This is a bit long winded.

It isn't really flushed, it's scrubbed. Hydo fill, drain will take some material out but cleaning is not it's purpose.

Transmission lines are laid on a production basis, ie the front end (pipe gang) lays all pipe that doesn't call for a road bore, creek/river crossing, PI (pipe intersection), or any other obstructions that will slow down the pipe gang. They are on a mad dash to lay as many joints as possible per day.
The pipe gang consists of a large group of people. Some buffing ahead, some preheating, several tractor drivers on side booms and tack rigs (crawler tractor with several mounted welding machines), several laborers throwing skids, the bead hands (putting in the root pass), hot pass hands, hot filler hands, inspectors on all phases. All of the above is a well oiled machine and moves as swiftly as possible with little or no wasted movement.
The front end has to leave a break in the pipe wherever there is a forementioned road bore, creek/river crossing, PI, etc (they don't lay slow moving sections, their job is high production).
They also have to leave a break in the line even on long straight sections because the crawler (internal tracked X-Ray machine) can only travel so far without new batteries. Plus temperature variations will cause the pipe to grow (or shrink) enough to come off the skids.
That means that the line pipe is in sections of various lengths. After the pipe gang, comes the firing line which welds out the joints partially welded by the pipe gang. The pipe gang welders walk, and weld off the tack rigs. The firing line works off their welding rig. Tie in and Lowering in tie in welders work off their welding rig. Tie in welders make tie ins at those road bore/crossings/PI's/etc. They leave the pipe in sections that can be hydro tested/pigged. Lowering in tie ins lowers the pipe into the ditch and the welders make tie ins for them, still leaving the line in sections for testing.
At test time, welders will weld test headers onto both ends of the pipe section to be tested. Hydro is then commenced. After hydro the sections may be pigged as they are, or tied together to pig longer sections.
After hydro, the test headers are cut off and fabricated pig launchers and receivers are welded on.
First pigs run are almost always scrubber pigs with aggressive wire brushes to remove rust, slag, dirt and debris. The owner or owners inspection makes the determination on how many pigs of each type are run. Pigs are inserted at the upstream end via the pig launcher, and driven by massive amounts of air pressure till they plop out at the pig receiver.
When the owners rep is happy with the scrubber pig process, they will start to run foam pigs for further cleaning, when happy with that process they will run drying pigs. Owners rep will often check the dewpoint inside the pipe to determine acceptable dryness. This is very important in natural gas and other service.
Drying pigs may be run for some time. It's not unusual to run several hundred pigs thru a section before approval.
At that point the section can be tied in to the next, assuming it has been properly pigged.
This process continues down the line until all is good, clean, dry and happy.
Final tie ins are made (usually hot) and viola, the line is now in service.

Smart pigs are run thru in-service lines, they usually use UT technology to identify dents, bulges, cracks and wall thickness reductions due to corrosion (internal or external), pits, and other anomolies. Those are than analized and dug up and repaired where necessary.

J