Solenoid valves are classified into three main categories based on their operating principle:
Direct-acting solenoid valves
Principle: When energized, the solenoid coil generates electromagnetic force that lifts the closing element from the valve seat, opening the valve; when de-energized, the electromagnetic force disappears, and the spring presses the closing element back onto the valve seat, closing the valve.
Characteristics: Can operate normally under vacuum, negative pressure, and zero pressure, but the nominal diameter is generally no more than 25mm.
Step-by-step direct-acting solenoid valve
Principle: It combines direct-acting and pilot-operated principles. When there is no pressure difference between the inlet and outlet, upon energization, the electromagnetic force directly lifts the pilot valve and the main valve closing element sequentially, opening the valve. When the inlet and outlet reach the starting pressure difference, upon energization, the electromagnetic force actuates the pilot valve, increasing the pressure in the lower chamber of the main valve and decreasing the pressure in the upper chamber, thus using the pressure difference to push the main valve open. When de-energized, the pilot valve uses spring force or medium pressure to push the closing element downwards, closing the valve.
Features: Reliable operation even at zero pressure difference, vacuum, or high pressure, but requires higher power and must be installed horizontally.
Pilot-operated solenoid valve
Principle: When energized, the electromagnetic force opens the pilot orifice, causing the pressure in the upper chamber to drop rapidly. This creates a pressure differential around the closing element, with the pressure lower at the top and higher at the bottom. The fluid pressure pushes the closing element upward, opening the valve. When de-energized, the spring force closes the pilot orifice. The inlet pressure quickly passes through the bypass orifice, creating a pressure differential around the closing element, with the pressure lower at the bottom and higher at the top. The fluid pressure pushes the closing element downward, closing the valve.
