No. of decimals in MAWP 2

[SilverMani]

Good morning friends....
I have one query regarding no. of decimals in MAWP of vessel as below:
“
In PV Elite, MAWP of vessel is specified till 3 decimals.
Should we take any of the unit systems, commonly used in the world, none of the pressure gauge can measure pressure up to 3rd decimal.
In other word, measuring pressure to 3rd decimal place is not practical.
Hence, a question comes to mind, why to mention MAWP which can’t be measured.
Other way round, no. of decimals in MAWP shall be same as those in design pressure, specified by user, which sounds logical also.
”
Request your review / suggestions.

 

[zdas04]

Regardless of what your program says, it is common practice to round that number down to hundreds (sometimes tens) so a vessel that PV Elite calls 297.241 psig would typically be called a 280 psig MAWP. If it says 638.437 psig most of the time it will be stamped and tested to a multiple of 600 psig. There are exceptions, but I see a lot of vessels at 280 psig, 600 psig, and 1440 psig to correspond with nominal values for ASME B16.5 Class 150, 300, and 600 respectively.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[marty007]

Some jurisdictions also place restrictions on decimals for MAWP. In Canada, the CSA B51 code specifically states that decimals are not allowed to be stamped on a nameplate.

I could see the use of decimals as opening up the potential for misinterpretation. For example, if someone glances at a nameplate that states 153.1 psi and for one reason or another doesn't notice the decimal (worn out/bad lighting/dirty/etc...), they might think they can pressurize the vessel to 1531 psi, creating a very dangerous situation.

 

[SnTMan]

In my class of work the practice is to drop the fractional part, if so calculated.

"Hence, a question comes to mind, why to mention MAWP which can’t be measured."

SilverMani, it's just software. It does the calculating for you, not the thinking

Regards,

Mike



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

 

[r6155]

SilverMani. May be the software show total weight 129996.99 kg
What do you think?

Regards
r6155

 

[SilverMani]

Thanks all for sharing opinion.
I am also in agreement with what u said.
But many designer simply follows MAWP whatever software shows.
e. g. For specified design pressure of 400 psi, if calculated MAWP is 405.650 psi, same has been mentioned on nameplate.
I strongly feel that it should be 405 psi only.

 

[dhengr]

SilverMani:
We are going absolutely crazy today, with out approach to real engineering of products and systems. Just because the computer programs which we rely on today to do all of our engineering thinking for us can/do calc. results to eight or ten decimal places does not mean that we can really design to that accuracy. We just don’t know enough. We are deceiving ourselves if we even think that for a second. The way we have modeled the system or structure incorrectly/inaccurately will cause a 10 or 15% error in the results. Imagine how vessel pressures or component stresses change every few minutes during the process. Imagine that some minor stress raiser, a weld termination or a poor fab. detail, something like that can raise the local stress by 50%, so what the hell are we trying to calc. operating pressure or vessel stresses to anything finer that 10 or 20 or 30 psi? The important thing, that we as experienced engineers should be doing is designing and watching for situations which make the whole system go crazy or blow up. Good clean detailing, good clean fabrication on the gross scale is far more important than a .5psi variation in stress or pressure.

 

[JStephen]

The program I use requires manual entry of static head, so automatically, nothing else is more accurate than that number. IE, whole numbers.

 

[jte]

But many designer simply follows MAWP whatever software shows.

Perhaps this is one issue which distinguishes a designer from an engineer.

The Committee recognizes that tools and techniques used for design and analysis change as technology progresses
and expects engineers to use good judgment in the application of these tools.

A well-grounded engineer recognizes the significance of significant figures. I maintain that in most pressure vessel applications, the notion of more than 3 sig. fig.'s for most values is fantasy once the steel is installed on site. Those who would argue for 8 sig. fig. weights are doomed to change the weight of the vessel every time a bird craps on it.

 

[JStephen]

jte, in a lot of my calculations (especially in spreadsheets), there's a lot of things shown to 8 digits or whatever. The problem is that once wind or seismic forces enter into the design, accuracy goes to +/-50% or more. So once you make the argument that your numbers shouldn't show more significant figures than what the accuracy of the analysis is, you don't have much of a number left to show in that case.

 

[zeusfaber]

To expand on JStephen's point, the actual proof strength of many pressure vessel materials varies significantly.

The material spec may well call out a minimum figure quoted to 2 significant figures, but in some cases, even the first digit can be a bit suck it and see when you look at achieved properties.

A.

 

[jte]

JStephen-

Agreed. But for most loadings, we can reasonably use two or three significant figure values and pass a red face test. The question here is reporting MAWP to the thousandth of a psi (in a plant which uses analog gages) or weight to the 10th of an ounce.


zeusfaber-

Also agreed... but with MTR values only being truly useful in a FFS environment, the actual UTS and YS are of less interest.

Bottom line, and circling back to the OP's query, I prefer 3 sig. figs for MAWP reporting, in some high pressure cases 4 may make sense. But for the vessels that I deal with, 8 sig. fig's on an MAWP is not sensible. When I see 8 sig. fig's, it is perceived by me to be a negative indicator of the engineer / designer who is signing off on the document. Don't get me wrong here, a "negative indicator" doesn't mean "this sucks" just that they haven't bothered to consider what they are reporting. And I recognize that others' perceptions can differ. That's what makes perception special!