Marine differential Check valve? 1

[Cmihov]

Good Day!

Im a starting engineer and I was set a task to design a flotation system for an unmanned device that drops to seabed, and has to pop up back to the surface. The flotation system is only engaged when the system has to ascend back to the surface
Now what I started off is 2 scuba airbottles that contain air at 3000psi. Then I have a pressure regulator that brings the pressure down to 150psi. After that I have a solenoid valve that lets the air into the airbags that inflate and increase the displacement of the system. Also a pressure sensor is installed for monitoring the pressure in the airbags. Now the problem is that the airbags can sustain only 3psi, and I need a mechanical check valve, that will monitor the pressure inside the airbags and prevent them from over-inflating. But because the system will be ascending, the surrounding pressure will vary. So my question is, will a check valve (with a cracking pressure of 3psi) work with airbags connected to the inlet of the check valve and the outlet being the surrounding water.

Thanx!

 

[Cmihov]

Ok this is what i came up with calculating the pressure needed for inflating the volume of airbag at different depths
The attached excel sheet

 

[BigInch]

Yes, ColB values are the pressures you will need to inflate the bag at the indicated depths.

If you are at 50 m, and use 6 BarA pressure to inflate the bag to a volume of 0.01 m3, you will have 10 kg of buoyant force.

Making no changes to the mass of air in the bag, if you move up to 40 m depth, the bag will have a volume of 0.0125 m3 and a buoyant force of 12 kg. At 20 meter depth, your buoyant force will be 22.3 kg. At 10 meters, buoyant force will be 31.2 kg. With those changing buoyant forces, you may be shooting up quite fast now, if you wern't releasing any air volume from the bag as you were ascending.

Now, if you didn't release any air, the bag material tension allowable might have been exceeded at some point during the ascent, for example somewhere between 50 meters and 20 meters, where the volume of the bag could have as much as doubled in size. If the bag didn't double in size, then the bag material was being tensioned up to some degree. That is why scuba divers must always exhale during an ascent. I ran out of air at 40 ft depth one time. It was really amazing to see all of that air coming out of my mouth as I went back up to the surface. A very personal lesson in Boyle's law. If you think that won't teach you to watch your tank pressure, nothing will.

 

[Cmihov]

So your saying that if the airbags will be inflated to ColB pressure, they will be inflated to their unstretched volume?

 

[Cmihov]

sorry more clearer:

If at 50m if the bags are inflated to 6.02bars, they will inflate to their maximum unstretched volume?

 

[BigInch]

If its a balloon, I would imagine that any volume you'd want to put into it is larger than its unstretched volume, so it would have to stretch, possibly up to the point it burst, if you didn't have that 3 psi relief valve in it. That valve will only allow whatever tension in the balloon will be caused by a net differential pressure between the inside of it and the water pressure outside of it of 3 psi. Hopefully the 3 psi max diff pressure isn't enough to burst it.

I don't know what the maximum volume of the bags are? But whatever volume of air you put into it will be at the pressure of the water depth where you do it + a max of 3 psi.

If you inflate the bag at 50 meters, it will be at the 50 m depth pressue of 6 barA + a max of 3 psi. If you move the bag, or inflate it at another depth, the pressure will be, or will change to, that depth's pressure + a max of 3 psi, etc.

 

[Cmihov]

The 3 psi differential pressure is just perfect for us.
Thanks again and that was very helpful.

 

[BigInch]

Good luck. What is this thing anyway?

 

[Cmihov]

its a system for taking measuring sensors down to the seabed