Fusible link actuator to be installed on 3" and 4" groves class 600 or if they have the complete valve +actuator that will be considered too, one feature I'm looking for is the ability to override the fusible link and operate the valve manualy.
There's many option available regarding this. When you use inteligent electric actuators such as INTELLI+, MX Series or IQ series (BERNARD, LIMITORQUE AND ROTORK), normally, you can set-up them in order to override automatically the fusible link. There are more expensive than normal equipment, but be sure that they will shut-off (or open) the gate valve automatically. Single restriction is ambient temperature.
Intelligent actuators has the option, when temperature becomes so high, to override the thermal protection and goes to failsafe position (open or close), but only "INTELLIGENT" series. These are non-intrusive equipment.
Maybe you can use to use an bimetalic thermometer (direc mount or with capillary) with microswitches. You can select trigger-point (or points) and arrange the circuit. Check at DeWIT or ASHCROFT options.

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