Steam/Condensate Piping

[cougarfan]

We have a problem where we have to replace steam piping which is buried under the ground every year. The piping is black iron and we get tired of replacing it. Any suggestions out there? We have tried stainless, still leaks - we tested the soil and was acididc a few years ago, , we put some lime down in the soil, it lasted a little longer but when the piping had to replaced, it was a mess...

 

[TBP]

Do you keep the line hot year round, or is it out of service and cold for extended periods? Low pressure lines corrode much faster than high pressure (and temp) ones underground.

What kind of insulation are you using? Something like Foamglas works well underground, as moisture has no effect on it. Fibreglass and cal-sil just suck up water and hold it against the pipe. The corrosion rate, when the lines are cold, is just spectacular.

There are pre-fab insulated piping systems, but I've always found them extremely expensive, long delivery, and didn't stand up very well. The field joints on the insulation jackets always seem to be a problem. Leaks show up in the jackets after a couple of years (almost always at a jacket field joint), and the vendor and contractor point at each other as to who's at fault. The owner of the line typically winds up eating the repair cost - big surprise.

We insulated the steam lines in Foamglas, then heavy roofing paper (mostly to keep the dirt out from between the cracks in the insulation), and direct buried them. These have lasted for years, going on decades now - as long as they're kept HOT.

The only other solution to your problem involves a tunnel, or a duct with some ventilation. That's expensive up front, and often there isn't room in the ground to allow for it.

 

[Metalguy]

Use something like 254SMO (super stainless) and your problems *should* end.

 

[cougarfan]

TBP ,
These steam lines run to our maint shop and only need to run about 6 months out of the year - what do we do now?

 

[TimboGMO]

We have had good luck with steam piping installed in concrete chases/trenches/small tunnels (whatever you want to call them) which are just big enough to place the piping in the trench, weld it up, insulate it, properly guide, anchor and support it. Some of these systems have lasted up to 50 years (maybe longer?). The piping is seamless A53 grade B, insulated with foamglass and covered with perforated aluminum jacketing. We are running at 60 psig, 420F and return all of our condensate. We have tried to stay away from preinsulated piping for the reasons mentioned by TBP, I will tell you though that we do have a 5000ft ricwil system that is about 40 years old, just starting to cause us problems because of the outer casing failing and condensate piping forming tubercules and failing from the inside out. This system doesn't even have any sacrificial anodes, that we know of. Today's systems do have sacrificial anodes which will help out alot. If you do go the preinsulated piping route, stick with a drainable-dryable class A system. Also would stay away from foams. Let me know if you want any of our standard specs for chase systems.

Tim G.
University of Missouri

 

[TBP]

didyjohn - you basically have 2 choices, you can run a duct system like Tim, or do it the way I have. If you have condenate that you'd like to get back, then I'd consider duct, if you have the space in the ground. But it all depends on payback. Sometimes condensate costs more to get back from some locations than it's worth. If it's direct buried, shut the steam off for the season in the maintenance shop, not in the main plant. Have a trap ahead of the shop isolation valve, so the main doesn't flood with condensate, and cool off & corrode.

The one thing about underground steam systems I can state with absolute certainty is - "local conditions dictate what's the right thing to do." What makes perfect sense in one location isn't phyically possible, isn't allowed by code, or just won't stand up someplace else.

Tim - do you leave your lines hot year-round?

 

[TimboGMO]

TBP,

Yes, for the most part our lines are hot year-round since we have quite a few steam absorption chillers. We do have some cases where steam is shut off during the summer at the building, but the buried line is left hot and the trap before the service valve takes care of the condensate. In a couple cases though we have left steam off for extended periods of time on the buried piping lines, this will lead to accelerated corrosion. The drain valves are kept open to prevent damage to the piping from condensing steam.

 

[saxon]

dideyjohn, The first question you need to ask is, "Why do condensate lines corrode in the first place?" The answer is, "When the steam condenses, the entrained air and CO2 reacts with the condensate to produce various dilute acid phases. What happens when CS/ 300 series SS pipe operates in an acidic environment? Right, it corrodes! So the solution becomes, getting the condensate out, so it doesn't lie around in the piping corroding everything. This all goes back to the fundamental Art/Science of proper piping layouts and trapping for condensate removal. Check in with the Armstrong Machine works or Spirax Sarco. They are the experts in field.

Hope this helps.
saxon

 

[Metalguy]

Except that 304 SS resists carbonic acid very well, and oxygenated water too. But Cl, that's a different story.

So, Dideyjohn, did your SS piping corrode on the OD, the ID or both?

 

[TBP]

saxon - Direct-buried condensate lines will almost always fail from the outside in.

 

[ChasBean1]

I've also seen these pipes eaten alive without any ground contact. There seems to be good advice above by protecting the outer pipe. Morpholine treatment (as long as there is no direct steam release into air systems, such as by humidifiers) extends the life of steam and condensate systems. It's a weak, volatile base (pH to maintain about 9.0-10). Rxn is:

C4H9NO + H2O = C4H9NOH+ + OH-
morpholine water morpholinium hydroxyl

The volatility is what makes morpholine effective - it carries over with the steam and is recycled in the condensate, maintaining alkalinity throughout, thus reducing corrosion. I would consider this (with the humidifier exception noted) and outer pipe protection as advised above. I agree also with Saxon's post above - deaeration by appropriate pipe design is key. -CB

 

[pazd]

woow...stainless steel and carbon steel corroded. It must be CHLORINE (CL2) components in there. Especillay when you get pitting corrosion (localized leaks) in stainless steel, you would probably suspect chlorines...

My suggestion is either one of the followings:
1) Really pay more attention to your deaeration systems (specs normally are 7 pbb for Oxygen and CO2) although this will not handle the Cl2 business but it'll improve the system.

2) Or you can inject chemical treatment compounds.
3) Or change your ground piping to NON-METALLIC piping such as RTR which is really super for underground water business

Wish ya luck