You should design the platform for an expected uniform live load (typical is 50 to 75 psf for elevated access platforms) plus any extraneous concentrated loads anticipated by the use of the platform (i.e., the 500 lb. valve).  Since the valve will rest on a relatively small area, you might consider the 500 lb. either as a direct concentrated load or spread on a very small area (4 to 6 square inches).

I agree with Ron's approach but don't forget to add in any moment effects transmitted into the floor structure of your platform from handrailing, deck self weight etc.  In the UK we allow 1.5kN/m2 (around 30psf)as a UDL for personnel and then add any materials as point loads in the worst possible place.

Andy Machon
Andy@machona.freeserve.co.uk
 
 

thanks for the input, it was helpful.

Try OSHA section 1910.21-23