Solenoid Valve Basics - A Quick Overview

The highly automated era, the precise control of fluids in many areas play a pivotal role, and solenoid valves is undoubtedly the key to achieve this goal "tool". Whether it is the industrial production line on the orderly material transportation, or home life in the convenient operation of intelligent equipment, or about life and health of the medical operation as well as municipal engineering, agricultural irrigation and other aspects of the solenoid valve are silently play an indispensable role. With its unique working principle and a variety of types, it can accurately and quickly control the liquid and gas and other fluids on and off, flow direction and flow, to meet the complex and changing needs of different scenarios. So, what is a solenoid valve? How does it work? And what are the common types and their respective scenarios? Next let us explore the mystery of the solenoid valve.

What is a Solenoid Valve? How Does it Work?

A solenoid valve is an automated control component that uses electricity to generate a force (usually called electromagnetic force) through a coil. With the electromagnetic force as the driving force, it triggers the spool to perform opening and closing actions, thereby controlling the on-off, flow direction, or flow rate of fluids (liquids, gases, etc.). Essentially, it is an "electromagnetically driven fluid switch/regulator", which is also the origin of its name. Its core function is to achieve precise and rapid control of fluids through electrical signals without the need for manual operation.

Key Components of a Solenoid Valve

The key components of a solenoid valve include: solenoid coil, spool, valve body, spring, and seals.

• Solenoid Coil: The core driving source that generates magnetic force when energized.
• Spool: The core actuating component that controls the on-off/switching of the channel through movement.
• Valve Body: The housing that accommodates the spool and fluid, with built-in fluid channels.
• Spring: Pushes the spool back to its original position when power is off.
• Seals: Prevent fluid leakage (such as O-rings, gaskets. The material needs to be suitable for the medium, such as rubber, polytetrafluorethylene, etc.).

Working Principle of a Solenoid Valve

When the coil is energized, the built-in solenoid coil of the solenoid valve generates a magnetic field to attract the spool. This magnetic force overcomes the spring force or medium pressure originally existing in the valve body. The iron core (or moving iron core) moves, thus pushing the spool to change its position. The fluid channel opens (closes or switches the flow direction), and the medium can flow along the preset path. When the power is off, the magnetic field disappears, and the attraction to the spool disappears as well. Without the electromagnetic force, the original pressure of the spring or medium will no longer be counteracted, pushing the spool back to its original position. The fluid channel closes (opens or restores the original flow direction), realizing the conversion of "electrical signal → mechanical action → fluid control".

Normally Open and Normally Closed Solenoid Valve

Solenoid valves usually have two states: normally open and normally closed. Depending on the medium and project requirements, a normally open solenoid valve is open when it is not energized (power off), and the fluid can flow freely. When the solenoid valve is energized, the valve core is pushed to the closed position to block the fluid flow. A normally closed solenoid valve is the opposite. It is closed when it is not energized or powered off, and it opens when energized.

Two Common Types of Solenoid Valve

Direct-Acting Solenoid Valve

The direct-acting solenoid valve has the simplest structure. The electromagnetic force directly pushes the spool to act without relying on the medium pressure. It has a small diameter (usually ≤ 15mm), a low pressure range (generally ≤ 1.6MPa), and a fast response speed (millisecond level).

Application Scenarios: For fluid control in low-pressure and small-flow situations, such as:

• Smart Home: The flushing water path of smart toilets, the filter element switching of water purifiers.
• Small Equipment: The on-off of the air path of electronic blood pressure monitors, the ink control of printers.

Common Types: Mostly two-way valves (with on-off function), divided into normally closed (NC) and normally open (NO) types.

Pilot-Operated Solenoid Valve

The pilot-operated solenoid valve consists of two parts: the "pilot valve" and the "main valve". First, the small electromagnetic force is used to control the pilot valve, and then the pressure of the medium itself is used to push the main spool to act. It has a large diameter (≥ 20mm), a wide pressure range (up to 10MPa or more), and is suitable for large-flow scenarios.

Application Scenarios: For fluid control in high-pressure and large-flow situations, such as:

• Industry: The compressed air main pipeline in the workshop, the chilled water loop of the central air conditioner.
• Municipal: The main water path of the automatic irrigation system, the water supply control of the fire sprinkler.

Note: It needs to rely on the medium pressure (usually ≥ 0.05MPa) to work and cannot be used in low-pressure or no-pressure environments.

