Relief valves and slam-shuts protect against unwanted pressure, but in different ways.
Relief valve
It opens to release a quantity of gas and limit the pressure.
Slam-shut
It completely shuts off the supply to stop gas inflow to the downstream network.
When to use each one
The choice depends on the design, pressure, permissible failure scenario, and safety requirements.
Setpoints coordination
The values must be correctly selected so that each device acts in the intended sequence and unwanted trips are not caused.
