carbon steel versus ductile iron 1

[knightquestion]

Have a petroleum waste water stream going into an existing treatment plant we are upgrading. All old below grade and exposed pipe is listed as ductile iron on old plans (late 60s, early to mid 70s). We will be replacing some pipe, and tieing into others. Some lines will be gravity, some are forced. Design life 30 years. Can someone give me a quick primer (or point me to one) on the use of each of these materials in ww jobs. What types of joints are best for below grade matches to the existing ductile. Thanks to the folks who can help!

 

[rconner]

The Ductile Iron Pipe Research Association (DIPRA) has much information available from their website at

http://www.dipra.org

that might be of help in your work. I know there is a brochure specifically talking about wastewater applications, and many also comparing/testing aspects of ductile iron piping to other piping materials e.g.

http://www.dipra.org/pdf/DIPvsSteel.pdf.

Once you have identified the manufacturer(s) of the existing piping, I think you will also find most 1960’s-‘70’s manufacturers are still around and some may even be able to help e.g. with specific identifications and suggestions and/or maybe even new gaskets (many different types of gaskets are available for ductile iron piping systems to handle various exposures, including various petroleum wastes e.g. per

http://www.dipra.org/pdf/gasketsForDIP.pdf

) to go into existing pipe and fitting sockets for cut-ins or continuations etc.

 

[unclesyd]

Here is another very good source of information on DI pipe. These people are great to work with.

http://www.acipco.com/adip/index.cfm

 

[RossABQ]

Various groundwater reg's may make it desirable to have piping without joints underground, if this wastewater can in any way/shape/form be considered hazardous. That was not the case when these lines were installed. Even cooling tower blowdown is considered hazardous nowadays. I would inquire if this is a concern before getting too far down the road, especially for the pumped lines. That said, ductile is used extensively underground because of its superior corrosion resistance.

 

[rconner]

I think I understand what you are saying and I realize there may be a perception out there, maybe even now cultivated by some steel and/or now plastic pipe salesmen, that their systems do not have "joints". In my opinion, however, in practical application this is of course not literally true and it might even be a quite dangerous perception. Of course all systems do in fact have joints, at least of the fused or welded if not gasketed varieties, as well as need for lateral connections, drains/bleeds, taps, future taps, repairs etc., and of course any types of joints can exhibit problems when not manufactured or installed correctly (this includes problems with welded joints of plastic and steel systems, that can be readily located with a good search engine). Of course also, hydrocarbon constituents can "permeate" in either direction across the walls of plastic pipes, as well as in very small amounts across the exposed extant of some rubber gasketing used in plastic and metal systems.

 

[RossABQ]

The concerns I mentioned come from our clients' environmental dept's. Groundwater contamination is a big deal, very bad PR and very expensive to mitigate. They postulate a joint leaks due to settlement of the soil it runs thru. 100% welded SS systems (in some cases double-contained) are used exclusively, if a line MUST go underground (their first choice is put it in a containment trench). This is in the context of some potentially very bad chemicals, which I would assume could be the case at a refinery.

I would not want to be on TV news, identified as the spokesman for a company that contaminated the acquifer supplying the town's drinking water. I suspect this motivates the PR and environmental people, too.

 

[rconner]

Interesting. Again while I guess I understand the perceptions, postulation and "motivations" involved, I am not necessarily sure of the ultimate technical superiority of all solutions described. Welded systems are rigid, and if the soil "settles", is it not possible that (some unlike e.g. the ductile iron systems with flexible joints) that the pipe beam/welds etc. (assuming same are welded right to begin with) could at locations be exposed to/at the expense of greater bending strains/grain distortions etc. in the pipe wall due to the settlement/relative movement, and unknown future, concomitant effects? Also, while I have also heard the perceptions of a rather bullet-proof nature of stainless steel (SS) pipe/material in general, in practice in at least some places it appears SS performance has not been entirely bulletproof nor lived up to expectations as anticipated -- e.g. with regard to buried SS pipe see the discussion at

http://www.eng-tips.com/viewthread.cfm?qid=96953

(and I think similar discussions on these and many other forums that could be pulled up with a good search engine), and I think there have also been failures of unlined SS pipe due to internal corrosion effects, perhaps even in water environments that would not have resulted in as precipitous failure of standard cement etc. lined ductile iron or steel pipes (e.g. see

http://www.corrosionsource.com/corrosioneering/journal/Jul02_Borenstein/Jul02_Borenstein_2.htm)?

I guess I have to agree, however, that if “very bad chemicals” are involved, very good shop and field quality control and testing and maybe even “double-containment” (along perhaps with leak warning systems?) may not be a bad idea with whatever piping materials are employed!

 

[knightquestion]

Thanks to all for your discussion. The issue of leaks is always a concern for my clients, as petroleum impacts to soil and groundwater are a huge liability. The fact this is an old facility means there probably already are existing impacts to both. Based on some additional research (some from these posts)we've decided to go with HDPE. Thanks.

 

[RossABQ]

rconner,

Yes, they do have leak detection system if it is double contained. Sometimes a membrane is also placed under the secondary containment. Belt and two pairs of suspenders. Thrust blocks and spread footings are placed along the line. The welds are 100% inspected and made to qualified procedures by qualified welders.

Since these are pipelines of concern, extra measures are taken compacting the soil to avoid settlement. True enough, SS is not perfect in its resistance to corrosion. Anything too agressive for plain 304SS is typically not direct-buried at all.