Maintenance platforms gaps between mechanical units

[Dan_summit]

I am working on designing some maintenance platforms that are around large cooling towers. The platform is greater than 10 feet off the roof below. I have guard rails all along the open sides of the platforms. However, I have a 6-to-9-inch gap between the mechanical cooling tower and the platform edge. This was requested for changing the filters. I know we will need kick plates to prevent falling tools, however, if the cooling tower extends 15 feet above the platforms, will a guard system still be required? What is the maximum distance between the solid unit and the platforms before just having a kick plate is not enough?