May 31, 2008

Shutdown Learning - Corrosion in Exchangers

In this shutdown, we openend many process exchangers, heaters, coolers etc for inspection and found different types of corrosions.
The most happening incident was in a cooler used as intercooler for ASU compressor. The strange thing was that all the baffles on shell side (cooling water) were gone. There was no indication of there existence except some marks on tube bundle as you can see in larger version of the below image.

Related references

The cooler is an interstage cooler in ASU unit for oxygen compressor. The cooling water flow is on shell side while gas was on tube side. Tubes were of SS & shell including baffles were of CS carbon steel. It seems that heavy corrosion has eaten away all the baffles.

If you can enlarge the photo & see it carefully then you will find marks of existence of those baffles.

Another example of similar type of corrosion is seen in other exchanger. This is a condenser for heavy glycol ether vapors. Cooling water is on tube side. So the corrosion is observed on tube sheet & partition plate.

The opposite side was also no better. In fact the damage is more here.

This is mainly due to electorlytic corrosion induced by presence of salts in cooling water. The easiest way to reduce it is to provide a Zinc Anode so that preferential corrosion or sacrificial corrosion saves your exchanger without much effort. One example is shown below.

These examples indicate that the cooling tower which is supplying cooling water to these exchangers doesn't have proper makeup water & has build up the concentration of salt beyond desired levels.

One should keep in mind that the contact of Zinc on the surface to be protected should be proper. In fact, one should prepare it by buffing or some other means to make it very clean.

Also keep in mind that there should not be any paint or coating on Zinc Anode as it is seen on some points in above image. This defeats the purpose of providing zinc anode. It is an usual thing that when you are in a hurry during shutdown you leave the job on contractor. The labourer does his best by providing coating on Zinc Anode also...............HOW Zzzzz THAT????????

So be careful, sometimes good thing are not so good to do.

Yes as an initial step we provided Zinc anode in these two exchangers.

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Anonymous said...

It is interesting that the exchangers reached such a malignant state without operations noticing the degradation in cooling efficiency ?

Was this exchanger outfitted with thermocouples and part of a supervisory computer monitoring and control system ?

If so, what was the trend in product quality, exchanger performance, effect on up-stream and down-stream systems and equipment ?

Thanks for the post.

profmaster said...

Yes you are 100% correct that this was a lapse from production people failing to monitor process as well as CW temperatures & that is the objective of this post that How we learn from misstakes.

Actually it was a air cooler is air separation unit (ASU). So the only impact was degradation in compressor capacity finally with higher power consumption.

The problem of lowering of capacity was noticed but not investigated thoroughly. Since downstream plant is the major product plant where oxygen is used in the process, we could not stop the unit & had to wear the losses till the annual turnaround. said...

We are facing the same tubesheet corrosion in our ammonia condensers due to electrolytic corrosion of salts in CW. Please share and advice the design and installtion procedure of zinc electrodes. Also tell us that where we can purchase these eletrodes?

Urgent reply would be appreciated.

Inspection Engineer
Engro Chemical Pakistan Limited

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