Carbon steel tubes with Aluminum fins 1

[2cool4school]

Good day All,

I am currently installing a HRSG boiler on a CHP plant. The designer opted for SS304L as the material of construction. Due to the high chloride levels in the water we decided against SS as it is prone to SCC.

When we asked the designer to switch to CS (A106 B) the claim is that aluminum fins cannot be used now due to the PWHT required on the tube to tubesheet welds. This is according to ASME BPVC SECTION VIII Part 1 UCS-56-1.

The lack of fins is increasing the area required for exchange by 230%

Is the designer interpreting the code correctly?

Tubesheet 25 mm
Tube diameter 25 mm

Thanks in advance

 

[SnTMan]

2cool4school, for the cited dimensions UCS-56 would not appear to require PWHT for P1 materials. Is it possible there is a service requirement for it?

I'm surprised that welded fins are not specified. If the alum fin is simply wrapped with no other bond I'd think subjecting the tube / fin assy to PWHT temps is likely not a good idea. My opinion only...

But then, what are the design temps?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[MJCronin]

Wrapped aluminum fins are only suitable for low temperature air-conditioning applications.

There are a few different extended surface tube designs available in the commercial market. See this document:

https://www.tps-technitube.com/wp-content/uploads/2017/05/inosindt_pdf_extended_surface_tubes_3.pdf

Robust welded carbon steel fins are the best choice for extreme and demanding HRSG applications

https://sciendo.com/pdf/10.2478/adms-2013-0009

If the client is insisting on aluminum fins and this is the first HRSG you have ever designed, this sounds like a disaster in the making

If your designer was going to use 304SS for the entire HRSG, this also makes me concerned ....

You say your HRSG will see high chloride levels ??...... WOW ...

Give us more details

MJCronin
Sr. Process Engineer

 

[weldstan]

Everything in this design appears crazy. Why SA-106B, a piping spec, instead of a tube spec? Al fins on carbon steel tubes also never seen before in the hundreds of HRSG units that we constructed over 20 years. Why 304L stainless steel originally specified? Again never seen before in finned tube sections.

 

[2cool4school]

Thank you for the this intriguing feedback.

The flue gas is around 450 Deg C.
The client (me) requires a HRSG capable of producing saturated steam at 16 barg.

The feedwater is around 130 Deg at 17 barg.

Designer proposed a SS unit with AL fins. I immediately flagged this design choice as the water had 71 ppm of Chlorides present. After multiple discussions the designer says they will switch to CS (I will confirm the exact material) but they cannot use AL fins because PWHT will be required as per code. The code requires temp of 595 Deg C.


The question I want to pose back to the design team is:

1.Why extruded AL fins and not welded CS fins?
2. Why do you require PWHT? Are they interpreting the code correctly?

 

[SnTMan]

2cool4school, 1) no idea, "economics" perhaps. 2) PWHT possibly required by UW-2(c) - unfired steam boiler?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

Typically plain C or a low Cr steel in the hot end of the HRSG and then higher alloys in the cold end.
I have seen these range from 5%Cr to the 9% and 13%Cr alloys, 2205, 304, and few other things.
And the temp drops you begin running into acid condensation temps (well above water condensation) and you need more robust materials.
If your cooling water is high in Cl then you do have to be careful with alloy selection and water treatment.
The hottest section often does not have fins, but as the temp drops they do use fins.
The fins are nearly always welded on and either match the tube alloy or similar (I have seen 5%Cr fins put onto 13%CR tubes).
PWHT issues are usually handled within each header section.
Often the lower temp section has no PWHT, it is joined by using transition pieces.

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P.E. Metallurgy, consulting work welcomed