HIC, SSC, SCC, HCL and HTHA 5

[0707]

We have an air cooler condenser in a Vacuum Hydrobon process unit. Operating temperature is 260ºC. Operating pressure is 55bar G. Hydrogen partial pressure is 670 PSI. Susceptible corrosion mechanisms are HIC, SSC, SCC, HCL and HTHA. The project specified for the air cooler tubes & headers, 1 ½ Cr ½ Mo steel with NACE requirements according MR 0175 & MR 0103 standards.

Because of difficulties in performing PWHT requirements for strength welds of 1 ½ Cr ½ Mo steel tubes into the tube sheets headers, we are suspicious about the materials project choice.

What proven steel materials in the above conditions would be an alternative to 1 ½ Cr ½ Mo steel?

An answer from the forum would be appreciated.

Best Regards

Luis Marques

 

[metengr]

At a 260 deg C service temperature, first glance it would appear that carbon steel could be an alternate choice over the 1.25% Cr- 1/2% Mo alloy steel. However, without further evaluation and review of the API 941 (Nelson Curve), the owner may want an upgraded material for other reasons.

The HIC resistance for Cr-Mo and carbon steel relates to PWHT, sulfur content and inclusion shape. This can be optimized for carbon steel plate or Cr-Mo plate.

 

[0707]

Metengr

Tank you for the emphaty.

Up stream of the air cooler condenser, we have a continuous water injection to dilute pH and remove ammonium chlorides.

I also think that a ½ Mo carbon steel NACE HIC resistant & according API 941, it would be enough against HTHA.

The problem is that because of HIC, SSC, and SCC even if we use ½ Mo carbon steel we have to PWHT strength welds of tubes into the tube sheet headers.

Unless we use expanded tubes into the tube sheet with a joint coefficient lower than 1.


Best Regards

Luis Marques

 

[NickelMet]

0707 (Luis):

From the operating 260°C (500°F) and 671-psi Hydrogen partial pressure you state, the most current issue of API 941 Nelson Curves Figure 1 (6th edition, 2004) put you very close but still below the carbon steel curve. The data point is within 20°F (7°C) of the bottom of the curve. From my experience, this would probably call for an upgrade to a higher metallurgy (after a good statistical evaluation of the operation temperature and pressure as well as a look at the design conditions).

Note that C-½Mo is not included in that curve. API 941 specifically calls C-½Mo steels out in an appendix to the main document. And in that document states "...most companies no longer specify C-½Mo steel for new or replacement equipment used for operation above the carbon steel curve in Figure 1 because of the uncertainties regarding its performance after prolonged use." I, myself, no longer specify C-½Mo steels in areas that are HTHA susceptible (with regards to carbon steel) and I consider anything within 50°F (10°C) of the Nelson Curve - without crossing it - as potentially susceptible.

Now, regarding the other damage mechanisms you mention. Do I understand right that hydrochloric acid is among them (from your first post)? And do I understand correctly that you'd like a material that: 1) meets MR0175 AND MR0103 requirements (which are mutually exclusive documents these days - one for oil/gas production and the other for petroleum refining); and 2) removes the need for PWHT for construction?

~NiM

 

[0707]

NickelMet

You understood quite well regarding the other damage mechanisms I have mentioned hydrochloric acid is among them.

For our purposes we would like to have a material, which doesn’t need to be PWHT, because of technical difficulties in performing PWHT requirements for strength welds of tubes into the tube sheet headers, and meets MR0175 AND MR0103 requirements

I don’t know if this technical contradiction is possible.

Best Regards

Luis Marques

 

[NickelMet]

Luis:

I must say, you have an intriguing problem there. Nasty service to plan for given the conditions, contents, and material requirements. If you could eliminate one or two items, I think the choices would become much clearer.

With that, let me tell you that I haven't run into this type of environment in my career - yet. So, I can't accurately give you an answer that is from prior experience.

How likely are you to recommend a high alloy upgrade for the project?

