What is the purpose of the safety valve (pressure relief valve) in the supply tank?

The SAFETY VALVE (pressure relief valve), usually installed in the supply (wet) tank, protects the air brake system from OVER-PRESSURIZATION. CDL air brakes. If the air pressure rises too high (typically the safety valve is set to release at around 150 psi) — for example, if the governor malfunctions and the compressor keeps pumping — the safety valve OPENS and releases (vents) air to prevent dangerously high pressure that could damage the system or cause a tank to rupture. NORMAL operation keeps pressure around 100-125 psi (controlled by the governor); the safety valve is a BACKUP that only activates if pressure gets too high. If the safety valve releases air, it indicates a problem (usually a faulty governor) that needs repair. Knowing the safety valve protects against over-pressurization (~150 psi) is commonly tested.