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The following key characteristics should be defined when selecting a solenoid valve:
Three pressure ratings should be defined for any solenoid valve:
In addition to determining relevant pressure ratings, a system designer should also identify typical operating conditions and potential transient pressure events that the solenoid valve may experience. Transient pressure events are generally caused by dynamic fluctuations due to disruptions within pressurized systems. These events may bring about extreme pressure levels and pressure rise rates that are more damaging than typical system pressure increases. Typical system pressure increases are protected against by methods which may not be reactive enough to mitigate pressure transients.
When open, the flow path of a solenoid valve should not impede the system by excessively restricting flow or by generating too large of a differential pressure such that downstream system performance is adversely affected. For example, if the function of a solenoid valve is to fill a cavity with fluid when open, the solenoid should be sized to flow the necessary volume within the required time period.
Flow rate may also be important to the functionality of the solenoid. Some solenoid valve configurations require a minimum amount of flow in order to open properly and avoid instability.
Valve leakage can be broken down into three distinctive categories, which are described below.
Leakage allowances may be influenced by a number of variables (such as whether the system is open or closed, system fluid volume, desired system efficiency, and the negative impact of fluid leaking into the downstream flow path). For instance, leakage may cause an improper fluid composition downstream of a solenoid valve whose function is to mix multiple fluids together.
| Note: In many cases, a system designer may request that a solenoid valve have “zero leakage.” However, the term “zero” is relative. It is important for a leakage requirement to specify both the flow rate and test fluid. For example: a solenoid valve that flows a viscous fluid and opens every few seconds may consider a leak rate below one drop per minute of water to be “zero.” Alternatively, a solenoid valve may control the outlet of a pneumatic fuel tank intended to operate for years may require leakage below 1*10-6 sccm of helium. |
Rated voltage is the maximum voltage level the coil may be subjected to without becoming damaged. For safety, the coil’s maximum rated voltage should be a higher voltage than the system is able to produce.
Pull-in voltage is the voltage level required to translate the valve from the de-energized, or resting position, into the energized position. It is typically specified as a minimum voltage to guarantee the point at which the valve is fully translated. Pull-in voltage may also be referred to as actuation voltage, or the energizing voltage.
Hold-in voltage is the minimum voltage required for a solenoid valve to remain in the energized position. A hold-in voltage is often used as a means of minimizing power consumption or limiting the stabilized temperature of a coil when a solenoid valve is in use. The magnitude of force provided by a solenoid coil on the armature is proportional to:
The coil force on the armature is strongest when the armature is in contact with the pole. In this state, it is possible to reduce the voltage provided to the solenoid while the valve remains in the energized position, thus reducing the force on the armature. The hold value may also be specified as a current rather than a voltage
| Note: A solenoid valve consumes power when voltage or current is applied to the coil. This power consumption will generate heat and increase the temperature within the coil. The coil temperature will reach a stable point when the heat being generated equalizes with heat dissipated to the environment. The temperature at which this occurs is knows at the stabilized coil temperature or self-heat temperature.
The design of the solenoid – including its envelope, power consumption, and the environment it is used in – will influence the coil stabilization temperature. Use of hold-in voltage is often employed to reduce the coil stabilization temperature. |
Drop-out voltage refers to the voltage level that will cause the valve to return to the de-energized position after being in the fully-energized position. It is typically specified as a maximum voltage; at any level below that level, the valve may return to its de-energized state. Drop-out voltage may also be referred to as de-energizing voltage.
The response time of a solenoid valve is defined as the time elapsed between electrical input and when the valve translates. Pull-in response time is the time it takes between applying the pull-in voltage and when the valve translates to the energized position. Drop-out response time is the time it takes between removing the voltage and when the valve translates to the de-energized position.
When selecting a solenoid valve, response time may be a significant factor in the decision-making process. Within specific applications, a faster response time may be crucial to overall valve performance. For example, faster response times can allow a solenoid valve to more consistently dispense smaller and more accurate volumes of fluid. A slow response time, conversely, may limit the use of pulse width modulation for proportional control of flow through a solenoid valve.
