if i put a dummy on a bend thre isogen fails extraction.
If i delete dummies, isoGEN can extract the isos.

example: vertical dummy on vertical bend:
Imagine you have an horizontal pipe, from node 10 to 20. in node 20 there is bend and there line go up to node 30.
usually i insert a rigid (having same lenght from 30 to 20) called 20-100. in 100 i insert the a restraint (rest).

but due to isogen problem i tried to put the restraint (rest)on node 20 directly. BUT result changes and in isometric (by isogen) The rest is on 30 or 10, not in 20.

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30
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20---10----

how can i solve it?

You can insert an element on the bend (let say29 to 100) and restraint at 100. Isogen will extract the iso showing also the dummy

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30
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20---10----

if you insert a dummy in node 20 (on bend, and what about this dummy lenght?) the calculus result differ from inserting a dummy in node 30 (having lenght 30-20).

You need to insert in the mid point of the elbow (normally 29) and to use pipe element not rigid element.
Many times the exact location of the dummy insert is not required but if you need to maintain the modeling technique as you describe, note that isogen is a piping software where doubling the elements in the same space is interpreted as an error.
You can use a copy of your normal file just for the isometrics purpose where you replace the rigid with this pipe element.
Regards,