Understanding the intricacies of Check Valve Operation can save time and money when creating current or future applications. At Smart Products, we customize every component of our valves – from body materials, o-rings, spring pressure, and more. This flexibility ensures accurate flow control and/or pressure relief to fit your specific design criteria. The more we know about your low pressure, low flow needs, the better equipped we will be in selecting the right components to create a precise and accurate valve exclusively for your application. 


Stage 1: Shut
The Active & Passive States of a Closed Check Valve


During the first stage, the valve remains shut and prohibits flow in either direction. If you just envisioned a dormant valve, think again. There’s a lot of action going on behind the scenes because the laws of physics continue to rule over the valve in what we call passive and active states.

All valves begin in a passive state. They are not yet in contact with any liquids or gasses, but the makeup of a valve – body material, o-rings, poppet, cartridge, and springs – remain in contact with each other. The laws of physics win again if certain body materials and o-rings remain together for long periods of time, and two factors should be considered in any application: 1) leak rate and 2) sticking. 

Leak rate can be easily understood when looking at a bicycle tire. If the tire loses 1 PSI over one week, it has a leak rate of 1 PSI per week.  In our valves, we are checking for tiny changes in pressure to assure that our valves are sealing well. We test our valves for leak rate in the check direction.  If the valve can hold pressure to our stringent requirements, it is accepted. To ensure maximum performance, we perform a leak test on 100% of our valves.  

Our next potential issue is sticking. Again, if a valve is inactive and sits for a long period of time, the o-ring can stick to the o-ring seat and cause an elevated opening pressure. This issue can also arise if you use soft o-rings. Awareness of these two issues is the first step to ensure a valve’s optimal performance when evolving from the passive to the active state. 

Once the valve makes contact with a liquid/gas, it becomes active and will never return to the passive state. While the valve is still closed, the o-rings play a key role when they first come into contact with the liquid/gas for the following reasons: 

  • Certain chemical components can make the o-ring expand
  • Certain liquids (such as water) can contain debris that will not allow the o-ring to seat correctly 

Application Environment
A final consideration is an application’s environment because it can also affect the passive and active states of a closed check valve. Will the valve reside in tubing? Or, will be outside exposed to elements such as temperature, humidity, sunlight, etc.? The answers to these questions could affect a valve’s performance based on the selected body material and o-rings. 

To ensure optimal valve performance every time, it is important to consider the following:  

  • shelf life of a valve
  • liquids/gasses used
  • application environment


Stage 2: Opening
The “In-Between” of Shut & Open Valves

OpeningStage 2 of Check Valve Operation is a balancing act between being shut in Stage 1 and open in Stage 3. The nature of check valves do not allow them to be switched on and off, and this stage should be noted in order to create the optimal valve for your current and future applications.

Once pressure has increased just enough to counter the force of the spring holding the o-ring against the seat, a small amount of flow will escape. Depending on your application, this stage can be viewed one of two ways: as an insignificant or significant amount of flow. Will the initial opening of a valve affect your application? The answer will help us create the ideal valve to meet your project’s requirements because we’re able to pair the right spring pressure with the right o-ring in order to control flow between stage 2 (barely open) and stage 3 (open). 

O-rings & The Cartridge Seat
Another consideration of Stage 2 is to understand the key role of o-rings and the cartridge seat. To the naked eye, o-rings appear perfectly smooth, but when examining them under a microscope they have tiny imperfections – it’s through these ridges and gaps that flow escapes during Stage 2. 

Our expert knowledge of o-rings allows us to have some control over a valve’s performance by: 

  • washing o-rings twice to remove any debris 
  • curing o-rings to affect their properties via various methods (i.e. sulfur, peroxide, etc.) 
  • prescribing a soak test (if needed) to determine chemical compatibility 
  • selecting o-ring materials and durometer, as well as spring force to be an ideal match for a specific design criteria 

O-rings are available in Buna-N, Ethylene-Propylene, Viton®, Silicone, Fluorosilicone, Aflas, and Kalrez®. We also offer an o-ring's durometer (hardness of rubber) on a scale of 50 (soft) to 80 (hard). Available durometer will vary for each o-ring material. 

An ideal environment for these components is different depending on how the valve is being used, but we can customize components to optimize a valve’s performance characteristics for any application. Once o-rings are selected, they are wedged into the cartridge seat to create a seal. 

Remember, no valve is completely leak free because of their physical properties, but we test 100% of all valves to ensure they seal.


