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Solenoid valves

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Typical laboratory scale 3-way valve (Cole Parmer)
Solenoid valves are control units which, when electrically energized or de-energized, either shut off or allow fluid or gas flow. The actuator takes the form of an electromagnet. When energized, a magnetic field builds up which pulls a piston or pivoted armature against the action of a spring. The piston is mechanically linked to a mechanical valve inside the solenoid valve. When de-energized, the plunger or pivoted armature is returned to its original position by the spring action and the valve returns to its resting state.

Contents

Valve Operation

Solenoid valves come in various configurations and sizes.  According to the mode of actuation, a distinction is made between direct-acting valves, internally piloted valves, and externally piloted valves. A further distinguishing feature is the number of port connections or the number of flow paths ("ways").  A two way solenoid valve has two ports and can be normally open or normally closed. A normally open solenoid valve allows a liquid or gas to flow through unless a current is applied to the solenoid valve. A normally closed valve works in the opposite manner. A three way solenoid valve has three ports; one port is common, one is normally open and the third is normally closed.

Laboratory automation makes frequent use of solenoid valves. The system controller can send an electrical signal (usually low voltage DC for small valves) to actuate the solenoid. In the case of a two-way valve, actuation can allow liquid to flow, and then remove the signal will close the solenoid valve and stop the flow of liquid. A gripper for grasping items on a device can be an air controlled device, powered by a solenoid valve to allow air pressure to close the gripper, and a second solenoid valve to open the gripper. If a three way solenoid valve is used, two separate valves are not needed.  Three way valves are often used in basic liquid handling syringe pumps, to change the valve in between “fill” and “dispense” positions.

The valve shown in the picture is a normally-closed, direct-acting (i.e. Two Way, two ports) valve. This type of solenoid valve has the most simple and easy to understand principle of operation. Image:Solenoid_valve_part_diagram.jpg

  1. Valve Body
  2. Inlet Port
  3. Outlet Port
  4. Coil / Solenoid
  5. Coil Windings
  6. Lead Wires
  7. Plunger
  8. Spring
  9. Orifice

The media controlled by the solenoid valve enters the valve through the inlet port (Part 2 in the illustration above). The media must flow through the orifice (9) before continuing into the outlet port (3). The orifice is closed and opened by the plunger (7). The valve pictured above is a normally-closed solenoid valve. Normally-closed valves use a spring (8) which presses the plunger tip against the opening of the orifice. The sealing material at the tip of the plunger keeps the media from entering the orifice, until the plunger is lifted up by an electromagnetic field created by the coil.

The video animation below shows the operation sequence for a direct-acting solenoid valve.



Direct acting 2-way valves

Image:Solenoid_two_way.gifTwo-way valves are shut-off valves with one inlet port and one outlet port. In the de-energized condition, the core spring, assisted by the fluid pressure, holds the valve seal on the valve seat to shut off the flow. When energized, the core and seal are pulled into the solenoid coil and the valve opens. The electro-magnetic force is greater than the combined spring force and the static and dynamic pressure forces of the medium.

Direct acting 3-way valves

Image:Solenoid_three_way.gifThree-way valves have three port connections, one being the "common" port and two valve seats. One valve seal always remains open and the other closed in the de-energized mode. When the coil is energized, the mode reverses.  This is the pneumatic equivalent of a single-pole single-throw electrical switch.

Direct acting 4-way valves

Image:Solenoid_four_way.gifFour-way valves have four port connections and two valve seats. When the coil is energized, one set of ports is connected straight through to the other set of ports.  In the de-energized mode, the connection is reversed.

Internally piloted solenoid valves

With direct-acting valves, the static pressure forces increase with increasing orifice diameter which means that the magnetic forces, required to overcome the pressure forces, become correspondingly larger. Internally piloted solenoid valves are therefore employed for switching higher pressures in conjunction with larger orifice sizes. In this case, the differential fluid pressure performs the main work in opening and closing the valve, as seen in this simple and entertaining video.





Materials

All materials used in the construction of the valves are carefully selected according to the varying types of applications. Body material, seal material, and solenoid material must be chosen to optimize functional reliability, fluid compatibility, service life and cost.

Body Materials

Polyamide material is used for economic reasons in various plastic valves. Neutral fluid valve bodies are made of brass and bronze. For fluids with high temperatures, e.g., steam, corrosion-resistant steel is available. In addition,

Solenoid materials

For laboratory environments, all parts of the solenoid actuator which come into contact with the fluid must be made of a material suitable for the given applications chemical, temperature and pressure enviroment.  Many materials are used, such as PEEK, 430 SS, ceramic, epoxy, Polyetheretherketone and Fluoroelastomers.

Seal materials

The particular mechanical, thermal and chemical conditions in an application factors in the selection of the seal material. the standard material for neutral fluids at temperatures up to 194°F is normally FKM. For higher temperatures EPDM and PTFE are employed. The PTFE material is universally resistant to practically all fluids of technical interest.

Selecting a solenoid valve

Consider your fluid type (liquid or gas) and its characteristics to determine compatible valve materials. PTFE withstands many harsh or corrosive chemicals. For safety reasons, always use metal valves for pressurized gases.
Determine the temperature, pressure, and flow rate under which your valve will be operating. In general, metal valves withstand higher temperatures and pressures than plastic valves.
For solenoid valves, consider response time and length of time valve will be energized. Continuous (100%) duty solenoid valves are best for frequent on/off cycling. Choose normally closed or normally open depending on the state the valve will be in most often.

Definitions

Breaking Pressure:

The minimum pressure required to produce flow through a valve.

Flow Patterns:

A diagram showing how flow can be directed using a particular valve. (See the diagrams above for further explanation.) Normally Closed: Valve stays closed in de-energized state; opens when energized. Normally Open: Valve stays open in de-energized state; closes when energized.

Pressure Differential or Pressure Drop:

The difference between the inlet and the outlet pressure through a valve. The outlet pressure is lower than the inlet pressure due to the restriction caused by the valve

Response times:

The small volumes and relatively high magnetic forces involved with solenoid valves enable rapid response times to be obtained. Valves with various response times are available for special applications. The response time is defined as the time between application of the switching signal and completion of mechanical opening or closing.

On period:

The on period is defined as the time between switching the solenoid current on and off.

Cycle period:

The total time of the energized and de-energized periods is the cycle period.

Relative duty cycle:

The relative duty cycle (%) is the percentage ratio of the energized period to the total cycle period. Continuous operation (100% duty cycle) is defined as continuous operation until steady-state temperature is reached.

Viscosity:

The technical data is valid for viscosities up to the figure specified for the valve. Higher viscosities are permissible, but in these cases the voltage tolerance range is reduced and the response times are extended.

Temperature range:

Temperature limits for the fluid medium are always detailed in valve specifications. Various factors, e.g. ambient conditions, cycling, speed, voltage tolerance, installation details, etc., can, however, influence the temperature performance.


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