Technical Note PP 831-TN
PE3408 Pipe in Compressed Air
or Compressed Gas Service
When compressed gases are to be conveyed in high-density polyethylene piping, the system designer must account for chemical and environmental effects, installation related effects and conditions, and other application-related limitations such as possible regulatory requirements.
Chemical Effects
Some gases such as argon, helium, methane, carbon dioxide and nitrogen have little or no known chemical effect on polyethylene. Other gases such as oxygen, oxygen in air, chlorine, fluorine and bromine can impair polyethylene to varying levels of severity by oxidizing or directly attacking the material. Further, when compressed gases are combined with other gases or vapors, deleterious effects from the combination may occur. For example, chemicals such as vapors from compressor oils that may be present in compressed air or other compressed gases can deposit and liquefy on the pipe surface and reduce material strength by chemical solvation. Overall chemical effects will vary depending upon the aggressiveness of the chemical gas and possible combinations with other chemicals and vapors. Chemical effects are increased by higher applied stress (higher internal pressure) and by increased service temperature. Degrading chemical effects act to reduce servicelife, and in some cases, service life can be severely compromised. Polyethylene pipe may not be suitable for use with some aggressive gases, especially in combination with some chemical vapors.
Installation
The system designer must address the mechanical safety aspects of compressed gas piping installations. The installation must not expose the pipeline to significant impact or possible mechanical damage, particularly in very cold conditions. Underground installation is recommended. When properly buried, the pipe is restrained against movement and protected against most potential sources of mechanical damage. Underground installations should comply with ASTM D 2774, and be buried below grade at least 12” or one pipe diameter whichever is greater.
Surface or above grade installations require special precautions to protect the pipe against mechanical damage.
WARNINGS:
- Severe mechanical damage can puncture HDPE pipe, and in some cases smaller pipe or tubing can be severed. If an unrestrained, pressurized HDPE line is severed, it can whiparound and cause property damage or injury or death to persons.
- Significant impact under extreme cold conditions can shatter pressurized PE piping. Escaping gas can propel pipe fragments that can present a hazard of injury or death to persons.
- Where surface or above-grade PE compressed gas piping may be subject to extreme coldand the potential for mechanical impact, the piping must be installed so that the line isrestrained and the risk of mechanical damage is minimized.
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