page_title: Solenoid valve
section_title: Internally piloted
hierarchical_section_title: Solenoid valve / Operation / Internally piloted
context_page_description: A solenoid valve is an electromechanically operated valve.
  Solenoid valves differ in the characteristics of the electric current they use, the strength of the magnetic field they generate, the mechanism they use to regulate the fluid, and the type and characteristics of fluid they control. The mechanism varies from linear action, plunger-type actuators to pivoted-armature actuators and rocker actuators. The valve can use a two-port design to regulate a flow or use a three or more port design to switch flows between ports. Multiple solenoid valves can be placed together on a manifold.
  Solenoid valves are the most frequently used control elements in fluidics. Their tasks are to shut off, release, dose, distribute or mix fluids. They are found in many application areas. Solenoids offer fast and safe switching, high reliability, long service life, good medium compatibility of the materials used, low control power and compact design.
context_section_description: While there are multiple design variants, the following is a detailed breakdown of a typical solenoid valve design.
  A solenoid valve has two main parts: the solenoid and the valve.  The solenoid converts electrical energy into mechanical energy which, in turn, opens or closes the valve mechanically. A direct acting valve has only a small flow circuit, shown within section E of this diagram (this section is mentioned below as a pilot valve).  In this example, a diaphragm piloted valve multiplies this small pilot flow, by using it to control the flow through a much larger orifice.  
  Solenoid valves may use metal seals or rubber seals, and may also have electrical interfaces to allow for easy control.  A spring may be used to hold the valve opened (normally open) or closed (normally closed) while the valve is not activated.
  The diagram to the right shows the design of a basic valve, controlling the flow of water in this example. At the top figure is the valve in its closed state.  The water under pressure enters at A. B is an elastic diaphragm and above it is a weak spring pushing it down.  The diaphragm has a pinhole through its center which allows a very small amount of water to flow through it. This water fills the cavity C on the other side of the diaphragm so that pressure is equal on both sides of the diaphragm, however the compressed spring supplies a net downward force. The spring is weak and is only able to close the inlet because water pressure is equalized on both sides of the diaphragm.
  Once the diaphragm closes the valve, the pressure on the outlet side of its bottom is reduced, and the greater pressure above holds it even more firmly closed.  Thus, the spring is irrelevant to holding the valve closed.
  The above all works because the small drain passage D was blocked by a pin which is the armature of the solenoid E and which is pushed down by a spring.  If current is passed through the solenoid, the pin is withdrawn via magnetic force, and the water in chamber C drains out the passage D faster than the pinhole can refill it.  The pressure in chamber C drops and the incoming pressure lifts the diaphragm, thus opening the main valve. Water now flows directly from A to F.  
  When the solenoid is again deactivated and the passage D is closed again, the spring needs very little force to push the diaphragm down again and the main valve closes. In practice there is often no separate spring; the elastomer diaphragm is molded so that it functions as its own spring, preferring to be in the closed shape.
  From this explanation it can be seen that this type of valve relies on a differential of pressure between input and output as the pressure at the input must always be greater than the pressure at the output for it to work.  Should the pressure at the output, for any reason, rise above that of the input then the valve would open regardless of the state of the solenoid and pilot valve.
caption: A- Input side
   B- Diaphragm
   C- Pressure chamber
   D- Pressure relief passage
   E- Electro Mechanical Solenoid
   F- Output side