Solenoid Valve Performance Trade-offs and Design Challenges

Engineer reviewed by:

• Sarah Charette, Technical Marketing Manager – Health & Science @ The Lee Company
• Tom Cleveland, Group Lead Engineer for Solenoid Group @ The Lee Company
• Mathew French, Technical Marketing Manager – Aerospace & Energy @ The Lee Company

Selecting a solenoid valve that meets all desired performance requirements across various environmental operating conditions may represent a significant challenge for a designer. Because the features of a solenoid valve are interconnected, modifying one aspect of the design may impact another performance element.

In some cases, performance trade-offs are obvious. For example, reducing the solenoid valve’s size may reduce its flow capacity. Attempting to make a component more durable by changing its material from plastic to metal will also increase the valve’s overall weight.

Modifying specific valve features to improve other performance factors may result in less obvious or more complex design challenges. Some trade-offs are easier to understand by considering the force balance equations of a solenoid valve design. Solenoid Valve Mechanics: Understanding Force Balance Equations is a helpful reference for evaluating a variety of solenoid valve design trade-offs. The list below describes common performance trade-offs regarding how fluid travels through each component of the solenoid valve.

Differential Pressure in a Solenoid Valve

In designs that are not pressure-balanced, the differential pressure between a solenoid valve’s inlet and outlet ports correlates to the force required to translate the armature and valve components. If the differential pressure increases, the force needed to overcome it must also increase. Additionally, indirect-acting solenoid valves require a minimum pressure differential of about 0.5 bar to function effectively.

To offset the force balance associated with an increased pressure differential, the solenoid valve will require:

• Increasing the coil forces. The coil must increase in size and/or power consumption to achieve a stronger coil force. An increase in coil size may lead to an increased weight, and an increase in power consumption may lead to a higher coil temperature.
• Reducing the distance the armature travels. The weaker the magnetic field gets, the further an armature travels from the center of a solenoid coil’s magnetic field. Increasing the pressure differential may limit the distance the armature can translate (also known as a stroke length).
• Reducing the area pressure acts upon. Force is related to pressure and area. As a result, increasing the differential pressure may be offset by decreasing the dimensions of the inlet or outlet port so that the pressure acts on a smaller area. When the size of these ports is reduced, the valve’s overall flow capacity decreases.

A sufficient pressure differential is crucial for the optimal operation of indirect-acting solenoid valves, allowing them to achieve high flow rates.

Leakage rate and cycle life are other performance trade-offs that a change in differential pressure requirements may also impact.

Leakage is determined by the seal quality between the valve seat and sealing element (such as a ball, poppet, or plunger) and the materials used. Higher pressure can sometimes reduce leakage by providing additional compression between these two elements, and in other cases, it can help fluid leak past the valve seat.

Pilot-operated or servo-operated valves utilize a small solenoid to control a diaphragm mechanism, effectively managing large flow rates based on pressure differences.

The pressure differential range determines the spring and coil force required to operate the valve. Expanding this range allows more force to be applied to the valve’s components under certain conditions. However, this force may cause stress and fatigue, reducing the solenoid valve’s operating cycle life. Using more durable materials may be an option to offset this issue.

Solenoid Valves: Flow Capacity

The flow capacity of direct-acting solenoid valves is related to their size. Valve components must be large enough to allow the required flow rate through the valve in the open position. As flow capacity increases, the valve must become larger (and, therefore, heavier).

The valve body plays a crucial role in controlling the flow, direction, and pressure of liquids or gases. It contains components interacting with an electromagnetic solenoid to effectively manage fluid dynamics.

As with differential pressure, the size of the flow path directly affects the forces acting on the valve components. Increasing flow capacity also increases these forces and may require a stronger coil design. As the number of flow passages increases, fluid is also at a greater risk of leaking across valve seals.

Internal Leak Rate of Solenoid Valves

Both differential pressure and flow capacity influence leakage. The internal leak rate—or the leakage through the solenoid valve—highly depends on the chosen fluid, the materials used within the sealing elements, and how those materials are compressed. A normally closed valve remains closed when de-energized, which can help minimize leakage. Fluids with lower density will leak more easily. Less porous or softer materials will typically provide lower rates of leakage. Materials have their own design trade-offs, including durability, pressure capacity, temperature range, and fluid compatibility.

It is also possible to reduce leakage by compressing the sealing elements to minimize gaps. This force may originate from the differential pressure of the fluid across the valve, the coil force translating moving components or the springs used within the valve design. Larger forces may impact durability.

A stronger spring will increase seat stress but requires additional coil power to overcome the strengthened spring force upon actuation. However, it is important to note that increased power consumption will also increase the coil’s temperature. When energized by current, the electric coil generates a magnetic field that enables the solenoid to control the movement of a plunger, thereby regulating the flow of fluids.

Alternatively, changing the materials of the sealing elements may increase the contact area, allowing for an improved seal. For example, an elastomer may deform in compression (increasing the contact area) or fill gaps between itself and the other sealing element. The sealing material and spring strength may also affect the operating life of the solenoid valve. A strong spring may cause plastic deformation of a soft elastomer over time.

Temperature

As temperatures rise, the solenoid coil’s resistance increases, making it less efficient. This reduces the strength of the magnetic field and the force provided by the solenoid coil. As a result, maximum operating temperatures may limit the maximum operating pressure differential.

Temperature impacts fluid properties such as viscosity and density, which may affect the solenoid valve’s flow and internal leakage rates. Temperature may also cause material properties to undergo additional changes. Temperature can potentially make materials brittle or soft, impacting the durability of a solenoid valve.

Pressure differential, flow capacity, leakage, temperature, and material selection impact one another. The correlation between each factor depends on many aspects of the solenoid valve configuration. There are also design trade-offs associated with the solenoid coil. Solenoid coil design and drive electronics help optimize response time and power consumption. These topics require a separate explanation and are beyond the scope of this document. However, they represent an important set of design trade-offs that should be understood at a basic level.

Power Consumption, Response Time, and Temperature

Increasing the current through a solenoid coil — either by increasing voltage or decreasing the coil resistance — will increase the force produced by that coil in a normally closed valve. This added force can improve response time, but the overall benefit may be limited by how much power the system can supply to the solenoid valve.

For example, a solenoid valve used in a dishwasher will “close” when the appliance is off to conserve both water and power.

Applying power to a solenoid coil causes it to generate its own heat, which may make the coil less efficient. Eventually, the solenoid may heat up to the point that it can no longer operate. It is important to understand both the maximum power that can be applied to a solenoid valve and the maximum length of time the solenoid can be continuously powered before overheating.

Sample Scenario and Performance Trade-Off Quick Guide Matrix

The sample scenario from the previously mentioned force balance reference guide can help illustrate one set of performance trade-offs in more detail. This article provides an example of a force balance equation to help explain how an engineer would design a solenoid valve capable of achieving the required force margins based on system requirements and a known valve layout.