Mafunctioning instrument air regulator incident

Instrument technicians were re-calibrating a control valve positioner. The feedback arm between the valve and the positioner was disconnected, a mA source was connected, and the valve was stroked using the flapper & nozzle arrangement inside the positioner. While doing this, due to a faulty regulator, full instrument air pressure of 8.5 bar was applied and the top of the diaphragm casing ruptured propelling it far away. On investigation, inside the regulator, three screws had become loose in service due to vibration. The regulator did not have full rated relief and no external relief device was fitted as part of the original valve supply.

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