A Process Incident Caused by Different Fail-Safe Positions for a Control Valve Under Loss of Signal and Loss of Instrument Air

It is essential to select correct and unique fail-safe positions for control valves in order to achieve a safe state under abnormal conditions. The implementation of fail-safe positions can become a bit complicated in case of split-range control loops, such as blanketing systems, where control valves operate in different control ranges. The fail-safe position of the pad and vent valve of a blanketing system, under loss of signal as well as under loss of motive utility, requires special design attention. If the design was flawed, or was compromised during maintenance, then the system can respond in an unexpected manner during certain operating modes. The incident under consideration involved an over-pressurization event of very large vessels during a shutdown and resulted in a loss of containment situation inside and near an occupied building.