SA 234 in firetube boiler - why not? 3

[RNDguy]

In Section II, A-234 grade WPB (being a carbon steel wrought short elbow) has an attached note G18. This note states that for Section I firetube boiler parts under external pressure, this fitting "may not" be used. My questions are:
1. Does this mean use of this material is prohibited under these circumstances, or is it left to the judgement of the designer?
2. What is the reason for this qualification (creep buckling maybe) ?
3. And where on earth would someone want to use a short elbow like this inside a firetube boiler?
Thanks in advance.

 

[GenB]

Is it curiosity?
If on earth you do not need it, well there is the answer.
May means you can under a variety of conditions. I do not know which ones.
ER

 

[RNDguy]

Thanks Generalblr. No it is not curiosity alone. I have an application of a steam jacketed kettle fired by natural gas with flue tubes (which is where the elbow is needed) passing thru steam jacket. Such a kettle can be built under Section VIII, there is an appendix for fired steam jacketed kettles. First temptation is to use SA-234 WPB elbow, the heat flux is not high, average (and I stress avg.) metal temperatures on flue tubes are expected between 400 to 450 degrees F range based on preliminary calcs. But then there is this Section II cautionary note. So I need to know more about the reasons/background of this note from ASME. Again, why did they put this note for an elbow (I personally have never seen any elbow within a firetube boiler)? I need some help here, thanks in advance to any one who can throw some light.

 

[deanc]

Where would one use an elbow for external pressure? The thing that comes to mind would be the "dry pipe" after the trottle in a steam locomotive. SA-234 parts are designed and tested for external pressure only,per the spec.

http://www.watercressline.co.uk/index.htm

Then go to the index,then to the"footplate courses" This should tell you how it all works.

 

[deanc]

TGIF:

Note for SA-234 that should be "internal" not external.

The English folks call the trottle valve a "regulator" which
may be more correct,depending on how you view it.

 

[RNDguy]

Thanks deanc. Still leaves me with the original problem as to the reasons behind the cautionary note against using this fitting in a firetube boiler in parts which are under external pressure. Does any one know for sure if this could be due to buckling danger under creep?

 

[17021978]

HI. I work in a power station with vapor on a project of determination of the thermal impact on a coal-fired boiler under pressure (150bar, 342°C). then, I wish knowledge how the heat flux is distributed on the walls and radiant surface overall in a coal-fired boiler also I want to know how to calculate the heat flux on the level of the hearth, the convection coefficient of the water-steam emulsion under these conditions of temperature and pressure, and the curve of variation of the thermal conductivity of carbon steel according to the température.that's very important.thanks in advance.

 

[unclesyd]

17021978,

Here is a good start on your quest.

http://www.babcock.com/pgg/tt/steambook.html

 

[GenB]

Sec I firetube min temp @ 700 deg F, regarless
of expectated less, so your fitting is out of range.
If you are allowed under sec VIII-1 unfired pv, you may be able to use it.
or why not forged steel?
'hope that helps. let us know.
ER

 

[RNDguy]

Hi Generalblr,
Your tips are greatly appreciated. Kind of supports my intuitive ideas about this. Since metal temps. are not high in my application, and since Sec VIII allows use of SA 234, I may go ahead with this in design. Thanks very much.

 

[RNDguy]

Hi 17021978,
In response to your earlier question, this site may be of use (although it is for fired heaters, but engineering principles remain same):

http://www.heaterdesign.com

Hope this will help you. Regards.