To change the selection list for AutoCAD P&ID to allow for stacked fractions the size selection list must be modified or a new list has to be created from within the project setup.
Project Setup -> P&ID DWG Settings -> P&ID Class Definitions -> Engineering Items -> Lines -> Pipe Line Segments
Select the 'Pipe Line Segments' and then click on the 'Size' row to select it. Then click the 'Edit'-button over on the right side. This will bring up the 'Selection List Property'-window.
You can modify the existing 'Pipe_Sizes'-selection list by deleting the original fractional values and adding the new values:
Or you can create a new list:
In order to get the stacked fractions into the selection lists you will need to use the Windows Character Map to view the unicode characters and copy them to the clipboard, for example: œ, Œ, Ÿ.
When the selection list is updated, click Ok and then Ok again to save changes and close out of Project Setup. No modifications are required to the default 'Pipeline Tag' annotations.
When the line is tagged with a fractional value, it should be displayed as shown below: