Limiting MAWP 1

[hplogger]

Is it still acceptable that when considering all the MAWP per UG-98, the limiting vessel component is the nozzle based on the limiting MAWP computed? Please advise. Thank you in advance.

 

[gr2vessels]

No, this is a preconception. The available software this days is calculating the MAWP for each element of the vessel and is selecting the weakest link. You could have a flange limiting the vessel MAWP or other component, or you can select one item and design it as the weakest link. However, most of the fabricators wouldn't bother with vessel analysis, select the design pressure as MAWP and be done with it. Long ago, my stu..id boss made me calculate all the MAWP (manually)...
cheers,
gr2vessels

 

[hplogger]

Thank you very much, Sir for the advise.

 

[vesselfab]

The limiting factor can not be the nozzle reinforcement, but can be the pressure temperature rating of flange.

yes, can remember figuring MAWP by hand, but software has been available for 25 or so years.

 

[SnTMan]

hplogger, from a Code standpoint there are no restrictions on which component limits MAWP.

Your contract may be another matter however. Recommend you study it closely.

Regards,

Mike

 

[TomBarsh]

I agree with SnTMan, there are no ASME Code restrictions (in Section VIII Div 1) that a nozzle or other "minor" part may not have the smallest MAWP of all components in the vessel.

However, it is common that vessel specifications from vessel owner, EPC firm, etc, do not allow a nozzle to govern the vessel MAWP.

So although permitted by Code there may be commercial restrictions.

 

[vesselfab]

yes, the code doesn't mention it, but I can say, I have never seen a well designed vessel limited by a nozzle neck or it's reinforcement. Pressure temp rating of a bottom flange..yes.

 

[moltenmetal]

"I have never seen a well-designed vessel limited by a nozzle neck or its reinforcement"

That's a bit of a value judgment, wouldn't you say?

Some folks design to have the flanges limit the MAWP always- that's their perogative. Some permit the shell, a nozzle or ANY part to limit. If the design pressure, relief pressure and corrosion allowance are specified correctly and the MAWP of each component is calculated correctly, what's wrong with having some part other than the flanges determine the MAWP in the hot and corroded condition?

It does get my goat when a vessel designer/fabricator sets the stamped MAWP equal to the design pressure instead of the actual MAWP of the limiting component. Guess it gives them business in re-rating vessels later...

 

[vesselfab]

I'm sorry, thought we were speaking about nozzles.

Normally a head or shell or transition or some major component would limit....except in the case of flange pressure temperature ratings.

as I said before....p/t ratings yes

nozzle neck or it's reinforcement no

if that is what is called a value judgment, then I guess it is.

If I want to give my client good value for his big dollars invested in capital equipment, I would NEVER limit a vessels mawp by a nozzle neck or it's reinforcement. I would consider that very bad design on my part.

 

[DG72]

Quoting vesselfab: "If I want to give my client good value for his big dollars invested in capital equipment, I would NEVER limit a vessels mawp by a nozzle neck or it's reinforcement. I would consider that very bad design on my part."

I respect your opinion, and I would agree that it is a common opinion, but I can't say that I agree that it has any reflection on the quality of the design. (It seems that this requirement is more common on the pressure vessel side of the industry, although we see it routinely on the S&T exchanger side as well.) As a mfg of shell and tube exchangers, this requirement is usually not a problem to meet...BUT, it does increase the cost of the exchanger, and can create excessive reinforcement, which can create fabrication challenges.

The notion that nozzle reinforcement is cheap, and thus should not limit the MAWP, is not always true. At times, I have seen situations where the pressure for nozzle reinforcement calculations had to be increased by a few hundred psi to meet this requirement--this can take you from a standard pipe nozzle neck to a custom bottleneck forging. I am not saying this is the usual solution, but I am trying to make the point that nozzle reinforcement is not always a "minor component". Even in more common, less drastic cases, where the usual solution is to add a reinforcement pad (which is sometimes an undesirable solution) or a long-weld neck, the increase can be costly, especially for larger diameter nozzles.

 

[vesselfab]

Never said it was cheap, just said it is the way we do it.

Corporate philosophy I guess. Give the client all he is paying for and if you want to be remembered for excellence, give him a little more.

We have put in thickened shell rings, shell insert plates, and large forgings to carry through with this way of thinking.

