Block valves, chain-locked open

Many relief valves pop open above or below their set point. Such valves have to be removed from the vessel and reset in a machine shop. Often, there are isolating block valves, located beneath the relief valve, that permit the relief valve to be pulled while the vessel is still in service. These block valves are perfectly legal, provided they are chain-locked open. It in unlawful to have an isolating valve below a relief valve that is not chain-locked open or sealed open in some positive manner. 

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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