Collaborative Roles of F3DG5S2-062A-220DC-50-DFZKV/B8 Isolation Valve, AST, and OPC Solenoid Valves in Emergency Trip System

I. The OPC Solenoid Valve: Managing “Minor Faults” to Avoid Unnecessary Trips

OPC, or Overspeed Protection Control Solenoid Valve, has a clear function: to handle “non-critical” overspeed events and minimize the need for unit shutdown.

For instance, if the unit is generating power and suddenly experiences a load rejection (the generated power cannot be sent out), the turbine speed will inevitably surge without the constraint of the load. This is where the OPC solenoid valve steps in. Its actuation momentarily interrupts the control oil to the governing valves. The governing valves immediately close partially, reducing steam flow and bringing the speed back down. Once the speed stabilizes, the OPC solenoid valve resets, the governing valves slowly reopen, and the unit resumes normal operation.

Essentially, the OPC acts as the “first line of defense,” dealing with “minor overspeed” with the goal of reducing the number of costly shutdowns. Without OPC, a minor grid fluctuation would cause a full trip, significantly decreasing the power plant’s efficiency and burdening maintenance personnel with constant startup procedures.

II. The AST Solenoid Valve: Responding to “Major Dangers” with Immediate Trip

AST stands for Automatic Shutdown Emergency Trip Solenoid Valve. When it operates, it signals that the unit has encountered a major fault requiring an unavoidable trip.

Examples include the speed exceeding a “critical value” (significantly higher than the OPC limit), bearing temperatures spiking to damaging levels, or lube oil pressure dropping to insufficient levels. In these situations, even a single-second delay can lead to catastrophic damage. AST solenoid valves are typically used in redundant groups of two or four to mitigate the risk of a single component failure during a critical moment.

Under normal operation, the AST solenoid valve is energized, maintaining the safety oil passage in a “blocked” state. This allows safety oil to be supplied to the main stop valves and governing valves, keeping them open. When a major fault is detected, the AST solenoid valve immediately de-energizes. The passage opens, and the safety oil is fully dumped (vented). Without safety oil pressure, all main stop valves and governing valves instantly close, forcing an emergency shutdown of the turbine.

The AST is the “last line of defense.” Regardless of the cost, its priority is to stop the unit immediately to protect equipment and personnel.

III. The F3DG5S2-062A-220DC-50-DFZKV/B8 Isolation Valve: The Safety Oil Path Gatekeeper

The F3DG5S2-062A-220DC-50-DFZKV/B8 isolation valve does not directly respond to faults like the two solenoid valves. It is the “dedicated switch” for the AST oil circuit, also governing safety during maintenance.

During normal operation, this isolation valve must remain open. Only when it is open can the safety oil flow out smoothly when the AST solenoid valve de-energizes and vents the oil. This ensures the steam valves close tightly, making the trip quick and complete. If the isolation valve is closed, the safety oil cannot vent regardless of the AST solenoid valve’s action, rendering the emergency trip system ineffective. The unit will fail to trip when it should.

The isolation valve is even more critical during maintenance. For instance, when replacing an AST solenoid valve, the entire hydraulic system’s oil should not have to be drained. By closing the F3DG5S2-062A-220DC-50-DFZKV/B8 isolation valve, the AST oil circuit is isolated from the rest of the hydraulic system. Maintenance personnel can then replace the solenoid valve without draining all the oil, saving time and cost.

Furthermore, the valve is necessary for unit restart after a trip. During the reset sequence, the isolation valve is opened first, and then the AST solenoid valve is energized. This allows the safety oil pressure to be re-established, the steam valves to slowly open, and the unit to start up. If the isolation valve is stuck closed, the unit cannot be brought back online.

IV. The Collaboration: Who Acts Under Which Condition

During normal operation, the status of the three components is fixed: the OPC solenoid valve is on standby (inactive); the AST solenoid valve is energized (blocking the safety oil vent); and the F3DG5S2-062A-220DC-50-DFZKV/B8 isolation valve is open (AST oil path is active).

• Minor Overspeed (e.g., Load Rejection): The OPC solenoid valve acts first, partially closing the governing valves. The AST solenoid valve and the isolation valve remain stationary. Once the speed drops, the OPC resets, and the unit continues to run.
• Severe Fault (Speed Exceeds AST Limit): The AST solenoid valve immediately de-energizes. Since the isolation valve is open, the safety oil rapidly vents, all steam valves fully close, and the unit trips.
• AST Solenoid Valve Maintenance: The F3DG5S2-062A-220DC-50-DFZKV/B8 isolation valve is closed to isolate the AST oil circuit. After repair, it is opened, the AST is energized to re-establish safety oil, and the unit can be restarted.
• Minor AST Oil Circuit Leak Inspection (Non-trip): The isolation valve can be closed to isolate the AST circuit for inspection without shutting down the unit. Once the issue is resolved and the isolation valve is reopened, power generation is unaffected.

V. Real-World Issues: Don’t Overlook Collaboration Failures

In many power plants, emergency trip system failures are not due to a single faulty component, but rather a breakdown in collaboration.

For example, if the isolation valve is stuck closed, and maintenance personnel want to replace the AST solenoid valve, they cannot shut off the oil flow, forcing them to drain the entire hydraulic system’s oil and causing several days of delay. Alternatively, if the isolation valve cannot open fully, the AST solenoid valve may de-energize but the oil vents slowly, delaying the steam valve closure. This causes the trip to be late, leading to a higher overspeed margin.

Another scenario: the OPC acts, but the speed keeps rising. Investigations reveal the AST solenoid valve did not prematurely actuate. Instead, the isolation valve was not fully open, hindering the AST oil path, slowing the safety oil vent, and causing the steam valves to close sluggishly.

Therefore, routine inspection must not only verify whether the OPC and AST are actuating. It must also check whether the F3DG5S2-062A-220DC-50-DFZKV/B8 isolation valve is correctly positioned and free from sticking. This involves simulating a minor overspeed to check if the speed drops after OPC action, and simulating a severe fault to ensure the isolation valve allows rapid oil flow when AST acts, confirming quick steam valve closure.

If your power plant has experienced issues like “speed still rising after OPC action,” “slow tripping after AST action,” or “stuck isolation valve,” the problem is likely poor component coordination or an improperly specified isolation valve model. Contact us to help you review the protection logic and test the compatibility of the F3DG5S2-062A-220DC-50-DFZKV/B8 isolation valve to ensure dependable protection.