Hello again ranayogesh, if you are seeing porosity in the root welds using the GTAW process this is likely due to insufficient preparation of the joint or if this is used pipe it could also include contamination from the contents of the pipe. Be sure to use new uncontaminated grinding wheels and flap-wheels on the bevel preparations and also be sure to flap the I.D.'s of the pipe joints at least a minimum of 1/4" back from the bevel edges. If you are using mechanical beveling units with any sort of machining fluids you will need to be sure to use a non-residual cleaner after making the bevels so as not to leave any contaminants. GTAW requires clean, clean, clean.
     As to porosity or slag entrapment with the E7018, generally slag entrapment is due to incorrect welding parameters(running too low on the amperage possibly), or incorrect rod manipulation by the operator(possibly incorrect rod angles, running the bead downhill and allowing slag to run ahead of the puddle and then become entrapped, keeping too short of an arc length which will sometimes prevents the puddle from forming a nice round or oval deposit as welding progresses). Porosity with E7018 can come from using wet or improperly stored electrodes(not keeping them in a holding oven), not paying attention to the ends or length of the flux coating on the electrodes(if the flux has been chipped away from the starting end of the electrode or portions of it are missing along the length these flux voids will possibly cause porosity). Keeping too long of an arc can introduce porosity in some cases. Making restarts with used electrodes and knocking too much flux back from the end of the electrode can lead to porosity at the point of the restart or tie-in. These are a few things that come to mind, I'm sure there are others that can apply and hopefully the others on the forum will chime in to include them for you. Best regards, aevald