Gas turbine Exhaust Duct Material

[waliq]

We want to install a duct at the outlet of Gas turbine. The temperature of Exhaust gases leaving the GT is 540 Deg C. There is also a duct burner installed, which when operated, raises the temperature of gases to 750 Deg C. Design pressure for this duct is 300 mm WC

We have asked the vendor if the following material is suitable for this duct

Outer surface : A36
Insulation : Rock wool (just beneath A 36 base metal) : thickness 120 mm
Insulation : Fiber mat (just beneath Rock wool insulation) : thickness 120 mm
SS 304 lining sheet, thickness 2 mm (inner side directly exposed to gases)

However, the vendor is saying that this is not possible as there is difference in thermal expansion of outer CS plate as lower temperature and inner SS place at higher temperature, which can damage the holding bolts/brackets for insulation arrangement and can distort the inner SS plate.

Now, I think that vendor's explanation is not applicable since SA 240 304 plate can be used till a temperature of 825 Deg C as per ASME Sec II, Part D.

It is requested to kindly advise if vendor's explanation is correct or not

 

[SwinnyGG]

The vendor is correct.

The stainless steel liner will get very hot - it will approach or equal the temperature of the gas - while the carbon steel shell will reach equilibrium at some much lower temperature. The difference in thermal expansion between the shell and the liner will break things.

 

[waliq]

@swinyy GG: Can you suggest an alternative? What is we use inconel alloy lining instead of SS?

Refractory is another option. However, the duct has a conical profile with flat bottom and hence refractory application is a difficult method here

 

[GD2]

Why don't you ask the turbine vendor for ducting construction technique? Ducting of exhaust is very common in turbines.

GDD
Canada

 

[waliq]

@GD2

Actually, the turbine was installed a number of years ago. Therefore, we do not have access to vendor now.

 

[GD2]

Waliq,
It shouldn’t matter when the turbine was installed. If you have the manufacturer name and model you should be able to drill down to their technical support. Just make sure that you approach the right company ( many companies have consolidated) over the years. The turbine industry is very common.
Else contact Solar Turbines, a caterpillar company, for help on the ducting. They do this a lot in gas compressor stations.

GDD
Canada

 

[Dhurjati Sen]

The duct burner is seldom used. I have seen SS 410 plates being used instead of SS 304, although they do get a little bent over the years.

DHURJATI SEN
Kolkata, India

 

[LittleInch]

A lot depends on how long the ducting is as to what the actual end variance in thermal expansion is.

How are you coping with thermal expansion in any event?

But you should be able to decouple the two and maybe need to use something other than rockwool or all the two elements to slide over each other.

The outer sleeve can be set up as sliding sections if needed, but is this duct 10m long or 100m long?

Any bends / elbows?

Is this exposed to weather or all inside / covered?

It depends what you've told the vendor. If they don't know the length then yes, they are correct, but clearly not being very helpful or coming up with any alternatives. Try another more helpful vendor?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

Often the liners are installed in sections overlapping each other.
They are only rigidly anchored at one end and allowed to slip at the other.
I would rather see 410 or 430 used for a couple of reasons.
First is lower thermal expansion.
Second is resistance to SCC.
When turbines are running at low power or are idle there can be a lot of condensation in the duct and lots of things can happen.

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P.E. Metallurgy, consulting work welcomed

 

[waliq]

@Ed stainless: You mean SS 410 or 430?. Also, I guess if we can provide slotted holes on the inner side, should it be helpful?

@Littleinch: The duct is 3 meters in length

 

[GD2]

Waliq,
Providing slots is a good idea for the liner thermal expansion, so that the hold down bolts are not bent/broken.

GDD
Canada

 

[SwinnyGG]

You want the liner to be continuous.. unless you want to pump 1500 degree combustion gasses into the insulation.

 

[EdStainless]

I have seen these done in sections but there is a lot of overlap between them.
This is how they allow for thermal expansion.
Designing these for continuous use is easy.
Designing ones that will survive heavy cycling is a very different matter.

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

 

[FacEngrPE]

Your vendor is correct about differential thermal expansion between the inner and outer casing creating problems for your design. How much is all in the construction detailing.

For your case inner duct 0c to 750c thermal expansion allowance for 304 is about 0.13 m per 10m, defiantly not negligible.

A web search for "gas turbine exhaust duct" finds a number of companies making a living in this market.