【2026 Actuators】Common accessories for shut-off valves (solenoid valves, check valves, check valves, quick-release valves……) – principles, selection, and common on-site issues

I. Positioner: The “brain” of the control valve. Function: It receives 4-20mA signals from the DCS/PLC, controls the pressure of the output air supply, thereby ensuring that the valve position corresponds accurately to the input signal. It is the most essential accessory for control valves; without a positioner, the valve can only be used for simple on/off control (or rely on the proportional action of the pneumatic diaphragm itself, resulting in very poor accuracy). Common types: mechanical (such as Siemens, Masoneilan), intelligent (such as Fisher DVC, Siemens PS2, Yamaki, STI, etc.).

I. Solenoid valve: The “on/off button” of the shut-off valve. The solenoid valve is the most critical control element of the shut-off valve. The DCS/PLC outputs a 24V DC (or 220V AC) signal; when the solenoid valve is powered or de-energized, the direction of the air flow changes, thereby controlling the opening and closing of the valve.

1. Two-position three-way vs two-position five-way “Number of positions and ways”: “Position”: the number of working positions of the valve spool. The two refer to two positions (one position when powered, and the other when not powered). “\"Throughput\": Number of interfaces (including intake ports, exhaust ports, and vent ports). Two three-way solenoid valves: have 3 connections (inlet P, outlet A, exhaust R). It is commonly used in single-acting actuators (spring-return). When powered, P→A, allowing air to enter the actuator ； In the event of a power loss, A→R, and the actuator vents (spring-return). Two five-way solenoid valves: they have 5 connections (intake P, two outlets A and B, and two exhausts R1 and R2). Used for double-acting actuators. When powered, P→A, B→R2 ； In the event of a power loss, P→B and A→R1, enabling two-way intake and exhaust.

💡 On-site selection: Single-acting with spring return → Two-position three-way, double-acting without spring → Two-position five-way

2. Single solenoid valve vs dual solenoid valve: Single solenoid valve: one coil; it operates when powered and returns to its original state when power is cut off (or vice versa). Suitable for ordinary interlocking logic. Dual solenoid valve: two coils (usually “dual coil” or “single coil + single coil”), one for opening the valve and one for closing it. Even if one coil burns out, the other can still operate the valve.

3. Series and parallel connection of dual solenoids. Series connection (safety type, 2oo2): The air paths of the two solenoids are connected in series; both solenoids must be powered simultaneously (or de-energized simultaneously) for the valve position to change. If any solenoid valve fails (the coil burns out, the valve core gets stuck, or power is lost), the valve cannot operate, which functions as a form of \"fail-safe\" mechanism. This configuration prevents accidental operation, but at the cost of availability (any failure will result in the valve failing to operate). Applicable scenarios: Interlock protection circuits require extremely high reliability; it is better to prevent operation than to cause incorrect operation. For example: steam turbine emergency shut-off valves; misoperation can result in huge losses due to plant shutdown ； Reactor feed valve; improper shutdown may cause an explosion.

Parallel connection (available, 1oo2): The air circuits of the two solenoid valves are connected in parallel; the valve position can be changed as long as either one is powered. A failure of a single solenoid valve does not affect the valve’s operation, but it may increase the risk of accidental activation (for example, if a solenoid valve is energized accidentally). Applicable scenarios: frequent operation of valves where high availability is required, such as programmatically controlled valves; accidental activation has little impact, but failure to activate can disrupt the production process.

SIL level considerations: For safety circuits with a SIL level of 2 or higher, the “1oo2” or “2oo2” architecture is commonly used, and the redundancy configuration of solenoids must be coordinated with the logic controller. Series (2oo2) is suitable for applications that require high safety (low false alarms) ； Parallel (1oo2) is suitable for applications that require high availability (low failure rate). Sometimes, three solenoid valves of the “2oo3” type are also used to balance safety and availability.

⚠️ Common error on-site: In a series circuit, the power supplies for the two solenoids come from the same fuse; when that fuse blows, both solenoids lose power and the valves fail to move. Correct approach: Provide separate power supply and include a fault alarm.

II. Positioning valve (lockout valve): Acts as a mechanism to maintain the current position in the event of a loss of air or power. Function: When the supply of instrument air is interrupted or the pressure drops below the set value, the positioning valve automatically shuts off the air supply and locks the valve in its current position, preventing it from opening or closing completely due to the lack of air supply. In cut-off valves, a hold-back valve is usually installed between the solenoid valve and the actuator.