Vessels on support legs

[Flash8111]

When analyzing the support legs on a vessel shouldn't one consider the loading
from the attached nozzles?
Thanks in advance.

 

[SnTMan]

Flash8111, strictly speaking, probably All external loads are of course reacted into the world. In practice nozzle loading is often not known with enough accuracy to make it worth the bother, at least in my class of work.

Regards,

Mike

On Edit: Expecting some different opinions


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

 

[XL83NL]

Well, nozzle loads are usually unknown, which is why nozzle load tables are used as an interface document between the vessel fab and piping guy - with this doc the vessel fab knows the minimum and the piping guy the maxmimum.

It's not a 100 perfect solution (are the loads e.g. primary or secondary loads, do they act together, what other combinations are possible, etc.) but it works out very well in most cases. With this document the vessel fab therefore has a theoretical upper limit (i.e. nozzle loads) to which he could check the supports. However, chances are the nozzle loads will never act in the most worst case situation but more likely in a situation where one cancels the other, or where the resultant vector is negligible wrt to the supports.

This is my experience, but my scope of vessels is usually rather limited in size (small volumes of a few millilitres to maybe a few cubic metres). Not sure what would happen with a 60 mtr column where the top nozzle loads gives a side load. Think of that (for a) moment ...

 

[JStephen]

The other side of this is that vessel flexibility should be considered in the derivation of those loads.
For example, small vessel, with big pipe attached. Is that pipe going to push the vessel over? Or does it hold the vessel up?

 

[NRP99]

Flash8111

We need to consider all the loads and load combinations for design of any component of pressure vessels as per code.

But as SnTMan, XL83NL and JStephen explained through their experience, you need to check how much impact it will have on the lug design. Here engineering judgment and logic will play key role(if one has no experience). Which is why I often say code is just guidebook but use your own logic and judgment to consider the best possible scenario of loading.

Another thing to note is the forces and moments due to Wind and Seismic will be much much higher than the nozzle loads acting on the vessel which will govern the design of leg supports. Also I think the piping loads acting on nozzles will impact locally rather than globally provided the size of nozzle is less or not big enough compared to vessel size.

 

[JohnGP]

I've also not come across a vessel where the nozzle loads had a significant effect on support design.

The difficulty with including nozzle loads in the support design is that piping connection loads are not generally known at the outset, and as noted above, the loads generally counteract each other anyway. Assumptions need to be made to ensure the vessel shell can handle reasonable loadings, and checked once piping design is firmed up. Such procedure could be used to check support design for unusual nozzle arrangements, but I find it difficult to believe this would be an issue.

 

[r6155]

During engineering works there are a lot of things difficult to measure. This is not a excuse. Good engineering practice must run to estimate it.

Regards
r6155

 

[Flash8111]

Over the years I have calculated the allowable nozzle loads on a vessel and applied these loads to the vessel nozzles when analyzing support legs (usually only the top nozzle). I use PVElite and apply this load at the top head to nozzle juncture using the external load feature. This loading most always impacts the leg design. I am thinking maybe this is too conservative as I am not taking into account any flexibility between the nozzle and top head. Any thoughts? Thanks for all previous replies.

 

[jte]

It certainly does not happen often, but I did run across a case around 20 years ago where I had to change the baseplate and anchor bolts to accommodate a piping load. This was for a large pipe (~1/3 the vessel diameter) which was supported off the side of the column. The weight and eccentricity, coupled with seismic, drove up the moment at the base. So, as others have said, you need to make a judgment as to the likelihood of having unusually high loads.

 

[KevinNZ]

Like jte found, we have larger pipe connections to our vessels. We include the vessel in the pipe stress model so the loads at the vessel support include loads from the pipe. This also gives us more accurate pipe to vessel nozzle loads and we can design the vessel around these.