This is getting into the realms of leak detection and how to control any leak.
Most liquid pipelines respond well to many different forms of leak detection and all respond well to large / medium leaks.
Remote controlled actuated block valves are the normal operation now, but not many are fitted with any type of automatic system, mainly due to the potential for false operation or having to set the parameters so high that they are ineffective in operation. Most pipeline operators distrust any automatic shutdown system based on even the most sophisticated leak detection systems and at best have a certain window ( 45 to 60 seconds) built in to allow the operator to override a shutdown.
On liquid lines you can get normal operational and transient activities which can result in pressure falling quite rapidly, e.g. pump start / stop, start up of the line, change of routing and flow to different places. To prevent inadvertent closure you therefore need to set the rate of fall really quite high and hence they may only work in extreme full bore rupture scenarios which are relatively easy to see on a control screen.
So bottom line, no I don't think they are effective but they do seem to give non technical people / Environmental consultants some comfort that there is a system there which acts "automatically" so sometimes they get fitted for that reason alone.
George - I didn't get the thinking behind the "cheap carbon steel" bit. I would hazard a guess that 99% of high pressure cross country pipelines are built from carbon steel. If they are suitably designed, constructed, tested, operated, inspected and maintained in the correct manner then their chance of leakage is very small. The risk from third parties remains the single biggest cause of pipeline leaks which could happen if the pipeline was built of Duplex, but then the cost would be horrendous.
I assume you mean small pin hole leaks, not ruptures. There's no such thing as a small rupture....
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