How to make a pressure maintaining valve?

[Toestr]

So I'm making a home fill setup with desiccant to remove the water from the air. In order remove as much water from the air as possible the desiccant itself needs to be at pressure (about 2500 psi) and gradually release into the tank. Without the desiccant being at pressure, the air will just pass into the tank without having the water squeezed out. It will then do so in the tank once it gets to pressure - which defeats the purpose of the desiccant until the tank is mostly full.

What I'm currently doing is pumping the system up to pressure (yes with a hand pump - it takes a while), then using a needle valve to release the dry air into the tank and equalize the pressure across the system, closing the valve repressurizing it. I continue to do this until the tank reaches that 2500 psi pressure when the system is equalized, then I just leave the valve open.

What I'd like is a valve that will maintain the pressure in the desiccant housing, but will release the air gradually over the course of pumping, once it reaches the desired pressure. Right now when I equalize the system, it causes condensation in the fill whip just from the flow due to pressure change.

I'd like something small and cheap (this is obviously a budget build after all), and preferably still be able to use the parts I have on hand for it.

I was thinking maybe using a fill nipple with some sort of spring behind it. I just don't know what springs would have the strength or what fittings would house them.

Any ideas would be appreciated.

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[BrickHaus]

Would a pair of check valves work? If you can find a 2500 psi, and something heavier. Like 5000 psi.

Have the 2.5k check to the tank, and the 5 k prevent back flow into your dryer.

The 2 combined should do what you are trying to do easily.

Unless they vaporlock eachother up or something.

Thats where my early morning brain went on this subject.

[BrickHaus]

Never mind. Need more coffee. But look at piloted check valves. You could put that on the dryer output. run either parallel air to the pilot or just have a small tank ready to toggle the pilot. (Maybe even balance it to the pilot triggers at 2200 psi or something. )

But the check that would hold air in the dryer assembly. Once to pressure trigher the pilot. Bam equalization. Release pilot. Recharge.

[the_hyren]

A pressure maintaining valve is a regulator but that's not really what you want. Probably the best thing you could do is make a cyclic control system. In this case use a relief valve set to a given pressure that actuates a valve that dumps air from the dryer to the tank for a specific duration.

Basically a relief valve set at your dump pressure into a regulator down to a safe actuation pressure into a 1 way flow restrictor into the actuation port of a normally closed 3way. Flow restrictor allows air from reg to 3way with no restriction using a check valve, then uses a needle valve to slowly vent the other direction which gives you on duration adjustment.

This is a very expensive option. High pressure handling valves cost hundreds of dollars. Your probably looking at $500 to $1000 to make such a control loop.

What you should do is get a centrifugal or cooled separator and just use the dryer as protection.

Probably the best thing to do is get a yong heng and a SCBA/scuba tank and keep bulk at pressure so that filling a tank never drops your bulk below whatever pressure you want and then run the yong heng after the fact to top things off. Which is exactly what I use at home and all together also cost only about $1000...

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[Toestr]

Thanks for the replies guys. Unfortunately these options are too big, complicated and expensive for me. I'm going to university next year so I can't justify having a setup that big and expensive that I'll barely use, and have to move a few times in the next few years.

I do agree though, the best option would probably be to have a bulk tank to fill off and just continuously top it up. That's the way I see most of the air gun guys with Yong Hengs doing it anyway.

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[grendal50]

The fill nipple on your tank is essentially a check valve, why not use a couple more of those?