ASME Sect. VIII Thickness Calcs.

By Mikeqc1 (****)

Date 03-19-2009 16:43

TRY HERE
http://www.onetb.com/forum.htm

MDK

By Arctic 510 (**)

Date 03-20-2009 01:01

Thanks! I appreciate it!

By G.S.Crisi (****)

Date 03-19-2009 20:58

A fin fan header box contains too low a pressure to be taken as a pressure vessel. ASME VIII considers as a pressure vessel that which is subject to an internal pressure of 15 psig minimum (it was 10 psig until a few years ago) whereas the air pressure inside a fin fan box is measured in inches on water column. What's your case? 20 - 30 inches of water column? Too low a pressure to use the formula for flat heads subject to internal pressure.
Fin fan boxes wall thickness is not calculated, it's adopted based on good engineering sense. Do you have the chance of visiting an industrial plant where there are fin fans installed? Go there and measure the wall thickness yourself. If not, as I said before, use your good engineering sense and figure out the thickness yourself. How much will it be? 1/8 inches? Or perhaps 3/16? It won't be too far from those.
Giovanni S. Crisi
Sao Paulo - Brazil

By Arctic 510 (**)

Date 03-20-2009 01:00 Edited 03-20-2009 01:04

We have 20-30 fin fan banks here. These ARE section 8 vessels, MAWP 450 psi. Again, they ARE actual "pressure vessels" as defined by section 8.

"Good engineering sense" (is that an oxymoron?-KIDDING!!! :-) I'm just an inspector, I don't have any of that...) won't help me determine a remaining life! I HAVE measured the walls myself, I am looking for t-required. Minimum required thickness. What I need to hold the ASME section 8 rated 450 psi MAWP, as stamped on the ASME data plate and the U-1 form.

Is there a formula? Appendix 13 applies here, but I can't find a formula for minimum thickness for internal pressure.

By G.S.Crisi (****)

Date 03-20-2009 17:20 Edited 03-20-2009 19:46

The only fin fans I'm used to (and I've erected quite a lot back in my days of erector engineer) are those installed in oil refineries and petrochemical plants. They consist of a steel frame with a bundle of finned tubes inside and a fan on top (induced draft) or in the bottom (forced draft). The fan sucks atmospheric air and pushes it across the tubes bundle, thus cooling the fluid which is inside them. That's why I said that the pressure inside the frame could be 20 - 30 inches of water column.
You're talking of a fin fan type I don't know. Would you describe it to me, so I can learn something new? Thankyou
Giovanni S. Crisi

P.S. By the way, what does "oxymoron" mean?

By Arctic 510 (**)

Date 03-23-2009 18:16

AN oxymoron is a statement that contradicts itself. (Like in my joke, an engineer having good sense is a statement that contradicts itself.) In my plant, engineers and inspectors banter back and forth with little insults like that. Welders especially give engineers a hard time. I really mean absolutely no disrespect to engineers! I'm glad I don't have to do all that math!!!

Our fin fans (I am in a pertrochem refinery) are forced draft type, with a header box at either end and finned tubes running between them. Here is maybe where our misunderstanding was... I was speaking of the internal pressure present inside the tubes and header boxes, not the pressure exerted on the bundle by the air inside the frame.

I never though of the air exerting its own pressure on the bundle, although that makes perfect sense. That's interesting to think about!

By G.S.Crisi (****)

Date 03-24-2009 00:37

Obeamweldor,
It's my fault. On your first posting you wrote "fin fan HEADER box" and I mislooked the word "Header". So, I thought you were talking about the pressure inside the steel frame crossed by the air draft. Here, the pressure very seldom is higher than, say, 20 inches of water column. Such a small pressure is negligible for the tubes and headers, so forget about it.
Now, let's go back to the headers you were actually talking about. The header consists of several flat faces, with one of them having the tubes attached to it. ASME VIII doesn't cover such kind of flat face. However, a shell and tube heat exchanger has a tubesheet that is exactly a flat plate with a bundle of tubes attached. Shell and tube heat exchangers are designed to TEMA standard (Tubular Exchangers Manufacturers' Association). Why don't you use TEMA to calculate your header box?
Giovanni S. Crisi

By Arctic 510 (**)

Date 03-24-2009 17:13

I've never looked into TEMA. I will, and thanks for the recource! I think Appendix 13 of ASME VIII covers these, but I may be mistaken. One way or the other, I can't find any t-required formulas in there!

By G.S.Crisi (****)

Date 03-26-2009 15:02

There are three classes of shell and tube heat exchangers coverd by TEMA.
Class R refers to heat exchangers suitable for the severe duty usually found in oil refineries and petrochemical plants.
Class B refers to heat exchangers suitable for the less severe duty usually found in process plants (chemical, pulp and paper and the like).
Class C refers to heat exchangers used for general purposes.
You choose the most adequate for your case.
Giovanni S. Crisi