How Solenoid Valves Work: Actuator, Control Valve Working Principle

This video demonstrates how to program a PLC to control multiple pneumatic cylinders in a specific sequence using solenoid valves, where a timer (T0) with 3-second preset and a counter (C0) with 6 preset values coordinate the sequential advancement and retraction of six cylinders through a series of conditional statements that activate specific solenoid valves based on the counter's current value.

A pneumatic solenoid valve assembly consists of a butterfly valve, pneumatic actuator, and feedback box with micro switches; the PLC controls valve operation by sending 24V DC power through a Digital Output (DO) card to the solenoid coil, which opens the valve by allowing air to flow to the pneumatic cylinder, while the Digital Input (DI) card receives feedback signals from the micro switches to confirm the valve's open or closed position.

This tutorial demonstrates how to program a CODESYS PLC to control three solenoids using three toggle switches through ladder logic, where Toggle Switch 1 energizes all solenoids, Toggle Switch 2 energizes only Solenoids 1 and 2 (using a normally closed contact to disable Solenoid 3), and Toggle Switch 3 de-energizes all solenoids using normally closed contacts; the video shows the complete process from creating a new project in CODESYS, drawing the ladder diagram with normally open and normally closed contacts, to simulating and debugging the program to verify the control logic works as intended.

In the stationary chain method, each relay controls specific outputs based on the process requirements. The instructor explains that double solenoid valves (like cylinder A) require two solenoids: one for advance (Y1) and one for retraction (Y2). Single solenoid valves (like cylinder B) require only one solenoid. The control logic follows: when a relay is activated, it closes contacts to energize the corresponding solenoid; when a relay is de-energized, it opens contacts to de-energize the solenoid. The last relay in the chain (K5) has the special function of resetting the entire chain by de-energizing the first relay (K1), allowing the cycle to restart.

A solenoid valve is an electromechanical actuator that converts electrical energy into linear mechanical motion to control fluid flow; there are three main types—direct-acting, indirect-acting, and semi-direct acting—each with distinct operating mechanisms: direct-acting valves use electromagnetic force to move a plunger against a spring to open/close the orifice, indirect-acting valves use a diaphragm and pilot orifice system where electromagnetic force opens a small channel to relieve pressure and allow flow, and semi-direct acting valves combine elements of both with the plunger attached directly to the diaphragm for efficient high-flow control; these valves are commonly used in industrial applications such as automated liquid level control systems where PLCs manage pump operation and valve actuation based on sensor inputs.

Valve selection follows established criteria and standards that ensure reliable and safe operation, including pressure rating, temperature rating, flow coefficient, and compatibility with the process fluid. Standards such as API and ASME provide guidelines for valve design and testing. Different valve types are suited for different applications, with globe valves providing good flow control for throttling applications, butterfly valves offering low flow resistance for large-diameter applications, and ball valves providing excellent shut-off capability for on/off service. The selection process involves matching valve characteristics to application requirements.

Hydraulic valves, comprising 20-30% of plant line costs, require careful characterization through resistance coefficient K (velocity head loss independent of Reynolds number), equivalent length (L/D ratio causing equivalent pressure loss), and flow coefficient CV (gallons per minute of water at 16°F through a pressure drop in PSI). Valve selection depends on function (closing, blocking, strangulation, or preventing reverse flow), service conditions (liquids, gases, solids, vapors, corrosion/erosion), and material compatibility with pressure and temperature requirements, with cost and availability being final selection criteria.

The flow coefficient (CV for imperial units, KV for metric units) is a critical parameter in solenoid valve selection that measures the fluid flow rate through the valve, calculated using different formulas for liquids (CV = Q × √(SG/ΔP)) and gases depending on pressure drop conditions; selecting the appropriate CV/KV ensures proper valve sizing to prevent issues like incomplete opening/closing, reduced efficiency, cavitation, and water hammer effect, with CV and KV being convertible through the relationship KV = 0.857 × CV.

Valve authority is the ratio of valve pressure drop to total circuit pressure drop. ASHRAE recommends maximum 5 psi total circuit pressure drop. Cv (flow coefficient) is calculated as Cv = Flow (GPM) / √(ΔP). Cv determines valve flow capacity and must match coil Cv. If Cv is too small, flow is limited causing cavitation and noise. If Cv is too large, control is inadequate. Cv is determined at factory and cannot be changed by disc replacement. Cv and Kvs are equivalent: 1 Cv = 1.17 Kvs.

The control valve flow coefficient Cv is defined as the number of US gallons of water at 60°F that will pass through a control valve and create a pressure drop of one psid in one minute; Cv values increase proportionally with valve size (e.g., 2-inch valve has Cv=47, 3-inch has Cv=120, 6-inch has Cv=490, 8-inch has Cv=750), and manufacturers recommend selecting valves where the Cv falls between 20-80% of the stroke length for optimal control performance.

