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.
Operation
There are many valve design variations. Ordinary valves can have many ports and fluid paths. A 2-way valve, for example, has 2 ports; if the valve is open, then the two ports are connected and fluid may flow between the ports; if the valve is closed, then ports are isolated. If the valve is open when the solenoid is not energized, then the valve is termed normally open (N.O.). Similarly, if the valve is closed when the solenoid is not energized, then the valve is termed normally closed (N.C.).[1] There are also 3-way and more complicated designs.[2] A 3-way valve has 3 ports; it connects one port to either of the two other ports (typically a supply port and an exhaust port).
The solenoid valve (small black box at the top of the photo) with input air line (small green tube) used to actuate a larger rack and pinionactuator (gray box) which controls the water pipe valve
Solenoid valves are also characterized by how they operate. A small solenoid can generate a limited force. An approximate relationship between the required solenoid force Fs, the fluid pressure P, and the orifice area A for a direct acting solenoid valve is:[3]
Where d is the orifice diameter. A typical solenoid force might be 15N (3.4lbf). An application might be a low pressure (e.g., 10psi (69kPa)) gas with a small orifice diameter (e.g., 3⁄8in (9.5mm) for an orifice area of 0.11in2 (7.1×10−5m2) and approximate force of 1.1lbf (4.9N)).
If the force required is low enough, the solenoid is able to directly actuate the main valve. These are simply called Direct-Acting solenoid valves. When electricity is supplied, electrical energy is converted to mechanical energy, physically moving a barrier to either obstruct flow (if it is N.O.) or allow flow (if it is N.C.). A spring is often used to return the valve to its resting position once power is shut off. Direct-acting valves are useful for their simplicity, although they do require a large amount of power relative to other types of solenoid valves.[4]
If fluid pressures are high and orifice diameter is large, a solenoid may not generate enough force on its own to actuate the valve. To solve this, a Pilot-Operated solenoid valve design can be used.[1] Such a design uses the pressurized fluid itself to apply the forces required to actuate the valve, with the solenoid as a "pilot" directing the fluid (see subsection below). These valves are used in dishwashers, irrigation systems, and other applications where large pressures and/or volumes are desired. Pilot-operated solenoids tend to consume less energy than direct-action, although they will not work at all without sufficient fluid pressure and are more susceptible to getting clogged if the fluid has solid impurities.[4]
A direct-acting solenoid valve typically operates in 5 to 10 milliseconds. Pilot-operated valves are slightly slower; depending on their size, typical values range from 15 to 150 milliseconds.[2]
Power consumption and supply requirements of the solenoid vary with application, being primarily determined by fluid pressure and orifice diameter. For example, a popular 3⁄4-inch 150 psi sprinkler valve, intended for 24 VAC (50–60Hz) residential systems, has a momentary inrush of 7.2 VA, and a holding power requirement of 4.6 VA.[5] Comparatively, an industrial 1⁄2-inch 10,000 psi valve, intended for 12, 24, or 120 VAC systems in high-pressure fluid and cryogenic applications, has an inrush of 300 VA and a holding power of 22 VA.[6] Neither valve lists a minimum pressure required to remain closed in the unpowered state.
A- Input side B- Diaphragm C- Pressure chamber D- Pressure relief passage E- Electro Mechanical Solenoid F- Output side
Pilot-operated
While there are multiple design variants, the following is a detailed breakdown of a typical pilot-operated solenoid valve. They may use metal seals or rubber seals, and may also have electrical interfaces to allow for easy control.
The diagram to the right shows the design of a basic valve, controlling the flow of water in this example. The top half shows the valve in its closed state. An inlet stream of pressurized water enters at A. B is an elastic diaphragm and above it is a spring pushing it down. The diaphragm has a pinhole through its center which allows a very small amount of water to flow through. This water fills cavity C so that pressure is roughly equal on both sides of the diaphragm. However, the pressurized water in cavity C acts across a much greater area of the diaphragm than the water in inlet A. From the equation , the force from cavity C pushing downward is greater than the force from inlet A pushing upward, and the diaphragm remains closed.
Diaphragm B will stay closed as long as small drain passage D remains blocked by a pin, which is controlled by solenoid E. In a normally closed valve, supplying an electric current to the solenoid will raise the pin via magnetic force, and the water in cavity C drains out through passage D faster than the pinhole can refill it. Less water in cavity C means the pressure on that side of the diaphragm drops, proportionately dropping the force too. With the downward force of cavity C now less than the upward force of inlet A, the diaphragm is pushed upward, thus opening the valve. Water now flows freely from A to F. When the solenoid is deactivated and passage D is closed, water once again accumulates in cavity C, closing the diaphragm once the downward force exerted is great enough.
