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Related Articles


What gases go through a CT stack?

What gases go through a CT stack?

What gases go through a CT stack?

I like to know if anybody knows what are gases or chemical go through a CT stack in a power plant.

Thank you

RE: What gases go through a CT stack?

SlideRuleEra (Structural)
Thank you for response. Since you are Structural, I wanted to ask you some direction. I am frustrated since I cannot find much useful information on design bases/criteria for Steel liners in CT Steel stacks.

I been asked to assist with repair of some cracks in a CT Steel stack liner.
The liner is hung from mostly lower part of stack.

I like to know what loads the liner sees and what these liners typically design for? How they come up with the thickness for these liners?

Any document, note, paper or information greatly will be appreciated it.
Also I like to know how the build these, are they welding them in field
Stack is almost 250ft

Thank you in advance for any help,

RE: What gases go through a CT stack?

The liner is hung at one elevation so that there is no restraint to thermal expansion. The liner "grows" up above the support elevation and down below the support elevation. If the chimney shell encloses the liner the only forces on the liner are it's own weight plus the weight of ductwork connected to and supported by the liner. Thickness is based on this weight plus a generous corrosion allowance since flue gas is corrosive. There may be an additional allowance for accumulation of any waste products on the inside of the liner or supported ductwork.

Buy the ASTM standard JStephen recommended in your other thread on this subject.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: What gases go through a CT stack?

SlideRuleEra (Structural)
Again Thank you for the direction and information.
Yes, I have already bought the ASME STS-1 and read the Liner section.

The liner of the project I am working is stainless steel and liner is circular.
So for the cracks on the liner, is patching an acceptable method? do I weld inside the crack and call it off? What are typical repair methods that is common industry practice?

I really like to learn it well.


RE: What gases go through a CT stack?

SlideRuleEra (Structural),
Do you know what's a "wet stack"? What stacks to be consider a "wet stack"? What's the characteristic of wet stack in term of liner design?


RE: What gases go through a CT stack?

CO2, SO2, SO3 etc. pass thru the stack. They form weak acids with H2O that may cause cold end corrosion. The stack top temperature is generally maintained above the dew point of water (generally 110C) so that condensation does not take place.
Cracking in the SS liner is quite rare and could be due to the following:

a. If it is a very old stack (approx. 10-15 years) then the SS has got sensitized.Hence, it has become prone to acidic corrosion.

b. If it is a new one, then poor workmanship or defective material could be the cause of liner cracking.

You may go for metallography testing of the liner to assess its condition. Otherwise, any repair would be temporary!

RE: What gases go through a CT stack?

Dhurjati Sen (Materials),

RE: What gases go through a CT stack?

STrP88 - Many fossil fuel coal fired power plants use a water-based flue gas desulfurization (FGD) system to remove SOx compounds. This is done just before the flue gas goes to the chimney. The flue gas is cooler and damper than it should be. In the past, this "wet" flue gas would be reheated before being sent to the chimney. Now, to save reheat energy costs, reheat is omitted. Therefore, a "wet" stack.

Quote (STrP88)

What's the characteristic of wet stack in term of liner design?

A complex compromise of variables best addressed by a specialist in this field.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: What gases go through a CT stack?

Hmmmn. SlideRuleEra, check your assumptions for stack gasses coming from a fossil fueled power plant vs the original question - which to my ear seems to be limited to a CT (combustion turbine) which is natural gas fired. Thus, I would expect very, very little stack gas chemicals other than the CO2 and H2O from the methane combustion, plus NOx.

On the CT exhausts that I've had to repair, biggest problems are stress risers from too rapid heatup/cooldown in the intermediate stainless steel transitions between turbine exhaust and the HRSG inlets, usually upstream of the vibration separator (gasket). Unfortunately, rarely successful repairs.

RE: What gases go through a CT stack?

Dear racookpe1978, your reply was very nice and appropriate and you might have hit on the nail.
Still there could be a few other alternatives that come to my mind.

Normally the CT stack would not be operated and the intermediate damper would remain closed as the flue gas passes thru the HRSG to extract the waste heat and produce steam. Only when the waste heat boiler is down then the guillotine damper is closed and the CT stack is in use. So, the nature of the cracks should be analysed and correctly inferred as it could be a case of environmental cracking even !!!

Also, there are many GTs that are dual fired, operating on NG & Naphtha / Fuel Oil as per availability and economics. So the possibility of SO2, SO3 etc can't be overruled.

Last of all I must say that based on the replies, StrP88 may like to share a little more data to come to a logical conclusion of the problem that he is facing.

I thank once again racookpe1978 and SlideRuleEra for their interesting inputs and making this forum a fascinating one.

RE: What gases go through a CT stack?

RA - Yes, I should have been more specific in answering the question about "wet stacks". Wet stacks apply exclusively (to my knowledge) to coal fired electric generating units. I'll edit that response to remove confusion. Although I doubt it will help with current project, the OP can read the Power Magazine article "Designing Wet-Duct/Stack Systems For Coal-Fired Plants".

As Dhurjati Sen kindly pointed out, combustion turbines can be either gas fired or oil fired. We have both in our generation mix, 6 gas fired units and 8 oil fired. None of them have wet stacks.

Six of our coal-fired units have wet scrubbers (water-based FGD systems). I believe four of these have wet stacks.

The OP obviously has limited knowledge and no experience on this type project. What I did try to do is guide the OP to a solution that would accomplish three things:

1. Solve the problem by hiring a qualified Consultant (such as Pullman). I expect an engineered solution, not a "best guess" is what the Client wants.

2. The OP can function as a technical liaison between the Client, the Consultant, and a Contractor performing the work. (Thereby earning his fee.)

3. The OP will be in a position to learn plenty... from a Consultant who knows what they are doing. (The OP says learning is one of his goals).

Unfortunately, the OP does not appear interested in this route.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: What gases go through a CT stack?

One issue that I have seen in such CT plants is that when the HRSG is operated the alloy the by-pass stack to get cold. And if the plant is near the sea this means salt laden fog every night. And then when you use it and it gets warm CSCC is a very real possibility. Know of a plant that lost one this way, they replaced the liner and bled a little steam to keep it warm. Then it turned out that the Cl level of the exhaust gas (because of Cl in the intake air) was high enough so that if they got any condensation they still got cracking.
(yes, eventually they had cracking in both CT and ST from the salts)

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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