2 replies to this topic

[cheminst]

When choosing a value for thickness of tubes in shell & tube heat exchangers,

Do I have to calculate the minimum thickness using ASME standards or are there any general standards which i can refer that have the minimum thickness to withstand the pressure?

 

I have to select a thickness for a tube which can bear 140 bar external pressure and 4.5 bar internal pressure.

 

If I have to calculate using ASME standards I have to set the design pressure by calculating the liquid height. To find the liquid height I have to find the total volume (Shell volume + volume of tubes). To find the volume of tubes I have to find the heat transfer area  and before that the overall heat transfer coefficient ( Q and LMTD are fixed). For that I need the tube thickness.

 

Can anyone tell me a way to solve this problem?

[MTumack]

Guess a tube thickness, iterate.

 

Ignoring, just do your initial design on your HEX so that MAWP is at the top of the unit; realistically with a column of fluid the stuff on the bottom will be the higher pressure due to Fluid Mechanics. Theoretically, you need to set your PSV lower, but even then the difference will be minimal. 9 times out of 10, even in large vessel sizes, the fluid head added or not added will have no bearing on which thickness of pressure retaining plate is chosen as this coincides with a standard size. Say you have a vertical  24"ID Cylinder x 30' LG filled with water to be designed at 1440 psig. If you take the MAWP as 1440 at the top, you have 1453 PSIG at the bottom of the vessel. If you take it the other way, your have 1440 PSIG at the bottom of the vessel and 1427 PSIG at the top of the vessel. For example: @ 1440 PSIG you have min thicknesses of 0.90301" in the shell, and 0.8703" in the head. @ 1453 PSIG you are looking at 0.9115" in the shell & 0.8782" in the head. Both times you are buying the same head and same shell. Now, do not misconstrue my words here, I am not saying you do not need to take it into account! This must be checked before final design! However, for initial design, this can be ignored. Where this gets you into trouble is when you max out your sizing on your standardized plate, or your minimum thickness gets pushed to a zone where supplier cannot feasibly get you material. Unless you are spending absurd amounts of money to get custom fab tubes.

 

I am confused as to what you mean by saying you need to calculate volume of tubes to calculate liquid height. This is quite frankly incorrect.

Edited by MTumack, 21 January 2016 - 04:51 PM.

[cheminst]

Thank you very much for the answer Mtumack. It is really an informative description and was really helpful.