~NiM

 

[johnnymist2003]

Hi,
first off, I am not a corrosion, nor materials engineer, but a vessel one.So forgive me if this post is useless. We are busy installing an air cooler with the following charcteristics.
Design pressure 5590 kPag
Design temperature 160 degC
Wet sour service-
H2 partial pressure 3730 kPaA
H2S partial pressure 384 kPaA
We have a wash water injection that reduces ammonium bisulphide levels to 5% wt.Chorides presence is negligible due to upstream treatment of the process contents.
We were not happy with the use of carbon steel (resitance to H2S attack), nor with Cr-Mo. Our metallurgy guys advised that 321 SS was an acceptable material to use. The steel was solution annealed and stabilized and had to pass intergranular corrosion testing to ASTM A262, Practice E.Tube-to-tubesheet joint was strength welded, and bimetallic finned tubes were used.No PWHT was necessary.This increased the initial cost, but in terms of the longer term benefits higher initial cost was felt to be justified.
Regards

 

[0707]

Johnnymist

Thank you very much for your input. In our case service, wet chlorides are not negligible. Chlorides are prone for austenitic steels even for the stabilized ones such as TP321. Our main corrosion mechanism is HTHA (high temperature hydrogen attack) but HIC, SSC, SCC and HCL are also present.

Regards

Luis Marques

 

[TomEun]

I would like to summarize the guidelines below;

You may have to consider all corrosion mechanisms step by step. Finally you may select the most noble material from each step.

(1) Salt deosit (NH4HS and/or NH4Cl) : CS ---> (Cr-Mo steels ---> 316L SS) ---> 2205 DSS ---> Alloy 825 / 2507 DSS/ 254SMO ---> Alloy C276 ---> Alloy 625 according to several factors, such as, pressure, temperature, Kp, velocity, pH, dew point, water washing, NH4Cl dispersants, etc. in API RP932-B-2004.
(2) HTHA : may be 1 1/4Cr-1/2Mo steel per API 941-2004
(3) SSC : for selected material per NACE MR0103-2005
(4) HIC : per NACE Publication 8X194-1994 if the selected material is carbon steels.
(5) High temperature sulfidation corrosion: material and corrosion allowance per NACE Publication 34103-2004

Note: 1. The PWHT for fin tubes to tubesheet weld joints is not recommended. See ASME Section VIII, Div.1, UCS-56 and Section IX. However the hardness control should be considered by appropriate WPS/PQR including mock-up test (for SSC resistance).
2. (materials) : not popular.

Hope this helps,

Thomas Eun
Corrosion and Materials Selection/Design Specialist

 

[0707]

Thomas

Thank you very much for your constructive approach.

Where does it state that The PWHT for fin tubes to tube-sheet weld joints are not recommended? Are you speaking in table UCS-56 for P nº4 note 2 (c?

Air-Cooled Heat Exchangers for General Refinery Service API Standard 661, Fifth Edition March 2002, also states:

“9.2 Post weld heat treatment

9.2.1 all carbon steel and low-alloy steel headers shall be subjected to post weld heat treatment. Welded tube-to tube-sheet joints shall be excluded from post weld heat treatment.”

Our main corrosion mechanism is HTHA (high temperature hydrogen attack) but HIC, SSC, SCC and HCL are also present.

After welding strength welds of tubes into the tube sheet headers, to meet MR0175 and MR0103 requirements we have to control the hardness of tube sheet to tube strength weld joints, in a way, to have less than 20 Rockwell.

Through Header boxes plugholes is very difficult to measure the hardness of tube-to-tube sheet strength welds. This is the reason why we are questioning the project.

If you have other ideas on this subject please share

Best regards

Luis Marques

 

[TomEun]

Luis, sorry I forgot to mention API 661 for no-requirement of PWHT of tubes to tubesheet joint.
However the requirements in Owner spec or sour service standards may overrule the exemptions in ASME Section VIII and API 661 because of the premature/catastrophic corrosion failure. Normally the most important requirement in SSC environment is hardness control (PWHT is only one of the ways), so you have to consider the best way for it.

For this reason, what I recommended is mock-ups test (but hardness measuring is one of the purposes of this test) because the actual hardness measuring is very difficult. It means you may be able to waive the actual hardness test by certified WPS/PQR and mock-ups if it is not available. The higher preheating (increase +50 to 100F) and/or temper bead technique and/or Ni alloy welding electrodes may be helpful to reduce the hardness.


Thomas

 

[0707]

Hi Thomas

Once again thank you very much!

Excuse me my ignorance but can you explain to me what do you mean about mock-ups test?

I understand that if we weld with inconel we can avoid PWHT, but Inconel welding is acceptable only for non-pressure parts, because along the time one can have cracks on transition zone. With inconel welding we also avoid excessive hardness but we don’t guarantee hardness lower than 20 Rockwell.

Best regards

Luis Marques