A solenoid valve is an electromechanical device in a pipeline that allows fluid to flow in one direction only when activated. The solenoid valve uses an electric magnet to open and close the valve, a direct-acting design. It usually has two ports and can also be used as an actuator or auto-closing/opening control device. Solenoid valves are typically found in pressure-reducing systems, oil pipelines, water supply networks, gas distribution networks, vacuum tube technology, and other similar applications.
Solenoid valves are used in pipelines to transport crude oil, refined products, and natural gas. The valves are used to control the flow of these liquids and gases and are fitted with flow meters. The valves are often seen in oil refineries for process control. A solenoid valve can be equipped with an electromechanical operator or a solenoid actuator that uses compressed air to actuate the mechanism that opens and closes the valve.
There are three main types of solenoid valves:
To open or close a direct-acting solenoid valve, its housing is energized by current passing through a coil wound around an iron core. This magnetic field attracts the armature, causing it to push against the stem and open the valve. In turn, to close a direct-acting solenoid valve, a current is passed through a coil wound around an iron core which attracts the armature. The armature pushes against the stem again and closes the valve.
The layout of a direct-acting solenoid valve housing can be found in US patent #4544110, granted to James F McCormick in 1985. This design allows for two or more ports and uses just one moving part, which is particularly cost effective compared with actuating rods and poppet valves.
The flow range of the solenoid valve is dependent on the size of its iron core. The flow rate is normally quoted as a maximum and minimum flow rate when the valve is open in both directions. Although the valve may remain closed in one order, it does not necessarily mean it will remain permanently closed in this direction. When the valve is closed, gravity forces act upon the trapped gas or liquid in the chamber, causing it to be forced out through one port or another when pressure increases. It could be possible to open a solenoid valve by
applying high pressure directly on one of its ports without its control device being activated.
Direct-acting solenoid valves are particularly useful in low-pressure applications requiring a small flow range. They can be found in water supply networks, gas distribution networks, oil pipelines, and vacuum tube technology. They can operate at high temperatures and pressure and withstand chemical attacks.
In addition to their advantages, direct-acting solenoids offer some disadvantages:
- The valves are relatively large due to their use of big, stronger copper coils, which need to be cooled by water.
- They can be damaged if they become stuck (often because of a broken connector or a partial obstruction) or if they are jerked too heavily while operating.
- Direct-acting solenoid valves are not suitable for spark ignition engines.
In contrast to their direct-action counterparts, indirect-acting solenoid valves have a much greater flow range than the rivals and can operate between a flow rate of 0 L/s (minimum) to more than 100 L/s (maximum). The advantage of an indirect-acting valve is that it can be easily adapted for other systems.
Direct and pilot-operated solenoids from Bürkert AG use a similar principle as direct-acting valves but provide a mechanical valve. The two arrangements are often confused and even mentioned together. However, the arrangement used is distinctly different as the solenoid in a pilot-operated valve is used to adjust the position of the moving parts rather than actuate them directly.
Pilot-operated valves allow fluid flow in one direction only and can be used as auto-closing/opening control devices when necessary. Fluid flow through this solenoid valve is regulated using an overlapping control element, such as a cam or plunger, which swings back and forth on its pivoting shaft. This control element disturbs the field of the moving part, so the valve is opened or closed. A pilot-operated solenoid valve usually has two ports and can use either an electromechanical operator or a machine operator.
Pilot-operated solenoid valves are used in pipelines to transport crude oil, refined products, and natural gas. The valves are used to control the flow of these liquids and gases and are fitted with flow meters. The valves are often seen in oil refineries for process control because they have a low-pressure rating (e.g. below 1 bar) compared with other types but also due to their reliability in chemical applications.
A schematic drawing of a pilot-operated solenoid valve can be found in US patent #4278264, granted to Samuel A Pastan in 1983. This design allows for two or more ports and uses just one moving part, which is particularly cost–effective compared with actuating rods and poppet valves.
To open or close a pilot-operated solenoid valve, its housing is energized by current passing through a wide range of electrical conductivity type of coils, which are wound around and insulated from each other by the spring-loaded arms or levers. This electrical field attracts the armature, which subsequently pushes against the lever or arm to close the valve. To complete a pilot-operated valve, the electrical field interacts with the spring and causes it to go back against the lever or component, which opens the valve.
Pilot-operated valves are almost always of indirect action but can also be directly actuated. This can be achieved by adding a valve to regulate the flow direction in each port. The additional valves are usually electromechanical operators, in which case they have a cam or plunger to control the opening/closing action. One of its ports is directly actuated in a direct actuated pilot-operated solenoid valve (using gears and levers). In contrast, a second port is indirectly actuated via another electromechanical operator using cams and plungers for control.