Pass Partitions with ANSI B16.5 flanges (for Section VIII-1 heat exchangers)

[Afox88]

I have been advised by my mechanical engineer that ANSI B16.5 flanges are generally not used for multi-pass heat exchangers as they would not pass Code calculations. After searching through the CODE Book, TEMA and ANSI B16.5, I did not find any reference to dispute or agree with this. I did not find any correlation between pass partitions and flange calculations. I know that ANSI B16.5 are not meant to be run with Appendix 2 calculations and are to be determined using the appropriate temperature/pressure charts (and some of them do not even pass Appendix 2 calculation without pass partitions). Do pass partitions have any affect on determining flange MAWP or is a flange acceptable as long as it falls under the right pressure temperature (accounting for static head of course)?

I have also tried running a heat exchanger with a 12" 150# flange at 140psig/600F in compress. This pressure and temperature is right at the border and obviously did not pass because of static head. It rated the flange for a pressure of ~139.5 psi, which sounds fine. I then made the exchanger 10 pass with tons of pass partitions, and this did not affect the MAWP nor did I get any errors when running the calculation program. This furthers my understanding that pass partitions do not affect Code calculations.

Please advise how pass partitions affect flange calculations (especially B16.5)or if there is any industry standard rule of thumb, or even some sot of equivalent pressure calculation to add on to the design pessure. I used to work for another, much larger, manufacturer and they used ansi flanges for body flanges with pass partitions all the time, with no issues. We are not talking 10 pass, but 4 pass units had ansi flanges all the time.

 

[prex]

You are right in your assumptions: there is no relationship between pass partitions, or just their number, and rating of flanges; and yes, B16.5 flanges are acceptable in Div.1, based on their temperature/pressure rating (using of course the local pressure at the flange, that includes the static head).

prex
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[Bobfromoh]

I think the pass partition "could" affect the MAWP?
Information about the pass partition can be entered
in the heat exchanger software. In the Appendix 2 calcs
for the body flanges, the bolt load from the partition
gasket is included in the flange calculations. This bolt
load can affect the bolt size, the flange thickness and
in turn the MAWP.

 

[fegenbush]

Bobfromoh brings up an interesting angle. Is the question relating to the nozzles connecting the piping to the heat exchanger or the body flanges integral to the heat exchanger construction?

 

[Afox88]

This is with regards to the body flanges. Nozzle flanges do not have pass ribs, except for weird circumstances that I cannot think of.

 

[gr2vessels]

Yes, the pass partition gasket load has to be added to the bolting loads, but also the nozzle loads have to be included, mainly as equivalent pressure loads.
Cheers,
gr2vessels

 

[SnTMan]

Lets focus on B16.5 flanges as body flanges, ignoring external loads. This is OK as far as it goes. B16.5 flanges have been used for body flanges, with and without pass ribs forever, with little if any consideration beyond B16.5 T/P ratings.

Part UHX requires that pass ribs be considered in establishing the required bolt area used in Appendix 2 design. And it is true that B 16.5 flanges often do not make under Appendix 2.

Reference to B16.5 Annex B "Method for Establishing Pressure-Temperature Ratings" shows nothing that would be useful to establishing a flange MAWP that accounts for the presence of pass ribs. For example required bolt area is proportional only to gasket OD and flange pressure class. No way to incorporate gasket properties and dimensions. Flange ratings are (for 300# and higher) are a function of allowable stress and again, pressure class. 150# ratings are a function of temperature only. No way to get at this either.

If you wanted some assurance you could show that the B16.5 flange bolting is adequate for the actual gasket properties and area used, including pass ribs, using Appendix 2 methods. Note that after calculating required bolt area you STOP. If bolting is adequate, you would use the flange at its rated pressure & temperature. Fortunately B16.5 flanges tend to be overbolted for a given design condition as compared to an Appendix 2 design. If marginal, you might go up a class.

Of course your software is going to rate the flange at its B16.5 rating, it's just a look up.

Regards,

Mike

 

[Afox88]

Thank you guys. Ths really clarfies a lot for me.

I ran 2 identical Appendix 2 body flanges with Compress, one having pass partitions and the other not having it. I found that adding pass paritions under reasonable amounts had minimal affects on the MAWP. 1 pass rib on a 12" flange had a 3 pound MAWP reduction at 600F. As such, in the future, I will be just be a bit conservative on my pressure/temperature ratings with pass ribs.