carbon steel vs stainless steel pipe 1

[kepharda]

I am piping cooling tower water supply (600 gallons per minute) header. Supply and return will be about 100 ft. long. I can get sch 10s Stainless Steel for about $59 per foot. Customer intially wanted me to investigate CPVC, but it would require a larger pipe to keep velocities down and sch 80 so it was pretty expensive for PVC at about $55 per foot.

The other option would be carbon steel, but I am more than a little concerned about corrosion, sure the water will be treated, but it is an open cooling tower, so in my limited experience (4 years) there is always corrosion.

My question is this is there a carbon steel pipe that would be economical to install instead of the Stainless Steel that wouldn't corrode quickly?

Also, this pipe will be welded, please do not suggest threading.

thanks for any help

 

[katmar]

Here in South Africa it would be most unusual to use Stainless Steel for a cooling water duty. I have seen carbon steel cooling water pipes that have been in use for 40 years. One thing I have noticed over the past few years is that installation labor is becoming a progressively higher percentage of the overall cost, so using a light gauge pipe (eg 10S) might save some installation costs. Also look at the supports you will need - you may not get the same span length with Sch10S as you would with Sch40 CS.

I agree with you that threaded pipes should be avoided at these sizes.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[bimr]

The water will be treated such that you will get satifactory service from carbon steel. Carbon steel is commonly used in this application.

If you are concerned about the corrosion, you can use a heavier pipe thickness or go with the stainless. Stainless would be considered a step up above the standard pipe material.

You are correct in that it is not a good idea to consider CPVC for this application. CPVC would be considered as a step down in quality.

 

[kepharda]

The one thing I failed to mention is that the system will connect to various pieces of equipment some having pipe connections as small as 1/2 in (12.7 mm) so while I am not worried about the pipe completely corroding thru the wall, my concern more is the accumulated pipe rot that might plug lines and equipment cooling jackets.

Does anyone have some similiar experience that could tell me if this will be a big issue or not?

thanks,

 

[rconner]

I know also cement mortar lined ductile iron piping has been used in many different cooling water applications in sizes up to at least 60" in the USA. In the size range you are perhaps talking about (I'm guessing ~10 inch diameter?), perhaps this pipe material might also be reasonably competitive with the other types of pipe you mention, and the standard cement mortar lining might well be helpful in the minimization of pipe "rot" you fear (also the maintenance of long-term flow properties). Restrained joints (and even field-adaptable restrained joints) as well as ready tapping/connection devices/procedures for all sizes of connections are readily available for such piping in such size ranges that do not require field welding.

 

[civilperson]

After designing your carbon steel pipe, add 1/8" additional wall thickness corrosion allowance.

 

[stanier]

Spiral wound stainless steel is commonly used in water treatment works in Australia. Flanged ductile iron is no longer the preferred material of choice due to costs of material plus labour.

FIled fitting and welding stainless is a lot more efficient. Having to wait weeks for DICL make spools is a risk and costly in time.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

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

 

[rconner]

Hi Geoff,
While certainly not denying the utility of steel, or for that matter stainless steel, pipes in e.g. some plant piping applications ductile iron pipes have a quite good record of application and durability in particularly large water, wastewater plants, pump stations, and other applications in the USA (that has not yet been matched by the thinner and unlined SS). It is possible as you may infer that the availability of substantial, normally responsive fabrication facilities in the form of large manufacturers in this country, as well as even local fabricators in many areas familiar with the material, may be a factor in this substantial utilization. However, I just wanted to clarify a couple points. As I attempted to explain in my first post, innovations over many years in even ductile iron systems have made it possible to accomplish e.g. “closures”, with lengths of piping “cut to suit in the field (“CTSIF”) or other modifications, without the need of ordering specially fabricated “spools” from the factory. This can actually now be accomplished in many different ways -- with various types of grooved pipe systems, rodding with sleeves or couplings, flanged adaptors and restrained flange adaptors from various vendors and with various restraint features (some for even quite high pressure ratings for even large pipes), and even innovative adaptation [for even very high pressures in all sizes of pipe] of very positive decades old practices of factory or field welding quite strong restraint rings and collars to the outside of the pipe some distance upon the barrel of suitably gauged pipes (to allow say for adjustable cutting of that barrel for shorter length adjustment and restraint with threaded rods). See an example of the latter structure at

