Combustion System Relief Regulator NFPA86 Code

[ccfoam]

I'm looking for thoughts on the sizing of a relief regulator for a natural gas 18MM BTUH industrial furnace combustion system. NFPA86 (8.7.1.10) states "If inlet pressure to main gas regulator exceeds the pressure rating of ANY downstream component, overpressure protection shall be provided." The SSOV's will handle the max system pressure. I have two scenarios.
1. Add an extremely large relieving regulator down stream of the SSOV's that will vent the entire gas supply if the main gas regulator fails wide open. The HI gas pressure switch will close the SSOV's on regulator failure. This will absolutely protect all down stream components. The cost of a large (5" vent line) relieveing regulator is prohibitive.

2. Add an small (1/4") relieving regulator down stream of the SSOV's that will vent the system. The HI gas pressure switch will close the SSOV's on regulator failure. The small relieving regulator will vent the over-pressure,but not as quickly. Some down stream components may get damaged in the brief period between when the regulator fails, the "HI" gas switch opens and the SSOV's close.

What's the right answer?

Thanks in advance!
Chuck

 

[dcasto]

Put two regulators in series, its called a monitor regulator.