Many larger gas trains use a double block and vent arrangement: two safety shutoff valves in series with a normally open vent valve between them, piped outdoors. If the upstream valve leaks while the burner is off, gas between the valves escapes through the vent instead of building pressure against the downstream valve.
The vent valve is one more valve that can fail, it needs a vent line run to a safe location outdoors, and it releases gas to atmosphere when it opens.
A valve proving system checks the shutoff valves directly. Before startup, after shutdown, or both, depending on setup, it pressurizes or depressurizes the space between the valves and watches the pressure. If a valve isn't sealing, the pressure changes, the test fails, and the burner doesn't start.
Whether a vent valve is required depends on which code applies to your equipment and which edition is enforced. Current editions of NFPA 86, which covers ovens and furnaces, accept either double block or double block and vent, and allow valve proving in place of a proof-of-closure switch on one of the safety shutoff valves. Boilers are covered by NFPA 85 or ASME CSD-1, depending on size, along with local rules and insurer requirements.
Several burner controls include valve proving, including Siemens LMV, Autoflame, and Fireye Nexus controls. Standalone valve proving units are also available.
Before removing a vent valve, confirm which code edition your inspector enforces, what your insurer requires, and that the valve proving device is approved for that use. Then confirm the test is wired in and running at startup.
We check valve proving operation as part of every burner startup and tune-up. If you're planning a gas train change, send us the details.
Related reading: Burner Management Systems Compared and Flame Safeguard Upgrades. See all of our services on the What We Do page.
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