By Jon Bittenbender
Crosslinked polyethylene (PEX) tubing is widely employed across plumbing and heating trades in residential applications, but many specifiers have yet to consider its use in commercial plumbing systems. Based on decades of successful installations and growing confidence in the trade, PEX is gaining ground as a high-performance, cost-effective alternative to copper and chlorinated polyvinyl chloride (CPVC) piping on large-scale commercial plumbing projects.
With more than 50 years of testing and quantifiable performance in applications all over the world, PEX tubing has proven to be a durable material that does not suffer from corrosion, electrolysis, mineral buildup, or water velocity wear—problems plaguing copper and other metal piping systems.1
PEX tubing is manufactured by extrusion in sizes from 6.3 to 101.6 mm (0.25 to 4 in.) nominal diameter. It is controlled to outside diameter (OD) dimensions in what is commonly called copper tube size (CTS). The ratio of the OD to the wall thickness of PEX tubing is 9:1 (based on the standard dimension ratio [SDR] of 9), which ensures the temperature and pressure ratings are uniform throughout the size range of the pipe.
Advantages of PEX as stated in the design guides of manufacturers include:
- minimization of fittings
- freeze-break resistance
- stress corrosion and pitting resistance
- scale and deposit buildup resistance
- minimization of noise (water hammer)
- notching damage and abrasion resistance
The primary advantage of PEX tubing—also available in coils and straight lengths—over copper and CPVC rigid pipe is its flexibility. Installers can route it through walls, floors, and ceilings with minimal fittings. The need for fewer fittings means less resistance in the pipe leading to reduced pressure loss.
The typical bend radius used by the installer is eight times the OD. The minimum bend radius is five times the OD for cold bends and three times the OD when heated with a heat gun. For pipe diameters greater than 31.7 mm (1.25 in.), broader bends are necessary to route pipe around obstacles and sharper bends may require a 45- or 90-degree elbow to accommodate these directional changes.
The flexibility of PEX leads to yet another advantage over rigid pipe: freeze-break resistance. PEX tubing expands if it accidentally freezes and returns to its original size when the ice thaws. When water freezes in a rigid pipe, it breaks open the piping and causes leaks and flooding, resulting in a financial headache for the building owner.
Pressure loss and flow rates
To meet standardized pressure ratings, plastic piping requires thicker walls than copper and CPVC, resulting in a slightly smaller inside pipe diameter (ID). The development of cold-expansion fittings has helped dispel the myth PEX tubing causes a loss of pressure due to its small ID and the use of insert fittings. In addition, the flexibility of PEX offsets ID impact on pressure loss by requiring fewer fittings. As mentioned earlier, fewer points of resistance mean less pressure loss.
Fittings for PEX plumbing systems are available in lead-free brass and polymers in two main styles—cold-expansion and insert. All fitting types must meet the performance requirements of CSA B137.5, Crosslinked Polyethylene (PEX) Tubing Systems for Pressure Applications.
There are two proven technologies for cold-expansion-style fittings, one under ASTM F1960, Standard Specification for Cold Expansion Fittings with PEX Reinforcing Rings for Use with Crosslinked Polyethylene (PEX) and Polyethylene of Raised Temperature (PE-RT) Tubing, and the other referred to as cold-expansion with PEX compression sleeve under ASTM F877, Standard Specification for Cross-linked Polyethylene (PEX) Hot- and Cold-Water Distribution Systems.
For insert-style fittings, standards include:
- ASTM F1807, Standard Specification for Metal Insert Fittings Utilizing a Copper Crimp Ring, or Alternate Stainless Steel Clamps, for SDR9 Crosslinked Polyethylene (PEX) Tubing and SDR9 Polyethylene of Raised Temperature (PE-RT) Tubing, brass
- ASTM F2159, Standard Specification for Plastic Insert Fittings Utilizing a Copper Crimp Ring, or Alternate Stainless Steel Clamps for SDR9 Crosslinked Polyethylene (PEX) Tubing and SDR9 Polyethylene of Raised Temperature (PE-RT) Tubing, polymer
- ASTM F3347, Standard Specification for Metal Press Insert Fittings with Factory Assembled Stainless Steel Press Sleeve for SDR9 Crosslinked Polyethylene (PEX) Tubing, brass
- ASTM F3348, Standard Specification for Plastic Press Insert Fittings with Factory Assembled Stainless Steel Press Sleeve for SDR9 Crosslinked Polyethylene (PEX) Tubing, polymer
There are three main crosslinking production methodologies for PEX, all of which can be used to produce pipe for potable applications:
- The peroxide method (PEXa), ASTM requires a minimum of 70 per cent crosslinked PE molecules.
- The silane method (PEXb), ASTM requires a minimum of 65 per cent crosslinked PE molecules.
- The electron beam method (PEXc), ASTM requires a minimum of 65 per cent crosslinked PE molecules.
PEXa is the most flexible, enabling the use of cold-expansion fittings as well as a tighter bend radius. Compared to insert fittings, cold-expansion fittings produce a more consistent connection due to the inherent shape memory of PEXa pipe.