Fisher 377 Pressure Sensing Trip Valve Product Overview The Fisher 377 Pressure Sensing Trip Valve is a robust, direct-operated safety device engineered to protect industrial processes from overpressure by automatically shutting off gas or liquid flow when a preset pressure threshold is exceeded. Designed for use in safety instrumented systems (SIS), it features a compact, inline construction with a proof-of-closure switch for remote verification. Certified to SIL 3 and