Boiler feed water pH to Boiler water pH 1

[CDNAR]

A new user:

Does some body know, why does the pH a boiler feed water of a pH of 8 to 8.5 will increase up to a pH of 10 to 10.5 when it is in boiler (boiler water). This has to do with cycling of the water, but is there a detail description of this? Which are the ions that will increase this pH?

 

[serendepidy]

Generally this depends on what chemical/s you are dosing and what type of plant configuration you have. If you have condensate polishers, Na slip on the cation resin will occur over time; if you are not regening them frequently enough NaOH will accumulate in the boiler water via the feedwater to increase the pH due to the slow concentration effects of steam creation. As the steam evaporates, it is very pure leaving concentrated water below, as you concentrate up the boiler depending on the ions present the pH will increase or decrease. Na is a classic for pH increases due to it being a weak acid and therefore a strong base NaOH forms in water. Ca and Mg may do this also but they are more likely scle in the boiler rather than increase the pH. The key is to keep a blowdown on.

Also your regens may not be occuring properly and you may be returning the resin to service with some residual caustic from the regen process. You can increase the blowdown if either of these are occuring as an interim measure to reduce the pH.

A boiler pH of 10.5 is quite high and I would look at increasing your blowdown to keep it below 10 and preferably around 9.5ish. Of course you need good instruments (calibrated ones to assure yourself of this)

A feedwater pH of 8 - 8.5 is quite low and I would recommend dosing of amine to increase the pH above 9, preferably ammonia. This is especially important as carbon steels at those low pH's will quickly experience flow accelerated corosion. Stay away from organic amines as they are too reducing and may exacerbate the problem.

There isnt much else that can cause the boiler pH to increase unless you are dosing chemicals into the boiler drums, in this case change the setpoints on your dose pumps so they come off when the pH hits 9.7 or so and run a blowdown to reduce the pH if above that. There is often a lag between the sample point and the analyser which can lead to overdosing so make sure chemical additions are not done too quickly. Any anions will depress the pH eg Cl SO4 etc and ammonia in the feewater or any other organic amines will partition/decompose quickly into the steam and not remain in the boiler long enough to increase the pH.

Really I need more info to answer this question.

Questions are;
What feedwater chemicals do you dose ?
What boilerwater chemicals do you dose ?
What conductivity is your make-up water and what type of water treatment plant ?
What materials are your feedwater system constructed of ?
Do you have condensate polishing ?
What sort feedwater flowrates do you have and what is the steam being used for ?

 

[25362]

thread1133-168273

 

[Truckdriver2]

Perhaps the most important piece of information lacking here is what type and pressure boiler are you running?

For 7 Bar firetube boilers the values you are experiencing are quite normal and within the "normal" range.

The feedwater Ph is lower as the treatment chemical stays in the drum as it should not significantly carry over and you are also diluting the feed with make-up.

The drum Ph is the most important parameter (don't be too concerned with your feed Ph unless it is abnormally high or low). For an LP boiler such as it sounds like you hve you should be aiming for 9.5 - 10.5 in the drum.