Dispensing solenoid valves are designed to dispense small volumes of liquid ranging from nanoliters to milliliters. In this image, the VHS® Series 2-Way Dispense Solenoid Valve delivers drop-on-demand performance to accurately regulate fluid flow.
Power is consumed as voltage is applied to a solenoid valve and the amount of power a system can support is dependent on many factors. In some cases, reducing power consumption provides significant cost savings. More often, it is related to both the power source and the power required by other electrical components within the system. For instance, a wall outlet or DC motor will supply a specific amount of power at any moment that must be shared by the entire system. If a system is battery-powered, power consumption will also dictate how long the battery lasts.
A solenoid valve is comprised of many subcomponents. Each component must be comprised of materials that are able to withstand the various forces that will be applied during the operating life of the valve. This includes pressures applied internally and externally to the valve, along with the associated pressure rise rates.
Subcomponent materials must also be compatible with their environment, including external fluids and the temperatures associated with both. For instance, a valve may be subject to extreme humidity or be incorporated in a system submerged in other liquids or gases. Valve materials should also be considered when determining how the valve will be installed into the system. Failure to consider material compatibility may create issues related to thermal expansion, corrosion, or general material degradation.
The envelope is another important factor to consider when selecting a solenoid valve. System designers should first consider the location of the valve within the system and desired flow path for the fluid. The system may require the valve to be located within a specific area, potentially limiting the external dimensions and overall size of the solenoid itself. The location may also dictate the flow path based on existing lines.
Further, the envelope must account for installation, retention, and maintenance requirements. Will the installation be permanent or removable? For example, some valve bodies are cartridges that must be inserted into a manifold and held in place by a retainer. To install or remove them requires considerable effort and may even necessitate disassembly of the system entirely. In-line solenoids, on the other hand, may only make physical contact via the tubing attached to each end. In this scenario, the electrical leads may be easily installed and removed.
Finally, designers should evaluate if the valve will be used in a system where weight is a serious factor (e.g., in a portable system). In addition to the physical interface, a solenoid valve must connect to an electrical power source. As such, the electrical interface should also be specified. Common examples of electrical connections include contact pins, lead wires, electrical connectors, and PC board mounts.
Solenoid valves are available in a wide range of envelopes and can be found everywhere – in space, deep underground, and in medical devices that you might encounter at your doctor’s office. BROWSE OUR SOLENOID VALVES WITH THE LEE PRODUCT FINDER TOOL
A solenoid valve may degrade over time and no longer meet certain performance specifications – or even fail to operate. Components may deteriorate due to natural material erosion or wear caused by impact or friction between moving subcomponents. It is critical to understand both the length of each valve actuation cycle and the maximum number of cycles the solenoid will survive. This allows the system designer to rate the life of the system or generate a scheduled preventative maintenance plan to replace the solenoid valves.
Contamination poses a serious risk to any pneumatic or hydraulic system. A solenoid valve may retain trace amounts of manufacturing fluids, test fluids, debris, or dust either on or within the valve itself. This contamination can derive from various manufacturing processes or the environment that the valve is subjected to during manufacturing, transportation, or storage. In most cases, the trace amount of contamination that is present on the valve after production is considered acceptable. If the system cannot tolerate the presence of even the most minute amount of contamination, however, the valve may need to undergo special cleaning and packaging processes. An example of such an application can be seen within a solenoid valve used in a ventilator to control the flow of oxygen to a patient. The space, automotive, and medical industries have each defined specific cleanliness levels that dictate relevant contamination prevention requirements for certain components or systems.
In order to ensure safety within a specific system, many industry associations have documented recommendations or requirements for the validation and verification of solenoid valve performance. Compliance to these specifications is typically required by governing bodies and passed down to suppliers of systems and components. When selecting a solenoid valve, it is important to ensure the valve meets the industry standards required for the application in question. Examples of industry associations that have created guidelines specific to solenoid valve performance are included in the list below:
The Lee Company designs and manufactures a wide range of valves from miniature plastic pneumatic solenoid valves intended for respiratory therapy applications to high pressure, high temperature hydraulic solenoid valves operating miles underground in oil wells. If you are searching for a valve to meet your needs and would like to learn more about solenoid valves offered by The Lee Company, contact a Lee Sales Engineer today.
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Always verify flow calculations by experiment.
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