Pump Sizing - Analyzing Siphon/Vacuum Effects

[wright44]

Hello,

I have come across a problem that has me running in circles. I have looked through previous posts discussing slack flow and siphon effects in various piping systems but I am struggling to apply it to evaluate my problem.

We are trying to determine a duty point to provide to vendor for a pump to feed 400 gpm from a source tank to a feed tank at a higher elevation. Both tanks are open to the atmosphere. Below I have attached a basic schematic of the profile and attached what I believe the hydraulic profile would look like. The cattle trough is connected to feed via float valve that keeps it from overflowing. The Source Tank is consistently at ~8 feet of water and Feed Tank at ~22 feet of water. At the high point (1,000 LF, 1450 ft amsl), there is a vent that is manually actuated to release air during start-up, then closed. The suction line in the Source Tank is 1 ft off bottom of tank (pump at 1,000 ft amsl). The inlet line for the Feed Tank is 1 ft off bottom of tank (1301 ft amsl).

So here is some conceptual problems I have.
- How would this system actually operate without installing a vacuum valve to intake air at the high point?
- Would the pump TDH be reduced by any siphon/vacuum effects from the flow downhill from the high point?
- For selecting a pump, do we actually need two points? One point for start-up (TDH = 1450' - 1007' + friction) and lower flowrates, and another point with lesser TDH at 400gpm to account for potential siphon effects?

General Schematic (n.t.s)

Hydraulic Profile

Siphon Effect (due to no air control?)

I believe the ideal solution would be a vacuum valve and eliminate any issues with air, but I want advice on how to analyze this system if it operated as drawn. I feel I am missing some fundamentals here.

Thanks.

 

[wright44]

While this application isn't potable, but a bulk of the work we do is potable. In those cases 1503-44 seems spot on, especially with the "low" pressure point. My states regulation says no point in the distribution system can ever be below 20psi or above 100psi. In most municipalities here the goal is between 40 and 80 psi. Fixtures are typically designed for 50psi(the anticipated flowrates on faucets for example) I think? A few of our city clients also demands velocities never go above 10 fps, which is reasonable but means you have to oversize to accommodate fire flows.

Definitely different world than the ranchers and mining clients.

 

[1503-44]

Always put the inlet upstream of the outlet.
Certainly the sizes of the networks and not allowing age limits of water in them to be exceeded is different. It can get tricky designing wide area networks covering territory with pronounced elevation differences, especially with regard to those tight pressure band limitation. Flow paths may not be what you think they should be and can vary with demand or pressure changes.

We have the oversize issues during early and late stages of regional scale field developments and overly optimistic, or secretive, geologists (100% of them). Otherwise its undersize issues on the demand dide.

And IMO their future is looking a lot brighter than oil and gas.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."