Nozzle Loads 3

[ellas1]

Problem: We have a vessel nozzle that is subjected to 3 thermal forces Fx, Fy, Fz and 3 thermal moments Mx, My, Mz with Fx the axial force and Mx the torsion. Even though all forces and 2 of the moments are much smaller than the given allowable's one of the bending moments exceed the allowable by about 40%.

Question: Is it possible and safe to combine the forces and moments and compare them with the combination of the allowable's as we do with normal and shear stresses in structures?

Thank you

 

[SnTMan]

ellas1, confusing, you refer to allowable forces and moments. Are you reading these from a table? You don't actually mean allowable stresses?

Have you done a standard stress analysis such as WRC-107?

Normally thermally stresses are considered self-relieving and are less concern that non-self relieving stresses due to mechanical loads.

More information needed.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[ellas1]

SnTMan

Thank you for your reply.

The allowable's given are forces (lbs) and moments (ft-lbs). The actual thermal loads are also forces (lbs) and moments (ft-lbs) from the stress analysis. Based on what we know from the MNFR's report the allowable's are based on the assumption that all three forces and moments are acting concurrently. Since only one of the bending moments exceed the allowable by about 40% and all the other forces and moments are well below their allowable, is it possible to combine the forces and moments and compare them with their respective allowable combination?

Thanks

 

[nickelkid]

Typically no. Each of the values you have given are probably values that cannot be exceeded. Not sure what type of vessel you have or the design criteria, but it is more often easier just to alter the pipe design, if not, you need to go back to the vessel supplier and understand their criteria for setting the limits.

 

[SnTMan]

ellas1, in general I'd say not. Best would be to have the manufacturer run the proposed load combination and check it.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[TGS4]

And most importantly, you need to combine the mechanical (deadweight) with the thermal piping loads. The stresses in the vessel shell attributable to restrained free thermal expansion will be classified as primary (same as for deadweight). Table 5.6 of VIII-2 makes this very clear.

 

[JStephen]

What SnTMan said about checking with the specific load combination would be the first choice.
The problem is that the stresses from the different loads add, but only to an extent. So adding a moment in one direction may reduce the moment capacity in another direction, or increasing axial load will reduce moment capacity. If you have a table of maximum forces and moments, it may be unduly conservative when used for just one force or one moment.

 

[SnTMan]

So adding a moment in one direction may reduce the moment capacity in another direction, or increasing axial load will reduce moment capacity.

This is why it is not really possible to calculate one unique set of allowable loads and moments referenced to the three axes.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand