Sulfuric acid injection for Cooling Water pH control

Question

Sulfuric acid injection for Cooling Water pH control

mjpetrag (Mechanical)

(OP)

18 Oct 23 12:01

All,

There is a project at our site to inject sulfuric acid into one of our cooling towers to help descale or prevent scaling of our HX tubes. The cooling tower is a 3-cell design, with each cell's water level equalized. Circulation rate is about 20000 GPM total for the tower. Construction of the tower is FRP, converted from previous construction of wood. The basin is constructed of carbon steel. The proposal is to inject acid into the water level of each basin cell, with a pH probe on the water circulation loop. This will be managed by our chemical treatment vendor, Nalco. I have some concerns

How will this impact the corrosion rate of the carbon steel basin? Is there a chance of localized corrosion at the basin where the acid is added? While the pH of the bulk fluid might be within range, at the points where the injection occurs, there could be a very low pH localized there.

When a cell is shut down say for fan maintenance, we shut down/lock out the fan, return water, and make up water to that cell. If the acid isn’t turned off, the pH will get extremely low in that cell’s basin. This is a risk. Obviously this should be controlled by LOTO, but if it’s missed this will be a big problem

When the tower is shut down for maintenance and basin is drained, any drips coming out of the acid addition tubing after the basin is empty will hole-through the basin. The tubing has to be free draining to prevent this from occurring. I could see this happening if we just block in the acid without clearing the line afterward.

These were just the main things I could come up with. I plan on speaking with Nalco to see if other customers with this setup have any issues, but also wanted to poll the forum here for their experiences.

Thanks!

-Mike

MJCronin (Mechanical) · Answer 1 · 2023-10-18 13:39:00

There are several types of water treatment methods for all of the various cooling tower types and constructions.
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Budget and chemical availabilty are also very important concerns ... Any scheme selected today may, of course, not be the best scheme for tomorrow !!

Sulfuric acid injection may or may not be the best choice for your facility

This paper may help:

https://www.cedengineering.com/userfiles/An%20Intr...

MJCronin
Sr. Process Engineer

hacksaw (Mechanical) · Answer 2 · 2023-10-18 13:46:00

Water test results?
acid strength?
degree of premixing?
pH measurement prior to injecting into tower basin?

This is a process design and control issue, so there is no quick answer

hacksaw (Mechanical) · Answer 3 · 2023-10-18 14:16:34

By the way, cooling towers recirulate the water and cool it.

Typically their water comes from pretreated water in a separate reservoir or basin, for sediment removal and water treatment, and premixing.

pierreick (Chemical) · Answer 4 · 2023-10-18 14:26:27

Hi,
Contact the supplier of chemical treatment in your facility. Not all treatments require Ph adjustment.
Check with Big Companies to get a meaningful answer.
Pierre

EdStainless (Materials) · Answer 5 · 2023-10-18 14:44:37

You don't do this by running full strength acid directly into the basin.
1. It is usually premixed/diluted to some degree. Yes this plumbing is usually made of exotic materials.
2. The injection needs to be at a location with significant turbulence, such as right after a pump in order to assure good rapid mixing.
3. You need to be careful where you are measuring pH for your control. Measure too close to the injection and the system will try to over correct. Measure too far away and the lag time will cause large swings in pH.
Get expert advice about this system and control.
Buy a prepackaged system to use. They are expensive and worth every penny.

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P.E. Metallurgy, consulting work welcomed

0707 (Petroleum) · Answer 6 · 2023-10-18 19:46:42

consult specialist in water treatement it is safer

mjpetrag (Mechanical) · Answer 7 · 2023-10-18 19:51:33

I found an error on the P&ID redline. It will go to a mixing chamber first. Nalco has designed the system. Once I get the revised diagram in hand I will update

-Mike

georgeverghese (Chemical) · Answer 8 · 2023-10-18 23:39:25

Why sulfuric acid ? Both MgSO4 and CaSO4 are insoluble in water, so carbonate and bicarbonate scale will remain on exchanger tubes. Even cheap acetic acid will do a better job of removing Ca and Mg scale, since the resulting acetates are soluble in water.

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Sulfuric acid injection for Cooling Water pH control