Slam Shut-off valve

Most gas pressure regulating stations include a secondary pressure regulator, called a Active-monitor system, which takes over if the primary unit fails to control. Should something go wrong with the primary worker regulator, the monitor, which is normally fully open, takes over to keep the system pressure at a safe level and maintain gas flow to the users downstream. But if something goes wrong with those secondary devices, then what? A slam shut valve provides an additional layer of protection in the event pressure regulation is lost from both the Active and monitor regulators. Although perhaps not always desirable as a first protection method, if the regulating devices, both primary and secondary fail, a slam shut system isolates the gas flow. Unlike most relief valve systems, when a slam shut activates, the system is immediately isolated and excess gas is not vented to atmosphere. Slam shut valves can be standalone devices or integrally designed into a pilot operated regulator, each option designed with their own sensing and control mechanisms. Its function is simple: upon sensing pressure exceeding setpoint (over pressure protection), or below setpoint (under-pressure protection), the internal mechanism is unlatched and an isolation flapper closes, stopping all gas flow. The flapper remains in that position until the regulator is manually reset. 

Low-pressure over pressure protection:  The slam shut controller is the device that actuates the slam shut mechanism. Since this controller requires a minimum of 10 psi to operate, an additional low-pressure pilot has been added, in this case configured in back-pressure mode. Inlet pressure to the over-pressure pilot is provided through the porting within the main regulator but is connected to the upstream pressure that the regulator is controlling. the sensing port of the over-pressure pilot is connected to the downstream system pressure and the outlet to the sense port of the slam shut controller. If the downstream pressure exceeds the set-point of the over-pressure pilot, it will open, allowing the upstream pressure (10 psi minimum) to flow through the over-pressure pilot to the slam shut controller which will actuate the closure mechanism and shut off the gas flow. As a safety precaution, a manual reset mechanism within the slam shut device is designed to require a manual reset to restart the flow of gas after an event. A manual restart should only follow rigorous procedures that ensure end-customer pilot valves have been closed and do not present further safety risk.

Low-pressure under pressure protection: A similar solution can be used to provide under-pressure protection. The schematic shows a basic under-pressure protection system for these very low trip pressures. This system is similar to the over-pressure protection system described above except the additional pilot is configured in pressure-reducing mode. Inlet pressure to the over-pressure pilot is again provided through the porting within the main regulator, connected to the upstream pressure. The sensing port of the under-pressure pilot is connected to the downstream pressure and the outlet to the sensing port of the slam shut controller. An isolation valve is added within the inlet connection to the under-pressure pilot to allow the system to be started up without instantly tripping due to low-pressure downstream If the downstream pressure drops below the set-point of the under pressure pilot, it will open, allowing the upstream pressure (10 psi minimum) to flow through the pilot to the slam shut controller, which will actuate the closure mechanism and shut off the gas flow.

Low-pressure over and under pressure protection: Combining both systems above all provide both over and under pressure protection. In this configuration, both the over-pressure (BPV) and under pressure (PRV) pilots are included. The downstream pressure is connected to the sensing ports of both pilots and the outlet ports connected to the sensing port of the Slam Shut controller. The downstream system is now fully protected from pressure excursions exceeding the MAOP, or below safe gas pressures to maintain safe gas appliance or equipment operation. In conclusion, federal regulations, codes, and company design standards will dictate the appropriate devices required to supply natural gas to end customers. Systems are built with redundant devices to prevent against a single point failure mode, but even then, systemic failures can cause secondary systems to fail under extreme cases. Slam shut regulators provide an incremental ability to isolate a system to protect downstream equipment and customers. Even where system operating pressures appear to be below the capabilities of standard Slam Shut devices, there are solutions available. The only difference is how the devices are configured together to provide the desired control functionality.