Prediction of internal corrosion in sewer pressure mains

[hello23]

hey,
i need to find some equations which predict where corrosion will occur in a pressure main. I understand that corrosion occurs at the high points but not always.

 

[bimr]

Here is a link to get you started:

http://www.roscoemoss.com/documents...catorsforCorrosionandScalinginGroundWater.pdf

Two indices are useful to predict the potential for corrosion or scale-formation of water. These are the Langlier Index and the Ryznar Index.

The American Water Works Association has a very useful calculator for the Langlier and Ryznar
Indices. It can be easily accessed using the following link:

http://www.awwa.org/Resources/RTWCorrosivityCalc.cfm?navItemNumber=1576

http://www.lenntech.com/calculators/langelier/index/langelier.htm

 

[rconner]

There is much guidance with regard to the conditions that promote corrosion, particularly in air or “head” space of some sewers including some forcemains (that can be significant without proper consideration) contained in ASCE Manual No. 69, “Sulfide in Wastewater Collection and Treatment Systems”. I believe this manual does in fact contain some formulas and programs, but I am not sure of their applicability to your specific conditions. There was also reference to corrosion issues and prevention measures in even earlier manuals such as ASCE MOP #36 and #37 [WPCF Manual of Practice No 9 (ASCE -- Manuals and Reports on Engineering Practice -- No. 37) [1970], "Design and Construction of Sanitary and Storm Sewers", pp. 124-127, 171, 175, 180-181.] There is also information now available in several WEF and EPA publications and websites, including

http://www.epa.gov/owm/mtb/force_main_sewers.pdf

and

http://www.epa.gov/nrmrl/pubs/600r09049/600r09049.pdf

etc. Basically, if pipelines do not flow full for extended periods of time (i.e. if they are not continuously rising mains, or with other profiles otherwise not dependably designed to flow full) and other conditions such as wastewater strength, H2S generation, temperature, length of main/detention time, flow rate etc. are amenable to the formation, there can be (without appropriate design provision) formation of extremely low pH (some authorities report pH of 1 or less) sulfuric acid produced by a succession of strains of bacteria on moist exposed walls in the head space. Effluent conditions in some sewer forcemains can result in copious hydrogen sulfide generation.

 

[hello23]

Thanks for you input!
I am more interested in the gas water phase of sewer mains.
When the sewer main stops pumping air goes to the high puts and starts corroding it. When the pump starts again i need to know weather the flow will be enough to pump out the air bubble from the high point

 

[sadochemeng]

Thanks to y'all, these are all very helpful information and articles.

 

[rconner]

From all I've been able to read/gleen over the years, the most severe form of corrosion (at least) does not necessarily occur immediately, but instead may require some significant duration of time with a somewhat stationary, accumulated air space (for the colonies of bacteria to "do their thing").

With regard to your restated or new question about moving the "air bubble", I know ASCE MOP No. 36, Wastewater Treatment Plant Design (1977) contains the statement, "A velocity of at least 1.2 m/s (4 fps) is required in the pipeline to shear the bubble and to keep it moving down grade." However, the subject of flow in various pipeline services for effectively scavenging, scouring, or shearing air bubbles from high points though without working air valves has been the subject of many other discussions on these blogs and others over the years (I think sometimes suggesting more complicated determinations and more/complex parameters). I suspect with a good search and some of these key words you could find much more information and other viewpoints.

 

[bimr]

There have been numerous posters on this forum inquiring about problems with force mains. If the velocity is inadequate to push the air downstream, you will have trapped air in the force main.

In general, you need a velocity of at least 4 ft/sec to force air down the force main.

Here are a couple of examples:
thread161-237329: sewer forced main problems: sewer forced main problems

http://www.eng-tips.com/viewthread.cfm?qid=270920

See the attachment for a detailed discussion.