Steaming a pentane line will be very easy to get hydrocarbon free. Generally this is prep work which can be done at night. A reasonable "in-plant" line can be steamed via a 1" steam hose for a few hours. A few hours before morning you can cut the steam and put under N2 purge to dry. My opinion is that 20% LEL at the vent is not good enough for cold cutting or hot work. With pentane you should be at zero shortly after steam appears at the end and that is what you need to be at when you sniff- anything else probably indicates a pocket of hydrocarbon residue somewhere that did not get benifit of the steam. Pockets can occure anywaywhere there was not a good steam flow, even high point branch lines (i.e. no high point vent usually means liquid won't drain well). Sniff in the pipe at a few low points, you can never be too careful with hot work.
The condensate created during initial warm up can be fairly significant in some cases, but your steam will keep it hot push it out of the way to the extent needed to get through. If you N2 dry then you can get the rest during dry out. Pressure and depressure at low points is the best way to dry, but operators rarely have the time for this so they usually leave it as a continuous blow which can take longer.
The two hazards with condensate are: 1) hot condensate can burn, so be sure to direct the discharge drain safely away rather than blowing wildly; 2) introducing large volume of uncontrolled steam into a cold line can create a long slug of fast moving condensate which can hammer and even rupture a pipe. The latter hazard is why you want to use a 1" steam hose, and warm the pipe up in a controlled manner. If you are steaming from a steam hose station which has a valve to control the steam flow, then you don't really need one at the pipe you are steaming.
These are just my opinions from general experience with preparing equipment, others may have different experience. best wishes as always, sshep
