MAWP Limiting Component 6

[truthseaker]

When the customer requires MAWP calculations. Is there an industry standard that dictates which component (heads, shells, nozzles) is allowed to be limit the MAWP for ASME code pressure vessels?

thanks

 

[vesselfab]

I don't know of any fabricator that will give away 20% excess shell thickness for nothing.

extra thickness/weight = extra cost

you buy steel by the pound

 

[boilerone]

I have seen both cases said by JStephen, option A, B plus in between. When I have a request for quoted of a vessel with just design conditions plus general dimensions and nozzle layouts, many times my fabrication suppliers come back with largely different designs. Some have thin shells but thick nozzles even with re-pads, some have thick shells and thin nozzles, just like or actually more extreme than JStephen gave above. The reason? It is very much related to manufacturer's most cost-effective methods.

For an exact example. It was a boiler steam drum: one supplier gave roughly 3" shell with almost all reinforced nozzles (forged material with thicker walls than the pipes they connect to), the other showed 4" shell with no almost reinforced nozzle at all. The prices offered by both vendors turned out to be very comparable so it was hard decision by us (technically we always like thinner shells for boiler drums because of thermal stress concern, by the way). The first vendor might has limited capabilities of its roller so was forced to spend money on nozzles. The second one might has large investments on its roller and even furnace so can save money on nozzles.

In all, it is just about the strength and compensation calcs if ever did once. When you look at cal sheets by both vendors, the first one has almost no extra thickness on the shell so had to use thick nozzles to compensate the lost of strength at nozzle openings so is limited on MAWP by the shell, the 2nd one has enough meat to allow for sizable openings without much need of compensation from the nozzles so the nozzles just needed to as thin as possible to just take care of themselves under MAWP but thus nozzles became the limiting components.

If either or both vendors are very frugal on exact thicknesses of both shells and nozzles (might never happen) then EVERY THING might become a limiting component. That's why I said the same in my earliest reply.

If the aboves are clear and now I say I jump to the customer's side of the fence, I would prefer the thicker shell option for possible future MAWP increase because you can add re-pads to the nozzles but you can hardly do any thing to the shells.

Boilerone

 

[truthseaker]

Good point Boilerone. Also don't forget while you think you are saving money buying a thinner shell or head most of the time you are giving it all back and then some when it comes to added labor for nozzle re-pads and extra welding. Especially if you have to add re-pads to a half dozen nozzles on a vessel just so that you can elevate their MAWP’s higher than the shell or head.

 

[jte]

Well, y'all make some good points. Although some are reluctant to take the responsibility, the person on the buying end who is playing the role of the Owner's Engineer generally has the power to waive parts of the specification.

So, although I tend to agree with vesselfab, I can appreciate boilerone's argument as well. In general, I strongly prefer integral reinforcement over re-pads, so that is a strong argument if I'm making the decision. I generally don't get involved at that level, though, with the exception of multimillion dollar orders.

So... I'd encourage all you fabricators out there to put your money where your mouth is! Start providing our project folks with dual quotes: One which meets the RFQ and is fully compliant with the spec, along with an alternative in which some minor component is allowed to govern MAWP. Show me the savings!

For what its worth, I've recently written the MAWP requirement out of some spec's that I'm working on. Introducing a new four letter acronym instead. We'll see how that goes with the second/third level internal review. Hopefully it'll get published with my changes!

jt

 

[MrBTU]

From a fabricator's point of view, why would we give any more than what is contractually agree to - meaning the design pressure? With shell and tube heat exchangers, a tubesheet is often machined to min required thickness, so MAWP equals P design.

 

[SnTMan]

thread794-180021

Nuff said.

 

[dgrande]

This is good stuff!

I'm a designer & fabricator of highly specified customer vessels & heat exchangers. We deal with a lot of customer specifications. I pretty much take the approach of "If it's in there, I will meet it. If it's not, ...."

For a fabricator, part of the engineer's job is to design a good quality vessel that meets all of the requirements of ASME Code and the customer specifications. Another part of their job is to design the most cost effective design that he/she can in order to ease shop load & maximize profits. A good engineer does both to the best of their abilities.

 

[moltenmetal]

Designing for future re-use is a mug's game.

I want vessels stamped with their actual MAWP, not just the design pressure I asked for. I want the designer to calculate and indicate to me what component limits the MAWP of their design. Other than that, I actually WANT them to do the leg-work to find out the most cost-effective way to achieve my design pressure within the limits of the code. That's their job.

If the limited component is a nozzle neck in the corroded condition, we'll probably change that!