Corrosion allowance with FBE lining water pipe

[BRIS]

We need to guarantee a service life of 50+ years for carbon steel water pipe carrying potable water. Specification is Fusion Bonded Epoxy lining plus a corrosion allowance.

Are we likely to get localised failure of the FBE and deep pitting corrosion, perhaps causing penetration, of the pipe wall or can we anticipate local failure of the FBE will result in undercutting and relatively large area corrosion ?

 

[gr2vessels]

Hard to predict, but the most likely failure would be the pitting due to holidays, pinhole defects in the lining. Once it penetrates the lining, it will be only matter of time the corrosion to extend between the lining and the pipe. Perhaps the concentrated pitting corrosion will penetrate the pipe wall before extensive damage will occur behind the lining. I just might be a sceptic...
gr2vessels

 

[rustbuster]

50 year guarantee, pretty tall order. If you can guarantee a holiday free coating it might work. Is it buried? what about external corrosion. Why not look at non-metallics?

 

[BRIS]

External protection is 3 layer PE system with cathodic protection. The majority of the pipeline length is GRP. Carbon Steel pipe is used in facilities and in high pressure areas.

On a previous project for the same client FBE plus a 3mm corrosion allowance was used. The 50 year life is total life. There is no residual value. The pipe can collapse at 50.1 years.

The question is does the corrosion allowance have any value.

Brian

 

[gr2vessels]

Theoretically, the lined carbon steel pipeline does not require corrosion allowance to the minimum calculated wall thickness required for the pressure containment. Also, the piping comes in schedules, not minimum allowable thickness. That is, the rest of the thickness is for mechanical strenght and corrosion allowance. Assuming that the external corrosion has been excluded by your external PE protection and cathodic protection, there is no reason for the pipe not lasting 1000 years, let alone 50 years.
Personally, I am sceptical of the above assessment and I prefer engineering judgement. In reality, the internal and external linig and cathodic protection will only minimize the corrosion and the piping break down.
I would start estimating a corrosion rate for the un-lined pipe, calculate for 50 years and add some percentage for unknown conditions. Add this to the pipe minimum thickness as corrosion allowwance, select the next up pipe schedule. Then provide the best corrosion protection (internal/external) for your pipe and work out some thickness monitoring system for the next 10 - 15 years, in order to have an idea of how the system works and maintains it's integrity.
However, as you mentioned, 3 mm corrosion allowance is a symbolic inclusion, rather than engineering calculation, hence no real value, unles confirmed by corrosion rate estimates as per above.
cheers,
gr2vessels

 

[brimmer]

A comment I would make is we don't know if FBE will stand up for 50 years, it has not been in service that long. My experience with FBE coating on pipelines, the coating will fail at specific locations, holidays or some form of disbondment and you will have localized pitting as opposed to large sections of the coating failing (assuming proper application). I would look at non-metallics as an option. If not, make sure external pipe surface is coated and has cathodic protection on as well. You might condsider intrenal corrosion management with coupons and probes etc., there are food grade corrosion inhibitors water distribution companies use for potable water systems if internal corrosion is a problem.