To trace the development of solenoid valves, both domestically and internationally, electromagnetic valves have been broadly classified into three main types: direct-acting, step-by-step pilot-guided, and indirect-acting. Further, based on structural differences and material variations, these categories can be subdivided into six types: straight-acting diaphragm type, step-by-step weighted type, pilot diaphragm type, direct-acting piston type, step-by-step direct-acting piston type, and pilot piston type.
**Direct-Acting Solenoid Valve:**
Principle: When power is applied, the electromagnetic coil generates a magnetic force that lifts the closing element off the valve seat, allowing the valve to open. When power is removed, the magnetic force disappears, and a spring pushes the closing element back onto the seat, closing the valve.
Features: This type operates effectively under vacuum, negative pressure, and zero-pressure conditions. However, it is typically limited in size, with a maximum diameter of around 25mm.
**Distributed Direct-Acting Solenoid Valve:**
Principle: This design combines elements of both direct-acting and pilot-operated mechanisms. In situations where there is no pressure difference between the inlet and outlet, the electromagnetic force directly lifts the pilot valve and the main valve’s closing component, opening the valve. Once the pressure difference reaches the required level, the electromagnetic force opens the pilot valve, increasing the pressure in the lower chamber of the main valve while reducing pressure in the upper chamber, thus creating a pressure differential that pushes the main valve open. When power is off, the pilot valve is closed by spring force or medium pressure, causing the valve to close.
Features: It can function at zero pressure difference, vacuum, or high pressure conditions. However, it requires more power and must be installed horizontally.
**Pilot Operated Solenoid Valve:**
Principle: When power is applied, the electromagnetic force opens the pilot port, rapidly reducing the pressure in the upper chamber. This creates a pressure difference across the closing element, which is then pushed upward by the fluid pressure, opening the valve. When power is turned off, the spring closes the pilot port, allowing the inlet pressure to bypass into the upper chamber, creating a higher pressure above the closing element. The resulting pressure differential forces the valve to close.
Features: It can handle higher pressure ranges and offers flexible installation options (with some customization needed). However, it requires a minimum pressure differential to operate effectively.
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