Pipe Supports

When a mass of water at significant velocity changes direction (e.g. at elbows), the acceleration entails significant forces, which can lift pipe off of supports, or cause shaking.

Water hammer (or fluid hammer).

Arrest the pipe with thrust blocks.

Hold the pipe on the vertical supports using "hold down pipe clamps". These will permit longitudinal movement but will not allow pipe to jump from supports.

There are many standards on Pipe support & Anchoring methods, you have to search. Pressure does not increase the load on liquid pipe.

"reason for the support-less pipe to fly off "

Yes it may fly off, if kept open to atmosphere on the other end. It can be compared with the behaviour of water hose left free on ground when pressurised water is charged. If both ends are fixed, it may vibrate heavily. This condition also can compared in similar way.

I have seen the effect in a newly laid 1500 m long 10" nitrogen pipe while charging nitrogen for blowing it. At the tail end pipe was laid on a 8 m high pipe rack in 'X' direction properly supported, takes a turn at point "A" in 'Y' direction for about 6 m length (not supported), turns downward in 'Z' direction for 7.5 m and then again turns to 'X- direction for 2 m with a free support at bottom without clamping and open to atmosphere for blowing. A 32 mm ribbed bar was welded one side with column of pipe rack to support the vertical pipe at 1 m hight on opposite side of blow direction. Once the pipe was charged with nitrogen for blowing from 1500 m distance, the tail end got twisted at point "A" by over 150 degrees after braking weld of 32 mm rod. I request members to comment on reasons for this too.