[BeanAnimal]

Originally Posted by Foxy Brown

You are absolutely correct about how a toilet works, but this not how a Borneman surge works, so it's clearly possible to get the "pop-up" with only a float to trigger it.

The difference is that Eric's surge used a toilet flapper. The flapper pivots past its negative buoyancy point

In systems similar to what you are proposing, a reliable trigger is made up of a float attached to a lever arm so that the motion of the float is amplified significantly at the valve. This help move the valve quickly past the equllibrium point where drag against the valve = upward force on the float.

What you're calling suction pulling it back down is actually drag (better to say blowing it down), and it's the shape of the valve (ball vs. flat plate) that changes things.

We can look at it from either perspective Drag is caused by the water moving past the valve, but there is a significant pressure (Bernoulli's Law) difference at the throat of the discharge piping We tend to try to avoid the actual physics when possible and keep things general for those following along

You are also right that buoyancy doesn't change with depth (water is incompressible, so depth doesn't matter). BUT the forces acting on the valve do change - depending on whether it is up or sealed down. When it is seated, the force pushing down on it is the pressure of the water column above it plus the suction of the water in the pipe below it (as long as it's not full of air). BUT when it is unseated and at the top of the cage, it is the drag force from the water flowing past it into the discharge tube that matters (there is not net pressure difference anymore).

Yes... but the problem is still getting the valve to pop into the position you want it so that the forces do not balance

To outline these forces, I have to give some details. You'll have to forgive me if I get a bit technical, I have an engineering PhD, and love equations. For my particular setup (120g) I was looking for a ~5g surge with a flow of 2000gph. I will be using a 2" dia. discharge tube, and the body will be a 12" cube placed about 1' above the top of the water line in the main tank. The pressure exerted by the water (in psi) = H/27.71 (H is the height of water column in inches), and downward force is given by F = P*A where A is the cross-sectional area of the pipe. Since H = 24" and the pipe dia. is 2", this gives a downward pressure on the ball of about 2.5 lbs! As for the viscous drag once water is flowing, the equation is a bit more complicated. If you calculate the drag coefficient (~7) for a ball this size (2.5") in water and given the linear flow rate (~1 in/s) in the body of the device, you get a drag force on the order of only 1-2 ounces.

So, once the ball is released, it will see about a 2lb decrease in the force pushing down on it... that's why it will pop-up, and why it will stay up even when the level drops and it's held only by the small float. Try it yourself with a bathtub, a ball and some string and you'll see what I mean clearly, but you need a pretty deep water column (couple of feet) to make it work. That's also why the flapper needs to swing to vertical so quickly... to reduce the drag effect.

I too like physics and math While I fully agree with the idea, the problem I see is the scale and sizing the pump properly. What happens more often than not is that the pump does not provide volume fast enough to move the float fast enough to allow the pressure gradient to "pop" the flapper and instead equilibrium is balanced and you have a slight ajar float allowing water to leave as fast as it comes in.

In an ideal system, you will be able to find the proper balance of pump/sub-float/main float. Most folks will give up in frustration long before they end up with a reliable trigger. That said, I would love to be wrong and see your setup work and be easily adjustable.

One thing to note: You mentioned the force acting upon the valve. Assuming the valve closes with water still in the discharge pipe (no air), you will have at least 1' of head @2" dia. being brought to an abrupt halt! That water hammer is going to be significant and hard on the discharge plumbing. I would avoid 90's and would use #80 pipe and fittings. A sealed air riser may help to buffer some of the shock with each valve cycle.



Let us know how things progress.