Operating a flowline made of duplex steel (A790 S31803) at a lower temp. than it was designed for.

[kjdush]

Hi!

This is regarding operating a flowline made of duplex material(A790 S31803) at a lower temperature than it was designed for.

This flowline was designed for a lowest temperature of -20C around 25 years ago for Oil production. Now it is producing only Gas. After the chock valve, because of the pressure reduction, the gas temperature is coming -38°C.

The question is, can we operate this line at -38°C? The material data sheet for the pipe says the Charpy Impact test is done at -20°C. The thermal stress analysis of the piping system has been updated to check for -38°C and it was found acceptable.

The new duplex pipes (A790 S31803) that are used in the Oil industry in Norway have a lower design temperature of -46°C. Note that these new pipe materials have criteria for Charpy Impact Test at -46°C.
I have read articles where it says, Duplex A790 S31803 materials are used for services up to -50°C.

Can we operate the line at a temperature of -38°C?
If we operate at -38°C, do we have to take any kind of mitigation and what kind of mitigation?

Hoping for some answers

Thanks

 

[BigInch]

It is true, the material should be fine for lower temperature service.
You should get samples of the pipe and test it at the lowest temperature it will see.

 

[SJones]

Weld zones will most likely be the location of highest susceptibility to fracture. Could be difficult to sample those. Try to examine the details to see if you really have a problem. Is this -38 deg C a measurement, or a model prediction? Is it a metal temperature, or a fluid temperature? What is the stress level in the pipe at -38 deg C? What is the NDT history of the welds? And so on. Unfortunately, fitness for service documents are light on duplex stainless steel, as compared to carbon and alloy steels, in terms of correlating Charpy energy results to fracture toughness to allow a fracture assessment. It could be a good ides to start a dialogue with a fitness for service specialist.

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.

 

[kjdush]

Thankas to both BigInch and Sjones for the answers

We thought of removing a small pipe spool and test it for Charpy at lower temperature. But, because of the lack existing documentation tracing of the Heat number for the different pipe spools, forgings and castings is a big task.

We are tested the well and found that the metal temperature of the pipe downstream of Chock is -38 deg C.
Since it is 25 year old platform, I am struggling to get the NDT reports for the welds. API 579-1 Part 3, Brittle Fracture analysis can be applied only for carbon and low alloy steels. We are in the process of dialogue with FFS specialist currently. I couldn't find any literaure on FFS for brittle fracture of Duplex stainless steel. If you come across any such literature, please pass on to me. Thanks.

 

[LittleInch]

Two different issues here.

Can you operate? - In practice almost certainly yes

Can you get anyone to sign it off and approve it for use without further tests - almost certainly no.

At the lower temperature, unless you do some specific tests at that temperature on a sample of the actual steel in place (sacrifice a small spool), then it will be difficult to get the data you need, especially with fracture toughness and you might need to do some weld tests as S Jones describes.

This might be better in the metal and metallurgy forum, but add in what your values are and some more details.

Although you said the thermal stress check had been updated, did any of the material properties change to reflect the new temperature or have they just run a stress check with new temperatures? What sort of percent maximum stress did they find?

you probably won't have a problem, but if you just change use and do nothing and the line goes snap, then you and your company are going to be in big trouble. Failure due to low temperature effects is a real issue and cannot be simply accepted "on paper".

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kjdush]

You are absolutely correct LittleInch. Practically we can operate at -38 deg C. But the problem is that we can not approve it with documentation without proper evaluation. As you said the low temperature is a real issue.

Regarding the thermal stress analysis:-The stress calculation is updated with the new temperature(-38 deg C). The newly calculated thermal stress range is 66%.

In your reply regarding the stress calculation, you have written"did any of the material properties change to reflect the new temperature" . What kind of material property changes one need to update in the stress calculation, apart from the temperature?

Thanks.

 

[georgeverghese]

Is this -38degC a startup transient ?

 

[kjdush]

No, -38 deg C is the metal temp during the normal operating condition.

 

[LittleInch]

Maybe the strength (SMYS) doesn't change much, but you would probably need to check that and the UTS value to make sure that your stress results don't change.

Probably not, but duplex does some strange things sometimes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Presume you have gone through ASME VIII, Section 1, UCS 66 to derive an envelope of permissible lower design temperatures and pressures based on the charpy impact test results you have now ? Will a lowering of the current design pressure for this flowline and associated equipment help to bring this back into the permissible operating P/ T envelope?

 

[SJones]

In case it wasn't noticed, the CS in 'UCS' stands for Carbon (& Low Alloy) Steel. Also, why would the OP go through ASME VIII for a pipeline? There are limited tools for an engineering critical assessment of duplex without destructive intervention on the equipment.

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.

 

[kjdush]

We have taken the dicision to replace the downstream piping with Duplex material according to the new Material Data Sheet (charpy at -46 deg C).

As SJones mentioned, there is not an easy critical assessment that we can do for duplex material without destructive intervention.

Thanks to all for the valuable comments and suggestions

 

[georgeverghese]

In that case, perhaps a search through B31.3 doesnt help with derivation of a P-T envelope for duplex either?