You can lock the nominal size of an existing pipe or fitting to ensure that it is not accidentally changed when you are modifying the pipe routing. For example, you might lock the diameter if a pipe run passes in close proximity to building elements or other systems, and the pipe diameter cannot be increased without interfering with other components.
To lock the nominal diameter
When locked, the nominal size list is inactive and the pipe or fitting diameter cannot be modified. You can, however, still modify the length or elevation of a pipe segment. To turn off a size, unlock this setting on the Properties palette.