pH problem on cooling tower

[djoko]

hi,

I have a problem with the pH of cooling tower at ammnonium sulfate (ZA) plant. The normal operation is 7.3-7.8. The make up water is 7.8. Starting from december 25th, the pH is decreasing until 6. There is an online pH analyzer that monitoring pH and a sulfuric acid pump that will work if the pH reach 7.8. I checked some possibilities that possibly lowering the pH as listed:
1. Leakage on heat exchanger services.
There are three heat exchangers that use cooling water as a cooling agent. CO2 cooler, carbonation (ammonium carbonate) cooler, and scrubber liquor cooler (a weak solution of NH3). I can't checked the pH of cooling water inlet/outlet HE because no sampling point. I checked supply and return of cooling tower and got the same pH (6.98 on december 31th) that indicate no leakage on heat exchanger services.
2. gypsum contamination
The cooling tower is located near the gypsum unloading. The gypsum is used as a raw material. The gypsum itself have a possibility to decrease the pH, but why the problem is arise now, not from the plant started (1984)? I also have two cooling towers located near to the gypsum unloading on phosphoric acid plant, and it have a normal pH.
3. Leakage from sulfuric acid injection.
The pump have not work since the problem arise.
4. Chemicals used at cooling tower.
Are there any possibilites from it? If yes, the cooling tower at sulfuric acid plant, phosphoric acid plant, ammonia plant and urea plant use the same chemicals and have no problem with pH.

Now i inject sodium hydroxide to increase the pH. Are there any other possibilities that could decrease the pH???

regards,


dj

 

[StoneCold]

djoko
I would not panic yet. Wait a few more days to see if the ph comes back up. My guess is that you have a small leak on one of your exchangers and it slowly bleeds into the cooling water. The rate is probably slow enough that you can not detect a difference by measuring the ph on the supply and return.
Can you isolate your heat exchangers and pressure test them?

Good luck

StoneCold

 

[25362]

A note in addition to what StoneCold has posted.
When speaking of carbonate species in water, at a pH near 8 the hydrogen carbonate ion HCO₃^- is most prevalent. A bit of CO₃⁼ resulting from partial hydration of CO₂ (a leak ?) would bring down the pH.

Referring to sulfuric acid injections, here is an example from nature: when a small amount of acid rain containing sulfuric acid falls into a lake containing carbonate ions, it gives rise to a CO₃H^-/CO₃⁼ buffer that can resist further changes in pH.

 

[djoko]

stonecold,
the corrotion rate made me worried..corrotion rate for carbon steel now is 3.49 mpy, usually i got 0.5 mpy. i can't isolate the HE, the only one way is to stop the cooling water system.

25362,
we usually check the M alkalinity, it is 50 ppm. before n after the pH decreasing, the M alkalinity is stabil.

regards

dj