Vacuum relief impulse line

With all due respect to the findings of the investigating team, the root cause appears to me to be a human factors situation. The new piping system looked simple but in fact this unique system was confusing. A check valve (valve no. 2) was installed on the vent header to prevent cross-contamination. A small block valve (valve no. 4) was installed on the vacuum relief impulse line to provide easy isolation. [5]... 

A small block valve is usually installed at a tank on any instrument impulse line just in case a leak develops on the small tubing. In this unit block valves are generally avoided on overpressure or vacuum relief devices. If a block valve that can defeat an overpressure device cannot be avoided, such a valve is chained and locked open or car-sealed opened and a protocol is developed to administratively control accidental closing of such a prime safety feature. 

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