3 replies to this topic

[marcouni08]

Hi,

I am writing this thread following my earlier thread: http://www.cheresour...packed-columns/ where the use of bubble caps against packed material was discussed with art montemayor.

 

in his last reply he discussed the particular design of cryogenic columns (integral welding and no manways) and offered his support in the case i would require more information.

I would be glad if art and anyone else available could expand on the topic.

 

as mentioned, i am a final year chemical engineering student in the Uk and as part of my final year design i am working on the design of an air separation distillation column (low pressure of double linde arrangement). as part of my design i am also required to provide mechanical details of the column (wall thickness, stresses, type of vessel closures, flanges, material of constructions etc). 

as a chemical engineer i am fairly lost on the topic and i would also be glad if you could help me by providing me with a short design guide that could help me achieving a preliminary design of the column. i do not require in depth design as the mechanical design component is not the main area of the project.

 

Thank you in advance for your help and support,

 

Marco 

[Bodhisatya]

Generally cold box inside doesn't have any flange connection,most of them are butt welded..Column thickness varies as per operating pressure. MOC generally is SS.

 

Can't get what do you mean by vessel enclosure.

[Art Montemayor]

 

Marco:

 

For some good, free mechanical design tools, go to: http://www.pveng.com...DesignTools.php

 

The outstanding points to look at and design for when you are mechanically designing a cryogenic vessel are the following:

• Expansion piping loops within the cold box – not only for piping that stays inside the cold box, but also for piping that is coming from the outside into the cold box as well as piping going out of the cold box;
• Piping and tubing that is used for instrumentation and has access outside the cold box;
• Allowance for equipment and piping shrinkage within the cold box as the equipment cools down to operating temperatures.  Piping expansion loops (or offsets) are essential to avoid stresses built up in the equipment and the piping;
• Allowance for equipment and piping expansion (growth) within the cold box as the equipment heats up during a “de-riming” operation.  This is a routine done every so often in order to do maintenance or repairs inside the cold box.  The equipment must be drained while cold and then subjected to hot air in order to bring all cold box components up to ambient temperatures;
• Proper design and installation of equipment supports and guides inside the cold box to lend stability and mechanical support while avoiding vibrations and movement; insulating material is required in order to isolate structural steel inside the cold box from the hotter, ambient exterior.
• The cold box must permit safe access to the cold box interior and permit installation and removal of cryogenic equipment inside.

[Art Montemayor]

Marco:

 

The reason you are obviously “lost” in the vessel fabrication arena is because you probably have never been in a fabrication shop, taken welding lessons, worked with metals and their forming techniques, or exposed to a real production process plant operation on a day-to-day basis.  That’s OK.

 

Even though you might lack the “hands-on” experience with pressure vessels, you still should know perfectly well what performance and operations you expect out of a fabricated pressure vessel – such as distillation column.  You also have the intelligence and foresight to be able to make a list of all those expectations (as far as you can rationally and logically identify them):

• The column’s working temperature;
• The column’s working pressure;
• The quantity and type of internals required in the column and what their purpose is;
• The physical size of the column;
• The required materials of construction;
• The potential mechanical hazards related to the column and its operation;
• The mechanical supports and foundations required by the column;
• The operational ancillary equipment required by the column for its operation and maintenance (ladders, platforms, railings, lighting, etc.);
• The insulation requirements of the column;
• The nozzles required by the column;
• The external stresses that the column must withstand safely (wind, tremors, fire, live loads, etc.)
• The maintenance requirements of the column.

The fact that you are a chemical engineering student does not divorce you from using your common, rational horse sense.  That is what a mechanical engineer would do – and exactly what you should also be doing when specifying and making the preliminary design of a process pressure vessel.  While you certainly cannot compete with a mechanical engineer is doing all of the detailed mechanical design, you should be able to apply the basic hoop stress equations that determine the shell thickness of a cylindrical shell.  Go to :

 

http://www.pveng.com...E/ASMEIntro.php

 

and download all the basic design information that they offer you free to calculate the shell and head thicknesses of pressure vessels as well as other details.  Use the free downloadable spreadsheets they give you and you can generate a fairly accurate mechanical design of your air separation column.  Of course you should also read all the rest of the material that you find at their website.  I am attaching some additional information to help you on external pressure (this is what mechanical engineers call partial vacuum).

 

As I have stated previously, you can eliminate any vessel closute or flanges for an air separation column within the confines of the cold box.

 

 External Pressure.doc   2.88MB   91 downloads

 Hand Calculations.doc   666KB   87 downloads