A valve for pressurized gas or chemical agent. The valve comprises: a body with: an inlet, an outlet, and a passage which connects the inlet with the outlet and which includes a seat]. The valve further comprises a sliding piston able to engage with the seat in order to close the passage and a telescopic extension sliding against the passage in order to guide the piston in the passage at least between the open position and the closed position. The pressure difference between opposite faces of the piston pulls away the piston and the telescopic extension from the seat in order to open the passage, the passage remaining closed under pressure equilibrium. Additionally, a fire extinguishing system.