Key Considerations for Selecting a Solenoid Valve in Health & Science Applications

Solenoid valves are used in countless applications to help control the flow of liquid or gas. In medical applications, solenoid valves are an essential design component to many life-saving devices or systems that rely on the solenoid to precisely control or direct fluid. In ventilators and anesthesia delivery systems, they blend gases and calibrate sensors. In diagnostic instruments, for example, solenoid valves can be used to control the distribution of reagents for sample analysis and disease detection. Solenoid valves help human patient simulators mimic normal and abnormal conditions – by controlling limb motor function or secreting bodily fluids – so that medical professionals in training can learn without the involvement of an actual patient. They are even used to control the vital flow of oxygen to patients in oxygen concentrators.

Since solenoid valves are available in a wide range of configurations based on the system in which they are used and their intended function, selecting the right valve for your specific medical application is dependent on several factors. It is important to understand your critical application requirements inside and out. A thorough review of the following considerations will help you choose the optimal solenoid valve for your unique application.

What Does the Solenoid Valve Need to Accomplish?

Functions of the Solenoid Valve

The most common function of a solenoid valve is flow control. When used for this purpose, the solenoid valve is either “on” to open the flow path, or “off” to close the flow path. A solenoid valve can also be used for pressure control, especially in systems that may be dependent on a particular pressure range or condition, or for proportional control to gradually meter volumetric flow rate. When precise flow volume is required, a solenoid valve can function to open and close during limited intervals to allow specified fluid volumes for use in dispensing, injecting, or dosing applications. Finally, a solenoid valve may be used in fluid mixing to control the direction that a fluid travels (especially in multi-port valves with more than one fluid channel).

Types of Solenoid Valves

Solenoid valves can be classified into four distinct subtypes: conventional, isolation, pinch-tube, and proportional. Each offers various advantages for use in medical applications.

After determining the specific function that your solenoid valve should perform, carefully consider the benefits and trade-offs of each subtype to determine the best fit for your specific application or system.

• Conventional Valve: A conventional (or control) solenoid valve is used to flow air or gas in pneumatic systems where versatility and reliability cannot be compromised for fast response times or low power operation. While conventional solenoid valves may also be used to flow mild liquids, their design makes them unsuitable for use with aggressive fluids which can attack the metal and/or non-inert plastic materials in the wetted flow path, causing corrosion and compatibility failures.
• Dispensing Valve: Dispensing valves are optimized for precision dispensing applications, often in the nanoliter, microliter, and milliliter ranges. Typically, they have an ultrafast response time, making them ideal for accurate fluid regulation and for dispensing repeatable and highly reproducible droplets, or distinct spray patterns.
• Isolation Valve: Isolation valves are used to flow aggressive fluids that could otherwise damage the system’s internal mechanics or impact fluid chemistry by triggering an unplanned chemical reaction. These inert valves have very few wetted materials – typically just an elastomer and an inert plastic. Diaphragm-style isolation valves commonly contain an amount of dead volume by nature of their design, but may be smaller, faster, and less expensive than other valve styles. Issues with carryover volume can occur in isolation valves with 3-way configurations, where an unwashed slug of fluid may become trapped between the active flow path and the seal of the closed port.
• Pinch-Tube Valve: Pinch-tube-style solenoid valves contain zero dead volume and are often categorized separately from other isolation valves. Fluid flows exclusively through a cylindrical flow path which helps to provide maximum flushability in the valve. This makes pinch-tube valves particularly helpful when flowing slurry mixtures or precious biological samples (such as cells). Similar to isolation valves, pinch-tube valves have very few wetted materials. If chemically or mechanically damaged, tubing can easily be replaced on external pinch tube valves. Longer valve life is a key characteristic of internal pinch-tube-style valves, which are designed to be compatible with specific fluids and last for millions of cycles, without the need for tube replacement.
• Proportional Valve: Proportional valves are designed to provide regulated flow output against a range of flows or pressures. They operate in a non-binary position in which the valve is neither fully open nor closed. Flow is metered by adjusting the valve components to change the overall level of restriction. Proportional valves may have limited wetted material options, making chemical compatibility a challenge.

