Control Valves for Power Generation

During a full-load turbine trip (sudden disconnection from grid), the boiler continues generating full steam flow for several seconds before the burner management system reduces firing. This steam must be rapidly diverted around the turbine — through the HP and LP bypass valves into the reheater and condenser respectively — to prevent boiler pressure exceeding the safety valve set point and to protect the turbine from over-speed on re-start. Fast opening (1–2 seconds) is essential: if the bypass valve is slow, boiler pressure rises rapidly and safety valves lift, causing a major upset. Pneumatic piston actuators with volume boosters are used to achieve this speed.

Noise in steam control valves is generated by high-velocity jet mixing as steam passes through the throttling restriction — acoustic energy is radiated through the valve body and piping. At high pressure ratios (P1/P2 > 2), steam reaches sonic velocity (choked flow) and noise can exceed 120 dB, causing hearing damage, piping vibration, and valve fatigue. Noise reduction strategies: (1) multi-stage trim — reduce pressure in steps, keeping each stage below choked flow; (2) downstream inline silencer/diffuser; (3) increase pipe wall thickness (acoustic lagging). Multi-stage trim is the most effective and is standard for turbine bypass and high-pressure reducing valves.