You have to know the pipe ...

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You have to know the pipe diameter. Can you find the pipe exposed where you can either measure the circumference or diameter to find the outside diameter? Assuming it is standard commercial pipe then look up the wall thickness in the pipe table and calculate the actual inside diameter.

v=21.22 Q/d^2 where

v = velocity is meters/sec

Q=Flow in Liters/Minute (convert your flow required)

d=pipe inside diameter in mm (Note: d^2 is inside diameter squared)