FBE roughness and degradation with time

I would certainly be comfortable with 5 micron for internal FBE.

Probably less so if there is any evidence of coting disbondment or removal of sections where patches of internal coatings have come off or become damaged.

If its running then I would just benchmark an analysis using 5, 10 or 15 micron to reality.

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

Most FBE has an inner layer that is pure resin and very smooth.
If this does start to degrade then you are looking at the failure of the piping.
I do like LIs suggestion, recalculate at various values and see what the impact is.

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P.E. Metallurgy, consulting work welcomed

I agree with the concept of doing a sensitivity analysis on the roughness but I disagree with the values suggested. Trebling the roughness from 5 to 15 microns will increase the frictional pressure drop by 4 or 5% for a water line. What you want to know is what will happen if the lining fails completely and you are faced with the typical roughness of steel pipe - typically 50 micron and 10x the roughness of your lining. Increasing the roughness 10x will increase the pressure drop by 15 to 20% and this is the effect that you need to evaluate.

But I suspect that the aim of your lining is not to reduce the pressure drop. It is more likely there to prevent corrosion and that is what you should be worrying about if the lining is damaged.

Katmar Software - AioFlo Pipe Hydraulics

"An undefined problem has an infinite number of solutions"

Katmar makes a good point. Using an internal lining for corrosion control purposes is high risk as you cannot inspect the lining during installation or after.

If you're happy that the internal lining is good and you haven't been transporting liquid with hard bits in it, then the lining should be as smooth if not smoother than it was when installed.

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