Seeking a better understanding of LSI/RSI ect. in practical cooling tower applications

[Mattyice]

I'm a technician at a powerplant working with one of my supervisors in an effort to save money on our cooling tower chemicals and improve our process. We currently spend roughly 250k per year on scale & corrosion inhibitors using a phosphoric acid as a corrosion inhibitor and HEDP for scale inhibitor - both are fed simultaneously around the clock. (Our facility has never faced issues with either phenomenon, but have always utilized both types of chemicals).

The question I have is, can a calculated LSI or other 'scale tendency' formula really be used to determine if our cooling tower circulated water is corrosive or scale forming -- Thus necessary to feed either the scale or corrosion inhibitors? -- and is there any 'dead band' in this range -- (For instance, using LSI, if we were to stay between -.2 and .2, do we need either chemical? -- do we need both?)

Our site has instrumentation on the circulating water for Temperature, pH, Conductivity, Alkalinity, and Calcium. We also monitor and control the make-up water pH, monitor the make-up conductivity, and monitor/control the free chlorine.

The doubts I have are from studies which suggest that 'all circulating water' is corrosive due to dissolved oxygen, web sites stating that water with an LSI between 0 and 0.3 can still be corrosive, -0.3 and 0 can be scale forming, ect. The fact that given the same chemistry values, LSI says the water is safe when the RSI calculation says the water is aggressive, ect.

I guess the real question I have is, can I use a calculated LSI number from our instrumentation to determine if our facility needs to be feeding a scale or corrosion inhibitor? Below is a simplified paint drawing of the system.

 

[EdStainless]

Are you doing the water treatment on your own or are you using one of the big chemical companies?
This looks like a power plant, base load or peaking?
This gets a lot more complex.
How much you have to worry about tight control will largely depend on what is in your water and what the materials in your system are.
Do you have a copy of the NALCO Water Treatment Handbook?
The pH is a balance between retarding scaling, causing corrosion, and optimizing the effectiveness of your chlorination.
For example this one advantage of SS tubing in condensers, this material doesn't really care about pH changes within a couple of points.
In general people set this up like any other system with statistical control limits.
You need to have your injection rates set low enough that you don't risk overshooting.
And the inhibitor should not need much feed other than to treat the makeup water.
Have any of you every gone to the Southwest Water Chemistry Conf?
There is also an annual water chem conf at UofI in Champaign IL.


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P.E. Metallurgy, consulting work welcomed

 

[Mattyice]

Our chemical supplier worked with us to establish our chemistry program, set control ranges, required dosages etc. The vendor also performs an annual condenser inspection and sends out a condenser tube coupon to have it examined. (They also sell us the expensive inhibitors we’re trying to reduce).

We’re a baseload operating power plant. Our cooling water is drawn from the Ohio River – its treated and disinfected in cold lime softening clarifiers. The make-up water pH entering the cooling tower is controlled with sulfuric acid.

The circulating water in the tower is continuously dosed with the inhibitors and bleach. The sodium hypo pumps are controlled by our DCS to maintain the free chlorine at 0.35ppm. The inhibitor pump doses are set manually but only need to be adjusted occasionally as our make-up requirements change. Blowdown rates are controlled automatically to maintain ~5 cycles of concentration. Water chemistry is performed twice a day to validate the pH, conductivity, free chlorine, alkalinity and hardness analyzers and test for the inhibitors.

The above ground piping is carbon steel, underground piping is HDPE, and the condenser tubing is 316SS.

I think I have seen that NALCO book around, ill try to find it and give it a read and I’ll also look into those conferences.

The idea I have is to use our instrumentation and DCS to continuously calculate the LSI value of the water entering and exiting the condenser. With these two LSI values we can control the make-up water pH in an attempt to keep the circulating water LSI as close to 0 as possible. Then, if either LSI value deviates too far from 0 (say +/-0.15?) then the DCS will start the corresponding scale or corrosion inhibitor pumps.

Is there anything glaringly wrong with that idea?

 

[Dhurjati Sen]

What is / are your objective(s) ?

Why you are so concerned with LSI?

Don't you have corrosion coupon racks at the inlet of the condensers which could be evaluated monthly?

You do not have any biological corrosion as there is no mention of biocides?

The tube material SS316L suggests that you do not face lot of tube side corrosion as others use Inconel tubes instead.

Study the following for better understanding:

Failure history of cooling water system and condensers.

Cooling water data at the sump, and at various sampling points like pH, TDS, phosphate content, chemical and biological oxygen content, etc.

Install the following for better monitoring

Corrosion coupon racks at inlet and outlet of condensers.

Corrosion coupons at low points and dead ends of the cooling water piping.

Unless you know the real scenario correctly, any short tern approach could lead to unforseen failures.



DHURJATI SEN
Kolkata, India

 

[EdStainless]

On the Ohio, that tells me a lot.
This must be 300-350MW.
You do need to worry about too much corrosion of the CS piping.
You don't need to worry about outright corrosion of the 316.
But you need to prevent scale formation, biofouling, and most of all Mn deposits (EPRI 10009597).
The biofouling and Mn may or may not be related.
As closely as you monitor I an guessing that you have some room to back off dosages.
I would start by looking to see what seasonal variations you have.
I know that the water will vary a lot so you might get some insight there.
Maybe a little less Hypo in the colder months (base it on inlet water temp).
And in colder months you max cond outlet temp should be lower so maybe a little less inhibitor.
Your costs are not bad though.

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P.E. Metallurgy, consulting work welcomed

 

[shvet]

@EdStainless
What does it mean EPRI 10009597?

 

[EdStainless]

It serves as the reference paper for Mn related pitting in SS condenser tubing.
I have 35 years of references on this subject.
I used to make and sell SS condenser tubing.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Mattyice]

Thanks for the EPRI reference Ed.
We haven’t seen any symptoms of Mn corrosion but it’s certainly something we’ll keep an eye out for. I think our clarification process should do a decent job at precipitating any MnO2 before the water makes it to the cooling tower, but I don’t have any data to back that up.

I think our plan now is to continue to gather additional chemistry data and set up additional coupon racks before experimenting. Then we’ll slowly lower our inhibitor doses while maintaining a fairly neutral LSI, and more frequent coupon evaluations until we feel we’ve proven the dose rate is safe. Then we’ll worry about adding some automation to the process.

Condenser outlet temps drop down to ~88f during low load operation during colder months. We try to maintain the basin temp at 80f year round.