Plant Engineering Advice 5

[Pumprin]

Hi all,

I have been working at a industrial plant for about a year now, and I have had some projects in specifying pressure vessels and tanks, as well as inputting piping.

I feel really unmotivated when I get these types of projects due to the fact that I am just acting as a liaison from the process engineers and materials engineers, putting the data in a spec sheet, and handing it off to the vendor/draftsman for design work. The most contemplating I have to do is dimensioning the size of the vessel. Since our company doesn't have any stress engineers, we just input the piping based on the draftsman's design.

I have a few questions:

1) Is this all there is to the vessel projects, or am I doing it wrong and should be incorporating more engineering thought to the selection process?

2) Why do engineering firms offer to perform pressure vessel calculations when the fabricator will have to perform the calculations anyways?

3) Am I supposed to do more engineering regarding piping projects in terms of stress analysis instead of just letting the draftsman design the system and putting the piping in?

The trend I am seeing from the more senior engineers is that we are just liaisons when these types of projects come along..

 

[GHartmann]

Most of what you are questioning are the corporate standards and procedures in place where you work. My experience (I am a ChE) is that the process engineer might do a sizing or preliminary spec and then the mechanical engineer would make a detail drawing or sketch and complete the mechanical part of the spec. How much detail depends on the corporate standards.

My own experience is that each vessel or tank has a pretty detailed drawing showing all the nozzle locations (for instruments, etc), where they are located and any special considerations. Design T & P would be provided along with the MOC for vessels, exchangers, and tanks. (All these would be done by the ME) For heat exchangers you might could get by with only a TEMA type data sheet depending on the complexity of the heat exchanger. (Process data by ChE / mechanical data by ME). If you are not doing your own drafting / CAD work then the ME would definitely check the final product. Your company should have some checking / approval procedures for internal and external work.

For pressure vessels the mechanical engineer might show the wall thicknesses based on his own stress calculations. The detail drawing would then go to a shop where they would prepare the ASME code calculations and paperwork. In ideal conditions the fabricator would submit these calculations to the mechanical engineer (along with any shop drawings provided) for checking and approval. This type of arrangement sort of covers the designer for good design practices (to avoid any potential legal consequences).

The other aspect is that sometimes when using "untried" fabricators, their calculations can have errors. Sometimes the shop engineers can get overloaded. The main reason a engineering company wants to perform the calculations is to mitigate their risk. Since they are "responsible" for the design.

As for piping stresses it depends on the size and temperature. If you are running a bunch of small cooling water lines these are usually not stress checked. Once you get to larger lines and higher temperatures these lines probably need to be checked. Even steam lines of long runs. Your company should establish when the stress calc's are needed or not. The piping can also require a review of the supports and in particular avoid imposed loads on pumps and similar equipment.

Most of the larger engineering firms will work in similar fashions.

Maybe a Mom and Pop company would have Jethro and Slim build their vessel in their garage.

Seems to me your senior engineers have abdicated their responsibility for the projects your mention. That could just be guidance the have been given.

I hope the above was helpful.

 

[Pumprin]

GHartmann,

I appreciate the perspective you have given me from another plant. Your advice has motivated me to incorporate more engineering thought and calculations into these type of projects despite that not seemingly being the norm for our department.

I can see the benefit of performing my own vessel thickness caclulations to get a quick verification on the vendor's calculations as well, and will try to perform nozzle load calculations for mechanical equipment instead of just putting the piping in.

I have always felt really empty/unpassionate when going thru these projects, so I hope that feeling will change.

 

[TehMightyEngineer]

3) YES! Pipe support spacing, pipe stress analysis, and overall just designing the pipe to be efficient and get the flows needed with minimal losses and efficient use of material are all critical steps for pipe design and are probably not being done by the drafter. Does the drafter know that the suction side of pumps should not use concentric reducers? Probably not. Does the draft person know that the thermal expansion of a pipe can cause some extreme loads? Probably not. If you don't know how to do these things then learn. You'll become much more valuable to your company (and any future company) the more you learn. Plus, when you know how to do something right then you can spot when someone else is doing something wrong which it sounds like your primary job function is.

Maine EIT, Civil/Structural.

 

[racookpe1978]

Does the "drafter" know when to explicitly "follow the rules"?
When to (intelligently and with forethought) "break" the rules?
When to write new rules?
When to force another branch to bend their "rules"?
When to change the current tank or pipe run to make a different division job's easier, faster, safer, or even "possible"?
The single part "drafter" (or even the single piece mechanical "designer" is not - often cannot! - able integrate his single tank (or pipe, or slab, or piece of steel) into the plant as a safe, economically efficient, can-be-built, can-be-run, whole.

 

[Pumprin]

All,

Thanks for the wisdom and motivation.

 

[GHartmann]

Pumprin:

The group really appreciates your acknowledgement of the help provided. This trait along with your "feeling" things weren't being done "up to snuff" will get you far in your career and will win much respect among your peers.