Superheater tubes corrosion or erosion 2

[niccolo_tu]

Dear all,
I'd like to ask an opinion about a power plant boiler.
The attached picture shows a superheater heat exchanger.
The material is partially ASTM A213 T12 and partially 14CD3 (Afnor standard, should be equivalent to a ASTM A213 T22).

The internal fluid is steam, internal pressure 85 [bar], average inlet temperature 310 [°C], average outlet temperature 460 [°C].
The external fluid are flue gas (whood biomass combustion), average inlet temperature 730 [°C], average outlet temperature 420 [°C], about 20 [%] water content (the rest is CO2, O2, N2. There are traces of HCl and SO2 (about 0.2 [mg/Nm3])) and fly-ashes content. The average flue-gas velocity is about 3.5 [m/s].

As you can see the the external tubes surface is affected by corrosion-erosion-(combination of both?) phenomena.
The white material attached on the external surface are ashes.

I accept any suggestions or advice.
Thanks.

 

[TugboatEng]

Erosion is typically caused by soot blowers which it appears this plant does not have? If there was erosion it would erode away all of that ash before the steel. This is certainly corrosion. Does his boiler idle or shut down frequently?

 

[MJCronin]

Hello Niccolo ...

Some questions for you:

1) Do you have sootblowers and was the erosion in the direct path of the sootblower travel ? Tell us more about the age and configuration of your boiler... How old is it ?

2) Since you have, essentially, a wood fired boiler, you are subject to the corrosive wood/biomass environment. You also have a marginally corrosion resistant alloy selected (ASTM A213 T12) for the superheater material. You are asking a lot of this boiler. Do you intend to retube this boiler with a more expensive, premium material ?(e.g a higher chrome alloy) ?? This can get really expnsive, very very fastt ...

3) How much longer do you expect to run this boiler ? Have you considered a complete replacement with a new boiler ?

An except showing a "more corrosion resistant" progression from a recent research document:

"Alloys used [49] include: carbon steel (no added Cr), UNS K11587 (1% Cr - T11), UNS K21590 (2.25% Cr steel - T22), UNS S30409 and UNS S34709 (19% Cr stainless steels - Type 304H and Type 347), UNS S31000 (25% Cr - Alloy 310) and UNS N08028 (27% Cr - Alloy 28)"

Note that more CHROME equals MORE CORROSION RESISTANCE !!

You have the LEAST CORROSION RESISTANT material of this progression, and it is probably fair to say that the SS group of tube materials are too expensive for nearly all re-work applications, in my opinion

(From: ORNL/TM-2013/276; Advanced Manufacturing Office' IMPROVING HEAT RECOVERY IN BIOMASS-FIRED BOILER)

You have noted that the T22 material is preferred from some bizarre standard ... in my opinion, T22 is not enough ...

Tell us more about your boiler configuration, your location and the owner's future plans for the boiler ....

Does you boss have an MBA ?

MJCronin
Sr. Process Engineer

 

[niccolo_tu]

1) I do have soot-blowers. the path of the soot-blowers is clean (and I would say smooth [pics attached]). Everywhere (except taht along the soot-blowers path) the ash crust is very strong (it must be removed with the grinder). So it also gives a sort of protection to the tubes.

2) First we need to understand the phenomena and, by now, we didn't have breaking due to underthickness. A SH ASTM P22 of this dimension is about 200 k€ (220 k$)..

3) Consider that the boiler itself is from 1962 (B&W). The ECO section, the membrane walls and SH(low temp) are the original ones. This Superheatetr (SH-high temperature) was first substituted in 1970 (ASTM A213 T22) and then in 2005 (ASTM A213 T12). The replacement of the SH.

4) The power station is a base load one, idle configuration.

Could you please attach the link of the paper?

Thanks.

 

[EdStainless]

I wonder if you have some low melting point salts in you ash.
It looks like it fusing and reacting with the mild steel.
Have you checked for Na, Ca, K and such? what about Si, Cl, and metals?
In bio-waste boilers this is common.
Sometimes related to composition of the fuel and sometimes related to firing practice (such as changing power levels).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[davefitz]

Normally the best way to analyze tube damage is to send a short section of tube, with deposits intact, to a metallurgist , who will analyze both the deposits and the surface metal, and draw a conclusion from the aspect of ash chemistry. Often times the corrosive damage is due to alternating periods of oxidizing then reducing atmosphere ( ie excess O2 followed by nil O2). Each atmosphere causes a different type of metal oxide to form on the surface of the tube, and the oxide formed during excess O2 periods can be easily removed by exposing it to a reducing atmosphere, and it only becomes an issue above a critical surface metal temperature . The root cause is then an issue with the combustion process, perhpas uneven air distribtuion in the burners or uneven fuel HHV values. If you cannot fix that combustion issue then you can either reduce the steam temperature ( which may reduce the surface metal temperature below the critical ash melting temp) or replace the tube metallurgy with a incolnel weld overlay or bimetallic tube. Let the metallurgist figure it out.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[0707]

You are experiencing oxidizing metal dust corrosion ( flue gas and ciclic steam soot blowing ) at 720ºc leeds your P22 material, to what you have, a material up date is required to extend your material life.

