Here's how I do it, I have plugs made for the pipe that seal and clamp so they dont blow out. One plug needs to allow air into it, I use @80-100 psi. Use a torch and start heating the outside of dent and it will start coming out, work it into center. There is a fine line here, get it hot enough to come out, but not melt the pipe. That's it. Please note that you are pressurizing a vessel that is not rated for pressure and has no safety relief, it can be hazardous. I do not recommend the above proceedure to be performed carelessly. Safety precautions should be taken.