Protective devices shall be installed as close to the equipment as practical where the design of the equipment connected is such that air, oxygen, or standby gases are capable of being forced into the gas supply system. Gas and air combustion mixers incorporating double diaphragm "zero" or "atmosphere" governors or regulators shall require no further protection unless connected directly to compressed air or oxygen at pressures of 5 psi (34 kPa) or more. [NFPA 54:5.10.1]
Protective devices shall include, but not be limited to the following:
- Check valves.
- Three-way valves (of the type that completely closes one side before starting to open the other side).
- Reverse flow indicators controlling positive shutoff valves.
- Normally closed air-actuated positive shutoff pressure regulators. [NFPA 54:5.10.2]