If the liquid nitrogen (LIN) is used for it's refrigeration value, then yes, generally the line is insulated.  If it isn't then there's little need except as you say for personnel protection.

If the LIN is used for refrigeration and the pipe is to be uninsulated, I'd suggest doing a thermal analysis using a coefficient of heat transfer of roughly 0.25 Btu/ft2-s.  That is, calculate the area of the pipe (OD*pie*Length) in square feet, and multiply by 0.25 to get the heat flux in Btu/s.  Then a simple first law analysis (which reduces to dH=Q) of the fluid will give you the quality of the fluid (or superheated temperature) at the outlet.  

Regarding supports, I could offer some distances, but I'd rather suggest a pipe stress analysis be done since supports are almost secondary to the stresses caused by thermal contraction.

PS: Most LIN lines use type K copper pipe here in the US as opposed to XS 304L which will be much more expensive.  There are also preinsulated type copper pipe systems which may be helpful.  I'd suggest discussing with your liquid supplier.

If there is personnel exposure it will have to be insulated. Also the line if not insulated properly will constantly drip water.  I would get with you supplier and check his recommendations as to the type insulation needed.  

Watch you valving arrangements and provide relief in the proper places.  No vents into the building.

Couple of tips, most come from experience.

Have a pressure relief device between any two sealing valves. Only used rated valves, valve stems must point up.
If you do not use insulation a thick layer of ice will form on the pipe, when it thaws out that ice falls down and can do some damage.
When you first start the liquid flow into the hot pipe, remember the expansion ratio is a hair under 700 to 1 volumetricly, you can get some high pressure spikes.
The pipe will contract due to temperature changes, design for it.
If you need insulation get the right type and size or it will not work.
If you do the pipe support design yourself remember the brittle temp of steel and how cold the bracing gets.
Know your safety rules the stuff is really cold, if you walk through enough of it your shoes will crack, wear leather.
Only vent it outside and do air sampling if entering tunnels or pits with the piping in it. I've seen cracked piping pour LN2 into a tunnel.  

Be safe the stuff can hurt you.

S

MikalT,

Piping for cyrogenic liquids is usually vacuum-jacketed piping.  A manufacturer of VJ piping, which the company I work for has used in the past, is "Quality Cryogenics."

Quality Cryogneics
425 Gennet Drive
Jasper, GA 30143
(706) 692-6167
http://www.vjpipe.com

MikalT
"processrob" is right on with his advice. Unless you are very, very good at applying insulation you will have nothing but problems with your pipe. I too, encourage you to think about vacuum insulated piping to your point of use.
There are several companies out there..

BTW We use copper and stainless tube (not pipe) for our short runs that are simple to insulate and maintain.

L8r
pennpoint