Siphon pipe-- maintain empty or full?

[mlib]

Should a siphon be maintained full or empty when not in use? I'm designing an 8" diameter, ductile iron siphon to be used on a small fresh-water reservoir. It will only be operated once a year. It will have a valve at both the inlet and outlet ends. Aside from priming considerations, should the siphon be kept full or drained empty after use? Which would result in less corrosion of the pipe?

 

[quark]

A full pipe would cause less corrosion. Dissolved oxygen in water cannot be more than 14 ppm at atmospheric pressure. Where as in air it is 21% of the volume.

 

[kenvlach]

Not quite so simple. Oxygen only causes rust in the presence of moisture. Also, galvanic corrosion could occur due to the heterogeneous microstructure of steel (ferrite + carbide). Any corrosion would depend upon a) the composition and pH of the water and b) the dryness of the empty siphon. I am reminded of the section on Painting & Corrosion in Design of Welded Structures, Omer Blodgett (1966): In fully enclosed, hollow welded sections, it is not necessary to use any corrosion protection since rusting ceases once all available moisture is consumed.
Having said that, I see 2 options:
1) completely drain and blow out all the water with dry compressed air before closing the valves, or
2) leave filled with water, but add some TSP (trisodium phosphate) as a corrosion inhibitor and use some lime to boost the pH to 10-11 (minimize the solubility of iron). Add via the priming system so as to not leave any air in the system.

 

[kenvlach]

Noticed ductile iron, not steel. Everything still applies, galvanic corrosion can occur between ferrite + graphite.

 

[quark]

From my faint memory of metallurgy I recall that for intergranular corrosion, carbide precipitation is important at grain boundaries. Will there be intergranular corrosion even if the Iron Carbide content is less (as is the case with non castings)?

Please note that this is just for my information and knowledge.

 

[mlib]

I don't think I can count on keeping air out of the system even if I leave the siphon full, because air will come out of the water over time. This system will not receive frequent maintenance. Likewise even if I were to dry it out after use it probably would not stay dry because the environment is humid. I can always size it up for corrosion. Would steel perform better than ductile iron in this application with respect to corrosion? It sounds like I should expect corrosion regardless of whether I store it empty or full.

 

[kenvlach]

mlib,
Don’t worry about the the small amount of dissolved oxygen. It may help with formation of a passive layer.
The point of raising the pH is to increase the stability of iron hydroxide vis-à-vis dissolved iron. See thread127-52975. This way, you have a stable, thin film of FeOOH on the surface rather than iron dissolving as Fe⁺³. Additionally, the TSP should create some iron phosphate protective film.
Incidentally, should you decide to drain the tube, it would be a good idea to first treat the water inside (raise the pH and add the TSP) to leave a protective film.

quark,
In this case (ductile cast iron), the carbon (graphite) is a primary phase. You could still get intergranular corrosion between the ferrite and graphite grains. And, since the graphite is very cathodic to the ferrite, it would be of galvanic nature.

 

[dicksewerrat]

Why not use a plastic pipe for the siphon. There are a lot of them approved for potable water.

 

[mlib]

Because of vandalism. It's a high risk site. It isn't a domestic reservoir anyway, just a recreation site.

 

[Ag93]

I agree completely with dicksewerrat. Plastic pipe would not corrode and eliminate your issue. Consider HDPE or pvc.

 

[cvg]

Excerpt from DIPRA website -

"The strength, durability, and long service life of Ductile's predecessor, gray Cast Iron pipe, are widely recognized. The first official record of Cast Iron pipe installation was in 1455 in Siegerland, Germany. In 1664, French King Louis XIV ordered the construction of a Cast Iron pipe main extending 15 miles from a pumping station at Marly-on-Seine to Versailles to supply water to the fountains and town. This pipe served the palace gardens for more than 330 years.

Cast Iron pipe was introduced to the United States as early as 1817, when it was installed in the Philadelphia water system. Today, more than 565 utilities (in the United States and Canada) have had Cast Iron mains in continuous service for more than 100 years. Additionally, at least 16 utilities have had Cast Iron mains in continuous service for more than 150 years.

Ductile Iron not only retains all of Cast Iron's attractive qualities, such as machinability and corrosion resistance, but also provides additional strength, toughness, and ductility. Although its chemical properties are similar to those of Cast Iron, Ductile Iron incorporates significant casting refinements, additional metallurgical processes, and superior quality control."


Point being, just put in ductile iron and forget about it. The corrosion will not be a problem in our lifetimes.

 

[bhwtam]

Mlib,

Corrosion should not be a big problem as you can internally lined the ductile iron pipe with concrete lining, epoxy or bitumen coating.

As your pipe will be operated once a year, another point you have to be awared of, if you choose to have a filled pipe, is water trapped inside the pipe will become septic. Make sure you can drain this water away from your system before operation.

 

[BobPE]

I agree with cvg. I don't like plastic for pressure service and even less for vacuum service.

BobPE