Two miners were in the fab shop making a repair on a WA700 Hydraulic Tank. EE began to weld with EE's left arm inside the cylinder, while other EE held a flashlight above with right hand. The first weld (about 1 inch) did not cause any issue, EE then began to weld again, and immediately the cylinder ignited causing a full flame blast up EE's arm. It also engulfed other EE's hand.