Depending on the brand and model, YES you can linearize an equal percentage valve. The cam can be used to modify the input signal/valve travel relationships to any characteristic desired.
As with any drawings, the cleaner the drawing with the fewer number of jogs in lines, the easier they are to understand. Make your main process lines heavier so the main flow is more readily apparent. Leave lots of space for future details/additions. Use standard ISA SP5.1 definitions for...
If the tank contains water and is in a cold climate, make sure the vent is heat traced, not just the tank. I worked on a water tower that collapsed when the vent froze over in the middle of winter. Big pain in the butt fixing stuff at -40 degrees!!
