Dear Sirs,

I'm new in pipe supporting. I have attached two type of supporting for a simple line. which one is the best and why?
I don't understand why most of engineers use type#1(trunnion).

What is the purpose of the vertical loop?
1. Dodge obstruction?
2. Expansion?
3. Maintenance?

I'm not sure I'd agree that most engineers would opt for example 1 as it would push the pipe up/down depending on temp-delta. I think we need more information to post an "informed opinion".

The purpose of the loop is to show a hypothetical situation. It does not matter why the loop is there. The object is to give an answer on supporting a configuration.

So, I would say:
a. Both methods are acceptable and are used.
b. Extenuating circumstances such as location(on-site vs. off-site) and the potential presents or absents of any overhead pipe supports will impact your decision.

Both methods are acceptable but on this particular case I prefer the first one.

If you look at the way the second one is set up, you will notice the system is not "balanced". That is, the loop is not supported axially in the vertical runs, so it tends to fall and cause bending moments. Usually, it is good practice that all piping be well supported and balanced. Such that if you run a SUStained case check and look at the Deformed Shape, there shouldn't be any region where significant sagging occurs. This is a quick way to check that.

A reason why not to use #1 would be if you are dealing with high pressure piping. I usually stay away from welding dummy legs unto thick schedule piping.

But as always, it all depends on your particular situation and on available steel.

IMHO example #1 is the better arrangement.

Ignoring the Z direction restraints, the supports add stability and reduce flange loads to a minimum. I suspect that a modal analysis would give better results too.

Example #2 will cause unwanted tension loads and bending moments to act on the flanged joints, due to weight.