That is the way I would read it. If it is safe to clean the piping, the flushing system may be used.
Interested in Language
The flushing system may only be used when it is otherwise safe to clean the piping.
I would like to know if my thinking is right.
I understand "it is otherwise safe to clean the piping" as follows:
it is, in other aspects, safe to clean the piping where "it" is a dummy subject (as, for instance, in "it is so easy to do this thing") and does not refer to "the flushing system".
The fragment is taken from operating/maintenance instructions for HS-ISO pressure valve.
What about the dummy subject "it"? Is "it" really the dummy subject? I think so. Could you confirm or deny?