For a large wastewater project in North America, for large pipes (diameter 30"- 48"),under low operating pressure conditions, is it cheaper to use steel pipes instead of ductile iron pipes?

Thanks in advance

 

Given the limited information, hard to say.
Ductile iron is probably cheaper for standard service.
Call some manufacturers and find out.

pressure?
corrosion?
underground?
length?
type of soil?

While I realize my following comments are anathema to some current "low bid" mentalities, IMHO "cheaper" should not be the overriding objective of pipe(line) selection and design. The job must after all be constructable as desired, and it must "work" at least reasonably with the parties and local conditions that may be involved.
The recent book "Trenchless Technology" by Najafi and Gokhale, in a section that discusses pipe materials in at least a little more depth than a great many prior textbooks and references, starts out with the interesting statement, "Trenchless construction methods cannot be successful without quality pipe." Frankly, I believe this is true of any construction methods and pipeline projects!
For a case in some points I am aware of a substantial 21st century wastewater project not long ago in the size range you discuss, with many accounts including what can be observed at http://northwest.construction.com/news/oregon/archive/0806.asp and http://www.portlandonline.com/bes/index.cfm?a=274242&c=51206 etc.
In short, a large wastewater project was initially designed with metal pipe, but was allowed to be actually built with cheaper alternatives/sections of new pvc and hdpe pipes, respectively offered up by contractor(s). There were issues, shall we say, with both of these types of plastic pipe sections in young service (some of which were quite nasty and also drew the less flattering attention of newspapers and bloggists). The problematic plastic pipelines were replaced, one section with new steel pipe and one with ductile iron. While I believe it was reported the re-building went more smoothly (although obviously at substantial additional time and/or cost), the chosen new steel pipe installation also promptly failed at least once, to kind of add insult to injury, with newspaper reporting at least one resident was again "fuming" (so to speak).
While I am not aware of any problems with the ductile iron part or line since, I guess I wouldn't necessarily put problems beyond such either!!

"Our lives begin to end the day we become silent about things that matter." Dr. Martin Luther King, Jr.
 

To answer cvg's questions;

pressure? 150 ft of head
corrosion?
underground? above grade but inside a building
length? 1000 ft
type of soil? Not applicable

I would normally use steel for this application

Cement lined or PE lined steel is the choice here.

The word "cheaper" has so many meanings. Do you mean the price of the pipe delivered to site, the total installed cost or the life cycle cost?

When you say "in a building" I presume if it supported above grade and thus the frequency of supports would rule out PE?

GRP could be considered but needs a high degree of design inconjunction with the manufacturer.

Have you considered spiral wound stainless steel? this has been successfully used in many plants carrying sewage or industrial waste water. The thinner wall thickness is adequate for lower pressures and thus costs can be kept down. The pipes can be welded or joined using Victaulic type couplings. SS pipes do not need to be painted in plant like steel or ductile iron. They cetainly dont need the restraints of ductile iron pipe to withstand pressure.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

While many other types of piping have been available for many decades, and perhaps not denying some local utility of other types of pipes and for special applications etc., by and large most plant water and wastewater process piping specified in the United States is either ductile iron and/or essentially mild, carbon steel (appropriately/dependably lined for the process involved). For various reasons, including the ready availability of the candidate fabricate pipe and fittings with the desired/compatible/proven joining, lining and coating systems, ductile iron predominates in particularly the smaller diameter sizes.

Depending on the type of process line involved and other factors, common contemporary linings for wastewater applications are cement mortar lining, ceramic epoxy, and glasslining (the latter for specific process lines like some primary sludge and scum etc., wherein no other piping material or linings have demonstrated comparable non-oleophilic nature, overall performance, and also maintenance capability e.g. with steam cleaning etc., if ever needed). Exposed process piping is traditionally furnished with special primers for compatible finish coatings of paint, in most cases not necessarily that it is "needed" for corrosion protection, but instead primarily to comply with process piping color-code requirements/practice and/or aesthetics.

You may want to visit with experienced personnel of plants that have been run continuously for substantial lengths of time, experienced engineers who have long been responsible for specifying the piping of such, and discerning review of standards and educational materials including these and other forums etc. for specific guidance concerning materials for specific processes.

For that pressure, inside building?
Consider PVC

FRP would do the job. BUT... When in doubt,make it stout with some stuff you know about = steel.

The original OP appeared to be more concerned with "cheapness" than making it with "stout stuff". As yet we do not have an explanation as to what basis the cost is being analysed. On this basis basis we as experineced engineers cannot help this poor sod!

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

1000 feet of anything is cheap.  You'll spend more for the contractor's mobilization charges.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

I noticed an incident some time ago reported at http://eng-tips.com/viewthread.cfm?qid=56942 where it appeared a designer took  a similar suggestion as earlier on this thread and attempted to use weaker pvc pipes in what appeared to be a common a plant application. There are other similar cases of which I am aware. The happenings indicate sometimes services are more demanding, and pipes more vulnerable, than perceived, and real safety factors afforded by a substantial and robust piping system (vs pressure and perhaps in less obvious fashion localized support stress/spans/deflection, and vibration/resonance etc.) are apparently quite helpful. It should be remembered also that plant personnel are often walking and working immediately around these exposed lines (if for nothing else than to exercise valves etc.), and many plants are also the scene of a lot of continual (and often quite close) outside visitation and also construction activity, where pumps and other heavy units are being added, removed, maintained etc. and pipe being adjacently routed etc., and fracture toughness safety/risk vs, collateral damages etc. can be a very real issue.

In the latter regard, I am also aware of an incident in per account at http://caselaw.lp.findlaw.com/data2/californiastatecases/B195724.DOC a worker was demolishing pipes in the basement of a college dormitory when a piece of iron pipe soil pipe fell on a pressurized pvc water line. That pvc pipe burst, thereafter knocking the worker knocked off the ladder and causing him to sustain "serious physical injuries". A nasty lawsuit and long legal wrangling ensued.

While it is admittedly not the same sort of "plant", I will also note the contemporary, real cost of accidents working around pipes can now be quite substantial, which is revealed in the similar sounding accident etc. report at http://www.galfandberger.com/verdicts/Products-Liability/76/ . Per this account a maintenance worker fell from a ladder onto a 6" pvc drain pipe attached to a acid tank, "The force of the maintenance worker's fall broke a six-inch PVC pipe nipple that was attached to a green chemical hose resulting in him being sprayed with sulfuric acid. A co-worker who had been holding the ladder for the maintenance worker came to the rescue and was also sprayed by sulfuric acid. The maintenance worker ultimately died from complications arising from his burn wounds.", "The injured workers also sued the tank manufacturer claiming that the provision of a six-inch long PVC discharge pipe rendered the tank defective and unsafe in that it should have been stainless steel, which is much more durable and stronger than PVC piping. The maintenance worker suffered third degree burns over 50% of his body. He was placed in a drug-induced coma and died from complications relating to his burn wounds 37 days later.", and as a result of lawsuits, "The total settlement to the family of the burned worker who died from his wounds was  $6,603,000.00 while the injured worker who returned back to work received $4,047,000.00. The combined settlement of $10,650,000.00 was one of the largest settlements ever in the Commonwealth of Pennsylvania." It appears sometimes a few feet of cheap pipe aren't really cheap!

Again, I am aware of a great many other unfortunate accidents etc. from working around/with weak pipes. While the actual cost and time/distraction to all parties and their management personnel who happen to be somehow around/involved, as well as who is responsible and/or pays for same, can be endlessly debated, I don't believe anyone really wants to be in such "Whauhauppened?" situations.