The aim of a pressure reducing skid is to regulate the NG outlet pressure to a setting value. One typical solution consists mainly of two reducing streams (2x100% capacity) with two regulators each: one is the main regulator and the other is used as a monitor/slam shut valve. Active and monitor pressure regulator shall be “Top entry type” in order to guarantee a simply maintenance of the equipment. The active regulator normally assures pressure reduction to the setting value.
To assure gas supply continuity in case of failure of the active regulator a second regulator (monitor) is installed in series. The monitor regulator is normally wide open. In the event of an accidental increase of outlet pressure, the monitor regulator will take over as control valve. During the normal operation, depending from the service, one line is working while the other line is in stand-by. Only in case of a failure of the monitor and main regulator, the standby line starts to work. An overpressure slam shut device, in the event the outlet pressure exceeds an acceptable value, protects the equipment downstream of the pressure regulators from over-pressure.
To minimize accidental intervention of the slam-shut device, a spring-loaded pressure relief valve is installed and designed to release relatively small quantities of gas only.