Corrosion, Erosion or Both? 5

[NPKresults]

Hello All,

Problem: My fan is being eaten alive.

Fan Description: 5' Diameter Radial Blades, 465fps tip speed, carbon steel blades

Process Description: The fan is downstream of a wet scrubbing system, complete with mist a multistage mist eliminator (chevrons and mesh). The mist eliminator seems to be removing some water (about one gallon per hour) but the fan is still seeing moisture as evidenced by removing the cover after shut-down. The catch is that the pH of the mist eliminator moisture is about 2.5. As of now we are not sure if it is the high-speed/water droplets causing erosion, the low pH of the mist causing corrosion, or a combination of both that is the cause. I currently do not have a way to measure the size of the water droplets or the amount entering the fan, advice on this measurement would also be helpful.

We are looking into adding caustic to the last wet scrubber stage which should rid us of the pH issue but will also make for a low concentration salt solution. My main concern is that the blades will still see the water droplets and would still be prone to erosion problems.

I would like to know if anyone has experienced similar problems with fans/blowers located behind neutral wet scrubbing systems that released a pure water mist/droplets.

Any ideas would be much appreciated,

DB

 

[strider6]

just an idea.. if you make a failure analysis of the blade you should be able to see if there are is a mechanical damage, erosion, of if there is also a corrosion component. I suggest you to search for a lab in your area and send the blade for analysis.

S

Corrosion Prevention & Corrosion Control

 

[TVP]

I am not very familiar with this type of process equipment, so can you tell me what chemical species are present such that the pH is so low? Also, what type of salt would be present if a neutralizer was used? In general, this sounds like a corrosion issue more than erosion, but perhaps the chemical environment is not as severe as I am imagining.

 

[NPKresults]

There is HF in low concentration, also H2SiF6. We have not decided the base at this point but figure NaOH for now. So the resultant would be NaF and Na2SiF6.

I know the acids will eat the heck out of carbon steel, especially in low concentration, however the blades appear to be more eroded than corroded based on the rather smooth wear and lack of discoloration.

I plan to get a lab analysis of the wear, but I have to wait until we can get another wheel fabricated.

 

[EdStainless]

I would suspect that corrosion is the biggest problem. With a fan the corrodent will not be distributed uniformly and spots will get wet and then dry. This would result in severe localized attack.
Can you get plastic blades, or composite, but make sure that there is no glass fiber fill in them.

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Plymouth Tube

 

[TVP]

Wow, the chemical environment is even worse than I imagined. I agree with EdStainless that this is likely to be primarily a corrosion issue, with perhaps some erosion component enhancing the degradation. This is likely to be a very expensive fan...

Maybe a carbon composite blade with a ceramic coating if erosion really is a problem?

 

[SJones]

At those speeds, would it not be unlikely that a corrodent is able to reside long enough on the surface to allow corrosion as the principal degradation mechanism? There may be an element of corrosion as the droplet crosses the surface of blade; but, the principal metal loss is likely to come from droplet erosion as it impinges the leading edge. A composite blade may also be subject to a similar mechanism and it could be that a hard facing alloy or a sprayed erosion resistant coating would be required.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[metengr]

I would tend to agree with SJones without a picture that with a tip speed of over 400 fps you are more likely to have water droplet erosion damage, similar to LP steam turbine blades. Stellite hardfaced blades might just work on a carbon steel substrate.

I would obtain a proper failure anlysis based on the various opinions offered.

 

[dik]

Any chance the damage could also be cavitation?

Dik

 

[NPKresults]

Thank you all for your thoughts. Attached is a picture of the damage, I should have posted it earlier.

Metengr- Is stellite a sprayed or welded coating? We wanted to try a paintable ceramic epoxy but the balancing issue continues to haunt us. I also spoke with our Sermatech rep about it and they told me of a similar application where ECTFE was used with success but we were holding off in hope that a cheaper solution would arise (about $50k for this option). This being a fertilizer plant we are economically limited and we are trying not to hand it over to the 'it is what it is' just yet if we can help it.

 

[metengr]

NPKresults;
Thanks for the picture and this confirms to me that you indeed have a water droplet erosion problem. Stellite 6 is a weld deposit versus coating. As long as the weld overlay is applied equally across all of the carbon steel fan blades in the high erosion areas this should not result in an unbalanced condition.

 

[EdStainless]

Yes, I'll agree that erosion is the dominant factor here.
While a Co based weld overlay is clearly the 'gold standard' for erosion resistance let me suggest an alternate.

I have run fans in acid gas service that were coated with epoxy. This was a 100% solids system with mineral fillers. And it was applied in three coats to a thickness of roughly 100 mils.
We did have some wear still at he leading edge of the blades so we used some thin (0.025") alloy 625 to make edge caps. These were put on after the first coat of epoxy so that they were well trapped in the resin.

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Plymouth Tube

 

[NPKresults]

Thank you all for your excellent recommendations and expertise...

Ed - Do you mind elaborating a bit on the epoxy? we had an epoxy on this guy to start with and as you can see it wasn't worth a nickel... I am not very experienced with epoxy options and I would like an outside opinion. Also, how did you fasten the edge caps that you described?

 

[dik]

If you're looking for a non-metallic coating, take a gander at some of the polyureas...

Dik

 

[corrosionman]

NPK I'm guessing the mist eliminator mesh is stainless steel and probably the chevrons also and these are in electrical contact with the carbon steel fan so you have a dissimilar metal corrosion problem. Change the chevrons for plastic and replace the mesh with a similar nylon mesh to solve this part of problem
Try the system with the fan coated with a high build epoxy coating. It might work and save you lots of £
Best of luck.
Cleveland Corrosion Co

 

[SJones]

http://www.google.com.au/search?sou...T4ACAW_enAU308AU309&q=+erosion++thermal+spray

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[NPKresults]

The Chevrons and mesh are polypropylene, housing is FRP.

 

[EdStainless]

NPK, I don't recall which resin we were using. It was a 100% solids system. The first coat was a light bond layer, the second was 80% of the total build, and then a light top seal coat.
The resin was loaded with ceramic filler to improve it's erosion resistance.
The leading edge strip was actually two pieces of SS. They were welded together (lengthwise, along the middles)the outer one was wider than the inner one. The inner one was formed tight to the bare blade edge and either welded or riveted down. The outer one was bent so that it was captured in the build layer of the epoxy.
All of this forming was craftsmanship, hammer and wooden block work.

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Plymouth Tube

 

[corrosionman]

NPK And dont forget that after the coating your fan needs balancing and the balance team will weld little weights onto the fan so you need to be able to overcoat the weights with the same resin so ideally the coating people must supply a patch repair kit. Edstainless suggested 100 mil maximum. You might be tempted to think of abrasion and go thicker but most resins shrink on setting and if you go thicker than 400 mils it shrinks and cracks so watch out for excess thickness in corners.
Corrosionman