In a new CFBC boiler we are facing economiser tube failure frequently. the failure type is, it looks like tube has got dent on the external surface and failing at that point. we have observed for the following reasons,
1) Failure due to corrosion No
2) Failure due to metallurgical issues of the tube - No
3) Erosion due to flue gas ruled out as each time only one tube is getting effected.
4) OEMs are also involved but no positive result.
Can any body suggest what may be the cause of dent formation and in turn tube failure.
One doubt is the dent may be caused during erection. Cannot be concluded as failure frequency is very high and boiler is one year old.
poor flow distribution: leads to overheating and build up of thermal stress sufficient to cause bulking
Economiser tube failure
the following details are needed for pin pointing the area of trouble. what is the operating pressure of the boiler? what is the steam output capacity and what is the circulation ratio in the boiler?
i am taking a guess here of what is happening. It seems that certain amount of steaming or bubble formation is taking place in the economizer this will lead to film boiling. what happens now is that the vapour film, which has a lower thermal conductivity will cause a reduction in the heat carried away, ie the heat transferred to the wall of the tube will not be equal to the heat carried away. this will cause the rest of the heat to be stored in the wall overheating it and causing a rupturing. (this i have seen happening mostly in riser tubes.) this scenario can happen in a econimiser; especially in the tube bends or near to the tube downside header due to
a. poor flow chara
b. over design of the economiser which results in higher heat gain
c. in older designs when there was horizontal a economiser
d. non ribbed tubes in economisers
CFBC is a relatively new tech in India and i am guessing that you are from India. is BHEL the supplier? though i have a high regard for BHEL as most of my work concerns with them, they usually keep quite about their design boo boos :P. you have to cajole the information out of them.
please check out if the heat gain in economizer is same as design and also if the tubes are ribbed or not. this will give you a fairly good idea to what is happening , and write back on what you find out.
How big is the boiler (MW)? What is the inlet and exit temp of the economizer. What is operating pressure of the boiler?
The economizer is in the lower temperature area of the boiler.
How fast are your startups? Are you following the manufacturer's recommendation for startup and ramp rate?
Is there any binding in the economizer and is everything free to move as the metal expands and grows during startup?
If the denting is not left over from construction, then something has to be "denting" it now. Either something is falling, a worker is making the dent (unlikely, the tubes are pretty darn thick), or the dent may a buckle or something similar from improper attachment of the economizer. If the tubing is trying to grow due to thermal expansion and it is being restrained, mechanical failure can occur.
Check pressure pulsation near the leaked tubes if any. Check also if any rapid movements of the corresponding steam control valve. The tubes fail faster if they are subjected to rapid change in pressure.
Rangacharya... is the 'one' tube always the same one, or a random one in the same manufactured lot?
Rangacharya... you might think it odd that an electrical engineer respond to this thread, but I want to share a similar experience with you. During startup of an LNG conversion plant we experienced several reformer-furnace tube failures.
Initially the failures appeared to occur randomly! For example: the 1st failed in the center of the north-header; the 2nd at the westend of the south-header, the 3rd... etc, etc. But, there was a strange sameness. No, they didn't fail at the tube-header connection, but instead they all failed at almost the same distance from the header!
At the time the world was awakening to an emerging corrosion menace... you might of heard about Polythionic Acid Attack! But no, that was not the problem.
Before I reveal the root-cause (no, you don't have to contact me off-forum) I await Namatimangan08's reply to, "You mean the dent was originated outside of the tube? If this is the case I think I know why."
Regards, Phil Corso
the "dent" is claimed to be on the external surface as if the tube tried to expand length wise and buckled, at least that is how i read the original query
> the "dent" is claimed to be on the external surface as if the tube tried to
> expand length wise and buckled, at least that is how i read the original query
You are right. The "dent" is claimed to be on the external surface. I'm interested to know his feedback to Mr Phil question too.
> the "dent" is claimed to be on the external surface as if the tube tried to
> expand length wise and buckled, at least that is how i read the original query.
Thanks a lot for such a good response to this post, yes it is very interesting and difficult to pin point the exact root cause of the problem. Here are some more inputs after going through various replies.