Configurations: Flow Paths and Fluidic Ports

Choosing the right solenoid valve for your medical application involves reviewing available configurations to find the best fit for your system. Important considerations include how many flow paths need to be controlled, the default state of each path when the valve is de-energized, and how you want the valve to interface fluidically to your system or the type of fluidic connections needed). How you ultimately intend to use the valve within your system’s architecture will help to further drive the design process.

Number of Flow Paths

Solenoid valves control fluid travel between various flow paths. To achieve this, they may have two, three, or even four valve ports. Meeting the goals of your fluid schematic may require a combination of multiple valve configurations. Selecting the right one depends greatly on what you need the solenoid valve to accomplish.

A 2-way valve, or 2-port valve, is the simplest solenoid valve design. This configuration style allows the valve to regulate flow with a single inlet and outlet (as it has two fluidic ports). A 2-way “on-off” configuration may be the appropriate choice if the intended function for that valve location is simply to start or stop the flow of one fluid stream.

A 3-way valve is typically a 2-position, 3-port design. This means that in the de-energized state, two of the ports are connected, allowing fluid flow through one flow path. When energized, the valve transitions. During this time, one of the ports is closed and the other opens to the third port, allowing fluid flow through the second flow path (or “position”). A 3-way valve can be used to mix fluids (with two inlets and one outlet) or to direct fluid flow or pressure from one channel to another (with one inlet and two outlets). Two 2-way valves can accomplish the work of a 3-way valve, with two separate streams controlled by two separate closures.

Depending on your device footprint or carryover requirements, you may consider utilizing this option to lessen potential cross contamination that can occur due to the inherent mixing nature of 3-way valves.

Normally Closed vs Normally Open

Solenoid valves are often referred to as normally closed (NC) or normally open (NO). When selecting a solenoid valve for use in your medical application, consider the frequency at which you actually need fluid to flow in your device or system.

In an NC configuration, the normal state of the valve is to block the flow of fluid. The valve will remain closed until such time that you choose to energize the coil to allow fluid flow. NC designs can help to save power as the valve only actuates in the minority case.

With an NO configuration, the valve is designed to remain open when de-energized (no power supplied) to consistently allow fluid flow. This configuration is typically used when the valve is required to be actively flowing fluid through it the majority of the time and only needs to shut flow off on-command. The flow path will remain open until such time that you choose to shut the valve off by energizing the coil. In the event of a sudden loss of power to the instrument, NO designs can be used as a failsafe requirement. For example, if the power supply to a blood pressure cuff is lost while on a patient’s arm, a NO valve will open up to allow the cuff to deflate.

Type of Fluidic Ports

You’ll also want to consider your preferred method of fluidic mounting. Solenoid valves are available with various types of ports, giving you flexibility over how you connect to them fluidically.

Do you want to connect to the valve using industry-standard threaded fittings? Or simple push-on tubing? Or will the valve be mounted on a manifold and integrated with other components?

In many cases, it’s easiest to start with threaded fittings or push-on tubing for bench testing, and progress to a manifold mounted valve as down selection occurs and a more finalized system design really begins to take shape.

To learn more about solenoid valve performance trade-offs and design challenges, read our latest technical insight.

What Fluid Does the Solenoid Valve Need to Control?

After determining what your solenoid valve needs to accomplish, you should also consider the fluid that the solenoid valve will control. Conventional valves are often used in pneumatic applications, as air or gas can easily circulate through the wide inner workings of the valve itself. Isolation valves are typically used to flow aggressive fluids via a chemically inert flow path. Understanding your chosen fluid will help you to select materials that both optimize performance and allow your solenoid valve to survive.

Fluid compatibility is the biggest driver of material selection.

