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Inexplicable pH drop in boiler blowdown water

A discussion started in 2004 but continuing through 2018


Q. This question pertains to drop in pH of blowdown water of a low pressure steam boiler.

In one of the waste heat recovery boilers, (operates at 180 °C), pH of the blow down water drops to 5 and even 4. The feed water is maintained around a pH of 9 and the water is treated with trisodium phosphate [linked by editor to product info at Amazon] along with other inhibitors. The pH of the feed water is adjusted with sodium hydroxide. (Note the process side is maintained at a very low pressure)

Analysis reveals that the total organic content of the feed water lies around 8-10 ppm.

What is the role of TOC? can this cause such a drop in pH?

If not what is the problem?


VS Raja
- Mumbai, MA, India


A. Without a treatment to keep the pH elevated in the condensate, you will always see a reduction in pH in the steam lines and consequently the condensate. This is due to a buildup of Carboxylic Acid as a reaction of carbon dioxide which is dissolved in the water. People will typically use amines as a steam line treatment because it volatilizes and keeps your pH elevated. You want your condensate to have a pH of 8-8.5. pH lower than that can lead to severe corrosion that you will not be aware of until your steam lines have been ruined. Get that pH up in the condensate as soon as possible. Depending on the pH and time and temperature, it can lead to catastrophic failure of your system.

The organic compounds have no effect on the pH of your boiler. They are naturally occurring in surface waters which is probably the source of your incoming water.

I would contact a water treatment chemical supplier in your area to get professional assistance which should be part of your chemical purchasing process.

Daryl Spindler
- Nashville, Tennessee

January 27, 2013

A. How many hours can you run boiler with the pH is 5 and 4?

Mathias Po
- philippines


Q. I would like to add on his question in relation to our plant. we have a high pressure boiler but we minimize our blowdown in relation to silica and pressure. in two months time without blowdown, we have observed decreasing pH in the feed and boiler water even with 2-3 ppm phosphate. our treatment if ammonia and hydrazine at the feedwater and TSP for the boiler. is this due to TOC or acid conditions at the boiler due to concentrating minerals.
thank you very much for your immediate response.

Rizza L. Ocsan
- Dagupan, Philippines

Ed. note: We are pleased to print your followup question, Rizza. But this is a public forum where those readers who are most capable of answering your question well may not even see it for weeks or months. Please contact your supplier or retain an independent consultant for urgent needs. Thanks!

November 30, 2009

Q. Sir,

We have operating 108 bar pressure boiler. Some times drum water pH dropped at level 7.5,but at the same time feed water pH, steam pH, and condensate pH were normal (above 8.4).

What is the reason for this problem.


siva kumaar
asst manager - erode, Tamil Nadu, India

May 31, 2011

Q. I am working in a boiler (17.5 Kg/Cm square) where boiler feed water pH is 8.5 but boiler blowdown pH is dropped to 7.5-8.0. My question is if boiler feed water pH is ok then what cause can be responsible to drop the pH of blow down.

Ajit Mondal
chemical engineer - Haldia, India

October 20, 2011

A. Dear friends,

I am also working in a 90 ksc boiler system. My experience and few consultants say that drum water (blow down water) lowering due to more iron content in feed water (during commissioning) where iron will eat away the phosphate (TSP hydrolysis will be affected and no caustic to raise the pH).

More TOC (> 1 ppm) in feed water will contribute to lowering pH due to formation weak acids (organic) due to decomposition of TOC at 350 deg. C. During this time if add caustic (2% solution at minimum level based on blow down) pH will increase; this is due to the formation of NaOH from TSP is consumed by weak acids and the external addition NaOH used for increase the pH.

Kumar Pavadai
- Salem, Tamil nadu, India

October 20, 2011

Q. Some of the power plants like super critical oxygen is getting injected in feed water to ACC corrosion and 150 ppb residual is being maintained in feed water. Is it advisable?

Because we are (90 ksc) force remove DO through mechanical de-aerating system as well as chemical treatment to achieve 7 ppb.

