3 replies to this topic

[Onurb85]

Hi,

On the past weeks a failure happened on a washoil drum (Vaccum unit) due to sulfidic corrosion (bottom area of the drum). This drum was never replaced (original from 1993 - P5 metallurgy) and until 2014 had very low corrosion rates even with high sulfur crudes (2.5-3.5% sulfur on washoil). Only after 2014, the corrosion rate increased up to 0.6mm/year. However, in the last 5 months, the corrosion rate increased to 6-7mm/year. Does anyone have any similar experiences on other refineries? Which kind of crudes were processed on those refineries? Any similar experiences on the HVGO section?

 

Based on the real process values (operating temperature, sulfur and acidity), the McConomy curves show a corrosion rate on that area of 0.5-0.6mm/year. What could increase the corrosion to 6-7mm/year?

Thanks in advance.

[PingPong]

I am not a metallurgist, but even if I were I don't think I could advise you based on the minimal info you provide.

 

You don't provide a PFD of the vacuum column and associated equipment.

You don't mention what changed in the operation of the unit in 2014, and again in 2016 just before the corrosion rate rapidly increased.

You don't mention which crude oil(s) you were processing before 2014, in 2016, and presently.

You don't mention the operating temperature of the drum before 2014, in 2016, and presently.

You don't mention the sulfur content of the washoil in 2016, and presently.

You don't mention the naphthenic acid content of the washoil before 2014, in 2016, and presently.

You don't provide photos of the corrosion.

You don't mention what the metallurgist of your company thinks about it.

Edited by PingPong, 25 March 2017 - 08:09 AM.

[Onurb85]

Sorry but I cannot provide a PFD of the unit, but I can detail a little more about the problem. However the real issue is to find out if anyone experienced this extremely high rate of corrosion due to sulfidic corrosion.

 

See below:

You don't mention what changed in the operation of the unit in 2014, and again in 2016 just before the corrosion rate rapidly increased.

 

Average %Sulfur processed

                 2012 2013 2014 2015 2016* 2017

Carga VV             1,52 1,90 1,90 2,20   2,66

HVGO -                1,10 1,40 1,45 1,80   2,25

Washoil -              1,52 1,90 1,90 2,20   2,66

Vac resid -            1,90 2,37 2,37 2,75   3,33

 

 

The design of the unit is : 4.7% on VAC RESID

 

You don't mention which crude oil(s) you were processing before 2014, in 2016, and presently.

Until October 2016, there were processed a wide variety of crudes. But since October is almost Ural crude and some Basrah Light.

 

You don't mention the operating temperature of the drum before 2014, in 2016, and presently.

Before 2014 - 320-340ºC;   2014-2015 - 350ºC     2016 - 335ºC      October 2016 to present: 355ºC    (operating design temp:385ºC)

You don't mention the sulfur content of the washoil in 2016, and presently.

See above

 

You don't mention the naphthenic acid content of the washoil before 2014, in 2016, and presently

 

   Average TAN (on processed naphtenic blends)

                 2012 2013 2014 2015 2016* 2017

Carga VV  0,88 0,92 1,13 0,98    1,13       not processed

HVGO       0,92 1,13 1,25 1,07   1,25        not processed

Washoil      0,60 0,65 0,58 0,38   0,62        not processed

Vac resid    0,28 0,32 0,45 0,30   0,53       not processed

 

 

The TAN of the crude and athmosferic residue analysed on the lab, is around 0.1-0.3
Not processed mean that feed of vaccum unit had TAN<0.5, so no naphtenic corrosion inhibitor was injected.

 

 

You don't provide photos of the corrosion.

 

You don't mention what the metallurgist of your company thinks about it.

The metallurgist thinks is sulfid corrosion for sure. But the rate is not explainable.

 

Im thinking more about the sulfur and how it decomposes in the drum. Do you know any article where mentions the sulfur speciation and how affects corrosion? Any experience where sulfidic corrosion happened at this rate?
Thanks

Edited by Onurb85, 28 March 2017 - 12:19 PM.

[PingPong]

I suggest you start with API RP 939-C and NACE 34103.

 

Without a PFD I was not quite sure where exactly that wash oil drum is located in your vacuum unit, but the operating temperature of 355 ^oC indicates to me that it must be for dirty washoil (overflash plus vacres entrainment) that is probably recycled to the vacuum heater.

 

Based on the real process values (operating temperature, sulfur and acidity), the McConomy curves show a corrosion rate on that area of 0.5-0.6mm/year.

Yes 0.5 mm/y according to McConomy, but such theoretical corrosion rate is in my opinion really too high to justify 5Cr material selection, unless they added an enormous corrosion allowance during the design in 1993. Nobody would select a material that corrodes at 0.5 mm/year for a new unit design, and nobody wants to continue operating an existing unit for years at such high rate.

 

Note moreover that API RP 939-C states about the McConomy curves: "There is significant scatter in the data, so these curves should be used with the knowledge that much higher or lower rates are possible (possibly by a factor of ten)".

 

I would expect that for such high sulfur contents the lower part of the vacuum column, and also this washoil drum, would be internally cladded with stainless steel. The drum apparently is not, so what about the column?

 

If the corrosion rate of the washoil drum is really in the order of 6 - 7 mm/year then there must also be severe corrosion of any other piping (especially bends) and equipment that is operating at similar temperature and liquid type, and made of similar 5Cr material. Verify also heater coils: are they merely 5Cr, or hopefully a better alloy?

 

What could increase the corrosion to 6-7mm/year?

If you are absolutely sure that that high corrosion rate measurement is correct, then there must have been a drastic change in the operation of washoil system or the whole vacuum unit in 2016.

I cannot guess what changed, your team should go through the operation history of the past year(s) and brainstorm about it. Look also at changes in heater outlet temperature.

 

Try also to verify whether the sudden corrosion rate increase was only a one time incident, caused by a change in operation that resulted in a sudden removal of an old layer of sulfide scale at the inside of then drum.

Edited by PingPong, 01 April 2017 - 01:42 PM.