corrosion problem on direct burried condensate return lines 1

[rhowe]

Basic system information: Steam distribution system at 125psi, return condensate ~200deg F. Systems typically run 24/7 with the exception of shutdowns for repairs/maintenance. There is over 10 miles of piping that interconnect (for redundant/ back feeds) in various sizes and various settings: Some piping in tunnels, some in trenches, some in racks, some through buildings and direct buried lines. Chemical treatment does occur for boiler feed water... but limited by FDA on film coating amines for carbonic acid protection. There is no satellite chemical treatment. The condensate return is a vented pumped return that uses a mix of both electric pumps and steam motive pumps. This pumped condensate return also collects the main drip trap condensate from the steam lines.

Environmental conditions NE weather -- snow and ice. Our plant uses far too much deicing salts on ground surfaces.

Current system problem / dilemma: in ~1000' section of direct buried lines we are seeing a huge influx on condensate failures -- from the inside out from carbonic acid, over the last year. The lines range in age from ~10 to ~30 years old. The existing piping is schedule 80 carbon steel (some in old style rickwell) encased in insultcrete. The rickwell sleaving was open to the inside manholes at the penetrations. Failures are tending to be within feet of entering/ exiting manholes. Failures have often been a couplings, or at direction change. This particular area is one of the farthest from the source generation. (We are looking at remote chemical treatment for this area --- but in the meantime we need to replace the damaged sections as needed.)

I forgot to mention we have pockets of high water tables in this area too.

There is a debate on the material that should be used when replacing these sections: 1. replace with schedule 80 carbon steel. Foamglass insulate covered by a roofing type paper. 2. Increase carbon steel to schedule 120. Same insulation. 3. Change to stainless steel. Same insulation.

I am concerned with using stainless steel with the huge amounts of deicing salts we use. I'm guessing that while we may slow down carbonic acid corrosion on the inside of the pipe, we'll be speeding up external corrosion of the pipe with the chlorides. Our existing lines are showing some external corrosion, but nothing like the inside.

We are also talking about seriously overhauling about a third of this area with a "pre insulated" piping product. The problem I can see with the pre insulated option is that when we do spring leaks in the future, the preinsulated has a relatively long lead time...

Aside from chemical treatments / overall system design... Can someone provide an opinion on the options listed above? Sched. 80 cs, sched. 120 sc, or ss. Other choices? options? for replacing sections of piping?

I am completely unfamiliar with cathodic corrosion protection, short of seeing it mentioned. Is this just for structural systems, rather than piping? If it can be used for piping, would this application be practical? Would this only protect from ionization from environment corrosion?

Any advice is greatly appreciated. My apologies, in advance, for being long winded.

-RHowe

 

[TBP]

Direct buried condensate lines typically fail from the outside-in. In any event, direct buried low pressure steam lines, and condensate lines in general are endless maintenance problems. As long as the higher pressure steam lines aren't shut down for any period of time (like over the summer) they'll last for a long, long time.

My experience with the pre-insulated piping systems is that the field joints on the insulation jackets are always a weak point. Once the outer jacket fails, ground moisture get through to the hot carrier pipe, and steams-off. This process typically causes the insulation to deteriorate very quickly, and just it just snowballs. And it's not like just one crew couldn't "get it right" - the problem spanned a number of installations all over North America. This is in addition to the brutal first cost plus with L-O-N-G delivery. We didn't suffer particularly from particularly high water table, so we had very good success with Foamglas & tar paper on the steam lines. The condensate system was abondoned years ago, due to endless maintenance issues. If I had to deal with any amount of ground water, I'd try Foamglas covered with pitwrap. (Pitwrap is a nasty, sticky wrap with heavy brown paper on one (the sticky) side that gets removed just prior to installation.) Once the paper is removed it sticks everything it touches whether you want it stuck there, or not. It'll be far cheaper than the pre-insulated pipe systems, and the lifespan just CANNOT be much shorter.

If you want any kind of lifespan on a condensate system for applications like yours, my experience is that you pretty much have to go with tunnels ($$$$$) or a duct system. (Still considerably more expensive than direct buried, but cheaper than tunnels.)

 

[EdStainless]

If you choose to go stainless you need to have the outside epoxy coated before it is insulated. Though, as with all systems the field joints are the weak link.
Let's look at this from another direction. Does recovering poor quality condensate really save you more money than the maint. costs? Would you be better off disposing of the condensate?
There is a power plant in Milwaukee WI that delivers steam to the downtown offices and Miller. The quit brining condensate back years ago. It greatly simplified their water system.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metengr]

For your application, I would not recommend stainless for condensate return lines. At one time we owned a combined cycle unit in New York that generated electricity and heating steam for Manhattan offices. The condensate return lines were necessary to keep the cost and usage of water as low as possible. Regarding the return lines, TBP has the correct recommendation.

 

[rhowe]

Thank you, everyone, for the responses!

TBD: Based on your feedback on 'preinsulated pipe,' I think we'll stay away from that. Unfortunately the area has a plethora of other utilities in the way for a tunnel. It's feasible, but would be very costly. I think we'll continue to use sched 80 carbon steel w/ foamglass in pitwrap.

EdStainless I feel so silly not thinking of epoxy on the pipe. We've recently started to epoxy ladders, steel supports, etc. seeing much better life. (non-epoxy galvanized ladders had been lasting less than 2 years.) Any recommendation of type of epoxy for piping? We've been using plasite 7122 for cs support beams, cs ladders.

As for determining to not return condensate... it's really not an option. We are in energy conservation mode here. Water usage, chemical make-up, & sewer costs. This option would be heavily frowned on from both a cost & environmental stand point.

Thank you all again!
-Rhowe

 

[EdStainless]

As to the type of epoxy, I don't have any strong feelings. 98% of the job is surface prep so that is where the work should be put. Please, always coat stainless when it is under insulation.
As I think about it I don't really see any problem with using epoxy lined CS either. The field coating of joints will be the weakness.
I understand about conservation, but I know of plants that spend more cleaning retured condenstate than it takes to make new water. Maybe this stuff has some other, more appropriate use.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[bimr]

This topic was discussed in another forum:

thread164-141213

 

[bimr]

Preinsulated piping systems are not generally recommended for steam piping because of the temperature limitations of the insulation. The ground water infiltrates the insulation materials and vaporizes on contact with the piping deteriorating the insulation.

You best option is to use a field installed instulation using cellular glass. The cellular glass can withstand the temperature, the glass structure also resists penetration from ground water.

It is recommended to use a perforated drain 6" below the bottom level of the pipeline to prevent the ground water from infiltrating the piping system. It is also important to very carefully consider the potential for damage to the insulation from uncontrolled pipe movement.