The following text is the transcript of the Air Logic educational video on issues with check valves.

 

INTRO

Alright, I want to make a video just about talking about cracking pressures and check valves here. Cracking pressure is the amount of pressure you need to open the check valve essentially. Not fully open it but where it just starts to crack open, where it just barely gets a little bit of flow going through the check valve. That’s the cracking pressure. Another way to think about cracking pressure is the amount of pre-load that’s internal to the check valve and wants to keep it closed. Normally when you have a check valve there’s some amount of spring force, and if there’s no pressure either way, the check valve is closed. That amount of force is the pre-load.

So that pre-loaded force has to overcome in the flow direction in order to get flow through your check valve. You can imagine there’s some relationship then between the amount of pre-load, the cracking pressure, and how well the check valve seals.

 

TECHNICAL TRADE OFF

The harder you keep that check valve closed, the better seal you’re going to have if everything else is constant. Generally if you need a really tight seal, you’re going to want a higher cracking pressure. But higher cracking pressures come with their own problems because generally as you start applying pressure in the forward direction you want flow to occur. So then there’s this relationship you have to decide on in your check valve of what’s that balance between how good of a seal do I need and how much cracking pressure can I get away with for my application.

In a perfect world a lot of check valves won’t have any cracking pressure. As soon as there was a little bit of forward pressure the valve would be completely wide open. That’s just not possible from a practical standpoint and so we have to make the tradeoffs again between that preloaded force and the cracking pressure and how good of a seal we get.

 

CRACKING PRESSURE TOLERANCE

One last thing I wanted to talk about then was the tolerance of the cracking pressure. There are some things that are just inherent to any check valve design. Things like:

  • mechanical stack up
  • the spring rate tolerance
  • the durometer tolerance inside a check valve

Those will all certainly impact the cracking pressure tolerance. But there’s one thing that is really going to impact it from an application standpoint. That is the fluid that’s going through the check valve. When we make a check valve, we test it with air pressure to make sure that cracking pressure is correct. But if you’re putting a different fluid through there (that could be a lubricated air, that could be water, that could be a chemical) it’s gonna get in between all the components inside the check valve. When the check valve goes to close, there’s gonna be a little bit of that fluid left there and that’s gonna impact how much the check valve wants to stick or not stick closed.

This isn’t such a big deal when you’re talking about some higher pressures but when you get to lower pressure, especially let’s say under one psi, those little sticking forces actually do add up to something and will actually impact how much it takes to open the check valve. Something to think about there and I’ll just encourage you to test it in your application with your fluid and just see if you get different results in air versus whatever fluid is going through your system.

 

So we talked about what cracking pressure is kind of some of the things internal to the check valve that affect that and then also some things that can affect the tolerance in there. We love talking about check valves especially for OEM applications. We provide check valve solutions to a lot of different industries, so reach out to us we would love to talk to you about your application and find something that works for you guys! Visit air-logic.com if you want some samples or some help with your application!

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