Mechanical Considerations Due to Process Changes

[GSDGoBucks]

I have been approached by my process engineer to consider lowering an existing S&T floating head exchanger's shell-side inlet temperature by nearly 250F. Tubeside conditions would stay the same. With this proposed change, the temperature differential between the shell-side and the tube-side would increase to ~450F. Both processes are hydrocarbon.

Due to this significant temperature change, what additional mechanical concerns should I consider? I plan to have a contract engineering firm perform an FEA to take a look at the probabilty of gasket leakage at the tubesheet joint as an example. Obviously, tubesheet distortion needs to be analyzed. Are there any other considerations?

Thanks in advance.

 

[MJCronin]

GSD...

Have you considered a formal "re-rating" (recalculation of performance) by the orignal HX vendor or has this already been done ?

Several computer software programs are available specifically for this purpose.

Discussions with a vendor would reveal any other mechanical/code possible problems that you may have regarding the particular TEMA style/ etc.

My opinion only...

-MJC

 

[Christine74]

If he's reducing the operating temperature then he won't need to re-rate the exchanger unless it drops below the MDMT.

Since this is a floating head exchanger, there really shouldn't be too many operational problems, even with a 450°F temperature difference. You could have some problems with the tubesheet gaskets due to the differential radial expansion between the channel and the tubesheet, but in my experience the tubesheet mean metal temperature closely tracks the channel temperature. If not, corrugated metal graphonics gaskets handle this quite well.

-Christine

 

[thermmech]

a) Christine, excellent point about suitable gaskets (I assume that you have Kammprofile gaskets in mind).

b) I would suggest more caution re the temperature of T/S closely following the temperature of the channel. Generally, tubeside fluid's effective surface area on T/S is greater than that of the shellside fluid. Hence it is reasonable to assume that the mean temperature of the T/S would be closer to that of the channel-side fluid. However, the actual mean temperature also depends on the actual heat transfer coefficients on each side of the tubesheet (i.e. fluid properties, phase, etc) especially with 2 passes on tubeside.

c) Finally, I believe that MJC was referring to the overall thermal performance of the S&T HEx; if you increase the dT in between the streams by lowering the inlet temperature of the shell side stream (COLD STREAM), you are increasing the overall heat duty faster than you are increasing CMTD. Hence thermal re-rating of the exchanger could be required to assure the process goal is achieved.

Cheers,
S.