Closer Look: Standard Proportional Valve

Standard Valve Operation:

Step 1: To obtain higher flows, the valve uses a large orifice.

Step 2: Fluid pressure pushes on the poppet.

Step 3: Because the pressure pushes on the poppet attempting to open the valve, a large spring is needed to keep the valve closed.

Step 4: In turn, a space-consuming solenoid valve must be designed into the application to overcome the spring.

To meet the needs of today’s healthcare professionals, medical instrumentation is getting smaller and made to be portable. That means the components in them need to be smaller, while still maintaining a high level of precision. This article will look at micro valve technology in portable medical devices.

When a design demands a proportional valve, developers are faced with two design paths, each with its own trade-offs.

Larger Orifice Size
This design path enables valve designers to obtain higher flow rates by increasing the orifice size of the valve. As the orifice size increases, the force required by the plunger to seal the valve when closed increases exponentially with the orifice diameter. This is often represented by the equation Force=Pressure × Area (F=P×A), where pressure is measured in Pascals (Pa), force in newtons/square meters (N/m2), and area in square meters (m2).

To control the valve, the preload force of the proportional spring increases; this provides the increased force necessary to seal the orifice. This, in turn, requires a larger, higher power solenoid to pull against the stronger spring. As a result, these valves are bulky (>800 gm), consume significant amounts of power (4 to 10 W), and have substantial cost (U.S. $300 to $500+).

Valves on a Manifold
The second possible design path connects several small valves in parallel on a manifold to deliver the necessary flow. In the case of valves ganged together in this manner, designers benefit from smaller, lower power valves, but are still left with a large manifold adding weight to the system. This bulk coupled with the cost of the manifold and labor to install all the components continues to frustrate designers. In general, solutions for higher flow rates are costly, leave a large size footprint, and are power hungry. These design limitations create unnecessary complications.

A Better Solution: Pressure Balancing
Pressure balancing can assist development engineers in their quest for higher flow rates without the trade-offs associated with the previously covered design solutions

The benefits of using a pressure balanced design are:

  • High flow rate
  • Small package size
  • Low power demand
  • Easy implementation

Left to right: Competitor valve A, Parker’s VSO-Max, competitor valve BHigh Flow Rates
Parker Hannifin created a new valve—the VSO-Max—to ensure the smallest possible package size for customers while delivering a significantly higher flow rate when compared to the existing products offered in the marketplace. To deliver this flow rate, a principle known as pressure balancing is incorporated into the valve design. This method uses a non-rigid component—an elastomeric diaphragm—to offset the flow forces created by the gas as it enters the valve. The flow force is balanced against itself, eliminating the need for a larger proportional spring preload to help seal the valve (see “Pressure Balancing Design” sidebar). This eliminates the requirement for a larger, higher power solenoid and is simply an efficient and effective design.

Smaller Package Size
The implementation of a pressure balance mechanism into the design of the proportional valve miniaturizes the dimensions compared to other proportional valves in the marketplace. As mentioned previously, the pressure balance principle eliminates the need for both a larger spring to create the sealing force and the solenoid valve. The overall result is a package size that offers 95% less volume and 92% less weight compared to competitors’ valves. This reduction in both size and weight is crucial for portable equipment. The smaller valve enables the overall system to be smaller and lighter.

Pressure Balancing Design with Elastomeric Diaphragm

  • The red lines show the path the fluid (air) travels when it enters the valve. When the valve is not actuated, the poppet is sealed against the valve seat. Fluid pushes on the face of the poppet with a force equal to pressure times area of the poppet face (F=PApoppet).
  • The thru-hole allows the fluid to travel up the poppet and out the side of the plunger, filling the cavity above the diaphragm. The fluid then pushes with a force equal to pressure times area of the diaphragm (F=PAdiaphragm).
  • The area of the diaphragm is set to equal the area of the poppet, so the force pushing up on the poppet and the force pushing down on the diaphragm cancel each other out.
  • In a standard valve, the force pushing on the poppet is not canceled out, so a big spring must be used to seal the valve and prevent the poppet from opening on its own.

Lower Power Demand
Incorporating the pressure balance mechanism into proportional valves allows OEMs to significantly reduce power demands compared to competitor solutions. For example, to achieve a flow rate of 200 slpm with a competitor’s valve requires approximately 5 W of power versus only 2 W with the pressure balanced valve. This reduction in power demand significantly improves the battery life of a portable product and also decreases the heat generated by the valve. The low energy requirements simplify fluidic system designs, improving flexibility in engineering. For healthcare products such as portable ventilators, the low power consumption and longer product life (due to less wear and fewer heat demands) are important considerations and increase the marketability of the final product. Ease of use, fewer battery changes, and durability give these portable solutions a competitive advantage, translating into costs savings not only for the OEM but for the end-user.

Easy Integration
Integration of the proportional valve should be hassle free. Looking for a design that is manifold mountable will minimize the connections needed, reducing the leak points, as well as simplifying the fluidics that minimizes the overall space taken up by the components.

The pressure balanced proportional valves increase the flow rate for applications while decreasing power consumption and overall product size. The VSO-Max is a great example of this technology at work, replacing multiple smaller valves or one large valve with a single, compact, efficient valve that reduces the overall space required for a system or sub-system.

As end users place more emphasis on portable solutions spanning from life science to production automation, the demand for compact, high-performance fluidic control persists. By incorporating pressure balancing technology, proportional valves can meet this need, delivering precision control without the bulk, power consumption, or cost associated with the traditional version.