Sharing of pipeline numbering and labeling rules.
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For the pipeline between two devices, regardless of whether its specifications or dimensions change, only one pipeline number is assigned; if there are pipelines branching off to other devices or pipelines in between, then separate pipeline numbers are assigned. For the venting and drainage of equipment, pipeline numbers must be assigned to any pipeline present. In the absence of a main pipe, a pipe number is assigned to the pipe leading to the farthest device with the same tag number, while the remaining pipes are given separate pipe numbers. Bypass pipes that connect two pipelines (not those of control valves), bypass pipes at the inlet and outlet of safety valves, as well as return pipes for equipment and pipelines, all require pipeline numbering.
Which manual is the original poster referring to? In my previous company, for some small cross-connects, as well as for systems used for draining condensate and waste, pipe numbers were not assigned; instead, only the pipe diameter was indicated