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NickelMet (Materials)
23 Jun 05 8:20
I have inherited the following situation with a heat exchanger:

Condensate (boiler feed water) on the tube side which is considered "pure water" with little or no contaminants.  It is at pH of 8.3 and Fe (iron) concentrations are well below ASME standards.

Treated Water (from the reverse osmosis unit) on the shell side which is considered an aggressive water.  First, it is not deaerated and has dissolved oxygen.  Second, it is not buffered and has a negative LSI (causing scale formation and indicating a corrosive water).  Third, there is normally <25ppm chlorides, but a large amount of brine (chlorides) will slip into the system occasionally.

Analysis showed failures on the tubes from the OD due to oxygen accelerated under-deposit corrosion.  The shell is also showing signs of localized oxygen pitting.  No problems have been noted on the tube ID, whether Duplex 2205 or carbon steel was used.

Carbon steel put in tube side service failed in 18 and 6 months (2 bundles) from the OD, not the ID.  A Duplex 2205 set of tubes with carbon steel support structure is currently at failure only where the carbon steel is (tie rods, baffles, nuts, bolts, etc.).  It is possible that the galvanic differences between the 2205 and carbon steel also helped accelerate the corrosion.

I know the Duplex 2205 is an acceptable material for the service just by the condition of the tubes.  I also know the correct solution is to deaerate and/or treat the water and fix the brine carryover problem.  However, I'm not being given the opportunity to do that at this time.  I am being asked to put another tube bundle into service for 18 months in order to re-engineer the entire water system and eliminate the problem.

I will not put carbon steel back in due to the non-reliability those have shown.  A Duplex 2205 bundle is expensive for such a short run.  However, I can't find any references to the performance of a 300-series stainless steel or Admiralty Brass material in this service, both of which may be less expensive.

My questions:
1) Could 300-series stainless steel or Admiralty brass survive 18 months in this service?
2) What GOOD references are out there concerning material selection for water service?  (Besides the DREW book.)

Thanks
~NiM
unclesyd (Materials)
23 Jun 05 8:38
Can you comeback with some temperatures and flows?
Metalguy (Materials)
23 Jun 05 8:38
Consider 90-10 Cu-Ni.  I would avoid Admiralty, but Al. brass/bronze should be OK.
NickelMet (Materials)
23 Jun 05 8:54
unclesyd:

Temperature (in/out) - shell side @ 85/145°F and tube side @ 294/175°F [both design sets]

Velocity [design only] - shell side @ 1 ft/sec and tube side @ 5 ft/sec.

Now, the reason I didn't list this earlier is that I don't necessarily trust the reported operating conditions (which say they are well within the design criteria).  I believe the exchanger to be operating near the maximum temperature difference.  And it is more the water chemistry on the shell side that is the problem.  I have had reports that "water hammer" effect can be noted in this area, maybe even in this exchanger.  When I asked the process engineer about the consequences of removing the exchanger from service (completely), I was told "bad things happen".  So, to aid them on their quest to replace the exchanger, I recommended a Duplex 2205 bundle (entire) and a Duplex 2205 lined shell.  Needless to say, maybe a bit overkill, but it got the point across.  The process team doesn't want to spend that type of money.  And, the engineered long-term solution would be to remove the exchanger and redirect the piping correctly, allowing the letdown of the condensate temperature in a mixing tank (maybe even use a fin fan cooler) and the redirection of the treated water to a deaerator.  That will take time, so they want a plug-n-play bundle to last 18 months.

~NiM
NickelMet (Materials)
23 Jun 05 8:56
Metalguy:

Do you think a 90-10 Cu-Ni bundle (entire construction)would be less expensive than Duplex 2205?

Also, why avoid Admiralty?  I have it in several locations for cooling water service and it performs well.  Is there something in this make-up that would be bad?

~NiM
Metalguy (Materials)
24 Jun 05 14:01
I'm not sure about the cost, but I'd suspect that 90-10 is cheaper than 2205.

In power plant condensers, Admir. has been used extensively and has had MANY failures.  By contrast, 90-10 has done very well.  Your clean water may be OK with Admir.-H2S polluted water will cause SCC very fast. (I can give you the reasons later if you want).

You should compare the costs of all three metals and see if you want to take a chance with the lowest cost Admir.
unclesyd (Materials)
24 Jun 05 14:54
We quit using Admiralty in refrigeration condensers quite a few years ago.  That was my big boss and Carrier's decision so I never heard the reason.  I'm trying to find out what they replaced it with.  I know we lost some Admiralty tubes in two large absorption machines running on condensate.  

Here is an alloy that is being used more and more.  I don’t know if it has made it to tubes yet but it was available and competitively priced with 2205 and others.  

http://www.hpalloy.com/DataSheets/255.htm
Helpful Member!  rmw (Mechanical)
25 Jun 05 15:38
First, you application terminology is puzzling.  Condensate and BFW ususally connote quite different process flow streams.  The temperature of your hot stream is such that it is difficult to determine if you are using BFW that has already been heated to 294°F, or if you are cooling process condensate that is at that temperature because of being at a higher pressure (either of which would explain the tendency toward water hammer due to flashing as the pressure drops.)  So it is unclear whether you are cooling the hot stream, whatever it is, or heating the treated water stream, and at what fuel costs.

