ISA S 5.1 does not show symbol for a pneumatic speed control unit for a pneumatic valve. However, it is common to see a variable restriction with a check valve orientation to depict speed controller ( metered in or metered out). Is there any standard document ( e.g., ISA) that provides direction on this?

 

There are different variations of pneumatic speed controllers so there is not one symbol that suits all, see the supplier catalog of the particular flow controller you want to use for the symbology