http://www.acipco.com/adip/pipe/restrained/gland.cfm

(and e.g. say that the 48” [~1200mm] joint size is quite formidably provided with 2.81”[70+mm] thick solid glands and 32 each 1-1/4”[~32mm) diameter high strength [normally ~50-55 ksi or 340-380 MPa Y.S.] steel rod restraints, that “are not going anywhere”!] If someone insists on field welding e.g. to accomplish field adaptable restraint or closures etc., even this can also be done with ductile iron systems as noted e.g. at

http://www.acipco.com/adip/pipe/restrained/welding.cfm

; however, a quite special welding material that is undeniably more expensive than that used to weld carbon steel is required (though I think this is also essentially true of SS pipe welding!)
[Incidentally, I understand labor costs are quite significant in many areas of the USA as well. While construction cost estimating is admittedly not my specific area of expertise I would be quite surprised if stainless steel pipe field welding production/cost could come anywhere close to the field productivity of particularly grooved ductile iron systems, that normally assemble in just a few minutes with a couple bolts!]

 

[kepharda]

Thanks for all the responses, I got quotes from the local suppliers for carbon steel, stainless steel, and ductile iron.

For the project, my recommendation to the customer was stainless because of the relatively short length of the pipe run (~200 ft) and small difference in cost (about $23/ft). I haven't recieved a reply back yet so we will see.

thanks again,

-dave

 

[PersianEng]

You could also get a quote for PVC coated carbon steel pipe. Probably it will be cheap enough and make the client happy that you think of cutting all possible expenses.

 

[rconner]

There is sometimes a perception, perhaps borne as a result of the material name (or some higher basic cost of the material?), that "stainless" is a more bulletproof material than other pipes when it comes to corrosion. Of course no piping material is completely bulletproof with regard to either aesthetics or actual performance failures, depending on specifics of the construction, application, operation, and exposure. In this regard I had noticed that the General Electric company presented a paper at a relatively recent NACE conference in Houston that is now available at

http://www.nace-houston.org/CAC Papers/Mel Esmacher, Behavior of 300-Series Stainless Steels.pdf

(talking it appeared about some behaviors involving various stainless steel pipes in various cooling water services). Much other information similar to this, and also limitations of some buried service etc., are also available on the web with a good search engine and some key words. If you have not already done so, you may wish to read such information that is available to at least familiarize yourself with these issues, as these piping systems can perhaps be more more complex in construction and application than the perception of just weld them up and go (and some installations as GE notes have actually failed quite quickly in some water services, due to internal corrosion --one wonders exactly how superior such systems were e.g. to a conventional cement mortar lined piping system?). Good luck with your project.

 

[kepharda]

thank for the tip, I read the presentation, but most of those problems will not be an issue with my project.

Temps will be below 100 F

The Pipe run isn't to be buried.

The minimum velocities that they recommended is a good tip, fortunately I had already designed for about 7 ft/s.

I have experience with designing piping systems, just not cooling water, so I do understand the complexities of installing welded piping.

 

[fergusx]

If you are considering use of carbon steel be aware of risksof accelerated corrosion at least 5 times normalfor sealed systems. In reality this can be a lot higher if water treatment regime is not adhered to correctly. I am currently looking at an open chilled water system inIreland with pipe wall failures after only seven years. If schedule 40 pipe is threaded the wall thickness may be reduced by up to 65%. Carbon steel is suitable as long as the correct water treatment is in place, the correct jointing methods are used (flanging) and teh correct pipe schedule is selected that is schedule 80 for 65mm dia and below.

 

[kepharda]

thanks fegusx,

I ended up at almost exactly the spec you proposed.

I have also made it clear to the owner the need for water treatment once the system is assembled and even put him in contact with a couple of treatment companies.

Hopefully this will be enough

-dave