My problem with compression fittings is that when I tighten the nut the pipe inside the nut also turns a little bit. This can be a big problem because it leads to loss of compression (and leaks) if there is a fixed nut in close proximity or to loss of geometry in the rest of the piping.

In the past I have been able to avoid it by holding the twisting pipe with a grip plier. The problem is that it requires "three hands" and leaves gripping marks on the pipe. Sometimes, to stop the twisting the gripping can lead to damage to the pipe.

I know I can use push fit fittings or soldered joints but I would like to know how to avoid this problem for future needs and for my particular situation now:

I am required to use compression fittings, with an existing compression service valve separated by 5cm of free pipe from a 90 degree compression fitting.

Thanks,

Antonio