Flap wheel for teh win! Home Depot has them.

 

Remember, its only charge piping. The amount of stress/load is so minimalistic its ridiculous. 15psi at 150*F is not even close to fatiguing stainless, aluminum, or mild steel. Also remember, there are couplers. The couplers relieve almost all the stress the joints would have. As long as the welds are half way decent and pressure test fine for 40ish psi for a couple minutes I cant see a reason why grinding the welds would cause any problems what so ever. Grinding other welded parts is a whole other story as I'm sure you know way more on that than I do.

 

when we do bumpers at work we use a 4" flex-lock disc with a 36 grit pad to take the welds down and shape them.

then we finish up with a coarse scotch brite pad that locks onto the same disc.

personally I run these on a 4" angle grinder.

 

4.5" angle grinder with a flapper wheel here

 

Don't forget that 15psi means 15 pounds PER SQUARE INCH. How many square inches of surface do you think are inside the charge pipes? Multiply that by 15 and you'll see how much force is in it. Puts it in a different light, huh?

As I said before, the amount of penetration will be a factor. If you have very little penetration, you could have problems. If you get good penetration, sure...you won't have any problems at all. It GREATLY depends on the quality of the welds. It's what's left after grinding that's important, not what's there prior to grinding.

 

its charge piping, its not a big deal man. grinding down welds? nah. id rather them be a little ugly and strong than be pretty and break. engloid is right (always) dont grind em down, let that flux/mig spatter show!