7 replies to this topic

[Arsal]

In petroleum refinery nelson said, desalting is required when Crude salt is above 10 PTB. it shows when salt is less than 10 PTB then these salt will not cause/impact too much to overhead corrosion.

But in our case(Arabian light crude)  we have always salt less than 10 PTB and after desalting we have salt in between 1-3 PTB but still we are facing too much problem in overhead system in which low thickness of overhead line frequently & condenser leakage problem etc. Although we are taking all measure to minimize corrosion like ,Corrosion inhibitor,NH3,Caustic,water wash etc.

My Question is that if 10 PTB is a thumb rule for desalting then why we are facing such type of problems?Either this thumb rule is not applicable?

 

 

[P.K.Rao]

Corrosion can be due to other reasons also such as presence of organic acids (carboxylic acids), ammonia. Please check copper in the condensate. Also check for organic acids using Ion Chromatograph.  This may help you.

Edited by P.K.Rao, 05 February 2015 - 10:53 PM.

[Arsal]

Thanks P.K Rao

But if we talk about Organic acid in the crude,organic acid (Naphthanic acid) corrosion will occur at temperature between 446-752 F and generally happens in a unit such as furnace tubes,transfer lines and side cut piping not in the atmospheric distillation overhead.

Kindly tell me the advantage to check copper in condensate.??it is for your information that we have a carbon steel piping at CDU Overhead.

 

I think it might be due to organic chlorides that are undesaltable. Please comments..............

Edited by rao arsalan, 07 February 2015 - 04:45 AM.

[P.K.Rao]

I mean carboxylic acids by organic acids. You are right. Naphthenic acids give corrosion at high temperature, usually at flash zones and furnace tubes. Carboxylic acids (formic acid, acetic acid, propionic acid etc) give corrosion at ordinary temperatures also.

 

Ammonia is detrimental to copper. Condenser tubes are made of brass (generally). So they can be affected by ammonia. If that is so, copper will appear in overhead condensate water. checking condensate (water) for copper will confirm condenser tube leakage.

 

The ionic strength of organic chlorides (example: carbon tetra chloride, ethylene dichloride etc)  is low or they are not ionizable  So they do not give corrosion. Generally crude oil does not contain these chlorides. They enter crude oil by contamination by fire extinguishing  materials. You can check organic chlorides in naphtha also for confirmation of their presence.

 

 

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[Napo]

Mr. Rao Arsalan,

 

I work in a little refinery in Ecuador (South America), the process charge is 3000 bpd, 20,5 °API (salt in = 20 ptb), and we have also problems in overhead system.

 

We don´t have steam (water vapor) in our system and we had a high chloride content in reflux acumulator, this moment we have onle 20 ppm. How much chlorides do you have in reflux drum?. What material is your overhead system?. What is the transformer voltage?.

 

We reach less 2 ptb, and we hope to stop our problems. We install a wash water system in overhead line, and we have put corrosion coupons before and after injection (neutralizar, corrosion inhibitor, water). We have done also other changes.

 

I think, 10 ptb of crude desalter is only a recommendation (Nelson, is an authority, but your book is from 1954, last edition).

 

Regards,

 

Napo.

[gegio1960]

Dear Rao and Napo,

in my experience 10 ptb of salts after desalter is an acceptable value unless you have catalytic units on the bottom-of-the-barrel processing.

Moreover, Arabian Light is not an acid crude.

In order to investigate this corrosion case, at least a couple of basic info should be known:

- temperature at top of tower

- configuration of overhead system (ie 1 or 2 condensing stages)

Good luck!

[Arsal]

Dear gegio1960

 

Temperature at tower top is 240-245 F

Although we don't have high chloride in tail water i.e always less than 10 ppm and Iron also reported less than 1 ppm.kindly suggest more to Minimize corrosion.

If we Check Naphthanic acid and organic chloirde in crude oil it may help to find out root cause??? 

Edited by rao arsalan, 10 February 2015 - 04:27 AM.

[gegio1960]

rao,

you need to know salt content in the crude before and after the desalter.

ARL is not an acid crude.

Is this a student exercise?

regards