Reboiler Tube Bundle Failure at Sour Stripping Unit 1

[Reformator]

Dear all,

In sour water stripping unit (SWS), reboiler had issue with pores/holes in tube bundle. In General Turnaround, this year, new tube bundle has been introduced to shell and after 4 month of operation pores/holes at tube bundle appeared. Material of tube bundle is Carbon Steel. Pores are in the middle of tube bundle near outlet of reboiler. In addition to that, it has been observed that in shell, from the middle to upper part, shell has small pittings. After taking analysis from stripped water it was determined:

Ammonia: 18 mg/l
Sulphur: 0.046 mg/l
Cl : 26.8 mg/l
Phenol index: 22.23 mg/l
Cyanides: 0.009 mg/l

Tube side - low pressure steam, Shell side - water from the bottom of stripping column.
Outlet temperature (shell side) of reboiler around 125 C.
Pressure in the inlet (shell) - 1.65 barg ; in outlet - 1.32 barg

Currently, we don't know what is the cause of corrosion, because it happened only in 4 month of operation. Do anybody had situation like this before?

 

[Dhurjati Sen]

I feel so inexperienced and naive when I answer your posts.

Anyway, since you and anyone from your organisation do not know the cause of corrosion, that's the best outcome ever !

Enjoy !!!


DHURJATI SEN

 

[cap1a79]

Have you checked oxygen content or possibility of the ingress?

SiHyoung Lee

 

[0707]

Corrosion of steel due to acidic sour water containing H2S at a pH between 4.5 and 7.0. Carbon dioxide (CO2) may also be present. Sour waters containing significant amounts of ammonia, chlorides or cyanides may significantly affect pH.

Primarily affects carbon steel. At a given pressure, the H2S concentration in the sour water decreases as temperature increases. Increasing concentrations of H2S tend to decrease solution pH down to about 4.5. Streams with a pH below 4.5 indicate the presence of a strong acid which would be the main corrosion concern. Above a pH of about 4.5, a protective, thin iron sulfide layer limits the corrosion rate.

In some instances at a pH above 4.5, a thicker, porous sulfide film layer can form. This can promote pitting under sulfide deposits which is maybe your case.

Critical factors include H2S content, pH, temperature, velocity and oxygen concentration. The H2S concentration in the sour water is dependent on the H2S partial pressure in the gas phase as well as temperature and pH. Other contaminants have a significant affect on water pH. For example, HCl and CO2 lower pH (more acidic). Ammonia significantly increases pH and is more often associated with alkaline sour water where the main concern is ammonia bisulfide corrosion. The presence of air or oxidants may increase the corrosion and usually produces pitting or underdeposit attacks in your case.

I think that un update of your tubes material will be required.

luis

 

[cap1a79]

Sour water corrosion occurs mostly at the overhead section, not most likely for the stripped water. Actually, reboiler located at the bottom of sour water stripper handles with "stripped" phase (corrosive component is off to the top). Also, regarding pH, the stripped water is not acidic for most of the case. Hence, I think it's better to check whether there is any possibility of oxygen ingress that can induce (general) corrosion by oxygen.

SiHyoung Lee

 

[Reformator]

Dear all,
Thank you for your answers

1. pH of stripped water is always at target between 6.5 - 7.5 and during period of 4 month of operation those parameters didn't change much
2. H2S is stripped around 99% in the column, so there shouldn't be any issue with H2S Corrosion and lab analysis proves that
3. Regarding oxygen ingression. We also thought that maybe the reason of failure is oxygen, but after checking the feed and water, no oxygen have been found.

Is there any possibility, that corrosion happened due to dual phase condition at the top of reboiler + highly concentrated Cl- at top of the tube bundle upon boiling?

 

[cap1a79]

Generally, corrosion failure for the stripped water is not common.
If you are confident that the oxygen has been controlled for sure, you need to carry out failure analysis to analyze the corrosion product. (By the way, I assume CO2 value is also negligible. If not, review it from CO2 corrosion perspective that can occur at the separated water phase.)



SiHyoung Lee

 

[0707]

Maybe you are having amonium cloride under deposit corrosion outside your tubes.

luis