1 Vacuum is always created when water falls freely in a drop pipe. When operating, the full "pull" of my well-return line is about 26" Hg. When the flow stops, the pressure drops even further, down to the saturation pressure of water at discharge temperature. 2. The tail end of the drop pipe is usually submerged in my application. This is standing column, so I've had occasions where the water table dropped below then end of the pipe, exposing it to atmosphere. No significant change in system performance was noted other than losing siphon at the end of the cycle. 3. The tail-end of the discharge pipe is, by definition, at ambient pressure. Ambient pressure means nothing is going to get actively "sucked" up the pipe. However, since air is lighter than water, "bubbles" can "float" up the pipe. Flow velocity keeps most of them at bay, faster flow, fewer bubbles. **** There are two simple design solutions to eliminate bubbles altogether. For a below-surface discharge well, keep the drop pipe submerged below the water table. For a non-submerged surface discharge, rotate the end of the pipe so that it discharges straight up (like a fountain). **** 4.Coincidently, my discharge pipe does step down from 1 1/4" to 1" at the pitless adapter. But that was to get everything to fit down in the casing, not to keep the siphon. I doubt this is a crucial part of the design. 5. Since the tail-end of my pipe is submerged most of the time, vacuum is not lost at the end of the cycle. If my well level drops below the end of the pipe, the siphon is lost when the flow stops. I notice when this occurs it's easy enough to reestablish the siphon, but it takes quite awhile to completely flush all that air back out. Since losing siphon was rare in my case, this has not been an ongoing problem. ** If designing for a surface discharge, add at least a foot of vertical stand-pipe to the end of the run. Since air can't "float" in a downward direction, the standpipe should be enough to maintain siphon. ** You don't need much of an elevation change in the discharge pipe to get some big benefits. If tail end of your discharge line is only slightly below the unit, a matter of feet, it's possible to pump at 0 psi and still have all the flow you want. |