Shell and Tube External MAWP

[posershadow]

I've noticed quite a few U-stamped shell and tube heat exchangers designed to BPVC.VIII.1-2017 that list the external MAWP of the inner chamber (tubeside) as either N/A or 15 psig on the U-1 form (line 16). I've seen it across the board with a wide range of shellside and tubeside design pressures. In addition, these heat exchangers were not manufactured by small no name manufacturers, in all cases these are TEMA members supplying these.

I figured I'd take one heat exchanger as an example:
Shellside internal design pressure 1100 psig, internal MAWP 1114 psig, external MAWP 15 psig and normal operating pressure of 750 psig
Tubeside internal design pressure 200 psig, internal MAWP 431 psig, external MAWP 15 psig and normal operating pressure of 50 psig

Is it acceptable to list the tubeside external MAWP as 15 psig? Doesn't that completely go against UG-28(e)? If this is permissible, what section of Code do I need to be looking at?

Lets say I flip the shellside and tubeside pressures in the example above. Now the tubeside normal operating pressure is much higher than the shellside. In that case I see many manufacturers listing N/A for the tubeside external MAWP. Is that permissible? I suppose it could be... so, the way I'm interpreting that is that under no circumstance am I allowed to let the tubeside pressure drop below the shellside pressure. We have instances where the tubeside might be depressurized due to some maintenance on surrounding equipment but the shellside remains pressurized. That scenario seems to be unacceptable in the eyes of UG-28(e) if the tubeside external MAWP is N/A.

 

[SnTMan]

posershadow, I am kind of picking thru your post, trying to answer as I go along, so...

I think your misconception is to consider the tubeside to be completely enclosed within the shellside. This is true for the tubes themselves and for the floating tubesheet and floating head if present. It is not true for the stationary tubesheet nor for the (normally external) tubeside chamber called the channel, which introduces tubeside flow from the piping into the tubes.

Keep in mind that the stated MAWP, internal or external, is the lowest of the differential pressures calculated for the various components.

So, for your example exchanger, and 1) ignoring operating pressures and 2) assuming full vacuum is a stated design case for both sides:

Shell cylinder assembly (includes cylinders, heads, nozzle reinforcement, etc.): MAWP, Internal, 1114 psig as you stated. As no external, differential pressure greater than 15 psig (full vacuum applied within the shell) can be imposed, MAWP, External is therefore 15 psig.

The common elements (tubes, tubesheets and floating head) must be designed for the greatest differential pressure that can be imposed upon them so for example the tubes must have an MAWP, Internal of at least 431+15 (446) psig, and MAWP External of at least 1114+15 (1129) psig.

The external component channel assembly however is in the same situation as the shell assembly. The maximum external differential pressure that can be imposed on it is again, 15 psig, full vacuum.

The lowest values then govern the MAWP's. It is true that either the shell assy, the channel assy or both may have a calculated MAWP, External higher than full vacuum, but since it is not possible for any higher pressure to be imposed, by convention the MAWP is simply taken as FV.

Hope you find this helpful,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[posershadow]

I agree that the channel and/or front head plus nozzles for the tubebundle are external to the shell, but... the code says exactly the opposite regarding differential pressures.

UG-28(e): The external design pressure or maximum allowable external working pressure shall not be less than the maximum expected difference in operating pressure that may exist between the outside and the inside of the vessel at any time.

UG-21: Each element of a pressure vessel shall be designed for at least the most severe condition of coincident pressure (including coincident static head in the operating position) and temperature expected in normal operation. For this condition, the maximum difference in pressure between the inside and outside of a vessel, or between any two chambers of a combination unit, shall be considered.

Regardless, back to my original question, I see what you're saying. Items 14 through 18 are not applicable to the tubes or tubesheet. So if that's the case, then where are my internal or external MAWPs of my tubes and tubesheet reported? Can't be item 16, it doesn't apply to those components. See the conundrum now? Maybe this is just a problem with the U-1 form and the way it's formatted. It would appear that we have components without any reported MAWPs. I have checked the remarks section (item 22) and there is nothing mentioned there either. I feel like the U-1 form could do a better job of displaying this, especially for something commonplace like a shell and tube, so that you aren't left to dig through pages of calculations trying to guess what the internal and external MAWPs are for the tubes and tubesheet.

 

[SnTMan]

posershadow, I don't see that UG-21, UG-28 are not complied with. The U-1 is set up to report MAWP's on a per chamber basis, not per component. See also UG-19 on combination units and so forth.

As far as finding the per-component MAWP, the calculation report is about all you got, although some software does compile it in one place. Many years ago when a shell and tube calculation package was 10 or 12 pages, it was not much of a burden. With the advent of "modern" software and packages of 50 to 100 pages, not so easy anymore

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[posershadow]

I think we might be on the same track now.

The reason I mentioned looking in the remarks (item 22) was because I was also looking at UG-19(a)(2). What I didn't realize is that UHX-19.3.1 is different than UG-120(b)(1)(-a). UHX-19.3.1 only requires a remark if the differential pressure design is less severe than the design conditions for each adjacent chamber. UG-120(b)(1)(-a) requires a remark for differential pressure regardless. The same is true for UHX-19.2.1(a) versus UG-116(j)(1)(-a) for the vessel marking.

Wouldn't it be better to align both UHX and UG so that differential design pressure remark/marking is always a requirement? I thought UG was applicable to all pressure vessels (UG-1). Seems a bit strange to me if UG-120(b)(1)(-a) and UG-116(j)(1)(-a) don't apply to shell and tube pressure vessels. Neither of those explicitly mention UHX.

 

[SnTMan]

posershadow, I think you are kind of splitting hairs here. Design of a common element is not considered a differential pressure design unless the (differential) design pressure is less than the gage pressure of the higher of the two chambers. In that case differential pressure marking is not required.

Having said that, it does seem the Code language could perhaps be streamlined. This wouldn't be the only such opportunity

The problem with sloppy work is that the supply FAR EXCEEDS the demand