How does a solenoid valve work

The term solenoid valve suggests two components, a solenoid and a valve. To best answer the question 'what is a solenoid valve' or to explain how a solenoid valve works, it is best that we describe each of these components before bringing our understanding of these components, the solenoid and the valve together to fully describe what a solenoid valve is.

As a solenoid valve has two states it can be referred to as a binary device. This definition lends itself well to an explanation as to why solenoid valves are predominantly deployed within industrial control processes where much of the automation is driven by binary control systems.

What is a Solenoid?

The most primitive description of a solenoid is 'a device that converts electrical energy into mechanical energy'. There are other devices such as electric motors and loudspeakers that also convert electrical energy into mechanical energy. Like the solenoid they also work by the same principles of physics and only differ in there design which targets different types of mechanical output. Electric motors predominately provide rotational mechanical energy where a loudspeaker converts electrical energy into oscillating mechanical energy that drives a speaker diaphragm. The solenoid generally provides a physical mechanical output about a single axis, either vertical or horizontal depending on the orientation of the coil of the solenoid.

What is a Solenoid?

The most primitive description of a solenoid is 'a device that converts electrical energy into mechanical energy'. There are other devices such as electric motors and loudspeakers that also convert electrical energy into mechanical energy. Like the solenoid they also work by the same principles of physics and only differ in there design which targets different types of mechanical output. Electric motors predominately provide rotational mechanical energy where a loudspeaker converts electrical energy into oscillating mechanical energy that drives a speaker diaphragm. The solenoid generally provides a physical mechanical output about a single axis, either vertical or horizontal depending on the orientation of the coil of the solenoid.

Components of a Coil

There are two primary components of a solenoid, the coil and the core, both of which can be seen in the image adjacent to this text. The function of a solenoid rely on the interaction of the physical properties of each of these components.

The Coil

The coil is made from a material that allows for easy flow of electrical current through it. As electrical current flows through the coil magnetic lines of force are created. The strength of these lines of force is proportional to the current flowing through the coil and the reactance of the coil to that current flow. The main component of the reactance of a coil when used in a solenoid is the coils inductance which restricts the rate of change of electrical current. The construction and shape of the coil are designed to focus these lines of magnetic force along the axis of the coil, thereby concentrating any electrical force along that same axis. These lines of magnetic force are polarised in much the same way that a permanent magnet are polarised to N and S poles.

The Core

The core of a solenoid is normally made from a ferrous compound. Ferrous or iron based materials have a natural magnetism as their molecules constantly try to align themselves with the earth's own magnetic fields which are generated from the earth's iron / ferrous core. This process results in a ferrous material having natural magnetic poles created by its own magnetic fields. It is important that the core of a solenoid has ferromagnetic properties as the poles created are a primary component of transforming the electrical energy into mechanical energy.

How a Solenoid Functions

If we consider the properties of the components of the solenoid, the coil and the core we will discover how mechanical energy is generated. We can see this mechanical energy when the core of the solenoid moves. As the core has mass and to generate movement of any mass a force must be applied therefore the movement of the core is our indication that mechanical force is being generated by the interaction of the stationery coil of the solenoid and its core which is having the generated mechanical energy applied to it via magnetic repulsion.

If we take two permanent magnets and we move the south pole of one towards the south pole of the other then we will see that the second magnet moves away from the first. This is because within magnetism opposing poles attract and like poles repel. It is exactly the same force that is being applied to our solenoid core. The core being made from a ferrous compound will have its own natural magnetic field. If this core is placed at the centre of our coil when no electrical current is applied there is no movement. When an electrical current is applied to the coil, the coil now generates its own magnetic field, with poles at either end. The magnetic field generated by the coil interacts with the magnetic field of the core in much the same way our two magnets did when we moved them together. As the coil is fixed the only movement allowed is by the core that follows the laws of physics and is propelled in a direction dictated by the opposing magnetic poles between itself and the coil.

How Does it Work?

A solenoid valve or solenoid actuated valve is basically an electrical valve that controls the flow of media either open/closed or diverting my means of an electro magnet or solenoid. The principles are based around a thin copper wire wound around a bobbin or core (The solenoid) in such a way that when electrical energy is applied a sufficient magnetic field is generated to provide a lifting force to a ferro magnetic stainless steel armature within the solenoid valve armature assembly which in turn will directly or indirectly change the position of the valve.

What is a Valve?

