Six months ago, there was a fault in our boiler. There was a failure in the Fuel Control Valve. The Valve opened 100% when the boiler load was maximum (Usually when the boiler load is maximum, the valve is open 40%). Suddenly the drum pressure increased. Drum level became very low. We opened the solenoid operated safety valve to reduce main steam pressure. MFT was avoided.

Our questions are:
1. Was our action (open the SV) correct?
2. Could you explain why when the fuel control valve opened 100%, the drum pressure increased and the drum level decreased?

Thanks a lot.

 

Dear datsacorn2511,
not knowing a lot about your plant my answer may not be 100% correct, but from what information you provided:

1: Yes, opening the steam vent valve was the best course of action, hopefully you were also able to reduce fuel quickly as well.

2: Opening the fuel valve from a nominal 40% to 100% would have increased fuel flow by a large percentage I assume. This large change in heat production would cause steam production to rise and thereby reduce water level in the drum, as well the increase in drum pressure would compress what water was left in the drum, creating an even lower level.

 

It's hard to know why the fuel valve went to 100%. Normally, a load-following boiler is controlled by a measured steam pressure or sometimes measured steam flow. The controller will open both the fuel and the air as necessary to maintain the pressure. In any case, the fuel valve should NOT have come open to 100% unless the combustion air also went to some high level to maintain the required fuel/air ratio. Fuel should be limited by the amount of available air in order to avoid a fuel rich mixture that can possibly result in a boiler explosion. Your action of opening the steam vent avoided a HIGH-HIGH condition that might have shut you down, although I'm surprised that you could act quickly enough to avoid this trip. But your real concerns are to find out if the air went up as did the fuel, and why the fuel valve needed 100% opening if its normal position is 40% for the same firing rate. These are your larger concerns.

As to why the drum level decreased on increasing pressure -- that is actually a normal response. Boiler steam drums are actually a mixture of water, entrained steam bubbles that are forming in the water, and steam in the upper portion of the drum. As pressure increases the boiling point of the water changes and the entrained bubbles collapse back to water state. This causes a fairly sudden decrease in water level, and the opposite effect also happens if the pressure suddenly drops. The drum level will increase. This action is commonly called the "Shrink and Swell Effect."