Your need to "let the temperature down" of the condensate is counter-intuitive to the need to pre-heat BFW for the best boiler efficiency purposes.

Second, while your "normal" chlorides number is low, the tendency for brine carryover is troubling for 300 series SS.  Although, for 'plug and play' for 18 mos, it might survive just fine.

If a temporary fix is all you are looking for in this instance, have you considered Plate & Frame type Hx's?  They come in 300 series SS as well as higher pedigrees.  They have pressure and temperature limitations, but sounds like you are in a range where they might work well.  You would also get much better temperature approaches than you are showing with the S&T Hx.

If a permanent fix is in order, and the heat recovery is important, have you considered a higher pedigree tube than 2205, say SEA-Cure with 2205 shell side components?

My comments re: admiralty are:  I have seen the admiralty tube problems mentioned by Metengr in large power plant condensers, but I have also seen admiralty condensers die of old age after 35-40 years of dependable service as well.  As a heat transfer material, the velocity limitations and tube wall thicknesses required for admiralty with its real good heat transfer coefficient are well offset by the strength and hardness characteristics of SS (and better) tube materials because of their ability to use thinner tube walls and utilize higher flows through the exchanger to offset the HTC penalty.  (your flow rates are low enough so that any material would be OK velocity wise.)

Most of the admiralty and/or Cu-Ni tube materials that I have seen replaced in the power industry were due to copper transport problems which showed up as deposits in boilers, turbines, and steam generators in nuclear plants.  I don't know if you have any consideration for this problem or not.

I hope some of this helps you with your problem.

rmw
NickelMet (Materials)
28 Jun 05 13:00
Sorry for the misleading terminology.  But you are right that we are trying to cool the hot stream.  The hot stream is listed on our documents as "Condensate"...and I was told to treat it as the same as boiler feed water (for composition).

So here is the update:
1)  Operations wants to run for 12-18 months on the least cost to them.  So, I proposed a stainless steel 316 bundle with carbon steel baffles, tie rods, etc.  This configuration should last the 12-18 months.
2)  We're going to use that time to engineer a solution such that the exchanger can be taken out of service.  Preliminary indications are that we can do this at considerably less cost than buying a whole new long-term exchanger.

The caveat to this, Operations has accepted the risk and knows that if they don't comply with the engineered solution within 18-months they have to buy the more expensive bundle.

THANKS to EVERYONE for the information provided, especially some of the insights on Admiralty and 90-10 Cu-Ni alloys.  I'm still learning about this new career of mine and all of you have helped further my education.

~NiM
stanier (Mechanical)
28 Jun 05 16:52
As for references Nalco have three excellent references on corrosion. If you cant get the titles from your local technical bookshop they are available electronocally from www.knovel.com.  If you were using chemicals Nalco would cough up the books for free.

EdStainless (Materials)
28 Jun 05 17:26
At todays Ni prices 90/10 should be more expensive than 2205.
With Mo prices where they are the 316 prices may be a shock.

I wouldn't have any trouble with internals (baffle/tierod) in 316, but you may be asking for trouble with CSCC if you use 316 tubes.
Are you using as light of gage 2205 tubes as possible?  You might be able to reduce cost a lot by using the correct wall thickness.  Tubing comes in other walls than 0.083".  If your original CS tubes were 0.083" you might be able to use 0.049" in 2205.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

stanier (Mechanical)
28 Jun 05 18:12
The nickel industry development institute has a consulancy service that you wish to refer your problem to for advice.  http://www.nidi.org/

The also have many papers and guides on corrosion. You can also download software on stress corrosion cracking predictions.

NickelMet (Materials)
29 Jun 05 7:46
EdStainless:
Thanks for the info on cost.  We've already gotten quotes and the 316 is the least cost alternative.  I did reduce the wall thickness to 0.065" on the Duplex 2205 to save some money, but this is a BIG bundle.  A mix of Duplex and 316 was $70,000+.  However, I didn't go through the TEMA calculations to see how thin I could go.

You mention Cl-SCC as a possibility.  The exchanger certainly meets the criteria of temperature (>140°F) and having aqueous chlorides availability (shell side).  There may be some residual stress, but I wouldn't think there would be applied stress, unless you are thinking of the U-bends.  Do you think the bundle may catastrophically fail within 12-18 months from Cl-SCC at those conditions?

Stanier:
Thanks for the data sources.  I currently have a subscription from Knovel and am quite familiar with NiDI.  Their information was helpful, but sometimes I like to get "live" feedback on subjects.  Different opinions sometimes lead to unique solutions.  And I appreciate/respect all the differing opinions on this website.  

Again thanks everyone.

~NiM

EdStainless (Materials)
29 Jun 05 10:28
The operating stresses, and even the u-bend stresses are not the problem.  The residual stress from straightening the tubes is huge.  Typicaly it is over 35% of the yield.  I have seen enough hot water lines in food plants and breweries fail to be scared of CSCC.  As soon as there is any corrosion initiation you can get cracking.  I have seen it in just days.
Why don't you double check the wall requirement.  Since 2205 is more than twice as strong as 316 you might be able to shave further.
The other possibility is to use AL2003, the lean duplex.  It has about the same pitting resistance as 316, but it has the same cracking resistance as 2205.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

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