Abdelkader,
This may be considered an unimportant side issue but I do not think your picture correctly reflects standard pipe fittings or a proper installation.

What size it the core pipe?
What kind of elbow is the core pipe (long or short radius)?

What size is the jacket pipe?
What kind of elbow is the jacket pipe (Long or short radius)?
What is the wall thickness (wall schedule)of the jacket pipe?

What is the dimension of the "Spider" piece?

What is the "Gap" dimension you are allowing between the "Spider" and the I.D. of the jacket?