Adding phosphate to Boiler feed water pump inlet to control Ph of the water

[Bahador_eshghi]

Dear all,
Is there any consideration about injection of phosphate at Boiler feed pump inlet ( for an HRSG)? Does it have any influence on life time of the pump or corrosion issues?
In HRSGs phosphate is used to control Ph of the water to avoid flow accelerated corrosion (FAC).
Now we are facing serious corrosion problems in the boiler tubes, therefore we are looking for any new ideas to control the PH in the lowest pressure level of the HRSG that feeds the Pumps,as I know, no manufacturer used phosphate at this pressure level up to now.
any advice will be appreciated.
Regards

 

[Dhurjati Sen]

Dear Bahador_eshghi,

You cannot get any proper analysis or solution with such little input.

Sodium tri-phosphate or the likes are dosed in the steam drum to neutralize salts like calcium carbonate, etc. that have inverse solubility. As the TDS in the drum increases intermittent blow downs are given in addition the the continuous blow down.

Hope you are making the Boiler Feed Water in de-aerators from De-mineralized water after dosing sodium sulphate / hydrazine and morpholene and not from potable water.

What are the serious corrosion problems?

I think you need to consult experts offline on this issue.

Regards.

DHURJATI SEN

 

[georgeverghese]

@Dhurjati Sen, minor spelling error, I think - you must have meant sodium sulfite and not sulphate - sulphate has no dissolved O2 scavenging capability.

 

[Dhurjati Sen]

Dear georgeverghese,

Thank you for the correction.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[EdStainless]

Why are you having pH issues? What is wrong with your system that is leading to this?

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P.E. Metallurgy, Plymouth Tube

 

This subject is far too complex to bring to this type of forum. I concur with DS that you should consult with an expert.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Bahador_eshghi]

Dear All
thanks for your kind replies.


@Dhurjati Sen , you are right, the problem is big enough to be followed by the top manager! I am sure that this information is not enough to make a proper vision.
but I really want to answer this question: Is there any corrosion issues for the pump itself regardless of the whole cycle in case of phosphate injection at its suction line?



@EdStainless, according to Carbon steel and Alloy steel (A192/A178/P22) materials of the tubes it is requested to maintain PH at higher values to avoid corrosion of the materials.

Regards

 

[Dhurjati Sen]

Dear Bahador_eshghi,

Still insufficient data!!!

Is the phosphate dosing a batch process, like you make a batch of the phosphate solution and continue dosing it till someone checks the phosphate concentration and adds more phosphate (solid / powder)???

Or is it a liquid dosing with the phosphate level in the batch coming directly in the DCS / PLC ???

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[Bahador_eshghi]

Dear @DHURJATI SEN
Second configuration !, that means amount of phosphate in the water is monitored in the DCS system.
I think I should check the pump material and its compatibility with phosphate and also gather additional data.
anyway thanks for your comments.
regards.

 

[springrise]

Hi,
phosphate isn't meant to be used for pH control, it's supposed to react with residues of water hardness.
Instead, sodium hydroxide is used to control pH. Check pH of your feedwater, as well as pH of the steam generator blowdown.
Concentrated chemicals should be preferrably injected to a tank (e.g. deareator) not to a pipe (pump suction).
Pictures of the corrosion can help identify it.

 

[Chumpes]

@springrise : phosphate is a pH buffering agent as it can be protonized by H+

@Bahador_eshghi : i am still investigating FAC (flow accelerated corrosion in boiler systems) but phosphate seems to be not efficient in water condensed from steam. initial boiler water pH may be in the suitable range (alkaline), but pH can drop to lower pH (more acidic = more corrosion) after boiler water vaporization and re-condensation.

 

[EdStainless]

Bahador
You need to look at what is driving your pH down in the first place.
Your feedwater should be ready to go coming out of the DA.
Much damage can be done by over-correcting or making additions to compensate for things that are wrong.

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P.E. Metallurgy, Plymouth Tube