CL 300 vs CL 600 Flanges

If this has been correctly described, you should be all right if you torque or tension the flange bolts only to the level needed for Class 300 applications.

[This is a simplified answer, which could possibly be more complicated in certain circumstances.]

Thickness of the raised faces has very little to do with the gasket sealing capabilities. The thickness of the gasket , the width of the gasket, the choice of type of gasket and material whether metallic or non metallic with internal and external rings are the main guiding factors for gasket sealing. Class 600 valve with class 300 mating flange is quite acceptable from sealing point of view since the pipe pressure rating is class 300. But this is not a common practice.

Also, for smaller valves of up to 1 inch , the industry wise 800 class valves are used without regard for the line pressure rating . This avoids wrong use, wrong selection of under rated valves, and is cost effective from manufacturing point of view.

This is in common practice where mostly flanged valves are used. Because this will reduce inventory in the stores and when big lots are purchased there is marginal diffrence in cost of class 300 and class 600 valves.

As gasket dimensions for class 300 and class 600 flanges are same below 3" size, sealing area is never a concerned. Even bolting sizes are also same.