Fluid Dynamics - increasing pressure

[Scorp1us]

I am working on hobby project with almost no budget.
I bought some "nice" (cheap & simple) electronic solenoid valves for water flow. I've since discovered that they need 3PSI to open. Which is ok for the supply lines. But the drain line requires that as well. Now I either need a column of water of 3x27.7" (about 7 ft) or soe other way. The valves have a 1/2" inlet, which I calculate to be .78 sqin. I'm not interested in the flow but I need a way to reduce the required height. I thought about putting a fat section of pipe to increase the water held above it.
A 6" PVC pipe has a area of 28", divide this by .78 = 36. Divide the 3PSI height (83") by 36 = 2.3" So a 6x2.3" canister should do it.

However, I know that pressure is not linear and weird things happen like velocity increases and pressure drops.

What can I do to get the height of the column of water down to less than 12" using my 1/2" valve and gravity?

 

[CWatters]

I thought about putting a fat section of pipe to increase the water held above it.

That won't increase the pressure at all. The pressure depends only on the height not the diameter. The pressure experienced by a diver only depends on the depth not the area of the ocean/lake/pool he's diving in.

Is there some sort of return spring in the valve you could modify?

 

[CWatters]

How much pressure is in the vessel feeding the output drain pipe? Suppose it was only 12" head of water... then the drain pipe would only need to be raised 12" to stop the flow... Perhaps you could arrange for the solenoid to allow high pressure water into another tank with a float that raises and lowers the drain pipe shutting off the flow. You might have an issue with syphoning but that's solvable.

or another approach..

Suppose the drain pipe was a soft silicon pipe that could be squashed. Perhaps you could arrange for the solenoid to squash the pipe by allowing high pressure water to flow into something resting on the pipe. If that "something" had a small hole in it then when the valve turned off the would flow out reducing the load on the silicon drain pipe allowing the pipe to open. Perhaps wasteful of water.

You need to hire Heath Robinson :-)

 

[SteamKing]

You also might want to check your calculation of the area of the 1/2" inlet.

Remember, the formula is A = pi * (D^2) / 4

 

[rollingstein]

An eductor?

http://www.amproduct.com/eductorPIC.jpg

The motive fluid is high Pressure water that you control using the solenoid. The educted fluid line comes from your drain.

Eductor is placed at a level higher than tank to prevent auto-draining. You'll need only a very low power eductor since all you are fighting is the low tank head.