This process is the opposite for a normally open pilot-operated valve. In that case, the pin is naturally held open by a spring, passage D is open, and cavity C is never able to fill up enough, pushing open diaphragm B and allowing unobstructed flow. Supplying an electric current to the solenoid pushes the pin into a closed position, blocking passage D, allowing water to accumulate in cavity C, and ultimately closing diaphragm B.
In this way, a pilot-operated solenoid valve can be conceptualized as two valves working together: a direct-acting solenoid valve which functions as the "brain" to direct the "muscle" of a much more powerful main valve which gets actuated pneumatically or hydraulically. This is why pilot-operated valves will not work without a sufficient pressure differential between input and output, the "muscle" needs to be strong enough to push back against the diaphragm and open it. Should the pressure at the output rise above that of the input, the valve would open regardless of the state of the solenoid and pilot valve.
Components
Example core tubes. Non-magnetic core tubes are used to isolate the fluid from the coil. The core tube encloses the plugnut, the core spring, and the core. The coil slips over the core tube; a retaining clip engages the depression near the closed end of the core tube and holds the coil on the core tube.
Solenoid valve designs have many variations and challenges.
The core or plunger is the magnetic component that moves when the solenoid is energized. The core is coaxial with the solenoid. The core's movement will make or break the seals that control the movement of the fluid. When the coil is not energized, springs will hold the core in its normal position.
The plugnut is also coaxial.
The core tube contains and guides the core. It also retains the plugnut and may seal the fluid. To optimize the movement of the core, the core tube needs to be nonmagnetic. If the core tube were magnetic, then it would offer a shunt path for the field lines.[11] In some designs, the core tube is an enclosed metal shell produced by deep drawing. Such a design simplifies the sealing problems because the fluid cannot escape from the enclosure, but the design also increases the magnetic path resistance because the magnetic path must traverse the thickness of the core tube twice: once near the plugnut and once near the core. In some other designs, the core tube is not closed but rather an open tube that slips over one end of the plugnut. To retain the plugnut, the tube might be crimped to the plugnut. An O-ring seal between the tube and the plugnut will prevent the fluid from escaping.
The solenoid coil consists of many turns of copper wire that surround the core tube and induce the movement of the core. The coil is often encapsulated in epoxy. The coil also has an iron frame that provides a low magnetic path resistance.
Materials
The valve body must be compatible with the fluid; common materials are brass, stainless steel, aluminum, and plastic.[12]
The seals must be compatible with the fluid.
To simplify the sealing issues, the plugnut, core, springs, shading ring, and other components are often exposed to the fluid, so they must be compatible as well. The requirements present some special problems. The core tube needs to be non-magnetic to pass the solenoid's field through to the plugnut and the core. The plugnut and core need a material with good magnetic properties such as iron, but iron is prone to corrosion. Stainless steels can be used because they come in both magnetic and non-magnetic varieties.[13] For example, a solenoid valve might use 304 stainless steel for the body, 305 stainless steel for the core tube, 302 stainless steel for the springs, and 430 F stainless steel (a magnetic stainless steel[14]) for the core and plugnut.[1]
Types
Many variations are possible on the basic, one-way, one-solenoid valve described above:
Solenoid valves are used in fluid power pneumatic and hydraulic systems, to control cylinders, fluid power motors or larger industrial valves. Automatic irrigation sprinkler systems also use solenoid valves with an automatic controller. Domestic washing machines and dishwashers use solenoid valves to control water entry into the machine. They are also often used in paintball gun triggers to actuate the CO2 hammer valve. Solenoid valves are usually referred to simply as "solenoids."
Solenoid valves can be used for a wide array of industrial applications, including general on-off control, calibration and test stands, pilot plant control loops, process control systems, and various original equipment manufacturer applications.[15]
History and commercial development
In 1910, ASCO Numatics became the first company to develop and manufacture the solenoid valve.[16][17]
A pump is a device that moves fluids, or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy.
A valve is a device or natural object that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil. The magnetic field disappears when the current is turned off. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet.
A gate valve, also known as a sluice valve, is a valve that opens by lifting a barrier (gate) out of the path of the fluid. Gate valves require very little space along the pipe axis and hardly restrict the flow of fluid when the gate is fully opened. The gate faces can be parallel but are most commonly wedge-shaped.
A ball valve is a flow control device which uses a hollow, perforated, and pivoting ball to control fluid flowing through it. It is open when the hole through the middle of the ball is in line with the flow inlet, and closed when it is pivoted 90 degrees by the valve handle, blocking the flow. The handle lies flat in alignment with the flow when open, and is perpendicular to it when closed, making for easy visual confirmation of the valve's status. The shut position 1/4 turn could be in either clockwise or counter-clockwise direction.
