Chloride Limits for Short Term Operation - SS347 & SS316 8

[PS6788]

I'm trying to determine the maximum chloride content we can accept in one of our hydroprocessing pilot units before pitting/crevice corrosion and CSCC becomes a significant issue. By significant I mean that we could see issues develop in the span of a few months of operation or expect to replace a significant amount of tubing and fittings in the unit after a run. Does anyone have some relevant experience to share?

We typically run for about a month or two before going into a turnaround. The metallurgy of the unit is predominately SS347 and SS316. The areas I'm mainly concerned about are the wash water injection point which is at elevated temperatures (~250F) and dead ends. Wash water is injected in large excess so there is no chance of drying out. Heat tracing maintains temperature above water dew point in the hotter parts of the unit. The wash water will have some dissolved oxygen. The H2S partial pressure could be as high as 25psi.

 

[PS6788]

He EdStainless and ironic metallurgist,

Thanks again for your help on this topic. From what I gather, our choices are to refuse chlorides all together, make significant metallurgy upgrades to our unit or take on significant risk of SCC. I don't think we can accept the additional risk, so that leaves us with metallurgy upgrades or refusing high chloride feed stocks.

Again, thank you for helping me fully understand the situation.

 

[EdStainless]

One reason that 2205 and AL-6XN are popular (each in their own applications) is that they can handle what normal 300 series alloys can't.
AL-6XN is virtually immune to SCC in neutral environments below 250F and resists pitting in horrific environments.
And 2205 has useful SCC resistance, being almost impossible to crack in neutral solutions <20ppm and it will handle up to roughly 100ppm at boiling.
See page 4

https://www.materials.sandvik/en/ma...l-datasheets/tube-and-pipe-seamless/saf-2205/

The real issue in 300 series alloys is that they might pit or they might crack.
The cracking might be a thousand tiny little cracks or one through the wall.
And cracking could take months, or minutes.
It all depends on local conditions.
The principal stress is the key, and most of the stress is related to fabrication and thermal expansion and not your service pressure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

In the Operating company I used to work in, 2205 duplex would have been the minimum for applications with chlorides, even at ambient temps, with or without any dissolved O2. Surely not 316L. Safety first.

 

[Chumpes]

hello PS6788

temperature 200F is kind of low temperature in the effluent of hydroprocessing units, and washing water injection was located here in order to dissolve (or to avoid formation in case of continuous wash water injection) of NH4HS solid salts that could condense at temperature lower than 200F in dry systems.

Be carefull that in presence of NH3 and HCl, the possibility of condensing NH4Cl solid salts at temperature higher than 200F should be anticipated, which would maybe require a wash water injection at a higher temperature to avoid fouling and pressure drop, and higher metallurgy too.

if operated for short time only, maybe you could avoid wash water injection without problem of fouling ? and clean / wash at turn around with clean or conditionned water.

see API RP 932B for details
NACE MR0175 part3 tables may also be helpful

SS 316L is not enough to resist your corrosive environmeent as soon as chloride ions and oxygen are present at quite "high" temperature (200F) for SS 316L

SS 904L is better but probably not good enough in your chloride H2S O2 service.

I would not use DSS 2205 or another DSS because, even if more resistant than SS 316L vs. chloride ions, DSS cannot resist the probable high pressure H2S you have in your system (NACE MR0175 part3).

Usual grade we use is Ni-Alloy 825 which to my knowledge never failed for similar application.

6%Mo grades are also a good compromise that should perform also very well in similar services.

If you cannot modify metallurgy SS 316L/347, then can you avoid wash water injection ??

pH calculations are possible and would lead to pH between 4.5 and 8.5 depending on the NH4Cl/NH4HS ratio

regards

 

[Guest102023]

The ‘3 Bad Amigos’ for metals are chlorides (or halides in general), hydrogen, and sulphur.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"