Control systems in safety-critical domains must operate within strict constraints, yet conventional pre-designed controllers such as PID controllers are difficult to modify once deployed and offer no formal guarantees. This work proposes a safety assurance framework that augments such controllers with a Control Barrier Function (CBF)-based safety filter. The framework is validated on two representative systems: a ball-and-beam system and a closed-loop anesthesia system. In both cases, the CBF filter successfully maintains system states within predefined safety regions. Specifically, the ball position and beam angle remain bounded in the ball-and-beam setup, while patient models in the anesthesia system consistently stay within safety limits. The safety filter intervenes by adjusting control inputs but gradually converges toward the nominal controller commands. While the approach inherently reduces overall control performance, the results highlight the fundamental trade-off between enforcing safety and preserving nominal control behavior.