Fundamentals of Solenoid Valve Selection and Their Structural Principles

Fundamentals of Solenoid Valve Selection and Their Structural Principles

I. Applicability

The fluid in the pipeline must be compatible with the medium specified for the selected solenoid valve model.

The fluid temperature must be lower than the rated temperature of the selected solenoid valve.

The allowable fluid viscosity is generally below 20 cSt; viscosities exceeding 20 cSt should be specified.

Regarding operating pressure differential: use direct-acting or semi-direct-acting valves when the maximum pipeline pressure differential is less than 0.04 MPa; use pilot-operated (pressure-differential operated) valves when the minimum operating pressure differential exceeds 0.04 MPa. The maximum operating pressure differential must be lower than the valve's maximum rated pressure. Since solenoid valves typically operate in one direction, check for potential back-pressure; install a check valve if necessary.

If fluid cleanliness is poor, install a filter upstream of the solenoid valve, as these valves generally require a clean medium.

Pay attention to the flow orifice size and pipe connection size. Solenoid valves typically offer only two-position (on/off) control. Install a bypass line if conditions permit to facilitate maintenance. If water hammer occurs, specify a valve with adjustable opening/closing times.

Consider the impact of ambient temperature on the solenoid valve.

Select the power supply current and power consumption based on output capacity; power supply voltage generally allows for a fluctuation of ±10%, but note that the VA value is higher during AC startup.

II. Reliability

Solenoid valves come in normally closed (NC) and normally open (NO) types. The normally closed type is generally selected (opens when energized, closes when de-energized); however, the normally open type should be chosen for applications requiring long periods of being open and short periods of being closed.

Service life testing is typically part of the manufacturer's type testing; strictly speaking, there is no specific national standard for solenoid valves in my country, so choose the manufacturer carefully.

Direct-acting valves are generally selected for applications requiring short actuation times and high operating frequencies; for large-bore applications, select "quick-acting" series valves.

III. Safety

Standard solenoid valves are not waterproof; if operating conditions require it, please select a waterproof model (custom versions are available from the factory).

The rated nominal pressure of the solenoid valve must exceed the maximum pressure within the pipeline; otherwise, the service life may be shortened, or other unexpected issues may arise.

For corrosive liquids, select an all-stainless steel model; for highly corrosive fluids, a PTFE (SLF) solenoid valve is recommended.

In explosive environments, appropriate explosion-proof products must be used.

IV. Cost-effectiveness

While many solenoid valves may be interchangeable, the most cost-effective product should be selected provided it meets the three criteria mentioned above.

Structural Principles of Solenoid Valves

I. Direct-acting Solenoid Valves

These come in two types: normally closed and normally open. The normally closed type remains closed when de-energized. When the coil is energized, electromagnetic force causes the movable iron core to overcome spring tension and engage with the stationary iron core, directly opening the valve to allow medium flow. When the coil is de-energized, the electromagnetic force vanishes, and the movable iron core returns to its original position under spring tension, directly closing the valve port and stopping the flow. This type features a simple structure and reliable operation, functioning normally under zero differential pressure and slight vacuum conditions. The normally open type operates in the exact opposite manner. Examples include solenoid valves with flow orifices smaller than φ6 mm.

II. Step-by-step Direct-acting Solenoid Valves

This valve integrates primary and secondary opening mechanisms, utilizing both electromagnetic force and pressure differential to open the main valve port in stages. When the coil is energized, electromagnetic force draws the movable iron core toward the stationary iron core, opening the pilot valve port (which is located on the main valve core). Since the movable iron core is connected to the main valve core, the pressure in the main valve's upper chamber is relieved through the pilot port. The combined action of the pressure differential and electromagnetic force then lifts the main valve core, opening the main valve to allow medium flow. When the coil is de-energized, the electromagnetic force vanishes; the movable iron core then closes the pilot valve port under the combined action of gravity and spring force. Consequently, the medium enters the upper chamber of the main valve spool through the balancing orifice, raising the pressure within that chamber. Driven by the spring and the increased pressure, the main valve closes, halting the flow of the medium. The structure is well-designed and operation is reliable, even under zero differential pressure.

III. Indirect Pilot-Operated Solenoid Valve

This series of solenoid valves consists of a pilot valve and a main valve spool linked to form a flow path. The normally closed type remains closed when de-energized. When the coil is energized, the resulting magnetic force draws the movable iron core toward the stationary iron core, opening the pilot valve port and allowing the medium to flow toward the outlet. This reduces the pressure in the main valve spool's upper chamber to a level below that of the inlet side; the resulting pressure differential overcomes the spring resistance, causing the spool to move upward and open the main valve port, thereby allowing the medium to flow. When the coil is de-energized, the magnetic force disappears, and the movable iron core returns to its original position under spring force, closing the pilot port. Medium then flows in through the balancing orifice, increasing the pressure in the upper chamber of the main valve spool; the spool moves downward under the spring force, closing the main valve port. The operating principle for the normally open type is exactly the reverse.