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Crude Unit - Material Of Constructions

cdu moc naphthenic acids corrosion

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#1 gegio1960

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Posted 12 July 2019 - 01:48 AM

Dear Experts,

I'm not very expert in little/modular Crude Distillation Units (CDU) and the relevant Materials of Construction (MOC).

For the big plants, I've always seen a big attention to the various corrosion aspects:

- wet H2S

- S at high temperature

- salts (eg water + chlorides)

- naphthenic acids.

As a consequence of that, very few parts of the CDU were made in Carbon Steel (CS).

Now, for the modularized CDU, several specialized vendors are proposing a 100% CS unit.

Could you kindly revert on your experience in this field?

Thanks,

gegio



#2 Napo

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Posted 12 July 2019 - 12:11 PM

Hello Gegio1960,

 

You need to have precaution, the presence of "contaminants" in crude oil is a very very important factor in the design stage.

 

You can put attention to "corrosion" problems. If you haven´t a good desalter of crude oil you will have many problems specially in the top tower and overhead system (air cooled, fin-fan).

 

In our Topping Plant (3000 bpd) we needed to change the overhead condenser (carbon steel) after 6 month of operation. We need to add a "aggresive" chemical treatment. We have high salt content after desalter (over 4 ptb). In our Plant we haven´t steam.

 

Also we have problems in the top tower (we have a monel cladding). The tray of the top are of Monel.

 

Napo.



#3 PingPong

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Posted 12 July 2019 - 01:02 PM

As you already suspect: the size of a CDU, or whether it is fabricated modular or not, has no impact on the MOC required.

 

It is not clear from your description what the scope of work for those "specialized vendors" is. 

Do they also size all equipment, lines, instrumentation, et cetera?

That could then also be a recipe for disaster.

 

 

Nowadays even full size process units are sometimes fabricated in big modules so as to save construction time. However the equipment sizes, line sizes, instrumentation, et cetera, and their MOC are specified by the licensor or the design company, not by the manufacturer of the modules.



#4 gegio1960

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Posted 13 July 2019 - 03:24 AM

Thank you Napo/Cesar.

I hoped in your answer :-) for your long and daily experience on this kind of plants joined to a wide knowledge of the "theory" behind them.

Anyway, it seems to confirm that people are selling this plants in CS. Please correct me if I'm wrong.

In my opinion/experience, desalters and chemical treatments are not enough to protect the CS against the various kinds of corrosion,

Monel in the top section and duplex in the overhead system are in line with my experienze.

ciao,

gegio



#5 gegio1960

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Posted 13 July 2019 - 03:39 AM

Thank you Ping Pong.

I completely agree with you.

But, anyway, there are a lot of people/organizations in the world going around to sell this kind of plants... and I suspect that the most part of the little units are made in CS, with all the relevant problems.

Unfortunately, the people responsible for the investments are very sensible to the money and some of them prefer "the egg today"...only because the egg costs less than the hen.

I think that also your motto "In theory, there is no difference between theory and practice, but in practice, there is" is well suited to this situation.

Kind regards,

gegio



#6 PingPong

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Posted 13 July 2019 - 06:46 AM

I have no experience with small modular CDU's completely designed by vendors.

Small units that we designed were based on our equipment sizing, line sizing, MOC, even when they were then built into modules.

 

I suggest you insist that all bidders supply, as a minimum, a sized equipment list, including mech design conditions, MOC of shell and internals (trays, packing, impellers, exchanger tubes, heater coils, ........),  and  corrosion allowance. Also performance guarantees and lifetime guarantees should be provided, or imposed by you. 

Not only so that you can verify MOC but also that you can see whether price differences between bidders are merely the result of smaller equipment sizes and/or cheaper MOC.



#7 Bobby Strain

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Posted 13 July 2019 - 09:12 AM

I always advise my clients not to invest in small refineries. So, if you follow my advice, there is no concern about MOC.

 

Bobby



#8 Napo

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Posted 13 July 2019 - 05:14 PM

Gegio1960,

 

Yes, you can buy a CS plant, but after you will have some problems.

 

There are a quotation (in spanish): "the cheap is expensive".

 

Napo.



#9 gegio1960

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Posted 21 July 2019 - 01:23 PM

Hi Bobby.

I made the same recommendation...but I'm only a technical consultant. You perfectly know that the business is driven by others.

Kind regards,

gegio






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