deaerator/feedwater problems

[lewie]

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.

 

[Sharik]

Maybe try blocking in the feedwater supply to the firetube boiler when it is taken off line. Or if possible, switch the feedwater level control manual and leave it closed See what happens to the deaerator and the storage tank.

If necessary, feedwater can be admitted manually and slowly to the boiler later to bring the water level back to normal.

 

[pmover]

first and foremost, i'd investigate if the existing da & surge tank are properly sized for the additional boilers.

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

 

[dfurey]

Morning, Having read this I don't understand how the boiler was secured. Once fires are secured the Main feed system to that boiler should also be secured. In addidtion the Main and Aux steam stops should be shut (not sure if you call them that or not). The boiler will continue to generate steam due to res. heat. If the stops are not shut and the feewater system is allowed to remian aligned, you will continue to feed the boiler until the pressure drops. The boiler water will start to shrink once secured, this is normal, you can fill the boiler by re-aligning the feed system to whatever level you need.
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.

 

[abcmex]

Lewie,
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

http://www.spiraxsarco.com

rgs

 

[smckennz]

There is another possibility which is a little embarassing, but happens from time to time. Some boilers do not have provision to isolate the combustion air/stack gas when shut down. If the boiler vents direct to a common stack under a reasonable stack draft, then heat will be rapidly sucked out of a shut down boiler. The hot water/steam in the boiler heats the air flow which is lost up the stack. This will cause the symptoms you describe. A damper on the stack breeching is the best way to overcome this, but partial relief can be obtained by positive shutoff of burner air.
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

 

[marada93]

To answer correctly requires additional information of your normal shutdownprocedures. I've see this happen on boilers that are shut down, and immediatley operators close thesteam outlet isolation. Since the boiler is still steaming due to the residual heat, pressure in the boiler increases above normal operating pressure cause safeties, ERVs, and or startup vents to open automatically. The elevated pressure drives the water level down into the drum, and the open safeties are steaming away to atmosphere further decreasin the already low level. This is all compounded by applying additionl feedwater, which is now lower in temperature than prior to shutdown, and collapsing the drum as you initially suggested.

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.