The importance of correct control valve fail-safe mode documentation in avoiding process incidents

摘要

Achieving and maintaining a safe state during abnormal situations is essential for the safety design and safe operation of process facilities. This applies first of all to process control valve loops. The selection of the different control loop components can become a bit complicated because there are several design options and at least two different signal-response actions per component. This is especially true for split-range control loops with exclusively sequenced control valves that operate in different control ranges. These valves will never be open simultaneously and their fail-safe position under loss of signal and 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. Such an unexpected response was a contributing factor to a recent incident in a heavy oil processing facility. It involved very large vessels and resulted in a loss of containment situation inside and near a building where more than 40 people were working. Quick operator intervention prevented worse from happening but some of the vessels needed to have their "fitness for service" reconfirmed.