I have a 700ft layout across the roof of a builing,,plus a 48m pump,any suggestions on on doing the blow out without killing someone???  or making to big a mess.

Each job is one in its own, make sure you have a pressure regulator,a bleed valve, to control the mess I would get a skid pan or concretewashout bin and I would blow the pipeline separate from the boom,make sure you have a pig catcher and the discharge end elevated so the concrete doesn't stack up at the end and just suck your sponge or pig or waterwash as you normally do with your boom,be careful air pressure is deadly make sure no one is in front of your discharge when using air,regulate your air enough just to keep your pig moving its tricky but it can be done safely,good luck and if you dont feel comfortable dont do it get someone with experience blowing line to help you,are get a bunch of laborers and hand dump and wash the old fashioned way  

What pudg told you is ok, but if you really want to take the "worry" out of this task, then get the concrete company to bring you a "Blow Out Mix". It will make everything a lot safer. And if you have any kind of problem, the mix will not set up before you can fix the problem. Blow Out Mix should be like a regular mix but with no cement and fly ash in its place.

We use it all the time on our high rise jobs, it blows out with very little force thereby making everything safer. Then blow another sponge ball through the pipeline to make sure the pipe is clear.

Just before pumping your last two yards or so, have the blow out mix standing by,(it won"t set up), then switch out trucks and finish the pour with the blow out mix.

Think ahead and you can save yourself a lot of grief and expense.