 

[mrfailure]

So you went from T22 installed in 1970 to a less corrosion and heat resistant T12 alloy in 2005? This is the type of negligence that causes litigation. I concur with DaveFitz that you should send one (or several) samples for metallurgical evaluation. Make sure to mark fireside and flow directions on whatever samples are sent.

 

[MJCronin]

Niccolo ...

A few more questions and some comments:

4) Quote: "So you went from T22 installed in 1970 to a less corrosion and heat resistant T12 alloy in 2005? This is the type of negligence that causes litigation".... I agree !!! .... Any explanation, sir ?... It was what the bosses wanted ?

5) So you have a SIXTY PLUS YEAR OLD BOILER ? Was the boiler ORIGINALLY DESIGNED to burn wood and/or biomass ?

6) Have you had problems with the cold end of the boiler ? Many wood/biomass boilers have economizer slagging and ash transport and fouling issues. Can you tell us more about your entire system ?

7) What is your feed system for the wood ? Are you co-firing fuel oil or Nat Gas during start-up ?

8) Where is the boiler located ?... How often is it cycled ?

Another paper that describes issues with superheater tubes in biomass boiler design:

https://digital.library.unt.edu/ark:/67531/metadc840465/

Are you aware of the superb French book on Boiler Tube Metalurgical Failures ?

https://www.amazon.com/Metallurgical-Failures-Fossil-Fired-Boilers/dp/0471558397

The Nalco Guide to Boiler Failure Analysis Second Edition is another classic

MJCronin
Sr. Process Engineer

 

[niccolo_tu]

Dear all,
thank you all for the replies.
For sure the one of the first step we did was to send a samples to 2 different metallurgic labs and to inspect the whole boier with a boilmaker.

4)5) The boiler was designed and manufactured in 60's to burn coal (it was a typical coal power station). In 70's it was converted to oil, the SH materail changed, but even the shape (probably due to the different temperature profile). In 2005 the power plant was converted to biomass (the bottom part of the combustion chamber was cut-off in order to install a fluidized bed combustion chamber, the SH changed (material and shape), it was added an additional ECO section, the HP-preheaters were removed etc..). In 2019 the power plant was sold to the company I work with, and now here we are.
Unfortunately there are few documents tracking the changes, it is very difficult to understand the re-design choices.

6) Actually I have the opposite problem, even with the additional ECO section the temperatures are still high. At the bag-house inlet we have 155 [°C] average (consider that in other power plants we record 15-20 [°C] lower). We also have gas recirculation (from the bag-hous outlet to the boiler).

7) 8) The fuel il 100% wood chips. Diesel oil is burnt only for the start up. The power plant is a base-load one (8000 h/y) at constant electrical power output (Italy).

Thank you all for the precios notes.

 

[MJCronin]

Sooooooo ....

We have an ancient (60 year old)FRANKENSTEIN and we are upset that he cannot tap-dance and seems to be coming apart ? ....

Is that what you are telling us Nicky ?

Time to once again remind your MBAs/Managers that nothing lasts forever ... ???

8) Where are you located ??

Time for a new boiler ???

Talk to us ....

What do you think ???

MJCronin
Sr. Process Engineer

 

[0707]

A star for you MJCronin

 

[EdStainless]

No, the boiler isn't Dr Frankenstein, it is his monster.
Yes, three re-design re-builds goes a long way to hide what is happening.
Looks like time to contact an engineering Co that specializes in biomass burners and re-design the whole boiler.
Biomass contains a lot of crap, not just the very high ash content but many minerals depending on the species of trees and where they grew.
I have seen metals and Cl values on biomass that would destroy about any boiler.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

Order a lot of spare tubes asap for all sections of the boiler, like superheeter, economizer, etc.

Repair as and when failure occurs.

You can still run this boiler for another 60 years.

Thr statutory inspectors (if any) could be suitably handled (may be by that MBA)!

DHURJATI SEN
Kolkata, India