Talk with any plumbing inspector and the one gray area in which there seems to be no agreement is when plumbing becomes process piping. Of course, this is a very important distinction. One part is regulated by the plumbing code; the other is regulated by engineering design. There are exceptions to that, such as certain hydronic systems regulated by the mechanical code. But I am not speaking about a building’s heating or cooling systems.
Let me give you an example. If you have a soft drink dispenser, is that plumbing or process piping? The answer is process piping. Once the water passes through the backflow preventer, it is no longer plumbing. It is water used for a soft drink.