BOLTING EQUIPMENT FOR THERMAL MOVEMENT

[IEngineer08]

Hello All,
This is my first post and I have to say that I am excited as I often come to this site to seek out answers to my questions. I have a situation where I have a reboiler that I need to mount and bolt down to structural steel. The reboiler is attached to a column and after thermal calculations, it was found that the reboiler must be allowed lateral movement along the same axis as the connecting nozzle in order to avoid overstress. The holes drilled on the structural steel are about the size of the bolts and wont allow much movement. The holes on the lug supports are slotted but (2) of the (4) have their slots in a direction opposite the movement. Should I make the 2 holes on the lugs bigger or should it be those on the structural steel? I tend to choose option one but I could use some guidance. Another issue would be how tight should the bolts be? Enough for the equipment to not vibrate but also loose enough to allow movement. Anyone ever dealt with this before?

thnk you,

 

[pennpiper]

IEngineer08,
Is it possible for you to share a sketch or a photo of your situation?

Sometimes its possible to do all the right things and still get bad results

 

[IEngineer08]

Here it is...

 

[EdStainless]

You need to mount it pads that are meant to slip.
The slots that you have are just to provide mounting alignment.
You can't just use slots like these and expect the unit to actually move.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[dhengr]

IEngineer08:
I’ll bet the column and reboiler can also move vertically w.r.t. the structural steel. Why not show some dimensions, thicknesses and sizes, some loads, weights, etc. on your sketch, so we get a real idea of the proportions and magnitudes of the problem. What is the temp. range and your calculated movements, in which directions? Slotted connections like you show tend to rust and fill up with junk and soon become inoperative, even if they did operate fairly well at first. Could you hang a reboiler off the column with some judicious hanging structure, repads, etc? Thus reducing the length of your connecting nozzle which is probably driving your thermal movement. Also, the closer the nozzle is in elev. to the sliding surface, the lower the induced moment will be on the reboiler shell. Usually piping and the like has to bend and be able to tolerate these types of movements without overstress. The problem becomes tougher to solve when the piping is the driving force. Maybe you should bolt the reboiler solid to a light structural frame which will carry the weight and other loads, but is flexible enough to allow the movement, within that structure, rather than the reboiler shell.

 

[MJCronin]

A common solution to this problem is to mount a small platform on the column and support the reboiler from the platform.

http://www.piping-engineering.com/heat-exchanger-piping-design-guide.html

MJCronin
Sr. Process Engineer

 

[dhengr]

MJCronin:
That type of platform is about what I had in mind. But, we don’t know the sizes and weights, and some of you guys know these typical solutions and this field much better than I do.

 

[MJCronin]

Here is another, more complex independent support scheme for a thermosyphon type reboiler...

http://www.whatispiping.com/vertical-reboiler-piping

Note that springs are sometimes required to accommodate the thermal expansion of the column/reboiler combination.

A "body type" expansion joint shown on the link and is sometimes necessary on the shell of a TEMA BEM reboiler to relieve internal tube stress.

Also note that sometimes a horizontal axis reboiler is selected instead of a vertical. This eliminates some of the thermal expansion issues. (but brings up other problems)

It seems that MBAs and PMPs usually assign a newbie to these complex and expensive chemical engineering design issues..... Newbies then post on eng-tips...

I believe that a comprehensive book should be written on reboilers, columns and their various design issues.

I have never found such a book.....

Anyone ?

MJCronin
Sr. Process Engineer