5 replies to this topic

[Hucky]

Hi there,

 

I am currently designing a Vacuum distillation column operating at around 10 kPa absolute and 80 degrees C. I am at the stage where I need to decide on the wall thickness and obviously need it to be thick enough to not collapse on itself. I am planning on using carbon steel as the material of construction, grade A285.

 

Everywhere I have read says that columns with an external pressure have a very complicated way of calculating the required thickness for a particular material. I was wondering if there are any rules of thumb or previous cases etc that I can use to avoid doing the full calculations?

 

Thanks, Will 

[Art Montemayor]

Will:

 

The calculations for determining the wall thickness of a vessel under vacuum are not complicated.  They are iterative and detailed if you employ the ASME Code, Section VIII.  The calculations for the vessel heads - or end closures are probably more complex, but what I've done in the past is apply the same wall thickness for the heads as the correct, calculated wall thickness for the cylindrical section.  The cylindrical section yields a thicker plate thickness requirement than that for the heads, so you would be not only safe in using the same thicknesses, but you would also make it a more efficient welding job when joining both types of components.

 

The way I've set up the calculation in the past was by using a spread sheet.  Today, you have programs that make these vessel calculations.  You can forget about any "Rules of Thumb".  There are none that I've ever heard about.

[Hucky]

OK I will give it a go, thanks for your help!

[Hucky]

Hello again,

 

I'm working through it at the moment, and you were right in saying it is not complicated. 

Would you recommend any head shape to use? Hemi-spherical or ellipsoidal etc etc 

[Art Montemayor]

Hucky:

 

You need to have more basic data to make a decision on which type of head is appropriate, correct, or better for your application.  You have to state the design temperature and pressure as well as the material.  You also have to know certain basic and common sense criteria.  For example, a hemi head is the perfect, and most strongest of all heads.  However, it is plagued with welding problems and manual labor costs.  Hemis are almost always used only on very high pressure applications and in large sizes (above 7-10 meters) where the cost and weight of the material is important.

 

Usually, the ellipsoidal head is the type most used in process vessels.  It fills a happy medium in process vessel applications.

 

Read and study the attached information to learn more about vacuum design in pressure vessels.  You don't have to study Mechanical Engineering, but it certainly doesn't hurt and it gives you great insight and engineering abilities to know ASME code and other important pressure vessel codes and information.

 External Pressure Design for Vessels.docx   2.81MB   70 downloads

 External Vessel Pressure.docx   37KB   50 downloads

 Pressure Vessel Heads.docx   381.08KB   47 downloads

 Cylindrical Shell 1.1.xlsm   38KB   53 downloads

 Ellipsoidal Head ver 1.1.xlsm   101KB   49 downloads

 Elliptical Head ver 2.11.xlsm   35.92KB   45 downloads

 Flanged and Dished Head ver 1.1.xlsm   123.49KB   42 downloads

 Process Vessel Design - Brownell 1959.pdf   9.26MB   82 downloads

[Hucky]

Thank you so much! You have been a great help