The line class specification for the system to be analyzed, calls for carbon steel pipe with 8mm carbide overlay (2 passes) with a corrosion allowance of 1.5mm.
Since the overlay has no benefit towards the pipe's strength and is there for abrasion resistance only, can I consider modeling the pipe with an increase in corrosion allowance? IMO that will account for loss of strength in the 'contaminated' cross-section of the pipe, where the first pass will modify the base material of the pipe. If so, what is a sensible value to consider? For safety reasons, I was going to suggest a total corrosion allowance of 3mm.

gabionex,

You did not provide system information and size of the piping that may give us an idea how sensitive the process is.

The piping may be very sensitive to the deflection, temperature and vibration due to the carbide overlay or other liners. I understand that you are using a flexible buffer zone with the first pass and the carbide pass on it. I would consider the full thickness with two layers with and without corrosion allowance (whatever the requirement says).

Some cases the the layer might require very low allowable to be able to keep the layer working under the operation conditions. You may find examples in the websites. One of them is the glass lined pipe. Glass lined pipes have very low allowable for the operating conditions.
You may need to find out the allowable composite section stress without causing problem in the layers by serial of tests approved by the autority.

If the process owner has the experience on the system without reducing the allowable stresses down, I would use the carbon steel properties in case the layer upgrade the allowables.


I would be very carefull with the catbide layer as well. In case there is a thermal gradient along the wall thickness and/or rapid variation in the temperature you may expect failures. If the downstresam has an important equipment you may need to put additional strainer to collect the failed carbide debris.

Hope it helps.

Ibrahim Demir

We would not use the overlay to take wall thickness credit for additional pressure containment, but the overlay will stiffen the pipe. We model overlayed pipe as:

Caesar WT: base pipe wt + overlay thickness
Caesar CA: line class CA + overlay thickness

This will give us the right loads due to the extra stiffness and weight of the pipe and also allow us to check that our SUS stresses are OK after the overlay and small CA are gone. The only issue is that the mill tolerance will be a little bit larger than it should be so you may run into the warning message about not having enough wt for pressure (in rare cases) but we have been ignoring that. (I guess you can tweek the mill tollerance so that everything works out but I don't think that is worth the effort.)

Regards,
Dave Clark / J. Zbitniff