Wetted materials are those that come in contact with fluid in the flow path (including the valve housing and elastomer seal materials). Your chosen fluids should be compatible with all wetted paths in the solenoid valve to prevent chemical attacks within your system. For example, if your fluid is incompatible with the plastic housing of a particular solenoid valve, microfractures or cracks can occur. More aggressive fluids may require the use of materials that are compatible with such chemistries (e.g., FFKM, FKM, EPDM, and PEEK). In certain scenarios, it may be appropriate to use a PTFE seal in lieu of an elastomer that has the potential to swell over time.

Consider the measured mechanical strength, chemical resistance, thermal resistance, and wear resistance of any given wetted material against your planned fluid and the system operating conditions of your application. Outside of chemical compatibility, fluid viscosity, specific gravity, and fluid temperature can all impact design choice. What are your flow rate requirements? A viscous, cold fluid typically allows for less flow than would a watery, warm fluid. Calculating the specific gravity of your fluid can help you understand how easily it can be handled.

What Size and Form Factor Does the Application Require?

When designing the system architecture for your medical application, consider requirements for size and form factor in your selection process.

How is your device meant to be used? Medical instruments typically have requirements for power and weight. Stationary medical devices plug directly into the wall, providing a steady power supply and ample space to run larger internal components. Size is often an integral consideration in portable devices, which are designed to be wearable and are typically powered using a battery. Small, lightweight components are required to ensure that the instrument can easily travel with the patient without weighing down the device. The battery must also provide a lasting charge so that the patient can remain mobile throughout the treatment process.

Tight envelope restrictions lessen the physical area that is available to design around in a given device. Finding the right solenoid valve for your specific medical application requires understanding how to appropriately position each valve to fit comfortably within the space allotted by your design. Solenoid valves can be made in a variety of orientations and form factors to save space and weight.

Integrating a manifold solution can help to further reduce instrument footprint and bulky tubing in your system. Although plug-in-style valves – also known as cartridge valves – extend vertically, their fluidic ports and part of the valve body are embedded within the manifold. This design can save vertical space and may reduce internal channel volumes. Plug-in-style valves are often useful depending on system layout, and they typically require less mounting hardware since they don’t always need screws or brackets. In contrast, face-mount-style valves are generally easier and more cost-effective to install, as their mounting surfaces are simpler to machine. They also sit flat and close together, which is ideal for systems with limited vertical clearance.

Consideration should also be given to design for manufacturing (DFM) as it relates to size, form factor, and production volume. Do you plan to optimize the design of your solenoid valve to aid in the overall manufacturability of the downstream medical device? As you plan your design, consider the production scalability of each element as it relates to the end use fabrication of the entire device.

In addition to size, a solenoid valve’s Cv rating should be appropriate to system flow needs. Flow is determined by the diameter of the orifice inside the valve, as well as the overall geometry of its flow path, and can be continuous or intermittent. Larger valves typically allow for higher flow. It is important to understand the duty requirements for your specific application to select a valve that is capable of achieving the desired flow rate. If flow is continuous, is it safe to run your device indefinitely? How will you define what intermittent flow means to you? Do you plan to turn on the device once a week or every two seconds? How much off time will occur between actuations? Determining the answer to these types of questions will help ensure that you select the appropriate solenoid valve for your specific needs.

What Key Features and Benefits Should be Considered in the Selection Process?

In addition to considering how you need your solenoid valve to function, what fluid it is meant to control, and the appropriate size/form factor dictated by your application, it is crucial to identify any required features inherent to the actual function of your medical device.

Power Management

Requirements for coil power and heat dissipation relate directly to your power budget and the projected duty of your solenoid valve (or the amount of time that the valve remains “on” during device operation). High duty is continuous duty. If you exceed reasonable power management and operate the valve outside of appropriate power and heat limitations, the device may malfunction if active cooling elements are not deployed.