There is also an electromagnetic valve that combines the advantages of the first two

Semi-Direct-Acting Solenoid Valves

The semi-direct-acting solenoid valve combines the advantages of the "direct-acting" and "pilot-operated" types. At low pressure, it relies on the electromagnetic force to directly push the spool (similar to the direct-acting type). At high pressure, it relies on the medium pressure for assistance (similar to the pilot-operated type). It has a medium diameter (10 - 25mm), a wide pressure range (0 - 2.5MPa), and has no low-pressure limitation.

Application Scenarios: For scenarios with large pressure fluctuations or possible no-pressure situations, such as:

• Water Treatment: The water path control of non-negative pressure water supply equipment.
• Medical: The gas loop of ventilators (low pressure and requiring stable on-off).

What is the Use of a Solenoid Valve?

Solenoid valves have a wide range of uses. In industrial automation, they can control fluid transmission and drive actuating elements. In daily life at home, they are used in smart home appliances and heating and cooling systems. In medical equipment, they precisely regulate the infusion and gas supply. In municipal engineering, they help with water supply, drainage, and fire protection. In the automotive field, they are involved in fuel injection control. In agriculture, they achieve irrigation and automation of facilities. With its precise, rapid, and automated fluid control ability, it plays a key role in many fields.

The following lists some common uses, application scenarios, and types.

Industrial Automation Field

Industrial production processes are complex and require precise control of the on-off, flow rate, and flow direction of various fluids (liquids, gases) to ensure the connection of processes and the operation of equipment, realize automated operations, and improve production efficiency and quality. Pilot-operated solenoid valves (for large-flow and high-pressure scenarios) and direct-acting solenoid valves (for small-flow and low-pressure scenarios) are often used. In chemical production, pilot-operated solenoid valves control the on-off of the raw material transportation pipeline, opening or closing according to the process requirements to ensure continuous production.

Home Life Field

Solenoid valves improve the convenience of life and the degree of intelligence of home appliances, precisely controlling fluids such as water and gas to achieve corresponding functions as needed. Most of them are direct-acting solenoid valves. For example, smart toilets, washing machines, etc. often use two-way direct-acting solenoid valves (to control the on-off of the water path). When the smart toilet flushes, the solenoid valve is energized to open and let the water flow out, and it is powered off and closed after flushing.

Medical Equipment Field

Medical operations have extremely high requirements for fluid control accuracy, which is related to the life and health of patients and requires precise adjustment of the flow rate and on-off of medicine and gas. High-precision direct-acting solenoid valves are often adopted, and some will use proportional control solenoid valves to achieve more precise flow rate adjustment. The direct-acting type directly acts on the spool through electromagnetic force to achieve on-off and preliminary flow rate adjustment. The proportional control solenoid valve linearly changes the opening degree of the spool according to the intensity of the input electrical signal to precisely control the flow rate. For example, in an infusion pump, the direct-acting solenoid valve controls the on-off of the medicine according to the setting to ensure the safety and accuracy of the infusion.

Municipal Engineering Field

Municipal facilities need to effectively manage the distribution of resources such as water and gas, realize remote or automatic control, and improve the efficiency of resource utilization and the level of public services. Large-diameter pilot-operated solenoid valves are used for the main water supply and drainage pipelines (large-flow scenarios). Two-way direct-acting solenoid valves are used for small irrigation systems, etc. The large-diameter pilot-operated solenoid valve uses the cooperation of electromagnetic force and medium pressure to control the on-off and flow rate of the water flow in the main pipeline to meet the regional water supply needs. The direct-acting solenoid valve simply and directly controls the opening and closing of a small range of water paths. For example, in automatic irrigation, the solenoid valve is opened according to the set time to spray water.

Agricultural Field

It improves the accuracy of agricultural irrigation, saves water resources, and at the same time realizes the automation of agricultural facilities, which is beneficial to the growth of crops. Mostly, direct-acting solenoid valves are used for small irrigation systems. Some large irrigation projects will use pilot-operated solenoid valves. The direct-acting solenoid valve is energized to open or powered off to close according to signals from soil moisture sensors, etc., to control the outflow of irrigation water. The pilot-operated solenoid valve uses the electromagnetic force and medium pressure to control the distribution of large-flow water in the main pipeline of large irrigation to meet the irrigation needs of different areas.