Deaerator water quality

[gaterine]

We are treating water exit a deaerator for use in steam raising equipment. The treatment spec. calls for subsequent condensate to be a certain quality (conductivity, iron, silica, chlorine etc) and any condensate loss is made up with demin water. We now wish to export some deaerator water elsewhere. The demin water quality is acceptable. What is the likely deteriation in quality as a result of condensate collection and passing through the deaerator? My gut feel is - very low! Can anyone confirm or otherwise?

Cheers

 

[TD2K]

Any impurities in your condensate typically either indicate corrosion (such as iron which means there is corrosion in your piping/equipment) or contamination due to a process leak, ground water contamination (which I've seen in below ground atmospheric collection systems) etc. I suppose silica could be an indication of carryover from your boilers but I would expect silica to adhere to the upstream surfaces (especially things like turbine blades) as the pressure is dropped. I've seen silica more of a control point in the boiler which then sets the maximum amount in the steam (excluding gross carryover of course) to prevent silica plateout from occuring in your turbines.

I'm excluding start up conditions during which I assume most of your recovered condensate would be dumped until the piping systems have been flushed out of construction material, dirt, etc. Unless the system has been well flushed and cleaned beforehand, the water quality could be really poor.

Pure steam condensate should otherwise have very little impurities in it as you thought.

Basically, when you condensate starts to exceed the return specifications or typically return values, you need to start troubleshooting. Now, that's my experience from a relatively low steam pressure plant (600 psig/750F). For some of the higher processing steam levels now (1500 psig) or utility system, there might be a lot of other factors I'm not familar with.

 

[gaterine]

Cheers TD2K.
I guess the pickup in the system should be low as that is the pupose of the additives!

 

[TD2K]

Hopefully no one is proposing adding untreated water to your condensate return to 'bring it up to specification' ;-).

 

[hrsgguru]

If you deaerator (DA) is operating correctly (under pressure with an open orifice/vent), you should see no deterioation of the condensate quality except for what is added by any make up water introduced to the DA. Since you're using demin water for makeup, the addition of hardness should be minimal and thus your export water will be similar or identical to the return condensate.

The treatment that occurs downstream of the DA prior to the boiler (or in the drum, depending on the design) is likely a phosphate or volatile type addition that helps remove the calcium and magnesium and control alkalinity and such. Buffers, such as ammonia, may be added to control pH, as well. And sometimes oxygen scavengers are added, but shouldn't be necessary if the DA is working correctly. Most of the byproducts of these treatments are eliminated at the boiler via the blowdown system.

Bottom line, given the addition of the treatment chemicals downstream of the DA, you're better off taking the water directly from the DA if you need condensate quality export.