Oxygen scavenger? 1

[uacar]

Hi Friends,

While boiler water treatment, what do you use to as oxygen scavenger? Some people use to HYDRAZINE,CARBOHYRAZINE,D.E.H.A., etc.

 

[EMKWR]

Uacar,

First thing one has to ask oneself is : do I need an oxygen scavenger? Usually the de-aerator is good enough to get a low oxygen level. (However if I had to choose for an oxygen scavenger I would go for carbohydrazine because of safety reasons.)

One can even choose a further extreme if a once through boiler is used. If high enough a water purity is used, one can switch to so-called combined treatment (keep some oxygen in the BFW in combination with ammonium). Higher oxygen levels lower the solubility of magnetite and even promote the formation of the more resistant haematite. Also one saves on the condensate polisher regenaration because less ammonium is used than in AVT.

Just curious, why this question ?

Best regards,

Edwin

Edwin Muller
KEMA Power Generation & Sustainables
Arnhem, The Netherlands
Internet:

http://www.kema-water.nl

http://www.kema.com

 

[MJCronin]

UACAR,

Mr. Muller is correct.....the selection of an oxygen scavenger depends on an evaluation of the entire steam/condensate cycle.

What he did't mention is that Hydrazine(and it's near relatives) are hazardous. Many boiler plants have discontinued use of these...

Try a "GOOGLE" serach on hydrazine and words like danger, toxic, hazard etc.....

Mt opinion only.....


MJC

 

[rmw]

MJCronin says, "evaluation of the entire steam/condensate cycle", and that includes metallurgy. Certain metallurgies don't like certain scavengers. Ammonia is death to copper alloy tubing, for example. Please put this as an important part of your evaluation.

I think MJCronin said that, but I wanted to be more specific.

And, EdwinKema, for those of us that are of the old school, where we were taught, as well as, were constantly fighting the tweaking of the hotwells, and the deaerators, so as to get them to make their minimum O2 levels, and fighting the process of the addition of the scavengers, and the damage they wrought, the concept of Oxygenated Treatment in the once throughs is (was initially) mind boggling. The very idea of 'adding' oxygen to BFW, after we had worked so hard all those years to get it out.

I will say, however, as a person who works both ends of the cycle, the top (boiler) and the bottom (condenser, fwh's) that the jury is still out in my mind as to what the OT, a boiler protection measure, is doing, if not controlled precisely, and it gets past the boiler and back around to the 'pre boiler' stream, the FWH's, BFP's, Condensate and booster pumps, etc. If the boiler doesn't turn it all into martensite protection, and/or haemitite, where does it go??? Downstream. And, what does it do there?? I am asking questions here, not giving answers any more. No process that us great engineers have been able to come up with yet is 100% efficient, so some has to get by.

The process is still too new for us to have seen all the 'unintended consequences.'

Or, said differently, NO GOOD DEAD GOES UNPUNISHED.

rmw

 

[GBENARD]

Hello,
I think everybody is right, depends on your plant. To avoid Hydrazine you can also use sodium bisulfite Na2SO3 as well, up to 20-30 bars, at higher temperature, you could see formation of H2S which could be quite detrimental to your steam condensate system. Anyway, it is a basic solution for the food industry (at least in europe), where hydrazine was banned a long time ago.

So at higher pressure, you can avoid Hydrazine, either by : Using nothing, that means you control O2 preferably continuously, and that your levels are less than 20 ppb. This is EPRI recommendations, and I am used to the same from few boiler manufacturers. My company has made extensive research on FAC, and operates quite a lot of nuclear plants, and I can tell you that FAC almost disappear above 10 ppb O2. That's a fact, bottomed by more than 15 years of operation. True only for single phase FAC.
But you may have other concerns than FAC.
So you can use DEHA, Carbohydrazide, well known also as Eliminox. This is OK as well and better operating than Hydrazine at low temperature, but : If you have downstream a 150 MW steam turbine, the manufacturer requirement on steam cationic conductivity will most probably something like 0,15-0,3 µS. In this case, you will have a hard time to get the right value. This organic compounds will produce CO2 or can even decompose in organic acid at high temperature. They will not be detrimental to the boiler whatever process conditions but will produce high cond. condensates and your turbine manufacturer won't like it. In the same way, at outlet of the last stage of LP turbine, the CO2/acids will preferently transfer to the water phase of the steam/water mixt. This will considerably acidify the condensates in this zone.
So if you are running a 500 MW combined cycle, be prepared to discuss with the turbine manufacturer before using something else than hydrazine.
hoping this may help

