The control valve associated with the start-up line, often referred to as a shunt valve or start-up valve, primarily functions to regulate and ensure a minimum flow of steam to the boiler during critical operational phases such as startup, shutdown, and light-load conditions. Its main purpose is to maintain a steady, controlled flow of cooling steam through the superheater coils, preventing them from becoming starved of steam during these transient periods. By ensuring that a minimum amount of steam is continuously supplied, the control valve helps protect the superheater coils from overheating and potential damage, thereby promoting safe and efficient boiler operation. The valve's operation can be manual, motorised, or pneumatically controlled, with modern boilers favouring motorised or pneumatically actuated valves for precise and reliable control. Positioned strategically on the main steam outlet before or after safety valves, the valve is extended to an appropriate height and fitted with silencers to minimise noise during venting, especially during venting of superheated steam. The control valve's function is crucial in maintaining the proper flow conditions, preventing flow interruption, and ensuring the boiler's operational stability during start-up and other transient states.

The control valve's primary role in starting the boiler is to guarantee a minimum flow of steam, which is essential for cooling and protecting the superheater coils, thereby avoiding thermal stress or damage during initial operation. During startup, the valve opens to permit enough steam flow to facilitate safe heating and warm-up of the boiler components, ensuring a smooth transition from cold to operational state. It also acts as a safeguard against sudden process disturbances or network failures that could interrupt steam flow, which might otherwise lead to overheating or damage to boiler parts. By maintaining a controlled flow, the control valve helps in stabilising boiler operation during critical phases, ensuring safety, efficiency, and longevity of the equipment. Its proper functioning is vital for preventing operational issues related to inadequate steam flow during start-up, shutdown, or low-load conditions.

Mahesh Wali

The boiler start-up vent control valve plays a vital role in maintaining the safe and efficient operation of the boiler during critical phases such as start-up, shutdown, and low-load conditions. Its primary function is to ensure a minimum continuous flow of steam through the superheater coils when the main steam line is not yet pressurized or operational. This is essential to prevent overheating and thermal stress in the superheater tubes, which could occur if steam flow is inadequate—particularly dangerous during early start-up or process disturbances.

By venting steam safely to atmosphere (usually through a silencer), the control valve allows operators to maintain proper steam circulation even when there's no load demand or during sudden grid failures. This helps preserve the integrity and lifespan of key components, especially in high-pressure boilers. Modern systems typically use motorized valves in tandem with pneumatic control valves for precise modulation and automated control, replacing older manual methods and improving responsiveness.

Additionally, the start-up vent valve acts as a safeguard to manage pressure build-up during non-steady-state operations. Though there’s typically no flow meter on this line, its sizing (often handling 30–35% of boiler MCR) ensures that enough steam can be relieved to prevent thermal damage. In summary, this valve is a critical control element that supports safe thermal transitions and protects the boiler's superheating system under variable operating conditions.