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Ph And Phosphate Concentration In Bfw And Steam Drum


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#1 aanita

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Posted 14 October 2007 - 10:20 PM

Boiler feed water (BFW) pH is to be maintained in the range of 8.0 to 9.5(guidelines) where as in steam drum pH maintaining guidelines is in the range of 9.5 to 11.0. Pressure of steam drum is 34 barg. Since dosing of PO4 (Trisodium phosphate is dosing in steam drum) cannot be a controlling media (although it will increase steam drum pH a little) of pH in steam drum, the guidelines seems to me is not correct. pH should be controlled in BFW. Can we re-evaluate the guideline of steam drum pH a little lower than the supply BFW? What should be the ideal pH and phosphate level to be maitained?
Is there any standard to be followed?

Regards.
aanita.

#2 Padmakar Katre

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Posted 15 October 2007 - 10:48 AM

Dear Aanita,
As the values of the BFW and steam drum pH you stated are ok i.e. 8.5 to 9.5 for your BFW and around 9.5 to 10.5 or 11 in the steam drum which measured in the blowdown sample.
I am not an expert in this issue but what I know is your pH is maintained by maintaing the sodium to phosphate reatio which we measure in the blowdown sample.Now as you asked about to mantain a lower pH in the steam drum than your BFW pH I think which is normally not an industrial practice the reason is that you are adding this phospahtes to remove the impurities in the form of slude or you can say dissolved solids at a point where phase seperation is there in the steam drum which we normally say as CBD (Contineous Blow Down) so any how you have to have the addition of the phosphates.Now your problem is that your finding pH is exceeding as there is no proper control on your phosphate addition rate as far as I remember we used to have analysis of the CBD once in a shift and according to the results we used to adjust the strokes of the Phosphate Dosing Pump this is purely a trial and error operation and over the period of time probably you will have a perfect idea of the dosing rate or the strokes of the dosing pump. I got some links where I found information regarding BFW treatment and a little bit related to your query .
I don't know at what extent it will help to you.Here are those links,

http://www.patentsto...escription.html
http://energyconcept...r_treatment.htm
http://www.sulphuric...W/treatment.htm


Regards,
Padmakar Katre

#3 Padmakar Katre

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Posted 02 November 2007 - 12:02 PM

Dear,
I am uploading the Boiler Feed Water Norms excel sheet.I hope it will help you.

Attached Files



#4 aanita

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Posted 03 November 2007 - 03:02 PM

Dear Padmakar

Thank you very much for your reply.

Regards.

aanita.

#5 Adriaan

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Posted 08 November 2007 - 07:53 PM

Boiler water pH and PO4 levels need to be in balance, a balance moreover that depends on the operating pressure. In the drum the concentration of chemicals is usually higher than in the boiler feedwater, especially for those (added) compounds that aren't taken out by the steam. This does, of course, have an effect on the pH.

Unfortunately most boilers are operated using additives supplied by specialist firms that do not provide their clients with all the information they'd want (because the competition might get the information I suppose) so as Padmakar said finding the right dosing settings is a bit of trial and error.

Attached File  phpo4.bmp   67.33KB   291 downloads

The attached pic is a screeny from a boilerwater quality data management thingy I made several years ago; it shows the separate pH and PO4 trends and a combined pH - PO4 trend. The diamond shape in the graph is where - for boilers operating at about 40 bar (with quite a spread in pressure) those points should lie (and indeed they do not, here).

Imagine the diamond extended into a diagonal bar and you have the area all data points ought to be in, with the target area moving along that bar deopending on the operating pressure.

Hope this sort of answers part of your question "What should be the ideal pH and phosphate level to be maitained?"; it depends on the operating pressure and the two have to be in balance.

#6 Satyajit

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Posted 15 November 2007 - 03:12 PM

Aanita,
I do agree with you on certain points. I would recommend following for improvement.
AA. Maintain BFW pH slightly higher: range: 8,8 to 9,2. Conductivity as low as possible.
i. Use hydrazine or eliminox for O2 scavenging. Keep dosing rate between 0,2-0,5ppm. Keep Dissolved O2 below 6 ppb( on line measurement)
ii. Use volatile alkali ( for example amine:Nalco's Triact-1800 ) for pH control.Don't use ammoniacal water as it would increase the conductivity and so losses due to higher blow down. Dosing rate should be based on pH indication by online pH meter.


BB. Maintain Steam Drum pH (measured in Blow Down:9,0-9,8. Phosphate dissolves all the suspended solids by making soluble complex compounds which may precipitate at pH higher than 11.
Phosphate: 6-8 ppm.
Conductivity: < 50 micro-siemens/cm.
CC. Run SPC for all water parameters and monitor all the parameters within a defined span.
This would help to maintain a healthy boiler water system and bigger reliability.
DD, Calibrate all dosing pumps and have a pumping curve for dosing rate at different pump strokes.
EE, Calibrate field instruments,maintenance of equipments on a schedule.
FF. Put alarms on low pH,high pH, high temperature alarm and low cooling water flow to analyser/sample point.
GG. Monitor chemical consumption per shift, cost per m3 etc.
HH. Checking of parameters by plant staff itself in shift on routine basis,monitoring,actions and reporting on water analysis.
II. Comparing Lab analysis with DCS or online indications and strategy to minimise the gap.
JJ. Put the controlling parameters in shift group/Shift Engineer's KPI ( Key Performance Indicators).

