Buoyancy force problem this is my first attempt at this forum, hope you can help me.
I'm trying to design a kind of water valve with inexpensive materials as a first prototype. 
The mechanism of the water valve goes like this:  the water flow from the pipe (1) reach the body of the valve and pass through an aluminum grid to the water tank. When the water level goes up pushes the ping pong ball closing the water intake at point (2).
How can I calculate the buoyancy force needed to stop the water flow? besides the density of the water, and the water intake pressure, which data do I need?
I built this prototype with a valve body of 63 mm diameter, and the ping pong ball couldn't seal the water intake, so now I will reduce the diameter of the body to 50 mm and with this increase the water volume displaced by the ping pong ball. Any suggestions?
Thanks]1
 A: It's pretty much obvious that the ping-pong ball is not enough to stop the water flow.
Let’s back to the basics; here I present the problem and some math that I been doing, I would like your opinion:


*

*I have a garden hose whit a water flow pressure of, let's say...49 kPa (I need to check this with a manometer), and I attached a 25 diameter and 0.5 meters long PVC pipe. The other side of the PVC pipe is tapped.
Please take in consideration that I'm not a fluid mechanic expert.
When I open the garden hose the PVC pipe starts to fill, so based on this situation:



P1+ρgh_1+(v^1 ρ)/2=P2+ρgh_2+(v^2 ρ)/2
If I took the height of P1 as a reference, h=0, and the diameter of the PVC pipe and the garden hose pipe are the same (25 mm), the water flow velocity is equaled:
P1=P2+ρgh_2
So, if the garden hose pressure is 49 Kpa:
49000  kg/(m s^2 )=P2+9.8  m/(s^2  )  x 1000  kg/m^3   x 0.5 m
P2=53900  kg/(m s^2 )
P2=53.9 Kpa
Ok, assuming this math is correct…now I have to calculate the force against the bottom of the PVC pipe at point 2:
P=F/A
A=πr^2=π(〖0.025〗^2 )=0.002 m^2
F=107.8 N
If the pressure of the water flow generates a force of 107.8 N, I need an opposite force with a higher value to counteract it. Is that correct?
I calculate the force that the water flow generates against the upper cross section area of the ping pong ball, using the same P2 = 53.9 kpa the force is F = 38.7 N, so the ping pong ball needs to generate a buoyance force higher than that, which is not possible.
My goal is to find a material that generate the enough buoyancy force to stop the water flow through the valve and seal the water intake, and when the water level goes down, the float valve will let pass the water flow to continue fill the water tank.