1) The Boiler capacity is 135 TPH
2) Boiler Pr Rating is 87 Ata and 510 Deg C
3)I am not sure whether i can paste the photograph in te forum.
4) The dent is external only and one tube affected at a time
5) Operating and start up procedure as per the OEM guidelines
6) 3 to 4 start ups in a month due to some teething troubles in the plant
7) Till Now it has happened in a particular zone and near to the casing area
8) No restriction in expansion, ensured.
Let us make a simple conclusion via direct observation of the facts. Since only flue gas is accessible to the "dent" area, therefore it is flue gas that did it. We only have to figure it out about the "how" part. It might not be as straight forward as it appears to be.
I will be right back to present my hypothesis.
> I will be right back to present my hypothesis.
Here is what I think.
In short, the dent might be due to the formation of "temporary" low pressure region (near vacuum) in the economizer. When the low pressure region appears it attract flue gas to flow onto it at a very high velocity. When the low pressure region inside the economizer collapses, the flue gas stream follows its normal path. The dent is formed due high velocity flue gas hits the external surface on and off. Over time, eventually, the outer surface will be damaged.
The low pressure region inside the economizer is formed due to transient movement of flow control mechanism. When the mechanism reduces the flow, during transient state, water far downstream from the mechanism tries to maintain its velocity due to inertia. For the moving water that is close to the mechanism but downstream to the flow, all of sudden, it has lost the driving force to move the water at similar velocity. Therefore the flow velocity around that area will reduce almost instantaneously.
Since the water velocities at various distances downstream are not all equal during this transient period where the "front" moves faster than the "back", therefore some portions of the economizer may achieve vacuum state (zero pressure) or "empty". The vacuum will collapse when the water reaches its new steady state flow.
Water hammer. I think you have heard about this phenomena. Water hammer causes explosion rather that implosion. What I'm predicting here is the opposite of water hammer. We can call it the formation of low pressure region near vacuum state. It can cause implosion since it creates almost vacuum state inside a constant volume flow conduit. This conduit is exposed to ambient air pressure. The nature of air is that it moves from high pressure to low pressure region.
Think about this simple experiment. Take a straw. Fill it up with water in full. Using your finger close the other end tightly. Now you can rotate whichever direction you want. The water in the straw remains there. Why? It is in pressure equilibrium with ambient air in all directions. No net pressure reacts on the straw.
Next, suck the other end. See what will happen? Yes. The straw turns flat. Why? When a quantity of water is removed from the straw says 2mm3, then an equivalent volume of vacuum state will be created inside the straw. Ambient air pressure "sees" this. Thus, net pressure will react to bring your straw-water system to a new equilibrium state. In this case, the straw is flattened...
I think this is the "physic" that caused one of your boiler tubes "dent"....
I am not convinced about the theory of erosion causing dent, since the effect is for one tube only and side by tubes are intact. As for as i know the erosion phenomenon may not create dent externally like somebody has made purposefully
> I am not convinced about the theory of erosion causing dent, since the effect
> is for one tube only and side by tubes are intact. As for as i know the erosion
> phenomenon may not create dent externally like somebody has made purposefully
Tell us a bit more about the nature of the dent. Is it looks like drilling mark or hitting by a blunt object?
> Tell us a bit more about the nature of the dent. Is it looks like drilling mark
> or hitting by a blunt object?
Yes it is like hitting by blunt object
>> Tell us a bit more about the nature of the dent. Is it looks like drilling mark
>> or hitting by a blunt object?
>Yes it is like hitting by blunt object
It does NOT look like hitting by metal right? How many spots?
How is the economizer supported? Hung from the top or bottom supported?
Is it possible that during startup and shutdown thermal expansion causes the tube to encounter some other metal? By thermal expansion I mean the movement of the entire backpass as the unit heats up.
Just a possibility.
Remy 21-May-11 (17:47) post about a similar experience.
The tube failures were traced to a heat-treating step in the last stage of fabrication. The tubes were placed on refractory bricks in an annealing furnace to keep them a few inches above the furnace floor. Unfortunately, a metallurgical imperfection occurred everywhere the brick contacted the tube, thus compromising its mechanical integrity!
Now you have heard... the rest of the story!
Regards, Phil Corso