Theoretical, how efficient is the siphon effect

Think of a long pipeline possibly starting in Alaska and ending up in Southern Ca, or it could be from the eastern US to the west over the great divide.

Suppose the water from Alaska was being piped from a point 1,000 ft higher than the highest peak it would have to cross on it's journey. If we were to devise away of filling a pipe with no air and had a way of bleeding off air as it contaminated the system. Would we be able to do this just on siphon, or 80% for instance without pumps.

The “method of bleeding off air” would be a pump.

A siphon isn’t magical, it just uses air pressure. That air pressure can be free, as in a short siphon. But air pressure can only lift the weight of liquid it displaces.

If the water source was 1,000 feet higher than the highest peak along its path, you wouldn’t need a pump at all.

So, up and down mountains numerous times during the trip would have no effect on maintaining a siphon, would the distance it traveled say 4000 miles, create enough friction off the sides of the pipe to slow down the volume quite a bit?

Water would flow, but it would be a very slow flow. Siphon really comes into effect when you want to lift water beyond it’s high using a further ‘fall’ on the outflow side to life the water.

It would create way too much friction. Think of a siphon as a chain draped over a pulley. The heavier side pulls the lighter side. But if the chain is too heavy, it just breaks.

Same thing with a tube of water. If the force is too great, the water just forms water vapor rather than pulling the rest of the water along with itself. For water, the maximum height is something like 10 meters. Siphon - Wikipedia

The point is, if the starting height was 1,000 feet higher than any point along the pipe, there would never need to be a syphon at all. A syphon is only required if there is a point in the middle of the pipe that is higher than the starting point.

Yes - if the starting point is higher than the end point, and there isn’t any point in the middle that’s even higher than both, it should just flow. The ‘head’ of pressure from the original height will push it through the pipe (ignoring viscosity, friction, leaks, blah)

This is the issue the OP needs to address. Are you going to be happy with a trickle? The losses due to friction make whatever water you’re getting too insignificant to make the construction worthwhile.

Note that you don’t want a pipe the whole way. Plan on almost all of it being open ditches. Less friction, less cost, but then evaporation and leaks take a big toll.

The pressure in the pipe at the low points is likely to become very great. So great that you may have trouble finding piping material to contain it.

For comparison, recommended Standard drain pipe slope is 1/16" of height per foot of horizontal run (for pipes of > 8" diameter). Your pipeline is at 0.000649" of height per foot of horizontal run. Think of how fast water flows in a typical drain pipe, and you can appreciate that water will not move very fast in your proposed pipeline.

As noted, no siphon effect required, since no point in your pipeline is higher than the source. But if you’re asking water to travel 3500 miles through a pipe with just 1000 feet of head, the flow rate won’t be very high unless the pipe has an extremely large inside diameter. Using standard pipe flow calculations, I came up with about 3-4 feet per second for a flow rate, assuming a straight pipe three feet in diameter (actual flow rate would be lower because of frictional losses in the numerous bends and joints). Considering the cost to build and maintain this pipe, that’s a pretty poor return on investment.

Suppose you slightly changed the layout where the pipe would have to cross elevations 3,000 feet higher than the starting point but starting point was still at least 2,000 feet higher than the ending point

Never mind the last question, syphon effect can only lift so far before it quits as mentioned above.

There’s a double confusion here.

I am quite used to “siphon” being used for a U-shaped pipe with the ends pointing up. My favorite example is the big one in the Owyhee Project (see under “North Canal”.) The water enters the pipe on the south side of the valley, goes down and across it and comes up the other side.

While the OP said “siphon effect” I assumed ordinary water siphons like above. But the upside-down U form seems to be presumed by others. That isn’t used all that much for big-time water transport. (OTOH it is used a lot for those little aluminum pipes used to irrigate crops. Setting those pipes is something I did sometimes, but not very well.)

I’ve heard that use of the word siphon too, and it may be a more common for large scale water projects, however the definition usually refers to the inverted U shape where water is lifted above the surface level - though wider usage is also indicated.

Read up on this, lots of info on the net.