Kumar Pavadai
Chemical engr - Salem, Tamil nadu, India

March 17, 2012

Q. I am working in a power industry of 500 MW unit and the drum pressure is around 190 Kg/cm2. I am facing the problem i.e., the drum pH is gradually in decreasing tendency i.e., around 4.4 and the phosphate reserve is < 1.0 and the pH and conductivity of the feed, condensate and steam values are in normal range i.e., pH - 8.9 and conductivity is 4.5 to 5.0. We are dosing tri-sodium phosphate to the drum, hydrazine and ammonia at feed section. Our limits for feed & steam are in the range 8.8 to 9.2 and for drum 9.1 to 9.7. We are in hurry. Please feel free to attend this problem at the earliest.

- Visakhapatnam, AP, INDIA

June 4, 2012

Q. Our (AFBC) boiler is 89 tph and pr is 88 kg/cm; we maintain TSP in boiler 12 ppm but our design limit is 20 ppm and maintain pH 9.5. So tell me about effect of low TSP dosing and maintaining low parameter.

Naresh Menaria
- Nimbahera, Rajasthan, India

pH of Boiler Blowdown Water

November 10, 2015

Q. Hello,
I am wondering how much will blow down pH be if we use raw water that has 6.9 pH to create 75% quality steam?


Abdu Ahmed
- Fort Mcmurray Canada

January 4, 2016

Q. Sir,

We have operating 45 KG/CM2 pressure boiler. Sometimes drum water pH dropped at level 6.5, but at the same time feed water pH, steam pH, and condensate pH were normal (above 8.4).

What is the reason for this problem?


February 17, 2016

• Check dosing chemicals quality
• Check 2P "M value" if it is too low, then that may also create low pH condition.
• Check hot well cationic conductivity; feed water hardness
• Check MB outlet to ensure that there is no accidental contamination of regeneration chemical

Ansuman Sen Sharma
- Vadodara, Gujarat, India

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pH rises between feedwater tank and boiler

March 31, 2016

Please I want to know what can really be responsible for increase in pH from feed water tank to boiler water because, where I work, the boiler water pH is always higher than feed water.

- Apapa, lagos, Nigeria

March 2016

A. Hi Thomas. Your question will probably be more answerable and draw better responses if you make it less vague, with data such as the pH of the feedwater and of the boiler.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

January 4, 2017

Q. Hi,

Anyone know the probable cause of different pH of the feed water and the boiler water? I'm getting a pH of more than 9 in the feed water, while in the boiler water I'm getting almost 11.80. Thanks.

- Jubail, Saudi Arabia

January 21, 2017

Q. We are Getting the drum water pH Below 9.0 always even the feedwater pH level is above 9.1.and Hydrazine residual levels Maintaining More Than limit for avoiding DO.

Venkatesh kumar
- Kakinada, Andhrapradesh, India

October 6, 2017

A. What I feel is that boiler pH can get down with entrance of exhaust in the boiler water as it totally contains CO & CO2; they can create carbonic acid which can contribute these amount of pH drop. So, better you check your exhaust gas lines whether there is any leakage which is allowing exhaust gas to enter into water.

Buckman, Water treatment chemical & solution provider - Dhaka, Bangladesh

December 29, 2017

Q. We have HRSG [Heat Recovery Steam Generator] with three drums LP [low pressure], IP [intermediate] and HP [high]. Pressure of HP drum is 120. We have facing problem of low pH (5-8) and Low PO4 in HP drum in spite of continuous dosing of Tri Sodium Phosphate. Conductivity of boiler water is also high, i.e., about 200 µs/cm. While Feed, Hot well condensate pH and steam pH is ok. We're dosing Ammonia and Hydrazine in feed water system.
Besides this, Sio2 in both IP and HP Boiler water is higher than Limit, i.e., above 100-200 µg/l. but SiO2 in feed and in our demi storage is within limit, i.e., less than 10 µg/l
Please solve our problem; what are the causes:
1. pH and PO4 is not maintaining in HP boiler.
2. High conductivity of Boiler water.
3. High SiO2 in Boiler water.

Fazal Khan
- Sukkur Pakistan

January 22, 2018

A. You are facing a problem called "phosphate hideout" and to solve this problem follow the following steps:

1. Increase blowdown up to 50%
2. Check Iron in the drum and the superheat

If you found Iron in the drum more than the limit, you must contact with engineers to schedule outage of station as soon as possible.
Most phosphate hideout problems have no solutions; the only solution is the station outage.

Ahmed Mahfouz
- Egypt, Cairo

February 22, 2018



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