Proportional valves use analog electrical signals instead of digital on/off control—the coil receives smoothly variable current that proportionally positions the spool. No signal means no movement; full signal means full movement; half signal means half movement. This allows infinite intermediate positions, acting like a flow control valve. Feedback transducers based on flow sensors or actuator position enable precise control of desired quantities. Servo valves use force motors to indirectly position spools, offering more linear force-current relationships than proportional valves, designed for high-performance closed-loop control of actuator speed or position. These technologies provide smoother and more precise actuator control than discrete valves, suitable for applications requiring extremely accurate positioning and dynamic response.

A proportional hydraulic valve uses a slotted spool for continuous flow regulation, with electronic control via integrated PID systems and position transducers that provide feedback for closed-loop control; for higher flow requirements, pilot valves are employed where a smaller proportional valve controls a larger main valve, with pilot systems operating at lower pressures (30-45 bar) compared to main systems (50-350 bar) to ensure proper valve operation.

Proportional control valves solve the complexity dilemma without introducing dozens of valves and hydraulic loops. Physically, they appear similar to on-off solenoid counterparts. The key difference is that proportional coils operate on DC current and produce varying voltages that generate variable force to shift the spool. The graphic symbol shows a solenoid slash in the operator box with a sloping arrow through the slash. The spool can be shifted against the centering spring force to any distance up to full shift by varying voltage and current.

A proportional control solenoid valve uses electromagnetic force to control fluid flow proportionally to input voltage; when voltage is applied to the solenoid coil, it creates magnetic force that moves a plunger against a spring, opening the valve orifice partially or fully depending on the voltage level, allowing precise flow control from fully closed (at low voltage) to fully open (at high voltage).

This video tutorial demonstrates how to program a PID control loop for a control valve in DeltaV DCS, covering essential concepts including reverse acting/direct acting control behavior, bumpless transfer for smooth mode switching between auto and manual, and back calculation to prevent reset windup; the practical implementation involves connecting an analog input (flow meter) to the PID block, configuring setpoint and control options, setting gain, reset, and rate parameters, and establishing output limits to regulate control valve position based on process flow requirements.

Pneumatic solenoid valves typically fail due to electrical issues (burnt coil, incorrect voltage) or contamination; to diagnose, verify voltage with a multimeter, test the solenoid by replacing the coil, and check for contamination by disassembling and cleaning the spool, while ensuring clean air supply with a filter regulator.

The solenoid valve has two main failure modes: electrical failure (coil short circuit causing continuous power draw) and mechanical failure (plunger stuck due to mineral deposits from hard water). To test a solenoid valve, apply 24V electrical power to the coil terminals. When power is applied, the coil generates a magnetic field that attracts the metal plunger inward, opening the valve. When power is removed, the spring pushes the plunger outward, closing the valve. The valve should respond immediately to power changes. To replace a solenoid valve, locate the valve on the RO water purifier system connected to the pump and pressure tank. The valve connector has a small release ring that must be pressed inward to release the valve. The new valve can be inserted directly into the connector without disconnecting electrical wires. After installation, test the new valve by applying 24V power to ensure it opens when powered and closes when power is removed.

The solenoid valve can fail in two ways: stuck open or stuck closed. When stuck open, fuel is continuously drawn from the reservoir to the intake manifold, causing the reservoir to become empty after extended driving. This is a common failure mode where the central continues to send negative to the valve even after the engine has warmed up. When stuck closed, the system fails to function entirely because fuel cannot flow from the reservoir to the intake manifold, preventing cold starts.

A valve solenoid operates by using an energized coil of wire to create a magnetic field that pulls an armature and push pin assembly, which moves the valve spool to direct fluid flow; the internal springs return the spool to neutral position, and a mechanical override allows manual testing of valve functionality by pushing the override pin to verify the spool moves freely, while holding the armature away from the coil center for extended periods can cause coil overheating and winding failure.

Diagnosing solenoid failures requires systematic testing to identify specific failure modes. A stuck open valve (continuously passing fluid/air) causes continuous fuel consumption or vapor release, leading to idle oscillation and difficult diagnosis. A stuck closed valve (receiving power but not actuating) often results from carbonization by fuel vapors and requires mechanical intervention. An open circuit (no resistance reading) indicates a broken coil winding anywhere along the coil, rendering the component non-functional. Each failure mode produces distinct symptoms: continuous fuel drain, idle oscillation, or complete system failure. Understanding these patterns helps technicians efficiently diagnose and repair solenoid-related problems.