 

[davefitz]

rmw:
regarding downstream implications of combined oxygenated treatment OT;

a) the turbine vendors have accepted it

b) if there are no copper alloy components in the downstream equipment ( ie heaters, condensers), there is usually no concern, however the OT method can only be used if the component has no stagnant legs . If zero fluid velocity is predcited to occur in the component , then one cannot use OT.

c) OT is usually provided by adding 200 ppb O2 to the feedwater downstream of the dearator, and lowering the pH to 7.5-8.5, and only used of once thru steam generators ( except some drum boilers in former soviet union)

Lowering the pH has the effect of extending the life of the demin resins by a factor of 5, and provides nearly infinite life to feedwater heater tubes ( eliminates spalling of magnetite and thus eliminates erosion - corrosion of tubes). The lack of spalling of magnettie from feedwater heater tubes nearly eliminates transport of iron in the feedwater to the boielr, thus there is no formation of "ripple roughness" in the furnace waterwalls and no need for routine acid cleaing of the waterwalls. Sine the new iron particles are heamatite, they are non-magnetic, so the iron particle filter should not be a magnetic type filter.

 

[rmw]

Davefitz,

I appreciated very much your input. I had forgotten that I knew about the copper material limitation. Where I am familiar with its use is in supercriticals with SS condensers, and SS and CS FWH's.

Question. Where is the ph actually lowered to the 7.5-8.5 value you state? The demin in the cycles I am familiar with is immediately after the condenser, before the first FWH, and there are a lot of things (FWH's, booster pumps, deaerators, BFP's, etc.,) in between there and the oxygen injection point. Typically, the only heater in the systems that I know that are CS are the HP's, after the BFP's. This should be a real benefit for them, as CS FWH's are not well thought of, although nobody seems to want to try to protect them.

Still, I wonder what happens to any O2 that is not magnetited, or heamatited. Like as in the opposite effect of a chelant. When there is nothing left to combine with, where does it go?

I know it certainly goes against everything I learned a long time ago, but I have a lot of clients who have gone to it in their supercriticals, and swear by it.

I still am "getting my mind right" about it all. Thanks for your input.

rmw

 

[EMKWR]

rmw,

It is not necessary to abandon the ammonia dosage completely. One can for example add ammonia up to pH 9.2 (50-100 ppb NH3)for example combined with 50-100 ppb of oxygen (VGB calls this the combi fahrweise). It gives the best of both worlds. It is even applied in the presence of copper.

The oxygen will probably leave the system via the steam to the condensor. As long as no other impurities (like Cl-) are present, corrosion will be limited. One has to notice that the oxygen not only promotes the formation of the protective layer, but also prevents it from dissolving again.



Edwin Muller
KEMA Power Generation & Sustainables
Arnhem, The Netherlands
E-mail e.f.muller@kema.nl
Internet:

http://www.kema-water.nl

http://www.kema.com

 

[davefitz]

rmw:
most once thru units require 100% condensate polishers; the only exception to this rule are the once thru HRSG's by Alstom- the use of a SS condenser and no feedwater heaters allows use of a temporary polisher during first month operation but no furhter polishing after system cleanup.

For all other once thru's, the polisher mixed beds need to be replenished based on the rate at which ammonia is contaminating the beds. By reducing the ammonia in the condensate ( which reduces the pH to 7.5-8.5) , you can extend the life of the beds.

