Using plastic for compressed air piping

Contaminant-free compressed air piping systems continue to become increasingly important with the advent of industrial automation. This article discusses the benefits to using plastic piping, the limitations of certain plastics, the do's and don'ts of installation, joint drying times for 100 percent pressure testing, provides a sizing guide for main air lines and more.

07/06/2001



Plant Engineering - January 2001

FLUID HANDLING
COMPRESSORS


Feature article : Using plastic for compressed air piping

S idebar 1:
Do's and don'ts of the plastic pipe installation process

Table I:
Joint drying times for 100% pressure testing

Table II :
Sizing guide for main air lines
Randy Doering, Industrial Plastics, NIBCO, Inc., Elkhart, IN
Key concepts
  • Plastic pipe eliminates rust and corrosion.

  • Not all plastics can be used for compressed air piping.

  • Plastic pipe reduces leakage because most joints are solvent cemented, not threaded.


"Plastics," the famous one-word line from the 1967 film "The Graduate," was said to be the key to the future. That prediction was never truer than today as it relates to compressed air piping systems. The technology for using plastic for compressed air systems ( Fig. 1 ) has been around for a number of years in Europe. Today, the use of plastic for clean compressed air systems has become widespread in the United States.

Contaminant-free compressed air systems have continued to become increasingly important with the advent of industrial automation. Assembly robots, packaging equipment, paint spraying, and pharmaceutical processing are just a few of the many applications that require clean compressed air.

Black iron or galvanized steel pipe commonly used for compressed air systems typically generates rust, corrosion, and other debris that can cause manufacturing problems. These contaminants are minimized or eliminated by using stainless steel or copper pipe. However, metal piping systems, particularly threaded systems, are still prone to leakage, which forces the use of larger, more expensive compressors. Also, stainless steel and copper, on an installed-cost basis, can be prohibitively expensive.

Low operating costs are possible with plastic in compressed air systems. Even if only a small percentage of threaded joints in a plant are leaking, this loss can amount to thousands of dollars per year, not counting any major leaks that may go unrepaired for long periods of time. The end result is lower efficiency and waste of valuable energy, which plastic piping can reduce.

Plastic materials
Plastic piping manufactured for compressed air systems is a specifically designed thermoplastic, a specially engineered formulation of ABS that has been extensively modified. The result is a homogeneous shatter-resistant piping system with outstanding strength, ductility, and impact resistance. The ABS used conforms to Cell Classification 54322 as outlined by ASTM D-3965. A failure from over-pressurization or severe impact would merely result in a crack or split that would release the pressure harmlessly. No fragmentation of the pipe or fitting would occur ( Fig. 2 ).

Other thermoplastic piping materials, such as PVC and CPVC, are recommended for liquid service, but should never be used to convey compressed air. The use of PVC and CPVC in a compressed air system could result in a failure that would cause the rapidly decompressing air to fling sharp fragments of plastic through the air.

Installation
Plastic has proven to be ideal for compressed air and gas, based on four factors engineers take into consideration when specifying a system:
  • Material performance

  • Material cost

  • Initial installed labor cost

  • Long-term maintenance cost.


    • Installation of plastic for compressed air systems is somewhat different from that of traditional metal pipe. Plastic pipe cannot be threaded because pressure ratings are dependent on wall thickness. Threads are used only for connecting transition fittings or flanges to metal system components. All plastic-to-plastic components, unions, flanges, and special fittings are joined with solvent cement ( Fig. 3 ).

      Plastic piping should never be connected directly to the compressor. Connect it downstream from the air receiver or aftercooler. Many compressor lubricating oils may be used with a plastic pipe system, though synthetic, diester, and polar-solvent-based oils are not suitable. Installers should read technical bulletins or check with technical service departments for a list of compressor oils compatible with a given system.

      Expansion and contraction of plastic pipe should be taken into consideration when installing the system. All piping materials, including plastics, undergo dimensional changes when the temperature rises above or falls below the initial installation temperature. The system should be designed so that it can expand and contract freely. Expansion loops are usually not required if the piping is in a plant ( Fig. 4 ). Technical manuals can assist the installer in calculating the amount of expansion to expect.

      Pressure/temperature ratings
      Pipe and fittings are pressure rated for continuous use at 185 psi and 100 F. Valves, unions, and flanges are pressure rated for continuous service at 150 psi and 100 F. The maximum service temperature for 1/2%%MDASSML%%2-in. fittings is 140 F, while the maximum temperature for 3 and 4-in. fittings is 120 F.

      —Edited by Joseph L. Foszcz, Senior Editor,
      630-320-7135, jfoszcz@cahners.com


      More info
      Technical assistance is available from the author's company for any questions relating to this subject. Call 888-4446-4226 or visit the web sit at nibco.com .


      Sidebar 1:


      D

      o's and don'ts of the plastic pipe installation process

      Do:
      • Use the proper applicator

      • Use the correct solvent cement

      • Apply the cement while the primer is still wet

      • Follow instructions completely


        • Don't:
          • Attempt to solvent weld if:


            • —it is raining

              —the atmospheric temperature is below 40 F

              —the product is under direct exposure to the sun at an atmospheric temperature above 90 F
              • Discard empty cans of solvent, primer, or rags near piping; concentrated fumes or dripping cement or primer can cause piping failure



                • Table I:


                  Joint drying times for 100% pressure testing
                  Nominal pipe size, inHot weather surface temperature, 90%%MDASSML%%150 FMild weather surface temperature, 50%%MDASSML%%90 FCold weather surface temperature, 10%%MDASSML%%50 F
                  1/2%%MDASSML%%11/44 hr5 hr7 hr
                  11/2%%MDASSML%%26 hr8 hr10 hr
                  3 and 48 hr18 hr24 hr

                  Note. The drying temperatures should not be confused with atmospheric joining temperature recommendations and limitations.


                  Table II:


                  Sizing guide for main air lines/Maximum flow of free air, scfm, when the psig is:
                  Pipe size, in.51020406080100120
                  1/23.54.46.29.713172024
                  3/45.56.99.71521273238
                  18.811152433425160
                  11/41418253953688296
                  11/2182333517089108126
                  229365181110140169198
                  36784117185253320388456
                  4110138194306418530642754
                  Note: Maximum flow of free air is based on a velocity of 20 fps.

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