Material selection for carbon steel piping and stainless steel vessel

[andygo11]

In our material selection diagram, the material construction for the separator vessel is specified to 316 stainless steel due to the high partial pressure of CO2. For the separated water line at the bottom of vessel, we specify carbon steel. Now, the problem is that carbon steel and stainless steel have significant electrical potential, and both of them are contacting the electrolytic solution (separtated water). This may cuase the galvanic corrosion issue. So, in this situation, what grade of materials should be selected? Or any mitigation methods to avoid the corrosion? Thanks.

 

[LittleInch]

If your co2 is that high it is difficult to work out how you're able to use carbon steel when the water phase is whet you get carbonic acid ??

Assuming this is ok, what I've done in the past is simply electrically isolate the two systems using isolating flanges. You can get rtj flanges with isolating ring joint flanges and isolating bolt washers. If there is no electrical contact, you can't get galvanic corrosion, but by the sounds of it plenty of co2 corrosion....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SJones]

An insulating flange kit should be the last, and least favoured corrosion control option. One of the higher order options is to use an internally coated stainless steel piping spool, of a length between 10 - 20 times the pipe diameter, between the vessel and the flanged transition to carbon steel. Essentially, you need to reduce the cathode surface area in the vicinity of the transition. Do not coat the carbon steel, even when you replace the piping after it has corroded!

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[andygo11]

Little Inch:

I understand that the produced water line can be also corrosive from the separator with high CO2. However, there is level control valve (piping break) for the water line. For the front part of the line which is connecting to separator, we think the CO2 corrosion should be evaluated as same as the vessel, while after the control valve, the fluid is normally no flow, so we specify carbon steel.

For this separated prodcued water from separator, if it is continuous operation, should we consider it has the same corrosivity as the separator? According to process simulation, there is no gas phase, CO2, available, or at very low level. The producced water line is full water service.

According to MR0175, for sour service, the dissolution of H2S should be considered and calculated based on the last separator. This can also be verified by process simulation. It is the same way to deal with CO2 corrosion? If it is, but why no CO2 can be derived from process simulaiton?

Mr. Jones, could you also comment on this point?

Thanks.

 

[LittleInch]

I admit I'm a little out of my depth there, but once the CO2 has dissolved to form carbonic acid, I don't think it magically goes back to CO2 and water again just because you've released the pressure. The corroison rate from the acid alone won't be as high as i the higher pressure seaparator, but it will be higher than just water alone. I don't think the process simulation works this out, hence it thinks all the CO2 will simply gas off once you release the pressure.

Also when you de-pressurise produced water you will get gas - either CO2 or methane or low C gases - think of a Coke bottle when you take the top off after shaking it a bit and that's what you get in your produced water line d/s of your control valve so it normally goes to a gassing off vessel / knock out drum.

You may get away with a high corrosion allowance (say 6mm) which because it is low pressure may be Ok from a wall thicknees point of view.

I think H2S works in the same way, i.e. it forms some other acidic compound which doens't reverse back to SO2 and water just becasue you reelase the presusre.

If this wate ris prodcued water from a well it can be incredibly salty, so in fact once you get it out into the open, salt water corrosion could easily be your key issue, not CO2 or H2S. You will get oxygen into the system somewhere so be careful

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SJones]

If corrosion modelling is being attempted, it is easier to use the separator pressure to predict the corrosivity of the downstream separated water. Beyond the valve, it will be a dead leg which will require different corrosion considerations as indicated in the Energy Institute Guidelines For Corrosion Management. Uncoated carbon steel could be one way of handling it, particularly if the fluid entering the separator has received correctly injected corrosion inhibitor. ISO 21457, sub-subclause 7.2.5 also gives guidance on materials selection. Ultimately, materials selection will depend upon the operating philosophy, maintenance philosophy, integrity policy and risk tolerance of the end user.

As to the validity of the process simulation software result, you would have to consult a process engineer conversant with its use.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.