Boiler Stack Steel Material

[jaypack]

We are looking at replacing the top section of a 75foot boiler stack. The stack is a self supporting stack and has a 3.5ft inner diameter at the last 50ft.

This section was just replaced 12 years ago. It was 7/16" thick A285 Gr C carbon steel. It is eaten away by the flue gas from the boiler. We have two quotes to replace the stack with A516 Gr 70 and A36 carbon steel.

Is there a particular steel that would be better suited to handle the acid from the flue gas that stack designers know about or is this the normal life of steel boiler stacks?

 

[metengr]

You might want to look at Corten steel, as an option. This is a steel designed for outdoor use, a weathering grade of steel. I would not use A 36 for this application. The SA 516 Grade 70 steel plate is acceptable. You could look at an option of installing Cr-Ni-Fe alloy steel liners to prevent exposure to corrosive flue gases, and extend the life of the stack metal.

Is the stack located in a cold weather climate where the stack metal temperature could be exposed to temperatures below -20 deg F (in the event of no flue gas during boiler outages)? If so, you need to assure adequate impact properties for the steel plate.

 

[Carburize]

I don't think Corten would help against the flue gas corrosion but the nickel base alloy liner sleeve is a good suggestion.
I have never found any record of brittle fracture in a stack even under severe low ambient temperature condtions- I don't think low temperature properties need to be considered for this type of application.

 

[metengr]

Carburize;
We had two metal stacks collapse in the 1950's (well before my time) at two (one is now retired) of our power stations in the Midwest. We have two metal stacks at a third station that require stack heaters during winter outages.

 

[Carburize]

Really - if you have any kind of literature reference I would be very interested to add these to my database.

 

[JStephen]

Note that A36 or A516-70 would normally be used for availability or strength or ductility reasons, and aren't normally considered any more corrosion resistant than other carbon steels.

Could you be getting condensate in the stack, where insulating that top part might help? Just a thought.

 

[metengr]

Carburize;
I have at least one copy of one of the actual reports, which I can retrieve. I am going on memory here but the steel used at that time was rolled and welded A7 steel plate, which is the predecessor to A 36. The stack collapsed on one of the coldest days in winter with a brisk wind, while the unit was off on a scheduled outage. The failure was attributed to brittle fracture from one of the shell course welds. The other stack failure occurred under similar conditions at a different site, and time.

I can make a copy and send it to you. I am not sure how to get in touch with you thru this forum without having any nasty messages regarding email addresses. Any ideas?

 

[rjoaks]

Corten is the best material. The only problem is that it will bleed and discolor any concrete foundations. Hi temp. paint will help to control the problem

 

[taffcad]

Just some thoughts of the top of my head. What is the sulphur content of your fuel? Can this be reduced. We ran for many years on <3.5% content with only one problem when some clown in purchasing thought they could save some money. 4.5% was purchased and we ended up with the back end of 2nd pass tubes being coked up and reduced bore of 50% causing chamber pressure problems and enforced cleaning. Are you using economisers, perhaps during cold weather you could divert straight to stack to increase temp which could limit corrosion problem.

 

[matay]

Jaypack.
I think before choosing material you should know the exhausht gas temperature and content of exhaust gas.Then you can choose materials.If acid formation occurs with SA285GrC the same thing occur with the SA516Gr70.If So2 content of flue gas is quite high, maybe you should try SA240 series stainless steels depending on the gas content.You can use 304TP or for worse conditions 316Ti.

 

[Carburize]

metenrg - I have e-mailed the forum web-masters to see if there is any way they can provide an upload area where reference articles, images etc could be deposited for a few days to enable additional exchange of information. We will see what happens.

 

[yates]

Matay has it. Before you choose materials, you need to know exhaust gas composition, flue gas temperature and thermal cycle. If sulphur levels can be kept low, Corten is an excellent contender. Flue gas temperatures should not be a problem with Corten - I have installed Corten stacks on Gas turbine exhausts, running at about 500°C, however, the stack was designed as a series of interlocking truncated inverted cones which allowed ambiant air to enter flue through an annular space, drawn in by the pressure drop from high-velocity flue gases. So, resulting temp. was around 220-240°C. Corten develops some degree of corrosion resistance by forming an adherant oxide layer. But this oxide layer will shake off unfortunately if the stack is subjected to too much vibration. If the stack is going to have a daily shutdown (like a peaking gas turbine) successive condensation cycles will force you to go for a stainless steel stack anyway, remarks for Corten are only valid at constant operating temperature.

 

[DowneastTech]

A steel plate specification that might be worth considering for your stack is ASTM A736 grade B. This material has great low temperature properties, high yield strength and is a P1 steel. It welds like melting butter. However, it is not any more resistant to corrosion than any other low alloy carbon steel. Perhaps Lukens can supply it with a nickel or other cladding that would resist the flue gas.

Regards,

DowneastTech

DowneastTech
Mechanical Engineer
Magnus R & D, Cypress, TX