While I agree that NFPA does not allow unions in the piping system, MRI is a different senario. Thus we find dielectric unions are acceptable to the AHJ's. The next edition of NFPA should address these fittings.

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MRI shielding companies are requesting that med gas installer put a dielectric union in the pipeline going to MRI rooms. This causes a problem with NFPA-99 that states that unions are not allowed in the piping system. Have you seen any installations like this in your area?