How Close Are We to Unmanned Supercritical Plants? A Discussion on Fail-Safe PLC Sequential Control Designs
"Automation" has become an overused buzzword in the industry, leaving most customers indifferent to this claim. In fact, many people mistakenly equate automation with installing a touchscreen, adding a few sensors, and enabling remote temperature and pressure monitoring. Frankly speaking, this is merely data visualisation, not real unmanned operation.
Two years ago, we upgraded the control system for a botanical extraction client. Their old equipment relied entirely on manual operation: operators adjusted extraction pressure and temperature by observing analogue gauges, manually regulated CO₂ flow, and judged the completion of solvent recovery based on experience. The plant ran three shifts with two operators per shift, whose daily work was to monitor pressure gauges continuously, resulting in severe eye fatigue after each shift.
After the system upgrade, the PLC sequential control system took over most operational procedures. However, the core reason the client deemed the upgrade cost-effective was not the sophisticated touchscreen, but a series of subtle and practical fail-safe designs.
First, pressure-balanced switching protection. Supercritical extraction requires alternating operation between multiple extraction vessels. When Vessel A finishes extraction and switches to Vessel B, uneven pressure during switching will generate instantaneous pressure differentials that damage equipment seals. We added a pressure comparison logic to the PLC sequence: the switching valve can only be activated when the pressure difference between the two vessels is less than 0.5 MPa. Previously, the client needed to replace seals every three months due to unregulated switching; after the upgrade, the original seals have remained functional for over a year.
Second, redundant emergency depressurisation interlocks. High-pressure systems require reliable pressure relief mechanisms, but blind venting is extremely dangerous. If the system still contains raw materials and the internal temperature remains high, sudden depressurisation will cause material splashing and equipment damage. We set up three interlock conditions for pressure relief: no venting when the internal temperature exceeds 60°C, no venting during stirring operation, and mandatory remote operator confirmation before venting. It is better to delay operation for three seconds than to trigger irreversible risks in one second.
Third, accidental start protection. The HMI start button requires a 3-second continuous hold to activate, and the system automatically conducts a full self-inspection of valve positions, pressure status and temperature ranges before startup. The client initially found this mechanism cumbersome. However, the design once prevented a safety accident: a maintenance worker accidentally touched the start button during on-site operation, and the system did not respond at all. That 3-second delay effectively avoided potential safety hazards.
Unmanned operation does not aim to eliminate manual labour, but to free operators from repetitive mechanical work, allowing them to focus on decision-making and exception handling that require human judgment. This is the core principle we adhere to in all control system designs.