This not exactly a Caesar question. But I am posting as there might be enough help available here-

B31.3 refers to ug-28,29 of pressure vessels code for vacuum design of piping.

However ug-28,29 strictly deal with pressure vessels and therefore some of the piping situations are not well covered.

For example elbows or change in direction is not addressed in ug-28,29.

While looking at a piping system and trying to figure out whether additional stiffening rings are required, how should we treat elbows?

The way I am looking at it is: every change in direction represents a stiffener at the end of straight pipe, similar to head of a pressure vessel as illustrated in ug-28,29.

Therefore,if the length of straight run between two elbows satisfy the requirement of max. length without stiffener, then no stiffener will be provided. The point that I am trying to make is : I do not have to add the length of pipe in perpendicular direction to get to the next saddle support or a rigid element.

Please comment.

I may have posted a similar reply before- give the search a try.

No. The elbow/stiffener limit is wrong and has been perpetuaded through the ages as a boundary condition. A flange is an excellect stiffener. Just think about it, the elbow thickness is essentially the same (in the stress calcs we use the same as pipe), thus the crushing capacity is essentially the same.

Look at some of the plots in the FEPipe brochures or the help files and the elbows/bends deform inward also.

Bob, Thanks for your response.
I could not find your earlier post on the same subject. If you can locate it please let me know.


Another question:

Ug29-1 shows arrangements for stiffeners. It also shows a saddle with min. 120 deg. coverage.

Now, can that saddle be treated as a stiffener then? I would tend think "no" , but it's appearance on that page creates a doubt.

Can someone reply to my last question above?

Dear All,

The effectiveness of a saddle support as a stiffener has not been studied to the best of my knowledge and this particular topic has not shown up in the literature (vessel design books) or journals that I am aware of. It would certainly seem that a saddle would help at least partially to keep the local cross section round, but ASME VIII-1 does not consider a saddle as a 'Line of Support' for external pressure.

Figure UG-29.1(K) illustrates a cylinder with a saddle support showing a minimum saddle bearing angle of 120 degrees. This figure is referenced in the cautionary note on page 26 of ASME VIII-1 2007 edition 09 addenda. The note brings to the attention of the designer the local stress that could be produced by using brackets or legs to support a horizontal vessel. The note does not seem to address external pressure at all.

In my opinion, it would be best to ignore the saddle support when determining dimensions for external pressure.