Protection of boiler tubes 3

[21121956]

Hello everybody:

We have in operation several waste heat boilers, installed after medium speed Diesel engines. They are water tube boilers and produce 3760 kg/h of saturated steam, @7 barg and @170ºC. They were put into operation 15 months ago.

After this period of operation, we have found that the schedule of the tubes (80 mm in diameter) has been reduced drastically, to the point of rupture.

Is there any treatment we can apply to these tubes? Maybe some especial paint or resin? or we should have to change the material of these tubes?

Your help will be very much appreciated.

Ricardo

 

[metengr]

This sounds like a possible sulfidation or reducing atmosphere corrosion mechanism on the OD (gas side) surface of the boiler tubes. Have you confirmed the extent of external wastage or is there internal wastage causing the wall thinning? How much of the boiler heating surface is being effected?

You need to perform wall thickness testing and decide how much of the existing tubing needs to be replaced or can be restored by weld overlays. I presume the tube material is carbon steel. If this is the case, you may need to evaluate the application of a corrosion resistant weld overlay - paints or coatings will not last in this type of service.

Surface treatment like chromizing of replacement carbon steel tubes versus weld overlay of carbon steel tubes may be an option. Right now, you need to confirm the actual corrosion mechanism that is causing the wastage.

 

[eyec]

offhand, after just 15 months, to my mind, there are bigger issues.

are the units being run within design parameters?

as metengr said, you need to identify the waste mechanism.

 

[21121956]

The boilers have vertical tubes, 3.20 m long, carbon steel schedule 40 (4 mm thickness), 126 at the entrance of exhaust gases @385ºC, and another 126 at the outlet @320ºC.

During the periodical cleaning process, the wet soot is partially deposited on the bottom of the tubes plate, because for design reasons it is not possible to eliminate the total amount of soot.

The damage in the tubes (loss in thickness) is almost uniform and is around 50 mm starting in the point were the tubes traverse the tubes plate. As a corrective action, we have eliminated the damaged parts and replaced them by pieces of tube schedule 80 (7.5 mm thickness).

We will await for the results that come up.

 

[metengr]

Ok, the additional helps. It would appear that you have some type of acidic corrosion problem with the soot. Is it likely that the soot contains sulfur compounds from the exhaust gas of the diesel engines? Given any moisture or condensation during off-line conditions of the boiler, soot that becomes moist or wet at the dew point could be creating sulfuric acid on the surface of the tubes.

 

[21121956]

metengr:

When you suggest to apply a welding overlay to protect the tubes, I wonder if you are thinking in the metalizing process, applying powdered metal alloy coatings.

If this is your idea, can you tell me what is the appropriate corrosion resistant powder alloy we can use?

Thanks

 

[metengr]

21121956;
I would not recommend using a metalizing process for a corrosion barrier in this application. Instead, a weld overlay that is fused to the tube substrate should be used for the distance necessary to alleviate the external corrosion problem with soot. An Inconel 625 weld overlay would work, but it is expensive.

If you are interested, I have recently come across an ASME Code case that is working thru our subgroup related to use of carbon steel tubing that has an intentional alloy additions of 0.15% antimony and copper for Section I construction. The copper and antimony combine to form a corrosion resistant oxide barrier. The tubing is made by Nippon Steel and it is called "Sulfuric Acid Dew-Point Corrosion Resistant Steel" (S-Ten 1). I believe the ASME Code Case number is 2494, and it will reference ASTM A 423 Grade 3 Corrosion Resistant Cu-Sb Carbon Steel Tube. I have seen the submittal for this Code Case and the material exhibits weldability just like carbon steel with no adverse effects of copper or antimony.

 

[21121956]

metengr:

Thanks for your suggestion, it seems at first sight that it can work out; but, I think that to obtain these tubes will be somewhat problematic.

On the other hand, we have thought about the possibility of replacing the section of tube damaged by acidic corrosion by one of stainless steel (same diameter of the original) and, to this one segment, to weld the original carbon steel tube.

For such a welding we think that electrode E-309 will be the appropriate.

What do you think?

 

[JimCasey]

Soot and moisture will cause galvanic corrosion.Take a look at your Wal-Mart mild steel barbecue grill after it sits outside all winter.

Diesel fuel contains a small amount of sulfur. Sulfides and moisture in the exhaust form acids. Also not good, and probably aggravate the galvanic corrosion from the soot.

