There's been an ongoing discussion among my peers as to why valve symbols are sometimes using numbers and other times letters to identify ports. It's been my understanding that pneumatic valve manufactures chose to use numbers while hydraulic valve manufactures use letters. (i.e. P=Pressure/inlet in Hydraulics vs. 1=Pressure/inlet in pneumatics). But some of my colleagues argue that it's an ANSI standard vs. ISO standard.

Can anyone offer any insight as to why you still see letters vs. numbers.

Wash your hands!!

I live in a state of confusion.....Oklahoma!

Thanks in advance for your replies.

All graphic symbology is now governed by ISO.  ISO is the officially recognized standard for symbology and that is ISO 1219.  This standard says that hydraulic valving shall label ports as per ISO 9461 standard to denote ports by alphabetic characters (A,B, P, T, X, Y, etc.). and pneumatic valve ports will be numbered (1,2,3,4,5, 12, 14, etc.)as per ISO 11727.