UHX 13.4 Fixed Tubesheet Exchanger and Vacuum condition 1

[OscarAb]

ASME Section VIII, Division I, paragraph 13.4(a)(7), sub-paragraph 2. Indicates; "When vacuum exists, each loading case shall be considered with and without the vacuum."

As the mean metal temperatures (Ts,m and Tt,m) have a significant effect on calculating the tubesheet thickness.

The mean metal temperatures for operation are readily available through the output file on HTRI, HTFS or calculated via TEMA T-4.

My question to the community is; what do you use for the MMT's when a vacuum exists and your reasoning for using these MMT's?

My initial thoughts would be to use the maximum temperature difference between the shell side process temperature and the tube side process temperature as indicated on the heat exchanger specification sheet.

Thank-you,

Trevor

 

[SnTMan]

Trevordw, the software I am familar with will apply any specified vacuum as one of the seven basic UHX cases, or as an additional case(s), both with and without thermal expansion. This occurs at the given MMT's. So, for example, if you are running what I would call a normal operating case, using those MMT's, vacuum will be applied at those MMT's only. So far, so good.

Does this reflect operating reality? Probably not. Shoud the vacuums cases use different MMT's? Probably. But the truth is, nobody but the owner (if even that) knows. Ideally, the process guys would supply ratings data for each unique operating case based on the applicable pressures, flowrates, temperatures, etc. In practice, this just about never happens. So, in my experience as an equipment manufacturer, what you look at is the one or two ratings cases normally supplied at those MMT's. Any specified vacuum is applied as I described above.

One of the programs I use allows the option of entering MMT's (and other data as well) for each of the seven Part UHX cases, or any other specified cases, one does not. So to accomodate other MMT's, other cases must be created and run.

Quote: "My initial thoughts would be to use the maximum temperature difference between the shell side process temperature and the tube side process temperature as indicated on the heat exchanger specification sheet." You could do that. I could not say it is wrong. Neither could I say, in the absence of a ratings case data, that it is right.

As you noted, the MMT's have a significant effect not only on tubesheet thickness but on the the other quantities of interest as well, tube-TS joint load, shell stress tensile and compressive, tube stress tensile and compressive, and so forth. In my experience the MMT's will dominate these calculations, the pressure effects are mostly secondary. For this reason I am reluctant to create cases with larger spreads between the MMT's than the spread between the supplied ratings cases, unless I can find a really, really good reason to do so. Otherwise you get uneeded tube strength welds, tube supports, expansion joints or something like that.

The owners really should supply ratings data for each case they are interested in operating at. Anything else contains, to me, an uacceptable degree of guesswork.

Regards,

Mike

 

[OscarAb]

MIke,

Thank-you for your insight and quick response. I agree with all of your comments, the reason for the question is that our local design survey office requires us to run the cases with the appropriate MMT in order to register the exchanger for a CRN and U-Stamp.

I certainly agree that using a large delta T, results in significant effects in all aspects of the design.

As you are probably aware it can be difficult to get the required information from the owner.

Thanks once again.

Trevor

 

[OscarAb]

Thank-you for your comments Mike and I agree with all of them.

I would only use a max delta T as a starting point to see if the design would be sufficient or would require additional work/material.

Our local design survey requires an appropriate MMT at the vacuum conditions in order to register the design.

I should go back to the owner/end user and ask them "Under what conditions will vacuum exist?" and get the process guys to model the exchanger that way to obtain the appropriate MMT.

Thanks again,

Trevor

 

[SnTMan]

Trevordw, I have generally found it impossible, not merely difficult

Actually, a strict reading of UHX13.4(a) requires that any present vacuum conditions be considered for each loading condition. By this reasoning, the calculations must use the MMT's for whatever case is under consideration.

Your survey office unfairly puts you in the position of having to either get the data from the owner, or make up the data, and in either case it may not represent a realistic operating case. Example, if the owner wants the exchanger designed for some incidentally applied vacuum while not in normal operations, it is still specified in the design data, and so becomes subject to Part UHX.

Unfortunately it seems your survey office is focused on "the letter of the law", rather than common sense

Getting the correct data would certainly be the preferred way to proceed, good luck.

Regards,

Mike