The J-2, commonly known as Rocketdyne J-2, was a liquid-fuel cryogenic rocket engine used on NASA's Saturn IB and Saturn V launch vehicles. Built in the United States by Rocketdyne, the J-2 burned cryogenic liquid hydrogen (LH2) and liquid oxygen (LOX) propellants, with each engine producing 1,033.1 kN (232,250 lbf) of thrust in vacuum. The engine's preliminary design dates back to recommendations of the 1959 Silverstein Committee. Rocketdyne won approval to develop the J-2 in June 1960 and the first flight, AS-201, occurred on 26 February 1966. The J-2 underwent several minor upgrades over its operational history to improve the engine's performance, with two major upgrade programs, the de Laval nozzle-type J-2S and aerospike-type J-2T, which were cancelled after the conclusion of the Apollo program.
A flow control valve regulates the flow or pressure of a fluid. Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges.
Hydraulic machines use liquid fluid power to perform work. Heavy construction vehicles are a common example. In this type of machine, hydraulic fluid is pumped to various hydraulic motors and hydraulic cylinders throughout the machine and becomes pressurized according to the resistance present. The fluid is controlled directly or automatically by control valves and distributed through hoses, tubes, or pipes.
A needle valve is a type of valve with a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates.
Diaphragm valves consists of a valve body with two or more ports, a flexible diaphragm, and a "weir or saddle" or seat upon which the diaphragm closes the valve. The valve body may be constructed from plastic, metal, wood or other materials depending on the intended use.
A thermal expansion valve or thermostatic expansion valve is a component in vapor-compression refrigeration and air conditioning systems that controls the amount of refrigerant released into the evaporator and is intended to regulate the superheat of the refrigerant that flows out of the evaporator to a steady value. Although often described as a "thermostatic" valve, an expansion valve is not able to regulate the evaporator's temperature to a precise value. The evaporator's temperature will vary only with the evaporating pressure, which will have to be regulated through other means.
A variable force solenoid (VFS) is an electro-hydraulic device that controls pressure proportionally or inversely proportionally to a signal obtained from the on-board controller of a powertrain. A low flow VFS is used as a signal level devices for transmission line pressure control or application of clutches. A high flow VFS controls line pressure directly or are used for direct shift clutch control. A VFS is a type of Transmission Solenoid. One or more VFS can be used in an automatic transmission or installed in an automatic transmission valve body.
A pressure regulator is a valve that controls the pressure of a fluid to a desired value, using negative feedback from the controlled pressure. Regulators are used for gases and liquids, and can be an integral device with a pressure setting, a restrictor and a sensor all in the one body, or consist of a separate pressure sensor, controller and flow valve.
A shutdown valve is an actuated valve designed to stop the flow of a hazardous fluid upon the detection of a dangerous event. This provides protection against possible harm to people, equipment or the environment. Shutdown valves form part of a safety instrumented system. The process of providing automated safety protection upon the detection of a hazardous event is called functional safety.
An angle seat piston valve is a pneumatically-controlled valve with a piston actuator providing linear actuation to lift a seal off its seat. The seat is set at an angle to provide the maximum possible flow when unseated. Angle seat piston valves are particularly suited to applications where high temperatures and large flow rates are required, such as steam or water. When used in reverse some models of angle seat piston valve will eliminate water hammer when operated.
The Camcon binary actuator is an electronically-controlled electromagnetic actuation device that physically moves in between 2 stable states. Unlike a solenoid the binary actuator technology (BAT) only uses energy when switching in between states. It does not require a constant current running through it.
Automatic balancing valves are utilised in central heating and cooling systems that rely on flow of water through the system. They use the latest flow technology to ensure that the design flow rate is achieved at all times irrespective of any pressure changes within the system.
The first automatic timer, the dashpot timer has been used in many different machines and has many variations. Pneumatic, hydraulic-action, and mercury displacement timers. Being used in a variety of things such as printing presses, motors, and even irrigation systems, the dashpot timer has seen many applications. Even in modern times with electrical and digital timers, these old mechanical timers are still in use due to their simplicity and ability to function in tough environments.
In engineering, a solenoid is a device that converts electrical energy to mechanical energy, using an electromagnet formed from a coil of wire. The device creates a magnetic field from electric current, and uses the magnetic field to create linear motion. In electromagnetic technology, a solenoid is an actuator assembly with a sliding ferromagnetic plunger inside the coil. Without power, the plunger extends for part of its length outside the coil; applying power pulls the plunger into the coil. Electromagnets with fixed cores are not considered solenoids. In simple terms, a solenoid converts electrical energy into mechanical work. Typically, it has a multiturn coil of magnet wire surrounded by a frame, which is also a magnetic flux carrier to enhance its efficiency. In engineering, the term may also refer to a variety of transducer devices that convert energy into linear motion, more sophisticated than simple two–position actuators. The term "solenoid" also often refers to a solenoid valve, an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid or linear solenoid. Solenoid bolts, a type of electromechanical locking mechanism, also exist.
↑ Skinner Valve 1997, p.128, stating "The tube is made of non-magnetic material to make certain that the flux is directed through the plunger rather than around it."
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