Wondering if there was equation to solve for pressure surges for gases (natural gas in my case) on slow closing valves.

The standards in object have been recently promoted from the "Committee Draft" (CD) to the "Draft International Standard" (DIS) status and, although still under development and at an inquiry stage

Pressure reduction of medium to low pressure saturated steam will always result in the steam superheating to a small degree.

The only thing I find is the time-honored "10% of set pressure". I know this is the typical value to prevent capacity/stability problems in conventional SV's, but I'm not convinced it is appropriate

It's normally a piping designer or engineer's job. The general considerations are:a. minimum back pressure or back pressure is considered in valve sizing.

Which Valve Manufacturers have been qualified in accordance with Shell SPE 77/312 Fugitive emissions specification? And for which valves have they been qualified?

Requirments for VR shops are contained in the licensing & cerification documentation of the National Board, Publication NB-23; the specifics are in section RA-220.

Typically control valve seat leakage is designated by the Fluid Controls Institute standard FCI 70-2 or IEC 534-4. A cage style globe valve complies with Class II or perhaps class III.