User Guide

2
Solenoid valves can generally be divided into two types:
Direct acting
•Pilot operated
Introduction
In a direct-acting solenoid valve, the pull of the coil opens the valve port
directly, by lifting the pin off the valve seat.
Direct-acting
solenoid-valve operation
Because a direct-acting solenoid valve depends on the power of the coil for
operation, its port size for a given operating-pressure differential is limited by
the practical limitations of the coil size.
Direct-acting solenoids are limited to smaller port sizes, below
1
/4-inch diameter.
Direct-acting port size
limitation
Figure 2 shows the parts of a direct-acting solenoid valve.
Diagram of direct-acting
solenoid valve
In a pilot-operated solenoid valve, the plunger does not open the main port
directly. Instead, the following operation occurs:
Stage Description
1 The plunger opens a pilot port.
2 Pressure on top of a piston is released through the pilot port,
thus creating a pressure imbalance across the piston.
3 With the pressure underneath the piston now greater than the
pressure on top, the piston will move in an upward direction,
opening the main port.
4 When the plunger drops, it closes the pilot port; thus, the pressure
above and below the piston equalizes, and the piston closes
the main port.
5 The pressure difference across the valve, acting on the area of the
main port, holds the piston in a tightly closed position.
Pilot-operated solenoid-
valve operation
Solenoid-valve types
Direct Acting
Figure 2
Pin
Plunger
Enclosing tube
Top plug
Return spring
Seal