Everything You Need To Know About Solenoid Valves - Part 1

Everything You Need To Know About Solenoid Valves - Part 1

What are Solenoid Valves?

A solenoid valve is an electromechanical control device that can be used to control gaseous or liquid media. The valve is actuated by an electromagnetic force which allows or blocks the fluid path.

Figure 1: Solenoid Valve Working Principle

VALVE TYPES

On/Off Valves Valves of this type use an electromagnet that moves a plunger directly connected to the sealing element. In a normally closed system, when the electromagnet is switched off, a spring keeps the valve closed by pressing the valve plunger and seal against the valve seat. To open the valve, the plunger must be lifted from the valve seat by the electromagnetic force; the drive must therefore work against the compression spring force applied to the valve plunger. The minimum force required by the electromagnetic drive to open the valve depends primarily on the spring force, the valve seat size, and the maximum differential pressure when the valve is closed.

Figure 2: On / Off Valve Working Principle

Proportional Valves

Compared to on/off valves, which can only be fully opened or closed, the opening rate of proportional valves can be controlled with current modulation. As the current increases, the plunger gets pulled further, allowing an increased passage opening and thus an increased fluid flow through the valve, proportional to the current applied. Proportional valves are used in hydraulics and pneumatics particularly where variable flows are required. The difference you see in the flow when you ramp up the current directly to a set point, compared to when you go past that flow point and try to return back to it, is called Hysteresis. This effect is inherent to any proportional valve due to its magnetic properties and is undesired as it limits the controllability of the valve. Depending on if you are increasing or decreasing the flow, the level of current required to get to a fixed flow rate will thus be different.

Figure 3: Proportional Valve Working Principle

Media-Separated Valves

In a media separated solenoid valve, the fluid is not in contact with the internal components, such as the plunger and spring. Media separated valves are also called membrane valves, rocker valves or isolation valves. These valves are particularly suitable for aggressive and sensitive fluids. The solenoid valves with media separation are divided into two parts:

• the flow part
• the drive part, which operates the valve components

A membrane inside the valve separates these two parts from each other so that the medium is only in contact with the valve body and the separating membrane. This prevents the internal components from corroding, as well as contamination of the media from detachable substances such as dust or metal due to wear during longer operating times. Moreover, media separated valves reduce the risk of cross-contamination between media. The use of chemically resistant materials such as PTFE, PEEK, FFKM enables a high chemical resistance. Because of this design, this type of valve is particularly suitable for analytical, medical or diagnostic devices. Main benefits are the protection from corrosion through aggressive media and the fact that the media such as blood, pharmaceuticals or reagents do not get contaminated. As an alternative to media-separated valves, pinch valves can be used, where the fluid flows through a tube and thus has a zero dead volume and no cross-contamination.

Figure 4: Fluid flow path of a media separated valve

Components and their task

Below you can find a basic description of the main components of a solenoid valve. The valve shown in the picture represents a normally-closed, on/off valve.

• Coil: The coil creates an electromagnetic field when the valve is powered on. This magnetic field pulls the plunger up against the spring force to open the valve.
• The spring pushes the plunger down towards the body to ensure the valve is closed when it is not powered.
• The plunger is the metallic part that moves up and down, allowing the valve to open and close.
• The dynamic seal is pressed against the body seat in the valve´s closed position. It is made of an elastomer and ensures there is no leakage from the inlet to the outlet when the valve is closed.
• The body is the main component in contact with the media. It includes the inlet and outlet ports.