Why are the induced draft fan and forced fan interlocked in the fuel valve?

The interlocking of the induced draft fan and the forced draft fan in the fuel valve is essential for ensuring proper operation and safety in a boiler system. The primary reason for this interlock is that the forced fan should not initiate until the induced fan is operational. This sequence is crucial because the induced draft fan creates a negative pressure in the combustion chamber, ensuring that flue gases are safely expelled from the system.

If the forced draft fan were to start without the induced draft fan, it could lead to the buildup of pressure within the combustion chamber, which poses a significant risk of unsafe operation, including potential explosions or the release of unburned fuel. By ensuring that the induced draft fan is running first, the system guarantees that there is adequate draft to safely handle the combustion process, thereby maintaining a stable and safe environment within the boiler.

In summary, this interlock provides a necessary safety measure, ensuring that the sequence of operation mitigates risks associated with combustion and pressure within the boiler system.