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!

 

[BigInch]

You need a relief valve set to 3 psig. It's not a check valve.

As the bag rises, outside pressure will reduce and the air will try to expand, so to maintain slightly positive buoyancy, you will have to dump air from the bag; just enough and no more. If you don't, you may go ballistic, or, if you dump too much, you'll start sinking again.

A 1000 lb vehicle with a volume of 100 ft3 would need 84 ft3 of air to reach neutral buoyancy, so you would have to hold the air volume constantly at 84 ft3 plus just enough more to give you some upward force, all the way up to the surface, dumping 406 ft3 of air by the time you arrived.

A potential problem is that, at depth, (say about 180 ft in the above example) the 3 psig may only give you a very small volume and buoyancy differential and resulting buoyant force to actually effect upward motion by buoyancy means alone.

 

[Cmihov]

Well Im doing testing right now and 3psi at depth of 1m is fine. As long as there is enough pressure to balance the surrounding pressure and a bit more(3psi) how the lower depths (higher pressures) would effect the inflated volume of the airbags compared to atmospheric conditions.

 

[BigInch]

Buoyancy control at shallow depths is more difficult.

a given volume at the surface would only have the indicated percentages of the original volume at the indicated depth, so would only provide that same percentage of buoyant force.

depth percent
0 m 100
2 m 83
4 m 71
6 m 63
8 m 56
11 50
13 45
15 42
17 38
19 36
21 33
23 31
25 29
27 28
30 26
32 25
34 24
36 23
38 22
40 21
42 20
44 19
46 19
49 18
51 17
53 17
55 16
57 16
59 15

 

[Cmihov]

so if i apply the same differential pressure, it would not inflate to the same volume? y is that?
U have any links on explaining the physics behind it?

 

[BigInch]

Right. The bag volume doesn't care about the pressure differential, only the pressure, its absolute pressure.

http://www.grc.nasa.gov/

http://WWW/K-12/airplane/aboyle.html

Atmospheric pressure at the water surface is 15 psia.
Add 0.43 psi for each additional foot of depth. That's P2.
use Boyle's law formula
P1 * V1 = P2 * V2
V2 = 15 psia * V1 / P2

 

[Compositepro]

An alternative is to drop some weights to come-up. Hard, high compression strength syntactic foams are used for flotation so the buoyancy does not change with depth. The a small ballast tank can be used for dynamic buoyancy control.

Submarines are designed to be slightly negative when the ballast tank is completely filled filled with water, so it can sink and submerge. They also usually have drop weights so that they can still surface if there is a problem with the balast tanks. Dropping a sandbag can be far simpler than filling a tank with compressed air deep under water.

 

[Cmihov]

BigInch, doesnt that apply to closed systems? Ive got 3000psi airbottle that has lots of air to supply. Wont the pressure in the airbags start getting higher only after inflating them all the way?

 

[BigInch]

The bottles are closed systems, since they are a "rigid" container, and closed valve, but as soon as you open the valve, they become open systems and the air bags are completely open to the surrounding water pressure, aside from the 3 psi differential pressure.

 

[Cmihov]

so I guess I have to take the total volume in the airbottles and the airbags as once the valves are open, it all becomes one big open system.
That would actually make sence now.

 

[BigInch]

An open fluid system refers to any part of the fluid that is exposed to "ambient" pressure. That commonly means "to atmosphere", but under water it would mean exposed to water pressure. The definition of a "closed system" implies a fluid contained within an ideally rigid container, any small amount of elasticity of the container being small enough to ignore completely. A balloon, or your air bag in water is, technically closed, but the elasticity of the container is great enough that it cannot be assumed as rigid to such an extent that external pressure controls the volume of the air inside, except for the very small maximum differential pressure of 3 psi supplied by the elasticity of the bag material. The air bag is not exactly equal to external water pressure; it can differ by up to 3 psi, but almost. Actually I would call this a semi-open, or semi-closed system; take your pick. In such a semi-open system, the fluid is contained physically, but is very much free to assume whatever volume it shall have due to the external water pressure.

The air that remains in the bottles, behind the regulator, is "contained" by the ideally rigid metal bottle, the pipe to the regulator and also by the pressure drop that is supplied by the regulator. Air after the regulator is basically exposed to water pressure, so that part of the system is open, or shall we say, semi-open.

 

[Cmihov]

To be honest Im getting a little confused here. The question is whether the bags will inflate to the same volume with the pressure differential of 3psi at depths of say 50m as it would inflate at atmospheric conditions with a pressure differential of 3psi?

 

[BigInch]

Of course not. OK. Just forget the 3 psi. It doesn't mean much anyway. Its a maximum, so it could just as easily be zero. The important point is that the air in the bag is nearly the same pressure as its surroundings.

The absolute pressure at the surface is 15 psia; the pressure at 50 meters is 15 + 50 * 3.28 * 62.4 / 144 = 86 psia. The volume of a given amount of gas at different pressures will be different. V2 = P1/P2 * V1

ReRead the above posts slowly.... very slowly.

 

[Cmihov]

ok, after reading more, i actually realized my mistake. My bad!
Now i have a another question. Because I have to raise the pressure in the airbags by a certain constant dependent on the surrounding pressure to completely inflate the bags, is there any mechanical way to prevent the airbags from bursting?
By the way i should mention that the airbags are actually tubes enclosed plastic shells with holes for the water to come in and out.

Thanx again!

 

[MikeHalloran]

Water ballast systems become a lot simpler mechanically, and a lot more reliable, if you just leave the bottom open to the sea.



Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

Exactly. Calculate, or by experiment, find the displacement volume you need and make a box with that volume. Now cut out the bottom panel. Anytime you want to go up, fill the box with all the air you want. Any excess air, or the excess air volume generated from the lower pressure as you rise up, will simply spill out the bottom. No fuss, no muss.

 

[1gibson]

Your other thread mentions magnetic sensing equipment... What about your solenoid valve? This obviously has a magnetic component.

Could you use pneumatics that are valved/actuated from the surface? Sorry, a bit off topic.

 

[Cmihov]

at 50m depths? I doubt it.
The solenoids will be actuated after the magnetic have done their job, so they will not affect the sensors

 

[Cmihov]

BigInch, how can i calculate the pressure needed to inflate the airbags back to its 100% volume. Because I will be adding extra volume of air to the airbags, the P1V1=P2V2 wont work.
I guess I can calculate the density of air at different depths/pressures, but then what formula can tell me the pressure need to inflate it to certain volume?

 

[BigInch]

If you have some volume V1 at some pressure P1, and you hold the depth, the P1 pressure stays the same, so V2 will be V1 + whatever volume of air you add at that same pressure, P1.

After you add that air, you can move to a new depth and you go back to using P1*V1 = P2*V2