Overpressure shutoff valves mechanically monitor fuel-supply pressure and compare it to setpoint pressure. If the sensed pressure exceeds the setpoint, the valve closes. Supply ressure is monitored via a sense line piped downstream of the pressure-reducing regulator and upstream of the safety-shutoff valves. The setpoint pressure is adjusted via a spring in the overpressure valve that should be set and tested by a qualified combustion technician. A slam-shut valve should be used in combination with a safety relief valve.
The safety relief valve is designed to reduce temporary pressure surges downstream of the pressure regulator caused by fluctuations in system operations. The setpoint of a safety relief valve is always lower than the setpoint of the lam-shut valve. In these cases, use of a small vent valve is required. Advantages. Using an overpressure shutoff valve has advantages. The obvious one is that it allows the use of less expensive safety-shutoff valves while still meeting NFPA 86 requirements. In addition to costing less, many of these lower pressure-rated safety-shutoff valves have dutycycle ratings in excess of one million cycles. They also are significantly smaller than high- ressure valves, which can lead to a much smaller footprint for fuel trains even when accounting for the space required for the overpressure valve.
A more subtle advantage in using an overpressure shutoff valve comes from another NFPA 86 paragraph (184.108.40.206.4): “A fuel gas regulator shall not e required to be vented if an automatic device shuts off gas upstream of the fuel gas regulator as a result of system overpressurization.” This requirement allows the system designer to eliminate the vent line to a regulator. If the system also incorporates a valve-proving device, the vent line ften required between the safety-shutoff valves also can be eliminated saving hundreds of dollars per installation.