depends what you plan to use it for. generally it is for flushing, but it would be difficult to flush a 2 meter dia pipeline with anything other than a very large blow off. if it is for draining, than how fast do you want it to drain? can you use a fire hydrant instead? not sure why you would design a blow off for either 600 or 2000 pipelines. you should loop the line to allow constant circulation and unlikely that flushing would be a problem anyway if designed with proper velocity