I am sure this is going to help all water users and increase their plant reliability and reduce operating cost.

Best regards,

Satyajit







QUOTE (aanita @ Oct 15 2007, 11:20 AM) <{POST_SNAPBACK}>
Boiler feed water (BFW) pH is to be maintained in the range of 8.0 to 9.5(guidelines) where as in steam drum pH maintaining guidelines is in the range of 9.5 to 11.0. Pressure of steam drum is 34 barg. Since dosing of PO4 (Trisodium phosphate is dosing in steam drum) cannot be a controlling media (although it will increase steam drum pH a little) of pH in steam drum, the guidelines seems to me is not correct. pH should be controlled in BFW. Can we re-evaluate the guideline of steam drum pH a little lower than the supply BFW? What should be the ideal pH and phosphate level to be maitained?
Is there any standard to be followed?

Regards.
aanita.


#7 VENKATT

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Posted 21 March 2010 - 01:07 AM

Boiler Drum water PH versus Residual Phospate,Alkalinity,Conductivity.

Boiler (100kg/sq.cm;510deg cel) feedwater PH,Conductivity and Silica levels are maintained normal.
LP dosing is Morpholine & HP dosing is TSP.
What will be the reason for result of Low PH of Drum water (lower than feed water PH)while other parameters are at the maximum level of its norms.
Further PO4 addition leading to High R.Phospate, high Alkalinity,high Conductivity and high silica levels in Drum water.
Can anybody suggest the method to improve PH to Norms.

#8 sugarmill apprentice

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Posted 28 March 2010 - 03:36 PM

Go to the ASME web page. They have standard limits for boiler water treatment. They only cost $10-20 US.
If you are in the power industry, go to the EPRI web site, some of the downloads are free.
If you operate above 1500 psig, Google "Phosphate hideout".
Robert Bartholomew, Sheppard T. Powell Associates, LLC

Boiler feed water (BFW) pH is to be maintained in the range of 8.0 to 9.5(guidelines) where as in steam drum pH maintaining guidelines is in the range of 9.5 to 11.0. Pressure of steam drum is 34 barg. Since dosing of PO4 (Trisodium phosphate is dosing in steam drum) cannot be a controlling media (although it will increase steam drum pH a little) of pH in steam drum, the guidelines seems to me is not correct. pH should be controlled in BFW. Can we re-evaluate the guideline of steam drum pH a little lower than the supply BFW? What should be the ideal pH and phosphate level to be maitained?
Is there any standard to be followed?

Regards.
aanita.



#9 kkala

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Posted 02 April 2010 - 12:02 AM

Boiler feed water (BFW) pH is to be maintained in the range of 8.0 to 9.5(guidelines) where as in steam drum pH maintaining guidelines is in the range of 9.5 to 11.0. Pressure of steam drum is 34 barg. Since dosing of PO4 (Trisodium phosphate is dosing in steam drum) cannot be a controlling media (although it will increase steam drum pH a little) of pH in steam drum, the guidelines seems to me is not correct. pH should be controlled in BFW. Can we re-evaluate the guideline of steam drum pH a little lower than the supply BFW? What should be the ideal pH and phosphate level to be maintained? Is there any standard to be followed?

To the posts already sent, I would like to add some more (having participated in the requisition of two 41 barg steam boilers).
1. Steam drum water pH was higher than BFW pH in the offers obtained (apparently due to phosphates).
2. We requested oxygen scavenger and phosphate injection into deaerator surge drum. Most vendors replied that phosphates should be injected into steam drum to limit phosphate deposits and subsequent valve clogging (apparently higher temperatures help in avoiding these deposits).
3. In spite of that, first of mentioned boilers have been operating for 9 years with phosphate injection into deaerator drum, without problems reported (the other has not been yet installed).
Note: In my opinion phosphates make a buffer solution to control pH of boiler water.

Edited by kkala, 02 April 2010 - 12:04 AM.


#10 farshad_process

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Posted 27 June 2012 - 04:31 AM

Dear friends, we have a steam drum working at 121 barg.according to the design,we have a sample point in the continuous blowdown line before blowdown drum for phosphate measurement.we see great difference between measured phosphate concentration before blowdown drum and in the outlet stream from it .Is it normal?which one is better for sampeling and why? another question:how can we calculate the best range for phosphate concentration?

Edited by farshad_process, 27 June 2012 - 09:49 PM.





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