I have found many pipe installations where "process pipes are supported by pipes".

In some cases there is a spring can support hanging vertically from one pipe to another pipe, and in other cases there is a rigid support (and short) vertical support between two pipes.

And it is also common to see small diameter pipes supported with horizontal "tie bars" to larger diameter pipes.

It is my understanding that this is not a "Best Practice". However there is nothing in ASME B31 to prevent this type of construction/design practice.

Does anyone here have experience or comments on "supporting pipes from pipes."

How important is it to avoid this type of design, and should existing occurences be re-designed?

It is not 'best practice' to support one pipe from another, but there are many situations where it is the only practical solution, particularly for small bore lines in open plot areas. Provided additional load in the supporting pipe is considered along with a study of relative movements, there should be no objection.

One reason for bracing (support with tie bars) a small bore pipe to a larger diameter pipe is to avoid unwanted vibrations and fatigue problems in branch connections.

In fact, I have clients that have made it common practice to brace all small bore pipes in such a way.