Importance of dissolved oxygen in power plant chemistry. Dissolved oxygen is always not a culprit for corrosion. To conclude dissolved oxygen is an important parameter like Cat Condy, Na+ etc. that need to be understood, monitored and controlled as per system requirement

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Dissolved Oxygen amitavadatta
Dissolved Oxygen is an impurity or a savior?
By Amitava Datta Date: 30-Oct-2019
When Oxygenated Treatment came in India for once through Super Critical Boiler there was an
un-easiness felt by the power plant professionals as the good old days belief get shattered.
As per the history to get rid of oxygen for boiler feed water initially sodium sulphite was used as an
oxygen scavenger later on mechanical thermal deaeration has come which have lot other benefits. The
concept of corrosion due to oxygen was so strong that above all chemical oxygen scavenger “Hydrazine”
was also dosed after deaeration.
Most interestingly those who are so cautious about removing the oxygen for boiler feed water are most
reluctant when they do initial filling of the boiler after shut down/ overhauling.
Fig: A
As the supercritical boiler having higher super heater and re-heater steam temperature exfoliation of
the tube started and as it came after some years of running, obsessed low oxygen group tried to link it
with oxygenated treatment. Some reports or papers tried to document it in a milder way.
[What is Exfoliation: If you Google it, you will get lot of information on skin care i.e. to remove dead
skin. So in this case also the inner tube surface of super heater and re heater tube gets dislodged due to
oxidation at higher temperature or marginal temperature nearer to the oxidation temperature of that
metal. The result is that the oxidized metal surface layers come out from the parent tube when unit
goes for shutdown and temperature drops. So same like peeling off the dead skin and when all this
debris clogged the super heater /re heater bends the steam flow obstructs and it becomes a vicious
cycle]
Napkin Science:
One of the easiest ways to explain
corrosion is that if there is oxygen
there is corrosion and with increase
of oxygen corrosion also increases.

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Dissolved Oxygen amitavadatta
But what is Oxygenated Treatment (OT)? Is it really putting oxygen in feed water that makes the system
better than what we were getting in previous system?
The answer can be it’s a better oxide film on the surface of the tube which is formed and maintained at
30 to 150 ppb of Dissolved Oxygen bracket and the purity of the water should be excellent i.e. it should
be polished by mixed bed resin i.e. CPU.
Now let us understand what the dissolved oxygen specifications want to tell us:
1st
the traditional low oxygen system make a layer correspondent to that low oxygen < 5 ppb oxygen.
2nd
the latest Oxygenated Treatment (OT) the specified Dissolved Oxygen bracket 30 to 150 ppb
oxygen.
3rd
our very pure Demineralized Water having nil or negligible mineral impurity but full of oxygen
impurity (~ 8000 ppb).
Fig: B
This Diss. Oxygen graph is plotted to make understand where is 8000 where is 150 and where is 5.
So OT does not mean lot of oxygen dosed; it is controlled oxygen to make appropriate passivation.
Around 8000 ppb is the saturated DM water Diss Oxygen which is used as make up water.
5 150
8000
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
1(Low Oxygen
Regime)
2(Oxygenated
Treatment)
3(Oxygen
Saturared DM
Water)
Dissolved Oxygen, ppb

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Dissolved Oxygen amitavadatta
To understand OT one has to remember it is not the Oxygen first, it is the purity of water which is most
important and as the purity of water increases then our boundary of thinking gets changed and new
thinking come in. As per this concept in high purity water (< 0.15µS/cm Cat Condy) controlled oxygen
dosing gives us a film that gives harder layer resulting low Fe dissolution.
Boiler Initial Filling & Boiler Clean up:
To discuss about the role of dissolved oxygen in thermal power plant an important area needs to be
discussed. Those who are acquainted with once through supercritical boiler they know about this
process called Clean up cycle and why it is needed. But this is not only required for supercritical boiler
this is applicable to drum type boiler too.
Clean up cycle is highly dependent on dissolved oxygen of water used.
At the initial stage of this write up I raised, those who bothered so much about low dissolved oxygen
forget to take care of initial boiler filling which is being done by oxygen saturated DM water having
dissolved oxygen in the tune of 8000 ppb.
We understand but sometimes forget that DM Water is De-Mineralized Water i.e. the mineral salts like
NaCl, Ca(HCO3)2 etc. has been removed but not de gassed/deaerated so like feed water we should feed
the boiler by de-mineralized as well as de gassed/deaerated water. Clean up cycle get delayed if not
done with deaerated water, at the same time lot of iron loss occurs during the process.
As per the reputed boiler OEM and reputed international consultant the dissolved oxygen value for
boiler filling should be < 10 ppb. If anybody want to check our drum type boiler also they can also find
the requirement of low oxygen water, but all this documents are overlooked. Practically it is difficult to
check dissolved oxygen in startup time as the sample for DO analysis needs to be cooled (online) and
come through an appropriate sample line which may take lot of time so the easiest process is to check
and monitor is Deaerator water temperature and that should be 100o
C +.
Another part is the boiler hydro test it is very difficult to make them understand that hydro test water is
also going to an operational boiler it is not the construction time boiler so this water also should be
deaerated. Some plants are already following it rigorously that no water should go to boiler starting
from hydro test without deaeration.
To conclude dissolved oxygen is an important parameter like Cat Condy, Na+ etc. that need to be
understood, monitored and controlled as per system requirement.
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