feet at a 60 degrees F Delta T (installed at 60 degrees F with an operating temperature of 120 degrees F). In this application, the piping will snake slightly in areas where it is not constrained, requiring the use of mid-story guides.

 

Fire-resistant construction

PEX manufacturers are required to test their products and assemblies to multiple ASTM fire-resistant standards if such products are to be installed in fire-rated applications. Some PEX-a manufacturers have been approved for installation in applications requiring plenum-rated pipe, as well as up to three-hour fire-resistive ratings. Most firestop manufacturers offer multiple “listed” solutions for through-penetrant applications using PEX pipe. Consult with the PEX manufacturer to verify that its specific fire-resistive construction ratings and listings match the intended application of products.

 

Direct burial

PEX pipe is approved for direct burial in soil or concrete. Some PEX fittings and fitting materials are also rated for direct burial; check with the manufacturer for approved applications and installation guidelines. Some manufacturers offer pre-sleeved pipe for direct-burial applications. This corrugated sleeving provides added protection for installations in concrete slabs or soil. Also, having the PEX inside the sleeve allows for quick and easy removal and replacement of the pipe, if necessary, without having to break up the slab or soil.