Piping Rejection Thickness

[Mariocarlo]

Hello Everybody,
a little question:
- Which standard parameters are used to define the rejection thickness in a pipe?
For instance, only the corrosion allowance is considered or evaluations on the fabrication tolerance (to be used both as available corroding thickness) are made before to reject the pipe?

Thanks in advance for your thoughts

Mario

 

[LittleInch]

Your question is unclear.

do you mean new pipes or corroded pipes?

New pipe you use the nominal thickness minus the agreed tolerance.

corroded pipe you do a fitness for purpose assessment and calculation. Often an acceptable metal loss is more than the CA.

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

 

[Mariocarlo]

Thank you for the reply.
I mean for corroded pipes.
In general terms, before any fitness for purpose analysis, in the data base a rejection limit has to be defined. Using linear regression I can predict the replacement date.
This limit could be considered conservatively the nominal pipe thk less the corrosion allowance or, increasing a bit the risk,using the portion of fabrication tolerance, I can extend the life of the pipe.
I woudl know which system is generally used.

Thanks
M.

 

[RVAmeche]

API 574 has criteria for determining minimum required thickness based on service conditions or structural minimum. It basically uses the ASME B31.3 thickness equation and you confirm the measured thickness exceeds this value, so it is acceptable.

 

[Mariocarlo]

Thank you.

 

[beaklio]

Older pipes will thicker rejection to prevent overflow. I agree that new pipes could be good for thinner rejection as it is still durable to use.