A polyolefin manufacturing system and method including polymerizing olefin in a first reactor to form a polyolefin, transferring the polyolefin to a second reactor, polymerizing olefin in the second reactor, and discharging a product polyolefin from the second reactor. The system and method including operating the first reactor with a first reactor pressure relief system and the second reactor with a second reactor pressure relief system, both pressure relief systems to discharge to a flare system, and wherein a relief instrumented system (RIS) is configured to direct at least one process interlock that mitigates an excess reaction scenario as an overpressure relief scenario.