material damage to SA 106 Gr. B piping 1

[dennjay]

A pipe carrying steam was exposed to over temperature event for approx. 2 hrs. (180 C above the design temperature). This happened due to malfunction of VHP (very high pressure) let down valve. Results of the external visual inspection revealed that no permanent damage to the piping. However, client concerns long term structural damage to the material. What are the metallurgical tests that I should conduct to see any damage to the material?

Do I have to perform accelerated creep testing?

material grade: SA 106 B
nominal size 6", schedule 80 Xtg.

 

[MOB1]

At the very least I would suggest a check on diameter (Pi Tape), a replica or site metallography to check the micro structure and a hardness check.

 

[metengr]

Agree with the Pi tape and visual inspection of the piping system, however, replica or hardness would not be of use for such a short term event.

The OP did not mention the actual design temperature for the piping system. Assuming the upper limit of design temperature for carbon steel piping is the temperature where time dependent (creep deformation) properties occur (650 deg F) an over temperature excursion event above this at 180 deg C would put you at around 1000 deg F. For a 2 hour duration, measurement of pipe diameter swell would be your best information. Keep in mind that you have a lag time during this 2 hour event where the pipe wall temperature has to catch up with the steam temperature.

 

[dennjay]

Thanks for every one for your valuble input. As metengr ask,

Pipe design temperature: 350 C (662 F)
Pipe design pressure: 10350 kPa (1500 psi)
Pipe opeating temperature: 330 C (626 F)
Pipe operating pressure: 9000 kPa (1305 psi)

I am conducting replica analysis, metallographic analysis and micro hardness tests on pipe sections we recevied. This incident occured 5 months ago. During that time one of our inspectors were on site and did checking for pipe bulging, sweling. No abnormality was found.

Do I have to conduct any test other than those mention above? Thanks for your reply in advance.

 

[metengr]

No.

 

[MJCronin]

dennjay..

Carbon steel piping systems can operate indefinitely at temperatures up to 775F, as per the ASME B31.1/B31.3 piping codes. The 662F for two hours is no big deal..

At extened long term operation above ~775F, carbon steel systems can exhibit cracking due to what is called "carburization"

The following document may be of help..

http://www.absa.ca/Forms/AB-507 Installed Fired Heaters Guideline.pdf

Please complete the thread and let us know of your final decision.....

-MJC

 

[Metcorr]

It is difficult to expect 'carburization' happening in carbon steel pipeline in steam service. May be you meant ' graphitization'.

 

[PipingMaterials]

First of all, the operating temperature mentioned seems to be incorrect sice at the pressure of 1305 PSI the steam will have a saturation temperature of around 500 Deg C. Secondly the thickness selected i.e.10.97mm Sch 80 / XS is undercalculated. If one designs the pipe line as per ANSI B31.1 the minimum thickness after considering the corrosion allowance and after allowing for the negative tolerence of 12.5%, works out to not less than Sch160. Hence there seems some apparent Design mistake.
Please be informed that A-106 Pipes are suitable up to a Design Temperature of Appx 400/425 Deg C and beyond this design temperature one must use A-335 Gr P-11. Hope I have tried to address the issue to your satisfaction.