Air Column Compression Measurement

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Alright - so this arrangement is going to be underwater by 1 foot to 23 feet, at times?

And you want to ensure water does not go up the tube to the check valve, right?

What exactly do you want to calculate? Pressure, volume, .... ?

"Could any body help me on how to calculate the trapped air inside the tube in inches"

How much the trapped air inside the tube get compressed in length (in/mm) per every feet of water inside the tank? So I can calculate at any increment in feet of water inside the tank how much the air inside the tube is compressed? The point is, that the tube is long enough so water will never enter the check valve.

What is the pressure on the inlet to the valve actuator?

At 2.31 PSI per foot of water, I think you will have a maximum pressure at the tube end of 53 PSI or 3.5 bar. If my fuzzy memory is correct, the change in air volume should be inversely proportional to the change in pressure.

The displacement of air would be approximately 75% of the original volume. If my math and assumption is correct. To be safe I would make the vertical tube length 5 or 6 times the horizontal length from the check valve.

I think for every 2.31 ft of water inside the tank we have 1 psi, hence if the tank is full at 26 ft then we have 11.25 psi at the inlet of exhaust tube. Now I have to calculate the volume of air trapped inside the tube as how much it will compress for every feet of water added inside the tank, then I can determine the length of tube required. This will be similar to compressing air inside a cylinder and as pressure in psi is exerted on the piston rod. Do you have a formula that I can use to calculate the volume compression?

The pressure in the tube is its depth inside the tank multiplied by the density of water multiplied by acceleration due to gravity.

What's the problem with raising the free end of the tube above the maximum level in the tank and doing away with the check valve?

.......and why is there a need for this actuated valve to be submerged in water anyway? Why can't it be positioned on the pipe that is leading to/from the tank outside it?

The pressure of 27.7" Water Column = 1 psig (approx.)

28 feet x 12 in/ft = 336" Then 336"/27.7 in/psi = 12.13 psig

12.13 psig + 14.7 psia = 28.8 psia then 26.83 psia / 14.7 = 1.83 atmospheres

Assuming that there was 1 atmosphere in the tube before it was submerged you have a 2 (1.83) / 1 compression ratio and the volume within your exit tube would be compressed (roughly) half way or 1.5".

This application sounds like Murphy is sure to show up.

In the very least I would extend the exit tube to, say 12" instead of 3".

A snorkel valve that would seal the tube from the entrance of water may have a better chance.

Thanks, very good answer, I assume the ratio comes from formula P1xV1=P2xV2, correct?

Boyle's Law, I think. The volume of a fixed quantity of gas is inversely proportional to its pressure. You were wise to verify my previous estimate. Since you are dealing with an approximate 2bar pressure, volume would be about 1/2 the original volume.

As Mr. Bush once stated, "Trust but verify." Check out any statment I, or anyone else on the internet, makes.

Mr. Slack does ask some relevant questions. Wouldn't this be simpler to keep the valve and discharge out of the tank?

Thanks Mr. Ried for helping me on this issue. Those comments about extending the tube to top of the tank for the exhaust air and placing the valve outside the tank are not possible because first this valve dumps water in pre-load tanks on an offshore drilling rig similar to ballast tank on a ship that dumps water to the sea by gravity and the pressure of the water column inside the tanks. The valve can not be installed outside of the hull of vessel. To extend the tubing to top of the tank there are twenty three tanks and the cost of installing tubing on tank bulkheads and all required tube fittings and mourning hardware will be costly so I was looking for a more economical installation. Once again thanks for your help and Mr. Kreher in helping me to resolve this issue.