when and how to conserve heatexchangers

[Yobbo]

In some of our powerstations auxiliary cooling takes place with surface water. River water as well as sea water. We are confronted with an aggressive kind of corrosion, that has been diagnosed as Microbiological Induced Corrosion (MIC). The cases, where this happened, were all preceded by a rather long idle period, where the apparatus concerned were filled with the cooling medium. No special measures had been taken. Apparently the damage has been done during these idle periods and even in an irreversible way. The applied pipe materials are Al-Brass. Application of chloration is not possible due to environmental regulations. I know that applying titanium pipes is a very
safe alternative, but I would like to know whether the existing heat exchangers can be operated in a safe way as well with a few extra simple measures.

My questions are the following:

1. Are there any guidelines for deciding when to conserve
a heat exchanger, when a certain idle period is
foreseen?
2. What type of conservation is considered best to
prevent MIC?
3. Are there any success stories available of alternative
protection methods to prevent or to decrease the effects
of MIC?



Karel Postulart, The Netherlands
Nuon Power Generation

 

[EdStainless]

The best is clean and dry.
Regrettably most of these units were not designed to be able to drain them fully.
This isn't an issue of chlorination wither. Even in plants that can chlorinate heavily, if they allow cooling water to sit stagnant in the system the Cl deplete and biological growth will start.
I don't usually see problems with outages of a few days, but when you are talking about 5 days or longer then a process is needed.
1. blow plugs or scrapers to clean tubes
2. flush with clean water (condensate or at least potable)
3. fully drain and dry as much as possible (use nitrogen or dehumidifiers)

These steps are commonly followed with everything from main steam condensers to small aux coolers.
We have supplied SEA-CURE tubing for many power plants specifically to provided them with a MIC resistant system (Ti isn't you only option).

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Metalguy]

You should perforn an eddy-current test on at least a sample of the tubes, if not the entire bundle.

If you can use it, ozone is a very powerful killer of bacteria. You only need ~1 PPM for 30 minutes, but it will probably harden any rubber gaskets in the system.

Also have all the water sources tested for sulfides and ammonia.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[unclesyd]

Good luck in your quest of spearing a windmill. I have worked on MIC problems since the late 60's early 70's when they took Chromium out of the water treatment business. FYI: Chromium salts are the only material that will mitigate MIC under all conditions. You will find that every problem area is different and what works across the road doest work under different conditions. Our biggest problems with MIC in the cooling water was resolved by very costly material changes for heat exchangers. We also have numerous heat exchangers that have to operate under conditions very similar to your conditions where we made extensive modifications to allow better drying during standby.


The first link is actually for a test kit for MIC but also has some good papers on same. Cotec has some VpCl system that work under certain conditions. The paper at One Petro has some very good information about the use of QA salts alone and with other materials.

http://www.mic-test.com/library1

http://www.cortecvci.com/Products/products.php

http://www.onepetro.org/mslib/servlet/onepetropreview?id=NACE-96290&soc=NACE

 

[MikeMet]

There is a great reference from MTI, I recommend you spend the $75

Guideline for the Mothballing of Process Plants
Ronald J. Twigg

Surveys major facility mothballing programs in chemical, petrochemical, refining, power generation, and oil exploration industries. Examines topics on economic considerations, protection systems and plant experience. Includes testing of protective compounds, inert gas purging and blanketing of equipment, protective programs, inspection, maintenance, and re-commissioning plants.

1989 by NACE for MTI, softcover, 128 pages, 2 figures, 3 tables. ISBN 1-877914-00-2

(AVAILABLE ONLY AS A PDF FILE ON CD)


MTI 9230-34$75.00