Saddle angle 1

[ama1234]

My client has asked us to design the saddle for one of their vessel. But due to nozzle arrangement (as shown in the link below, the saddle angle (120 degree) is unequal from both the sides.
i.e. with respect to 180 degrees, I am having 22 degrees + 120 degrees + 38 degrees.
Please help as to can I accept this sort of configuration. Design can be to ASME, PED or Good engineering practice. I have attached sketch for further refrence.

 

[vesselguy]

You should move the saddle to clear the interfering nozzle or move the nozzle to clear the saddle. Pick one. Not a good idea to make a uneven saddle like that. But if you really must insist to do this weird thing then do a zick cal for each saddle angle to check your horn stresses. Bet you it won't be good for the side with the small contact angle. With a nozzle in the way, you can't even put in a reinforcement ring. overall, just a bad design.

 

[jte]

ama-

For a relatively small vessel, you might be ok. From the sketch it seems as though the diameter is ~2m or 6' while the shell thickness is ~40mm or ~1.5". This relatively low D/t helps. While I agree with vesselguy that this type of situation is best avoided, it might well pass easily based on the high shell thickness. Keep in mind that Zick is an approximation (his work is based on steel vessels on concrete saddles).

If you are considering an FEA, keep in mind that that is also an approximation. To be faithful to the physical situation, you'll have to use solid elements (are you using SW?) along with contact. Alternatively, consider eliminating the wear pad (which is there to keep the steel shell from wearing on the concrete saddle as it moves due to thermal expansion). Again, with such a high shell thickness, you may find that it is unnecesary.

Nozzle reinforcement will also be an issue... Perhaps reasonably solved via the same FEA model.

Though you didn't state which code is governing your design, consider that ASME VIII Div. 1 App G suggests that the saddle angle not be less than 120 deg "for other than very small vessels." I strongly doubt that you would find anyone to state that that is not meant to be 120 deg equally distributed about the vertical centerline. But... also keep in mind that App G is a non-mandatory appendix and that the thickness of the shell is not a consideration in that suggestion.

Let us know how you proceed... We can all learn from your evaluation.

jt

 

[JStephen]

I would take the side which was shorter, and calculate it assuming both sides were like that.

 

[waskillywabbit]

I have run into this before and when push came to shove the customer really didn't have to have the nozzle there as bad as they thought they did (money). Move the saddle or move the nozzle.

 

[chicopee]

I wonder if an opening thru the saddle could be an option to maintain near symmetry?

 

[ama1234]

Thanks for the reply guys. I cannot get the nozzle angle changed, but only the good thing is saddle are gonna be just used for transport.

As per calculation (Henry H Bednar), the thickness of the saddle comes as acceptable.

Sound like nothing to worry about but still difference in the angle is still a bit of worry. Other thing instead of anchor bolts I have to use straps to hold the vessel in place. From calcs I had a rough idea of placing the straps closer to base plate. Straps can hold 5 tons each. Does anyone has different views on it ? Vessel weighs 8 tons and had a diameter of 2000 mm x 1650 long (shell)

 

[TomBarsh]

Just my opinion, but I wouldn't worry about it for transport saddles. The vessel will not be pressurized during transportation, will it? If no pressure, then circ stress is essentially zip.