The threading action of the mandrel into the expander draws the seal against {HEAD ~ the tubesheet} to seal {TAIL ~ the testing apparatus} and allows the rib plug to be in situ pressure tested 
to seal
The threading action of the mandrel into the expander draws the seal against { HEAD ~ the tubesheet } to seal { TAIL ~ the testing apparatus } and allows the rib plug to be in situ pressure tested
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH