Effective ID comes into picture only to calculate Pressure thrust , primarily for an untied joint or to evaluate forces at tie rods (if they are input in expansion joint modelling).
It is difference in id of pipe and covolutions and that only should be considered to calculate thrust forces on nozzles or anchors in line of thrust.
For slurry service, you probably need to have a lined below , if at all you can't avoid it altogether.
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SJ