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Posted by Tony Conner on January 07, 2002 at 07:38:14:
In Reply to: Deaerators posted by Afdmello on January 05, 2002 at 00:20:37:
Ideally, you want to take the water to the boiling point to drive off the dissolved gases, particularly oxygen. Most DAs are pressurized, but some are vacuum. I've only ever seen one vacuum unit.
The higher the pressure you run, the heavier the vessel must be designed and built. This means it's more expensive to buy. There is also the steam loss out of the vent. This means it's more expensive to operate. Normally, you try to run with as low a pressure as you can, and still get proper deaeration.
An often overlooked factor in DA selection, installation and operation is how they match up with the feedpumps. A lot of chronic, long term, annoying, and expensive problems result from not properly matching the DA with the feedpumps and/or installing the equipment correctly. A prime example of this is not quite enough suction head for the pump. Not so little that there's obvious cavitation, but just a borderline situation where a small amount of air gets drawn in around the seal or packing arrangement on the feedpump. This air, which of course includes oxygen, procedes to corrode the feed water piping, and the boiler.
Most powerhouse people will never design a DA, or pump, or boiler. The equipment manufacturers have people that do that for a living. However, while the DA guy may be a real wizard at that particular aspect, he most often knows very little, if anything about pumps, boilers, etc. It is the job of the plant people to make sure that all of the individual components and systems match properly.