deaerator/feedwater problems
deaerator/feedwater problems
(OP)
We recently added 2 new 220 hp steam boilers to our existing 125 psig steam system consisting of 2 400 hp water tube boilers. Also utilized is a 30,000 lb/h deaerator and surge tank.
Under normal operation both of the new firetube boilers and one of the 400 hp boilers operate simultaneously. We are having a problem when one of the fire tube boilers is taken off line. The offline boiler experiences what I think is shrinkage from collapsing steam bubbles and regulates its feedwater control valve to wide open to makeup the water.
When this happens the deaerator and surge tank completely drain of water. I'm not sure where all the feedwater is going and how to prevent future alarms from happening. The system works fine under steady state operation. Any tips would be appreciated.
Under normal operation both of the new firetube boilers and one of the 400 hp boilers operate simultaneously. We are having a problem when one of the fire tube boilers is taken off line. The offline boiler experiences what I think is shrinkage from collapsing steam bubbles and regulates its feedwater control valve to wide open to makeup the water.
When this happens the deaerator and surge tank completely drain of water. I'm not sure where all the feedwater is going and how to prevent future alarms from happening. The system works fine under steady state operation. Any tips would be appreciated.





RE: deaerator/feedwater problems
If necessary, feedwater can be admitted manually and slowly to the boiler later to bring the water level back to normal.
RE: deaerator/feedwater problems
otherwise, consider gradually lowering firing rate to idle and maintain for a minute or 2 and then shutdown the boiler. also consider manually raising water level in boiler to be shutdown a couple of inches or so before shutting down. monitor da & surge tank water levels.
-pmover
RE: deaerator/feedwater problems
Feeding a secured boiler will drop your DFT (DA tank)level, however not to the point where your DFT is empty. What kind of Make-up Feed system do you have? Is the Make-up Feed vacuum dragged from a tank through an air operated regulating valve? If so what was the position of the valve during evolution.
I have operated marine steam plants for a long time and found that most of the problems with the plants are caused by system mis-alignment, operator error.
RE: deaerator/feedwater problems
It sound like you are probably rigth. It can be happening that when your boiler is disconnected (load control) the level drops and the level control reacts to increase the water level. This is a typical problem of a simple boiler level control. It can also be a bad control loop adjustment.
Usually it is recommended that the level control receive information from the produced steam flow sensor (cascade) to avoid this. When the flow is high the level should be higher and when the flow is low the level should be lower
You can find more about this in www.spiraxsarco.com
rgs
RE: deaerator/feedwater problems
Also check that continuous blowdown is disabled on shutdown, although this will not be your problem.
And sack the design engineer if gas path isolation has not been provided :)
Normally, an isolated boiler cools down without straining the feedwater system.
Cheers
Steve McKenzie
RE: deaerator/feedwater problems
I've found that to avoid these complications, after the fuel is removed from the boiler, simply allow it to "drift off line. With no outlet steam flow indicated, then proceed with normal boiler isolation procedure.