Character is doing the right thing when no one is watching.

 

[tlcos]

Ouch, this touches a raw nerve for me as well. Right now I am designing some low pressure alloy exchangers with MAWP's not allowed to be limited by reinforcement or nozzle ratings. These sub 100 psig designs are jumping to 300 plus pound designs with 300# flanges and more difficult to find components at increased cost, labor, and lead time for my customers account. What does he get, an overpriced, over designed, beefy unit to do a low pressure job.

I don't consider that to be value, but simply a waste of money, much like buying premium gasoline for most cars. In addition to paying more for the equipment, this cost sensitive client will also have to increase at least the flange ratings of all piping connecting to the items. Hmmm, would that extend to all connections in the pipe system?

As for rerates, I have worked for one of the largest and oldest HX companies for over twenty years and must say those are rare, maybe <0.1% of all units ever sold.

Simply put, overdesigning a unit needlessly says nothing about one's character or ability to design well. It is wasteful and inefficient and the customer pays the price. If a customer knows their process will operate at a higher pressure (or temperature or flow) in the future, then that should be adequately reflected in their stated design conditions.

I hope my tone is not harsh, but MAWP "not limited by"'s have always been a pet peeve of mine. So please, lets get rid of this requirement.

 

[moltenmetal]

In my experience, many engineers confuse preferences with necessity or "best practice". Much engineering has been replaced with the blind application of company specs, at great and unnecessary cost. There are diamonds in some of those specs, distilled from years of hard learning by operations and maintenance folks- but the trouble is that the specs are also laden with preference issues and @ss-covering overdesign.

As to the need for re-rating: this is minimized by stamping equipment with its ACTUAL, calculated MAWP, rather than with its design pressure. With computers running the calcs, there's no legitimate reason to avoid stamping the vessel with its actual MAWP hot and corroded.

 

[vesselfab]

AGAIN TICOS

I never said NOT to limit by nozzle pressure temperature ratings. I said should not be limited by nozzle wall or nozzle reinforcement.

THAT IS A HUGE DIFFERENCE

in vessels pressure temperature ratings often limit because of thicker shells and heads due to forces other than pressure. this is not so true of heat exchangers, but i imagine it happens at times.

there are many tricks to limiting mawp by a major component so that it does not rise to ridiculous proportions. this comes with experience and knowing will and will not affect the value of the product.

Let's just say, some conditions are different.

perhaps you have just not yet figured out how to lit that HX so as not to have the design run wild. And there is always the ""RFI"" to the client to request a deviation or clarification.

 

[SnTMan]

vesselfab, when designing low-pressure, pipe sized units the MAWP of heads and cylinders is often far above the design. It can easily present the situation ticos refers to, if only heads or shell are to limit.

Also, I generally agree with Moltenmetals' opinion on specs.

My bottom line, if it is not in the Code, if it is not specified, it is NOT a REQUIREMENT.

Regards,

Mike

 

[vesselfab]

FULLY AGREE

if it is not in the Code, if it is not specified, it is NOT a REQUIREMENT

YES, as to the pipe sized units.

Never think about those much. Moved way past those 25 years ago and try to avoid them by letting the smaller fabricators fight over them. I refuse to get into a price struggle over those little things.

 

[SnTMan]

vesselfab, agree about the pipe size units. Most HX fabricators have to take some with an order to get the "good stuff".

Regards,

Mike

 

[DG72]

Quoting vesselfab: "Character is doing the right thing when no one is watching"

It's a shame that a simple discussion regarding differing design philosophies has boiled down to cheap shots--I am proud of the emphasis that I (and my company) place on quality and the satisfaction of our clients. We go above (and beyond) ASME code minimum WHERE WE FEEL IT IS IMPORTANT--So please don't suggest that it reflects on my character.

It sounds like you have a client base that appreciates this approach, and I applaud you for incorporating this into your standard practice. We too have some clients that value this approach. For these clients, we apply this principle, but many of our clients agree with my previous comment and would consider the extra cost unnecessary.

 

[vesselfab]

wasn't meant to be a cheap shot, but I suppose if you take it out of context, it could be construed as such.

I was talking about our corporate philosohy from the beginning to the end of the post.

 

[DG72]

No problem here, vesselfab.