Problems in tubes of a Recovery Boiler 2

[MedicineEng]

Dear All:
In my plant we have a recovery boiler system that burns Production byproducts to produce steam. This system has been working for around 15 months and we are experiencing a problem that I would like to share with you:
The system is composed by a combustion chamber where the byproducts (sometimes extremely acidic with a HCl content of 10%) are burned at a temperature of 1100C, and then these gases go to a double pass boiler to produce steam. The exit temperature of those gases to the gas treatment part of the system is around 400C.

My problem is that the tubes of the boiler are breaking. This failure always happens in the first pass of the boiler and there are already 3 broken pipes (that were closed with a metal disc). The first pipe broke in Feb last year, so 5 months after the start up, then the other two pipes broke around one month ago.

The system is stopped and cooled down to almost ambient temperature in average 1 time/week to clean the boiler due to salt formation in the tubes

Any clue of what is provoking these failures?

Thanks a lot for your help

 

[vpl]

MedecineEng

A bit more data would be helpful:

What are the tubes made of?
What is the chemical composition of the salts
Where is the largest deposition of the salts in the tubes (front, mid-tube, tube sheet?...)
How rapid is the cooldown to ambient?
Describe the cooldown process. For example, does the boiler portion continue to run after the heating system is shut down?
Have you ruled out manufacturing defects?
Have you pulled tubes or done eddy current testing to find exactly where the failure is occurring?

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[davefitz]

The best way to narrow down the cause of the failure is to send the failed section of tube to a metallurgist who has accesss to a metallurgical lab.

There are dozens of causes of boiler tube failures. From the limited info provided above, the failure mechanisms of (a) OD corrosion (b) tube overheat due to inadequate water circulation (c) ID corrosion (d) manufacturing or construction defect are possible. send the failed tubes to a metallurgist and read their report.

 

[MedicineEng]

Thanks a lot for your feedback.
I got some more info that can help:

1-Salts: I believe that this is a form of NaCl since we have in the stream HCL and Na that are cooled down in the boiler from 1100C to around 400C;

2-It seems to me that the deposition of salts is even along the tube, but I am not sure. Visually, it seems that there are more salt in the second pass of the boiler, what it would be expectable, as far as I know;
3-The cooling is average 150C/hour;
4-In the beginning, I tought about manufacturing defects, since the first failure was only around 5 months from the start. But since we didnt'have any more problems until one month ago, that means around 10 months after the first problem, I don't think that it is a manufacturer problem.
5-I am not a boiler specialist. Can you please explain what is "pulled tubes" or "eddy current test"
6-The boiler continues to work after the combustion chamber is switched off until the gases are cold

 

[vpl]

MedicineEng:

Since you say this is a fairly new boiler (15 months is NOT a long time for a boiler to be in service), you need to get back with your manufacturer as the system should still be under warranty. The manufacturer should be more than willing to help find out what the cause is.

Pulling a tube (which DaveFitz suggested and I absolutely agree) means to remove a failed tube (or a portion of the tube) from the bundle. There are a number of companies that can do this, including the original manufacturer. Once you remove the tube, you send it to a reliable metalurgical firm who can tell you exactly what caused the failure.

Eddy current testing is a method used to detect which tubes have failed and whether other tubes have cracks which would make them likely to fail. I was looking for information on how your company found which three tubes failed and if you knew what type of failure occurred. The difference is that pulling a tube would tell you why a failure occurred whereas eddy current testing would tell you where in the tube it failed and if more tubes are close to failure.

I suspect you have some stress corrosion cracking going on. You didn't say what the boiler tubes were made of, but stainless steel would be a very bad choice. At this point, you would be much better off talking to the manufacturer and having a defective tube pulled and looked at rather than for some one to try and guess.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[rmw]

The 'pipes' (tubes) you closed with the metal plate, if they were able to be removed from the boiler would be candidates for sending to the lab, especially if you can get the failed section out without disturbing the area where the failure occurred.

Like VPL, I would like to know the metallurgy.

The paper industry operates chemical recovery boilers with some nasty salts day in and day out without 'breaking' tubes. You have some other type of mechansim going on here. I was thinking SCC too, but without the tubing metallurgy it is difficult to say.

rmw

 

[MedicineEng]

Dear All:
Thanks a lot for your help. Just one important remark that I missed regarding this boiler. This boiler in fact has 20 years old, but before being installed to this new system, it was completely revamped.

 

[Welchita]

So, some new factors are present into your system, after it has been revamped.
What new factors, new boundary conditions came along?. What is the boiler working record before, for more than 18 years ? I think you have to make a cause effect relation process.
Good luck!

 

[chicopee]

Where are the tubes failing? and what type of failure are you experiencing --longitudinal ruptures, pin hole leaks, loosening flared tube ends in steam or mud drums,cracked welds, erosion or corrosion of tubes, or is failure occuring on the waterside or fireside?

 

[MedicineEng]

Dear All:
Some more information:
The tube that I saw damaged look like it was "collapsed", that means visually seemed that it couldn't stand the pressure difference between interior and exterior. I only saw this in the tip of the tube but I don't know if this was the cause or the consequence of the damage.
We only noticed that becasue there was water coming out from the fireside (hot gas side).

Regarding the record failure, I spoke with the persons that were in charge of this boiler before and they becasme surpreised because they couldn't recall any happening like this in the past.

 

[chicopee]

If the tubes were collapsing inward then there was a partial vacuum created,unless there were damaged during their installation and then leaked afterwards.