Pipe Rack vs sleepers

Yes safety and accessibility are two of the main reasons.
You also will increase the cost of the plant because you lose the space under the sleepers for other important needs.
These needs include:
-Space for pumps
-Space for underground main electrical duct banks
-Space for mains for the underground drain systems

For more insight into Pipe Rack and Pipetrack (Sleepers) go to the following web site.
www.red-bag.com
On the left side of the screen go to "Library", then "Design Guides", then "Bn-dg-C1b"[/list][/list]

In addition you will find that many if not most Refinery, Petrochemical and Chemical plants have multi-level pipe racks. Sometimes Fin-Fan Coolers are mounted on top of the pipe racks.

If you went with multi-level sleepers you would need stairs (going up to the level of a typical third floor) to a stile to cross to the other side of the rack.

When there are Fin-Fans they are set on an elevation based on critical process conditions that would not change. So you would still have that high structure down the middle of the unit. You might as well go ahead and raise the pipe way decks up so you can walk and drive under them.

There is also the problem of all the branch lines entering and leaving the pipe way (Sleepers). Lines would be pocketed when this should be avoided and there would also be a "waterfall" wall of piping on each side of the rack and running the length of the unit.

All in all I think that, with some thought you will decide that "Sleepers" in a complex process unit would not be a good thing.

You asked, (Re. Sleepers vs. elevated Pipe Racks)
"What do you think could be the guideline for this kind of combination? "

The difference is the geographic location within the plant site. By this I mean elevated Pipe Racks are the choice for Onsite Units (complex process units) and Sleepers are the choice for Offsite areas (Tankage, Flare and transfer areas).