Ever since man first rolled a rock into a stream and noticed the water flow stopped and started when he removed the rock we have understood the principles of a valve. What makes a valve unique is that can control media that 'flows through it'.

How a Valve Works

A valve is a simple device that provides a route for media to flow through it. Within this internal route an obstruction to media flow can be placed thereby stopping the flow of the media. This obstruction is normally controlled by an external mechanical device such as a handle or screwed thread shaft. The external mechanics of the valve allow for the obstruction to be removed or reduced thereby allowing the flow of media through it. This generic description fits well with the vast majority of valve components used within solenoid valve. There are many other types of valve that are not used as components for solenoid valve, these valve types are beyond the scope of this content.

How Valves are Used

In the context of solenoid valves the valve component is placed in series or in-line with a media control system or pipe network. Assuming that this media control system or pipe network is complex with many branches and junctions then valves are placed at locations within the media control path to allow for the management of the flow of the media under control. In such complex systems there can be many valves who's state or position are logically added together to dictate the flow path and direction of flow of the media under control. In such systems two or more valves may need to have their state changed simultaneously to affect the desired control of the media. As one could imagine such changing of valve states within complex media flow systems can be complex in both terms of sequential combined states and in terms of timing when these states are made. Many media control systems are too complex to allow for manual valve operation or have valve change timings that could not be practically achieved without increased man power and availability. This situation leads us to the next section, combining the solenoid and the valve.

Types of Valve

Apart from the basic valve operation many more techniques have been developed to create many valve types based on how they achieve the same basic goals of a valve, to control the media flowing through it. Some of these valve variants are listed below.

Diaphragm Valve

A diaphragm valve consists of a typically flexible elastomer sheet material such as NBR (Nitrile Buna Rubber), EPDM (ethylene propylene diene monomer) or FKM (fluoroelastomer) that has been installed across the flow path through a valve. This flexible diaphragm elastomer seal will typically rest against a sealing face within the valve to prevent flow and can be lifted or moved away from this sealing face to allow the flow of media through the valve. Actuation of a diaphragm valve can me via manual operation by hand wheel or lever, pneumatic operation via an air actuator typically a 3 to 8 bar air supply required, electric motor actuator or solenoid operated utilising a control voltage either 12 volt, 24 volt AC/DC , 110 volt or 240volt AC 50/60Hz.

Ball Valve

A ball valve consists of a stainless steel or chrome plated brass ball with a lateral central hole through it that controls the flow of media. The ball can rotate through 90 degrees (quarter turn) within a ball valve body by means of a connecting metal stem or rod that connects the ball through the top of the ball valve body to an external control lever or actuator. When this central ball hole is rotated so that the central bore (flow path) is in-line with the inlet and outlet ports this will allow flow through the valve, but when rotated at right angles to the flow path (no hole visible to the ports) this will prevent flow. Sealing around the ball is typically PTFE (Polytetrafluoroethylene) and the stem seals are typically either PTFE, EPDM or NBR.

Actuation of a ball valve can be manual lever or hand wheel, quarter turn pneumatic actuator with external air pilot control of 3 to 8 bar, external hydraulic control for larger ball valves or electric motor.

Proportional Valve

A proportional valve or modulating valve offers controlled degrees of opening and closing, unlike other valves that offer simple On/OFF or fully open or fully closed function. In this case a proportional valve will open in proportion to an applied electric control signal of either 0 to 10 volt DC or 4 to 20 mAmp.

For example for a 2 way valve with a proportional or modulating voltage control signal of:

0 volts valve fully closed - 1 volt 10% valve open - 2 volts 20% valve open - 3 volts 30% valve open - 4 volts 40% valve open - 5 volts 50% valve open - 6 volts 60% valve open - 7 volts 70% valve open - 8 volts 80% valve open - 9 volts 90% valve open - 10 volts valve fully open.

Positions between the single volt step voltages can also be achieved so that in theory there are limitless open and close positions available, but only in accordance with the quality of voltage modulating or proportional controller installed.

