Two independent POSRVs are housed in the same body. The first valve ensures pressure safety and relief. The second valve provides fail safe protection in the event of failure of the primary valve. Both autonomous piloting systems use dedicated and independent pressure impulse sources of fluid in the protected process (No external source of energy is needed for operation). No-Flow piloting system. Compact design. Excellent leak tightness. (The disc and the nozzle are leak tight up to the point of opening and immediately again after closing). No erosion of sealing faces; no chattering. Proven perfect repeatability and reliability. System insensitive to back pressure. Stable system: no mass spring effect.