We are using a close-loop coling water (CLCW) as a coolant media for
our plant. This CLCW is also circulted through another plant. During
last turnaround (T/A), we wanted to isolate our header in order to
clean some exchangers. However, there was a problem. The header
isolateion valves were passing and we could not isolate CLCW header to
our plant. The only solution is to shutdown CLCW to both plants which
is not feasable and have been rejected by management.
My Question:
Is there a way to have some kind of mechanical or whatever method that
will allow us to isolate the header to our plant only?

I am reading you have isolating valves which have failed or are not
closing properly.
There is a method used on smaller pipes which freezes the pipe and
creates a "frozen plug" as long as you keep it cold. This method could
use dry ice, co2, liquid nitrogen whatever is available.
The down side is that certain pipe materials might bust and it is done
mostly on smaller pipe sizes. Also it is difficult to keep freezing
for long periods of time. It is meant for emergency repairs were a
total shut down is not feasible.
You would freeze the pipe, do a cut behind the plug and imediatly
install a new valve (press fit). In your case on a closed loop you
would have to do it 2 times.

Is it possible to freez a flowing stream?