If you switch to stainless tubing in the HX, you will reduce the corrosion but SS has a much lower coefficient of heat transfer.

If you install a particle separator upstream of the HX, it should help remove the sooty particles. Just be sure to insulate it so the efficiency of the recovery system is not reduced.

Probably the best way to deal with it is to tweak the injectors to reduce smoke from the diesel engine. Smoke is unburned fuel. Heating the fuel can reduce the viscosity and cause better atomization, or adjustment or replacement of the injector nozzles may be appropriate.

 

[metengr]

21121956;
I would NOT recommend use of butt welded austenitic stainless steel in a water-touched circuit of the boiler. This material can develop stress corrosion cracking due to sensitization from welding. It also has a different coefficient of linear expansion in comparison to stainless steel. You also have galvanic corrosion issues between the dissimilar materials.

If you are going to install stainless steel tubes, I would order carbon steel tubes that have a corrosion resistant spiral weld overlay. These are common items for boiler components and can be easily installed.

 

[rodofgod]

Hi All!

21121956, I think you have a definite chemical problem!
Have you considered using co-extruded tubing in the affected areas?
Inco 625 overlay is quite common in domestic waste incinerators and quite effective as well!
If you choose to use S/S to C/S inserts, choose an Inconel filler instead of 309!
Regards

 

[chicopee]

Cardon dioxide from the exhaust is becomes carbonic acid when mixed w/ water vapor and is the probable cause of tube corrosion on the fire side of the tubes.

 

[21121956]

Hello everybody:

Thanks to all you for your commentaries.

metengr: I ask myself if your suggestion is, if instead of using stainless steel segments, we would have to use segments (not the complete tube) of carbon steel tube that have a corrosion resistant spiral overlay. Am I in the correct thing?

If that were the case, which electrode would be appropriate to achieve the butt weld?

 

[metengr]

21121956;
Correct, do not use stainless steel tubing. My thought is to have the carbon steel tubes spiral welded to the desired length that would afford necessary corrosion protection in service. Extend the length of the carbon steel tube to assure a carbon steel to carbon steel butt weld in the field. There is no need to complicate your efforts by terminating the weld build-up at the butt weld location, just leave enough extension of carbon steel tube material, say about 1".

 

[21121956]

metengr:

Thank you very much for your help.
I beg your pardon for going over and over on the butt weld subject, but we have at least to possibilities:
1. To achieve the spiral weld overlay by ourselves (I think this will be very difficult to do) and of course, leave enough extension of carbon steel tube.
2. If we buy the tubes with such special corrosion resistant weld overlay (this is more affordable) we will have to achieve the weld of two dissimilar materials, that is the reason for the question about the appropriate electrode.

 

[metengr]

21121956;
If you need to weld one end of the spiral welded tubing for a dissimilar metal weld, I would use Inco A or Inconel 182. I would purchase the spiral welded tubing instead of attempting to do this operation on your own in the boiler. Also, just to be sure, you may need to have a National Board R- Certificate of Authorization to perform any weld repairs to this boiler. Check with your local Jurisdiction.

 

[21121956]

Hello everybody:

In order to solve this problem, a decision was taken.
1. The damaged segments of the tubes will be eliminated and replaced by carbon steel segments of the original tube.
2. The existing water spraying system will be replaced by steam soot blowers. These new soot blowers will be manually operated rotating type.
We have to wait to see the results.

 

[JEB66]

so your problem was sootblower erosion not fire side corrosion?

 

[21121956]

Hi JEB66:

The new steam soot blower system has been proposed by the boiler´s manufacturer.
The disadvantage of water spraying cleaning is the high risk for sulphur corrosion of the gases side of the tubes. Sulphur corrosion is extremely heavy in the temperature range of 100-120ºC.

We have been told that, with steam, the cleaning effect is based on the velocity and turbulence of the cleaning agent, which removes soot from the heating surface.
And more, there not exists sulphur corrosion risk, because the cleaning agent is DRY.

 

[JEB66]

steam blowers are dry when everything works properly. The traps are removing condensate and the poppet valves are set to the correct pressures...

If you have not had the failed tubes analyzed for the failure mechanisum, you really should do so. If the cause of the tube damage is fire side corrosion, then it does not matter what type of blowers you have.