DIP vs PVC for 36" FM 2

[Bleh]

I'm doing some research for a new project, and would like to get some input.

The project is calling for a 36" sewer force main to be constructed INSIDE a large underground transport/storage structure (~17'w x 30'h). Tentative plans are to place the new FM atop the 2'x2' struts that run at 12' on center about halfway up the structure. Should I be looking at one particular pipe material over another? Pipe pressures, as I've been told by the mechanical guys, should in the 50-80 psi range.

From the research I've done already, floatation is a real concern if PVC is used, but can be easily remedied by proper supports.

DIP may just be too heavy during construction.

 

[stanier]

There is always carbon steel steel or spiral wound stainless steel pipe. You get away with 4mm wall SWSS.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[RWF7437]

Lots of other questions to answer ?
How long is this force main ?
Wow ! Is this transport/storage structure really 17 feet by 30 feet ?
What does it store ?
What do the "struts" look like and what are they made of and what is their purpose ?
Can you maintain a positive grade from the pump to the outlet to avoid trapping air or other gas in the force main ?
Is raw sewage the fluid carried by the forcemain ?

Why is flotation a problem ?
Is there access for men, equipment or vehicles to repair or replace the force main if it fails ?
Are there other pipelines, or utilities in the structure ?
Is the forcemain straight or does it have bends or other fittings ?

Seems like the choice of pipe material is only one of many questions to be answered.

good luck

 

[Bleh]

Thanks for the response.

Depending on alignment chosen, length varies between 3000' and 10000'

The structure is really that large, width is fixed, height varies from about 8' to 30', it's for combined sewage (dry- and wet-weather)

Struts are 2'x2' reinforced concrete, for structural support of the structure, I would assume.

We cannot maintain a positive grade throughout the entire run.

Force main will carry raw sewage.

The transport/storage fills up at times (above the struts), which may cause the new FM to float.

There is adequate access for men and equipment.

There are other utility/pipeline crossings. Clearance should not be an issue.

Force main will bend horizontally and vertically.

 

[rconner]

I agree there could be many questions. There is however a quite detailed comparison of ductile iron and pvc pipes, including various comparison testing of actual pipes in some large sizes (e.g. at least up to 24”), available from the Ductile Iron Pipe Research Association at

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

There is a document also concerning design and other aspects of ductile iron piping on various types of supports available at

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

.
36” Pressure Class 150 ductile iron pipes and for that matter 36”( or larger if needed for comparable flow?) pvc pressure pipes and fittings etc. (with strength anywhere close to being able to dependably handle such a situation?) for that matter would both be “heavy” in air (but float if empty in water). However, I know this size ductile iron piping and even much larger have been installed by experienced contractors e.g. within even much smaller tunnels etc. The introduction to this area of the DIPRA literature reads, “Ductile Iron pipe is stronger, easier to tap, requires less support, and provides greater flow areas compared to pvc pipe. These are just a few of the advantages that Ductile Iron has over pvc pipe.”
However, standard cement mortar linings etc. for ductile iron piping are not adequate if the does not flow full in any areas for extended periods of time in certain severe sewer conditions, due to extremely low pH acid that can be formed. In these cases alternative special lining/coatings suitably resistant to such exposures should be applied (see also the DIPRA sewage application publication also available at the aforementioned site).
More detailed information and/or different takes, and how to adapt various length pipes and joints etc. to available supports and performance needs, could perhaps be obtained from experienced consulting engineers, contractors, the various pipe trade organizations, and/or pipe manufacturers etc. (of course given design/operating conditions and adequate drawings or sketches etc. of the structure to which the pipe will be affixed).

 

[dicksewerrat]

I'd use HDPE pipe. To counteract the floatation, strap the pipe down to the supports. Ductile will corrode due to the H2s in sewage. HDPE is lighter and can be butt welded inside your structure without much difficulty.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[CivEnvEng98]

I'll add my $0.02.

No offense Richard, but I would be skeptical to hang large diameter HDPE for any type of job. I have no experience doing that and it just seems like the wrong application to me.

Stainless steel would be wonderful but to me it is so expensive compared to PVC and DIP and it not considered industry practice for sewer transmission lines, at least in my opinion. However, I would highly recommend ALL of your ARVs be 100% stainless tapping saddles with brass or stainless nipples. ARVs need to be extended above the anticipated high water line by a foot or two. Include ample ARVs if you plan on avoiding floatation.

I would take into consideration PVC due to corrosion issues. One could imagine that at least one good nick in the DIP exterior coating will occur during installation that doesn't get caught. Couple that with intermittent submergence (saline or fresh?) and that just seems troublesome to me.

Only other piece of advice I would give is that obviously, you'll need to make sure that the occasional strut placement near a joint is moved to accomodate enough room to work on it/turn a wrench.

Good luck!

 

[civilperson]

HDPE requires continuous linear support, DIP can be bought with a tar coating for H2 applications, PVC has a shorter life because of internal scour/wear.

 

[BRIS]

What about GRP (Glass reinforced plastic) seems ideal to me - you could have the manufacturer laminate a support beam into the pipe fabrication.