Stage 3: Open
Opening Pressure is Assigned

OpenStage 3: Open involves a significant chain reaction of events:

  • pressure builds
  • the o-ring lifts off the cartridge seat
  • a significant amount of gas/liquid flows through the valve

After this series of events, we are now able to assign the valve’s opening pressure. 
With Smart Products, you can choose an open pressure to suit your design criteria. Valves are rated at the pressure where the "open" stage occurs.

We use specialized equipment to measure the effects of pressure on a valve’s performance including the open PSI assignment. During this test, pressure builds up within the test volume until the valve opens. The valve opens far enough to allow a large amount of flow to escape, which causes a sudden drop in pressure. The test equipment records the point of this occurrence. All of our valves are rated in this manner. 

Even more noteworthy, Smart Products has the ability to customize the open pressure of valves ranging from 0.09 to 20 PSI. And, very soon we will be able to offer higher pressures because we are currently testing new spring designs to allow even higher opening pressure of our valves. 

If you’re looking for relief valves, Stage 3: Open is a critical component to understand because an accurate open pressure ensures your device will function exactly to your device requirements. 


Stage 4: Fully Open
Regulated Pressure Occurs

Fully OpenDuring stage 4, the pressure moves fluid through the valve with such velocity that the poppet travels to its limit. At this point, the valve has achieved a FULLY OPEN state. Once the valve is fully open, more flow can still be achieved, but at a diminishing return because the poppet cannot travel any further.

For customers who want to maintain a level of pressure in applications, being aware of this phenomenon is important. Once fully open is reached, a large pressure increase will be required to marginally raise the flow. That is why it is always good to make sure the valve you order has sufficient flow at your operating pressures. There are no operational penalties for a larger valve other than your space constraints.

Note: The minimum pressure, at which flow is possible, will surprisingly occur at a pressure lower than the initial opening pressure. This happens because more pressure is required to overcome stiction, which can occur if a valve is inactive and sits for a long period of time. However, we have found that by applying a parylene coating to o-rings, they are able to remain idle longer without the risk of an elevated opening pressure. Smart Products tests every series of valves to establish its performance in liquid and gas applications beginning with the OPENING, progressing through OPEN or minimum regulated pressure (SSRP), and ending with RESEALING. 


Stage 5: The Descent & Stage 6: Touch Down
Pressure Decreases, Internal Components Begin to Move Back

DescentStage 5: The Descent is when pressure in an application decreases, and the spring begins to pull the poppet back towards the shut or re-seal position. Watching the process closely, the valve’s poppet begins to shut following a different path. Note – the valve does not instantly return to the shut stage – a slight delay, hysteresis, occurs due to internal friction or a change in forces acting upon the valve. This lag does affect an application’s performance, but for most users it is so subtle that it is not even detectable. The journey back to re-seal is not instantaneous. These factors make a valve have a difference in their opening pressure and reseal pressure. In fact, the reseal pressure is around 20 percent below the opening pressure. 

The poppet plays a key role during these stages because it is the connection between the spring and o-ring seal. The poppet sets the spacing between these two components – thus, it directly contributes to the actual pressure the valve operates at during all stages. 

When the internal components (poppet, spring, & o-ring) continue to travel, they eventually make contact with the o-ring seat, also known as Stage 6: Touch Down. The valve will not seal well at this stage because there is no force pressing the o-ring back into the seat. As mentioned in Stage 2: Opening, o-rings and other components appear perfectly smooth, but when examining them under a microscope they have tiny imperfections – it is through these ridges and gaps that flow escapes. Therefore, the valve will not fully close until Stage 7: Resealing. 


Stage 7: Resealing & Stage 8: Reset
Internal Components are Back in Place & Valve is Ready to Repeat All Stages of Check Valve Operation Again

ResetIn Stage 7: Resealing, the o-ring continues its journey back to the seat. Previously, the o-ring was only touching the seat, but now the spring force (via the poppet) has compressed the o-ring enough to hold it into place and create a seal against every gap. 

This stage is particularly noteworthy for customers with applications that cannot drop below a certain pressure because reseal pressure will always be lower than the open PSI.  For example, if your device requires a reseal pressure of 10 PSI, then you will want a valve with an opening pressure around 12 PSI. In addition, reseal pressures under 1 PSI may not be repeatable due to the low seat pressure. Thus, the more we know about your specific pressure requirements, the more precise your check valve’s performance will be. 

To ensure accurate and reliable flow control, all of our spring-loaded valves have been extensively tested to determine their open, regulated, and reseal pressures. 

Once the valve reseals, it is in Stage 8: Reset. The valve has returned to its original position and is ready to go through all eight stages of operation again.


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