Polyethylene is not one of the materials listed in 4.4.1 of Standard 22. What is the rationale for the omission of this material?
Most likely no one has successfully proposed to the technical committee to include polyethylene as an acceptable tank material. Also, there are no listed polyethylene tanks for fire protection and I don't think any manufacturer is willing to fork out the cash or spend their resources to be the first.
Even if the above is accomplished I don't think they will be viable in other than supplying sprinkler systems for light hazard or residential occupancies,
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