b5a5d860_for_any_fuel_gas_fired__boiler__Item
[Safety] Two automatic block valves with a vent valve between them are used to stop natural gas from reaching the burner when it is off. The block valves have position switches so the control system knows when they are close. [Reason] A single automatic block valve in good condition would be enough to stop natural gas flow, but a second automatic block valve provides insurance. A vent valve between the block valves pipes any leakage gas to atmosphere when the block valves are closed. [Item] 
Natural Gas Block & Vent Valves
Safety controls for any fuel gas fired  boiler
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