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I am currently designing a small pneumatic system that pumps air into an airbag, holds the air in, then releases it to the atmosphere. Each pump in the system actually feeds two sets of airbags and solenoids.

I have decided to use normally closed (NC) solenoids as the air has to be held in the airbag for a long time (and I don't want to deplete my battery by energising a normally open solenoid for all of that time).

The set up is as follows: I have a dc pump, connected to a 2/2 NC solenoid valve, which feeds into the air bag. The second solenoid is connected to this line (using some kind of T connector) in order to release the pressure from the airbag when necessary. As mentioned before, the dc pump actually feeds two sets of airbags and solenoids.

My question: The first solenoid in the set up only needs to let air pass in one direction, however when it is closed it will likely experience a pressure differential in different directions at different times. Can a NC, single direction solenoid valve stay closed when the pressure differential is in either direction?

Here is a link to one of the solenoids that I am considering (warning - its a chinese site, so may run a little slowly)

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That datasheet is a little light on details, so I can't answer for sure how that valve will do, but in general, when a solenoid valve is closed it will not pass fluid in either direction. This should be true up to a reasonable pressure and usually there is a rated leakage rate. So if I understand your situation right that you only need it to pass in the 'correct' direction, but you need it to not pass in both directions, you should be OK.

It's worth noting that many solenoid valves will not pass in the 'wrong' direction when they are open. In some cases they won't pass at all because they rely on air pressure in addition tot he force from the solenoid to actuate. In other cases they will pass, but much less efficiently because the geometry of the valve is designed to let the medium flow smoothly the other way, and it is not optimized for reverse flow. Usually there is not an additional check valve in line unless it is explicitly listed in the diagram, so it would be a bad idea to count on the valve not passing, even if that's the usual behavior.

In the first case, where the air pressure is used to actuate the valve, you'll also need a minimum pressure differential for the valve to operate even in the manufacturer's intended direction. So while it will hold the back-pressure just fine, you won't be able to switch it's state until you establish a few PSI of forward pressure. These kinds of valves are sometimes advertised as 'energy efficient' because they use less electrical power.

Overall, this is probably a case of you get what you pay for. If those are the only valves you can afford or fit in your project or whatever, buy them and test them (or try to contact the manufacturer if you dare.) But if you have options, it's well worth a few more dollars to buy a valve that is from a 'main line' vendor that has more detailed documentation and support. Like most industries there is a lot of formal knowledge in the fluid power industry, and also a lot of people hacking things together and getting unpredictable results. The more you know about your components, the more you know about your system and the more you know about your system the better your chance of getting your product to work reliably.

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    $\begingroup$ Thanks for answering! It's a very ambiguous case and question. In an ideal world I would be buying the valves from a company like Parker, but the cost is too dear for us ($25 per piece for their miniature solenoid valves), hence looking to the far east. $\endgroup$ Sep 20 '16 at 14:24
  • $\begingroup$ In both cases of the solenoid, I will be using an electric circuit to actuate the solenoid, so I won't need to have a forward pressure to open the valve. I don't think this system would work properly with air actuated valves. Your comment mentions a check valve, which could be a solution if the valves in fact only block in the high pressure to low pressure direction - so thanks for that. $\endgroup$ Sep 20 '16 at 14:44
  • $\begingroup$ There are a lot of valves that use electricity for switching, but internally redirect the air pressure on the supply side to help operate the valve too, so they need both. $\endgroup$
    – Ethan48
    Sep 20 '16 at 14:53

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