replace process piping.

[Inspector_1998]

Hello ,
I have process piping is high critical due to stream fluid ,constructed material is carbon steel ,maximum operation pressure 20 bar and temperature 30C.
piping class 3103 , size 6'' Sch.80 NT=10.97mm . as per the thickness measurement the minim actual thickness 6.0mm and the next opportunity for replacement after three years corrosion rate 0.9mm/year.this is unform corrosion along pipe (sulfidation).
Now, i want to understand when the replacement of process piping constructed as per ASME31.3 REQUIRE , Is there any critical or philosophy behind it ?
when i reviewed the history of process piping in area where i worked , i could not figure out the rule of thumb of replacement .
some piping replaced for the same reason when the corrosion allowance consumed and others not.
API570 and API574 are not covering this point.


regards,

 

[Dhurjati Sen]

It seems you are not having any inspection software which could tell you whether a replacement is required.
You can always calculate the remaining life as per ASME B 31.3 which would give you quite a margin.
However, piping replacement philosophy depends on other things like opportunity, economics, criticality of service and as you have mentioned, rule of thumb. Some call it experience, though.

 

[LittleInch]

is there critical assessment and thinking? Yes- it's called assemement of fitness for purpose and is a careful approach to lookigat the metal loss, the forces on the pipe and deteremining whether the pipe is still sufficiently safe to continue operating.

A corrosion rate of 0.9mm/yr sounds far too precise. Corrosion rarely occurs uniformly and is often concentrated in certain locations


There are simply too many factors to consider to come up with any single ROT on these issues.

Looks to me like you have the wrong material for safe and effective operation.

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

 

[KevinNZ]

What was the corrosion allowance in the original design?

If the corrosion is indeed even then, once down to the original allowance then the pipe will have to be completely design checked for all stress not just hoop stress. The pipe wall is not just required for hold in pressure. It also supports the pipe and resists earthquake and wind loads etc.

 

[GD2]

I agree with KevinNZ.That's what API 570 will lead you to determine 'minimum required thickness' in Section 7.0 (i dont have the latest version). A stress analysis with all loading conditions is a must.
Further,ASME B31G (which is much simpler to analyse than API 570) will guide to determine whether a corroded piping can remain in service, repaired or replaced.

Ganga D. Deka, P. Eng
Canada

 

[Ehzin]

Following up on the comment made regarding how specific 0.9mm/yr is; did you back calculating the corrosion rate that will get you to 3 years?

Thanks,
Ehzin

 

[weldstan]

Sulfidation type corrosion may be greater in weld locations. Have you checked the welds as well as the pipe?
Determination of replacement is based on safety (fitness for service) and cost.