While there is no "Standard" CA, commonly used CA in the petrochemical industry for carbon steel pipe is 1/16" for pipe in mildly corrosive service. Greater CA is mandated when greater corrosion rates are well established.
Not being an expert of corrosion or process system, just wanted to say that I've seen quite many times that CA is specified for pipelines in water projects. But I guess it is common practice inherited from many years ago. Nowadays we implement different lining and coating for the pipeline. AWWA M11 (for steel water pipeline) says:
I am not sure about the lining and coatings in the process engineering though.
