life expectancy of steel casing pipe 2

[cvg]

I have a boring and jacking project in Phoenix, Arizona. Client is concerned about the life expectancy of the 48" diameter steel casing pipe. We are proposing to use the casing pipe as a storm drain without a carrier pipe. The City is requesting a life expectancy of 75 years. Field resistivity of the soil is 4711 (ohm/cm). Will a 1/2" thick casing pipe be thick enough?

 

[Metalguy]

Don't know what "boring and jacking" is, but there are a lot of corrosion-related variables to consider. Are you going to have any corr. protection methods? Will you have a lot of wet/dry conditions? SRB-related MIC can perforate steel pipe very fast. How much leakage can you have?

 

[cvg]

this will be a large diameter gravity drain, not under pressure (less than 3 psi at centerline of pipe). Leakage is not a significant concern. However, it will be near several large sewer outfalls and infiltration of any leaking sewage into the pipe would not be good. The joints would be welded, so the only possibility for leakage would be a full perforation in the casing due to corrosion.

Jacking and boring is the process for installing this underground line. A tunnel is bored with an auger and then a hydraulic jack pushes the pipe in the tunnel. The inside could be coated with coal tar or epoxy. The outside could be coated, but the coating would probably be damaged during the jacking process. We don't envision using any cathodic protection.

 

[Metalguy]

Untreated sewage can be loaded w/bacteria, and the soils of the AZ deserts are loaded with Cl (I lived there for ~20 years). You may get lucky, but the underground corrosion problems I've been involved with there make me think that you don't have much chance considering the 75 yrs. Wetting and drying are also two huge factors.

How long is this pipe and what is the dia.?

 

[cvg]

length is about 110 feet, 48 inch ID, minimum thickness 1/2 inch steel plate.

We have 12 gage corrugated steel culverts here that are designed to last 75 years. Granted, they are usually galvanized. But still, one scratch through the galvanizing and they would begin corroding.

My thought is that you could install it and inspect periodically to check for corrosion. If corrosion is progressing, then either slipline it or provide cathodic protection. Otherwise, just leave it alone and continue to monitor.

 

[YLWEA]

There are two considerations: internal corrosion from the storm water and external corrosion from the soil side. I have been doing a lot of work on external corrosion of buried steel piping, including bare (no coating) without C.P. on piping installed between 1915 and 1970. While the average of all the deepest pit depth measurements on all the various pipe samples works out a long term average corrosion rate of a couple MPY (1 mpy = 0.001"/year) the standard deviation is quite high (higher than the mean values). NACE RP0502 recommends using an assumed external corrosion rate of 16 mpy when no details of the soil environment are known and no prior history of corrosion performance in the specific soil is available or known. The 16 mpy is the 80% confidence level of worst pitting based on available published corrosion studies in soil (mainly work published by NBS in the early to mid 20th centuty). The 16 mpy is generally considered to be grossly conservative, but does include consideration of some very high corrosion rates for some steel samples that were buried for several years in some very corrosive soils.

Comments from others regarding MIC are correct, particularly on the water immersion side. Also, be careful about using soil resistivity alone for estimating corrosiveness of soil. The mean of the corrosion rate data follows changes in soil resistivity nicely, but once again, the standard deviation of the data is huge.

 

[stanweld]

We could barely get 20 year life for noncoated, non cathodically protected, approximately 0.2" wall steel pipe in desert like soils (corrosion only from soil side). Essentially full wastage of pipe occurred. Almost double that life was experienced for pipe manufactured prior to 1954 - pipe contained apreciable amounts of S and P. It is not uncommon to coat both OD and ID surfaces for casing subject to the conditions that you described and not cathodically protect or to additionally protect with al-zn or zn type anodes.

 

[cvg]

I agree, that if we do this we would recommend coating the pipe inside and out. This is a desert environment, with generally less than 7 inches rain per year. Most of this coming in a few storm events. It isn't uncommon to see relative humidity in the summer at less than 10% with temperatures ranging from 110 to 120 F. I would expect the inside of the pipe to be dry much of the time. A sanitary sewerline currently being installed below this storm drain is cased in 1/2 inch steel with no CP. I wouldn't be surprised if the inside of this casing is just as wet as the storm drain (due to infiltration at the casing ends as well as leakage from the carrier pipe)