How demanding is your planned duty cycle? Excessive heat can have a negative effect on other electromechanical components within your system, especially as heat migrates through the tight enclosure of the shield as the solenoid is energized. If heat is a concern in your application, a solenoid valve with minimal power consumption and low heat dissipation may be a good option to consider. Spike and hold circuitry may also help to save power by providing a brief actuation voltage prior to switching to a lower voltage in order to keep the solenoid in an energized state for an extended period of time.

A magnetically latched solenoid valve is engineered to hold its flow state without power. Unlike traditional solenoid valves that require continuous power to hold a position, a latching valve only needs a momentary electrical pulse to switch between states. Once the valve changes state, it remains latched in that position without further power input. This design eliminates the concept of “normally open” or “normally closed” configurations, as the valve does not default to any state when unpowered. This makes latching solenoids especially beneficial in applications with long duty cycles where minimal power usage and low heat dissipation are critical.

Sealing Performance

Each solenoid valve is rated with a unique leakage value. The solenoid valve may malfunction or leak if you provide too much pressure without accounting for that higher pressure by balancing the valve with enough coil power or spring force. To determine if your solenoid valve will behave as intended, identify all forces that will be applied to the valve components over its intended life cycle using a force balance equation.

When designing the internal architecture of your device or system, you should also examine your requirements for sealing performance. The importance of preventing leakage in your system will vary based on the needs of your specific application. Choosing a zero leak valve may require you to trade off on other desired features in your design due to the associated costs and testing requirements needed to ensure high sealing performance.

In applications that require zero leak tolerance, look for a solenoid valve with very good sealing performance efficiency. When utilizing a medical device to inflate a bladder, for example, you generally need pressure to hold steady for a certain period of time. If components in that device leak, pressure will decay, and any data collected may be inaccurate. Alternatively, if your application can accommodate a commodity leak rate, the sealing performance of a solenoid valve may not factor as highly when compared to other elements of the design. Does the device have a means of recovering pressure loss as a result of potential leakage? Will leakage cause minimal damage to the functionality of your application? If so, you may not need a zero leak valve.

Response/Switching Time

A solenoid valve’s response time (or “switching time”) is the time it takes to transition from one flow state to another. Response time plays a role in the total machine function time needed to execute the intended output of your application. How fast do you need your solenoid valve to run? A high speed solenoid valve may be appropriate in precision dispense applications where the speed of the valve dictates dispense volume. In other applications, the solenoid valve may only need to be as fast as the slowest component in your system.

Balance the importance of response time against other must-have design features to determine if a high speed solenoid valve is appropriate for your application or if there are other ways to drive the valve to increase speed.

Operating Temperature

Operating temperature differs from fluid temperature. The temperature of air surrounding a solenoid valve can negatively impact performance. If the valve becomes too hot, it can lose the ability to remain open when energized. High operating temperatures can also affect elastomer efficiency and battery life. As a designer, you should work to understand all potential temperature conditions related to your application – from the environment where the device will be stored to where it will operate most frequently (and any other place it may travel in between). By fully understanding the environment in which your device will operate, you will be better able to select a solenoid valve that can function successfully within the applicable range demanded by your application.

Control of your environment allows for more reliable valve performance. Ensure that your preferred wetted materials are rated for use at certain operating temperature levels. A material that works well at ambient temperatures and is compatible with your chosen fluids, for instance, may not work in lower or higher temperatures. Similarly, materials that work well at higher temperatures may not work at lower temperatures. You can typically run a device “harder” at a colder temperature because the environment allows for better heat dissipation. Operating temperature may be more of a concern in portable devices that travel with the patient to different locations (e.g., in the car, hospital, and home), especially when compared to a stationary device in a temperature-controlled location. In portable applications, active cooling may be needed to help control variances in operating temperature.