Ammonia was originally added to a pH of 9.2 in conventional AVT alkaline systems, which also used hydrazine to scavenge oxygen, because those systems generated magnetite and the high pH is needed to reduce corrosion of CS feedwater heaters if magnetite is the primary protective layer. Once you switch to a heamatite protective layer ( as with OT), the need for a high pH dissappears ( as long as the fluid is flowing and not stagnant legs exist)

As EdwinKema says, the noncondensibles ( O2, CO2) are vented in the condenser dearation zone or when you occasionally burp open the dearator vent. The O2 is added at the dearator outlet using either bottled welder's O2 or hydrogen peroxide.

 

[rmw]

I guess I was driving at getting someone to admit just what EdwinKema stated, and that any uncombined O2 would end up in the condenser, (I worry about the "burping" process, and have seen some even quit venting the DA, to up the O2 levels) and knowing that condenser air off take systems have their own particular limiting issues, (a site search will show posts I have made on such issues) to address the question, 'what is this O2 doing to the condenser itself, and what is any dissolved O2 in the condensate, there due to subcooling, liquid ring derates at high CW temperatures, etc., etc.. doing to downstream equipment??'

The dissolved O2, while quite benificial past the deaerator, and the last heater, might not be such a good actor in the low end of the cycle, in the, let's say 'between condenser, to deaerator' part of the chain.

That is where I have concerns. I have seen plenty of condensers that struggle just to get the rated air inleakage back out to the atmosphere, before it does it's incipient damage, much less throwing some elemental O2 into the mix. Hello carbonic acid.

Can you allay my concerns?? I am in a learning mode here.

And by the way, just to add a comment to a fact that you presented, I have seen condensers in the summertime, with high temp CW, operating at turbine back pressure upper limitation, that were sending condensate to the polishers at 137F, while the temperature limit on the particular resin used was 140F. The plant operators at this point were less worried about the back pressure limit, and very worried about the potential destruction of the polisher resins. So what does a little ammonia, or O2 add to this nasty picture?

I guess I am saying, that I don't think we haven shaken out all the issues that might come to bear with this new OT process.

rmw

 

[davefitz]

rmw:

Well, maybe you can't teach an old dog new tricks, but it is a fact that nearly every base loaded once thru steam generator on the planet earth (which does not have copper alloy components) has converted over to the OT method, and they all sing praises.

 

[rmw]

Oh, I hear the praises, and have even had the fun of enlightening some industry "old dogs" who weren't up to speed yet on the new technology, but I have also seen some MTBF on IP FWH's in particular shorten, (304SS heater ready for its third retube in 28 years, and its second in 9 years) and I wonder if there is a connection, since there is known cl from road salt in the river water (river drains 2/3 of my country, which enters the system through the leaky old condenser, which is also near the end of its useful life.

I know of one two unit, both supercritical station, where one condenser is brass tubing, and they are sick that they can't get the benefits that they get on the other unit with the SS condenser.

And, I guess, part of my problem is, that in our area, these are gas and oil fired supercriticals, and the words base loaded and supercritical no longer fit in the same sentence. The original designers would roll over in their graves if they knew how low on load these things get at night, and on weekends. Hence, my problem, because, with high river CW temperatures, and LR vacuum pumps, using CW as the cooling medium for the seal water loop, the derate on the LRVP's is horrenduous, like up to 75%, so venting at low loads is almost non existant because the condenser operating pressure at those loads drops off to a point where it is lower than the operating point on the curve of the LRVP, which is dictated by the CW temp, which does not drop off at night, so the LRVP's essentially quit evacuating any air, and any potential O2 that might have gotten by the process in the boiler, blanketing the condenser with a pretty corrosive element. Hence, I say again, my reservation.

I'm open, but I'm from Missouri, figuratively speaking, or course. Time will tell.

rmw

 

[sistizol]

Don’t wase your time friends. Try Helamin.

http://www.helamin-usa.com.

I’m trying hard to find something against their product. I have a book about corrosion inhibitors edited in 1982. In all parts of water-steam cycle, polyamines are mentioned. The Swiss Filtro seems to find the complete product a few years later.

 

[Yordif]

Also, be aware of the uses of your steam. In a syngas plant where the steam would be introduced into the process, it is not advisable to use sulfur based oxygen scavengers. In the event of any mal operation / carry over the sulfur would poison the down stream catalysts.