Tubesheet subject to thermal loads in Section I boiler

[pietkoning]

When I have a tubesheet in a boiler, it will be subject to both pressure loads and thermal loads. Pressure loads are well covereed in Section I by applying rules for flat plates and ligaments. However, there don't seem rules to cover thermal loads on the tubesheet.

Consider the following very standard case: a cylindrical shell closed with two heads forms the boiler vessel. Tubes run from one side to the other side of the shell and connect to tubesheets at both sides. Simply said, the boiler has the layout of a Fixed Tubesheet Heat Exchanger as defined by Figure UHX-3 (a) in Section VIII div. 1. Since the shellside fluid and tubeside fluid are at different temperatures, the expansion of tubes and shells is very different. This imposes large loads on the tubesheet, the tube-to-tubesheet welds and the tubesheet attachment. However, I don't see rules in ASME Section I to cover this.

Comparing with Section VIII, in UHX-15.5.5 you account for the axial differential expansion of shell and tubes by computation of the parameter gamma. Since in a steam boiler it is all about temperatures, I would expect a calculation that accounts for differential expansion, but can't find it.

How is the differential thermal expansion covered by Section I? And why does Section VIII account for the thermal load on the tubesheet and its attachment, while Section I does not?

Thanks for your comments and response.

 

[pietkoning]

After doing some more reading, it seems to me that everything that is not covered by Section I, like the thermal loads on the tubesheet, is covered by PG-16. This paragraph tells that when design rules are not given in Section I, you have to come up with an appropriate analysis method yourself.

In practice for a tubesheet, I interpret this as using Part UHX from Section VIII div. 1, since it covers the thermal loads on the tubesheet well.

Agree or disgree with this?

 

[Fizza453]

I agree with Pietkoning, same is written in second paragraph of Preamble of Section I in addition to PG-16.
I will add, please see UHX-10 that are general conditions of Applicability, you can use UHX rules if UHX-10 is satisfied. When rules are not given in Section I you can use Section VIII-1 rules subject to your AI satisfaction.

Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[marty007]

One detail to consider when it comes to a Section I fire-tube boiler is the magnitude of any differential thermal expansion.

One of the nice properties of water is that the boiling heat transfer coefficient is very very high. Compare this to the heat transfer coefficient of a gas inside a tube, which could be a combination of convection and radiation depending on your temperature. In many cases with a Section 1 fire-tube boiler, the boiling shellside heat transfer will be orders of magnitude higher than the tubeside gas heat transfer coefficient. In this situation, the differential thermal expansion will be minimal, and the impact on tubesheet thickness will be minimal.

I don't know the details of your design, so please do your own due diligence, but this is something to keep in mind.

 

[pietkoning]

Thanks for your contributions Fizza453 and marty007.

Especially the point made about the heat transfer coefficients is a good point. Indeed, the tubes will have a temperature close to the shell and differential expansion will be minimal. I'm curious to find out the maximum temperature difference between, but that remains to be seen.

 

[GenB]

The boilers will only be exposed when unsafe conditions and on shut downs, A lot of this is left to the firing, level controls, unsafe conditions as shut downs, and to the operators. It does not happen with Sect VIII-1 Hxs'