Fluid Management

Internal volume refers to the total wetted fluid volume within the fluidic component – in this case, a solenoid valve. This includes both the volume of fluid that is not easily flushable (dead volume) and the volume of fluid in all flow paths (swept volume). When designing your system, consider your fluid volume needs. Large internal volumes increase the quantity of fluid needed for operation and can also raise the risk of cross-contamination between fluids. Smaller internal volumes minimize these issues but may reduce maximum flow rates.

In diagnostic applications, having as low an internal volume as possible can help to reduce the cost of goods by minimizing reagent usage, waste, and general fluid volume. In these scenarios, a pinch-tube-style isolation solenoid valve may be a good option to consider as they allow full flushability and laminar flow. Fluid volume may also impact dispensing applications, where it is important to minimize downstream dead volume to prevent dribble or drool from passageways as the valve shuts off.

Maintenance

A medical device is only as good as the components that comprise it.

As such, it is important to choose a reliable, robust solenoid valve that meets or exceeds the useable life cycle of your device in order to minimize preventative maintenance or downtime.

What is the calculated cycle life of your medical application? How hard will you run the device? A dispensing application that operates 10,000 times per day may require preventative maintenance at more frequent intervals than one that only operates 10 times per week. Device downtime signifies a loss of productivity. Wear and tear from constant use can cause elastomers to degrade and leak; friction between the moving and stationary parts of the solenoid valve can impact operating efficiency. To verify the reliability of your solenoid valve for use in medical applications, make sure that you select a design that has been qualified and rated up to a certain cycle rate to coincide with the projected maintenance schedule or overall life expectancy of your device.

To learn about solenoid valve performance characteristics and key factors that impact operation, access our engineer-reviewed breakdown.

Avoiding Common Issues When Operating Your Solenoid Valve

When operating your solenoid valve for use in medical applications, make sure to avoid the following issues.

• Contamination: Contamination can have a profoundly negative impact on the performance of a solenoid valve, especially as it relates to sealing performance. If a contaminant or particulate lodges itself in the sealing surface of your solenoid valve, it can create a leak path that ultimately worsens system performance. Consider filtration requirements and employ proper system cleanliness practices to reduce the likelihood of contamination in your system.
• Corrosion: Ensuring compatibility between your chosen fluids and the wetted materials of your solenoid valve helps to prevent chemical attacks within your system. Corrosion can occur if fluid is incompatible with any metals used in the valve. This compromises the integrity of the solenoid valve and may impact its ability to switch or actuate.
• Incorrect Voltage or Current: Make sure that the current or voltage to the solenoid valve is within the specification range called out by the design. The power supply must be sufficient to power all valves in your system.
• Exceeding Pressure Conditions/Ratings: Operate within all rated conditions of the design. Exceeding listed pressure conditions or ratings can cause the valve to operate incorrectly.
• Improper Installation: Are you using the valve outside of the recommended environment or conditions that this product can safely and reliably operate? Review and abide by all manufacturer recommendations relating to installation.

For more information on five common failure modes to consider when designing a solenoid valve, explore our expert guide.

HOW CAN THE LEE COMPANY HELP?

At The Lee Company, we specialize in transforming complex challenges into innovative solutions. As a world leader in miniature precision fluid control technology for more than 75 years, we’ve delivered millions of products worldwide for industries including diagnostics, medical equipment, scientific instruments, aerospace & defense, space, oil & gas, motorsports, automotive, power generation, and off-highway equipment. Lee products are recognized worldwide for superior quality, reliability, and performance.

We offer a wide range of solenoid valves to meet your fluid handling needs, providing you with engineered solutions to overcome key challenges such as conserving space, weight, and power – without sacrificing system-level performance requirements. Whether you are designing medical equipment to treat patients or train healthcare professionals, our goal is to help you safeguard patient care and improve their quality of life.

The Lee Company has a team of Technical Sales Engineers available around the world to work one-on-one with our clients to solve their unique fluid control problems. Contact The Lee Company today to learn more about our solenoid valve family and how The Lee Company can customize a solution for your unique needs.