For example a 2 way actuated valve with proportional or modulating Milliamp current control signal of:

4 mAmp valve fully closed - 5 mAmp 6.25% valve open - 6 mAmp 12.5% valve open - 7 mAmp 18.75% valve open - 8 mAmp 25% valve open - 9 mAmp 31.25% valve open - 10 mAmp 37.5% valve open - 11 mAmp 43.75% valve open - 12 mAmp 50% valve open - 13 mAmp 56.25% valve open - 14 mAmp 62.5% valve open - 15 mAmp 68.75% valve open - 16 mAmp 75% valve open - 17 mAmp 81.25% valve open - 18 mAmp 87.5% valve open - 19 mAmp 93.75% valve open - 20 mAmp 100% fully open

Positions between the single mAmp steps can also be achieved so that in theory there are infinite valve positions available, but only in accordance with the quality of voltage modulating or proportional controller installed.

Combining a Solenoid with a Valve

Following on from the situation we described in the previous section it should be obvious that as valves became more widespread within industrial processes and media control systems became more complex then a new means of operating the valve had to be found. The requirement for a more efficient control of valves came about and in fact was driven by the birth of industrial automation and its need to control industrial processes by a central intelligence. The combination of a solenoid attached to a valve was the breakthrough that enabled the birth of modern industrial automation and control of media used within industrial processes.

Combined Functionality

The basic functionality of a solenoid valve is derived from using the mechanical energy from the solenoid to operate or actuate the valve. We also know that a solenoid will convert electrical energy into mechanical energy. This leads to a situation where we can say that we can control a solenoid valve by electrical energy. As we can now control a valve via electrical energy this removes the need for manual operation. Initially this may not seem like such a big deal, but consider a media control network that extends over several miles, if not ten's of miles and more. We now have a system whereby an electrical signal can be transmitted by a flick of a switch, this electrical signal then being used to operate a valve that is many miles away from where the decision is made to flick the switch. Imagine the man power and time that would be needed if the valve was manual and not controlled via a solenoid.

The Physical Connection

As we discovered from our investigation of the solenoid, a solenoid has two states, energised / de-energised, activated / none activated, on / off, or however you want to refer to them, but two states is all a solenoid has. The valve also has two states, open and closed. This symmetry of operation bonds both of the components of the solenoid valve, the solenoid and the valve together from a functional point of view. However the physical realisation of this union is dependant upon the type of valve operation that the solenoid is controlling. For example smaller solenoid valves tend to have the solenoid mounted directly above the valve control obstruction which is directly attached to the solenoid core. With this configuration the movement of the core gives an exact movement of the obstruction used to control flow through the valve. Whereas with larger solenoid valves or those having larger pressure differentials across them the mechanical force needed to move the controlling valve obstruction becomes impractical in terms of electrical energy and solenoid size required. In such instances the internal workings of the valve are arranged whereby the pressure differential across the valve itself is used as part of the force needed to control the valve. Where a solenoid valve is configured in such a way the function of the solenoid is somewhat different. The action of the solenoid will normally be used to release a smaller or pilot valve that itself will either reduce the differential across the controlling components which will allow flow of media through the valve, or via control of an internal pilot valve allow for the pressure difference across the valve to to act on the valve obstruction thereby stopping the flow of the controlled media.

The Future of the Solenoid Valve

At first glance it may be difficult to imagine how such a basic function as a solenoid operating a valve can be developed further. Yet over the years since its inception mans creativity has found news ways in using solenoid valves and by design how solenoid valves operate. There are solenoid valve suppliers who have available over 20,000 different makes and models of solenoid valves, many of them having unique operational qualities. As we have previously wrote, many of the solenoid valves we see today have been developed to perform a specific function, a specific media or within a certain environment. As the media a solenoid valve is asked to control expands there will be a natural development of solenoid valves to control that new media. The same will be true of new and more challenging environments such as space and beyond.

While commerce will take care of mans efforts to develop the solenoid valve to meet the needs of new media and working environments the basic function will always remain the same.

Future Solenoid Valve Enhancements

Where as the basic functionality of the solenoid valve will not change, what we consider the job of the solenoid valve will. As control automation evolves it would seem reasonable to suggest that solenoid valve's may well gain intelligence to assist in the efficiency of the controlling system. As a practical example consider a control system that monitors a flow rate through a valve. If that flow rates exceeds a certain amount then the control system sends an electrical signal to the solenoid valve that changes its state that has the effect of changing the flow rate being monitored. This is a simple and routine application where a solenoid valve is deployed. What if the solenoid valve itself could monitor the flow rate and be set to change its state at a set flow rate level. While the process is identical to the first description, the complexity is far less. It's yet to be seen how much control intelligence will be included with solenoid valves of the future, but it is an area that is sure to grow.