Fixed tubesheet with drain/vent drilled out the side

[piaengr]

Hello all. I am looking at a vertical shell and tube heat exchanger built to ASME code Section VIII, Division 1 and stamped. The shell is 20" in diameter. The tubesheets are extended as a flange for bolt-up of the tubeside channels.
Each fixed tubesheet has a drain/vent drilled into it.

To elaborate, a 1/4" diameter hole is drilled into the shellside face of the tubesheet to a depth approximately 1/2 the thickness of the tubesheet. A 1/4" diameter hole is drilled radially in from the OD of the tubesheet to meet the other hole. At the OD of the tubesheet, there is a larger hole bored out to fit a 1/2" pipe. (socket weld connection). The purpose is to drain at the bottom and vent at the top.

So my question... has anyone ever seen a reference that addresses this? I've seen it done before, but I have never seen it addressed in Section VIII or any other reference out there for tubesheet design. I'm just curious how someone goes about accounting for this in their tubesheet design.

 

[SnTMan]

piaengr, this is a very common practice, and as far as I know, no one makes any special provisions or analysis of this, you just do it, as long as the holes are small compared to other dimensions.

The Code (S VIII, Div 1 anyway) recognizes that areas of stress concentration exist, and design rules are generally conservative enough that these concentrations can be ignored.

Regards,

Mike

 

[unclesyd]

As I've stated before we have numerous fixed tubesheet SS heat exchangers each with 4 tubesheet vents the same size as those in your OP. I've never seen anything in any code about such vents, only that they are recommended where chlorides are present in the shell side fluids. In our case we used them to mitigate both SCC and MIC of the top tubesheet on the shell side. In some discussions I have heard the vents are considered in the same vain as the holes for tie rods. We have built a considerable number of this type Hx in house but have numerous ones built in Hx shops and I've never seen a question concerning the vents, even when they just on process specifications and not on prints.
Our design codes for this type heat exchanger are data form the 50's and were copied from earlier design codes from DuPont.

 

[piaengr]

Thank you both for your replies.

 

[rmw]

The tubesheet thickness is determined by the pressure boundry requirements in the in the area of the tube field where there are obviously lots of holes drilled. In the area in which you are describing the vent holes being drained, the tubesheet thickness exceeds that which is needed for pressure boundary considerations in that region of the face of the tubesheet so there is plenty of stock for the drilling you describe with thickness left over.

rmw