Horizontal Vessel Supports 1

[Sharik]

I have found four large, horizontal propane storage vessels, 10 ft diameter, 35 feet in length, which are only sitting on the saddles (2). The saddles are not fillet welded to the vessel. I know this isn't a good practice and is allowing external corrosion to occur between the saddle and the shell.

Is there anywhere in ASME Section VIII, Div. 1 which requires large horizontal vessels to have the saddles welded to the shell?

The only way to convince this Owner to fix these supports is to quote a Code paragraph.

Thanks

 

[codeeng]

Nothing in the code about fixing saddles as long as there are no issues with seismic, wind or piping loads. Rubbing due to thermal growth could be a problem plus the corrosion you mention.

 

[jte]

codeeng's right. There is no requirement in Section VIII. For a lack of design references, most folks use Zick's method for evaluating stresses in horizontal vessels. Don't forget that Zick's paper was based on a vessel with wear plates resting on concrete saddles. Obviously a design in which the vessel is not attached to the saddles. When you start to think about it, there are a lot of potential issues about extrapolating the Zick method to modern welded steel saddles...

So as long as the vessels in question have a wear plate or have adequate remaining thickness in the shell, I wouldn't worry too much about it.

jt

 

[pve5]

ASME code Section VIII div 1 Fig ULW -22, acceptable supports for layered vessel show welding of supports to shell. UG -54 specify that supports shall be arranged or attached to the vessel in such a way as to provide for the maximum imposed loadings.Supports are designed for prefixed location of saddles. In case saddles are not welded, then due to free movement of shell, thee will be change in location of the saddles and can overstress the shell.
Pressure vessel hand book by Henry H. Bedner ( page 180) recommends saddle to shell continuous weld.

 

[NozzleTwister]

I've seen these large, usally long, storage bullets resting in their saddle supports without being welded.

I don't see the logic behind 'fixing' existing vessels that aren't causing problems, and there are always risks when welding to existing hydrocarbon vessels. You would also probably have to modify the existing saddles so that one or both ends could slide and be sure the increased moments from friction aren't over stressing the shell. I'm sure there are other ways to deal with the corrosion.

Where seismic is not a concern and aside from the potential corrosion problem, there are, in my opinion, advantages to this type of design. Due to weather extremes and/or product temperatures, there is going to thermal growth/shrinkage of the vessel between the saddles.

If the saddles are welded to the shell, the saddle support will have to slide (or bend) putting large moments on the saddle to shell juncture. I have seen saddle to shell welds crack where saddles were supposed to slide but didn't.

When the vessel is free in the saddle, the moments due to friction will be in the support structure and not in the vessel shell. Except for slight expansion and contraction, this vessel isn't going anywhere. Assume the midpoint between saddels as the nuetral point for thermal growth. I would have to think this design would be more forgiving due to settlement also. Support locations are marked on the drawings, it's not a choice construction makes. I can't see it being a concern if a support gets shifted an inch or two one way or another.

That's my thoughts on the subject.

NozzleTwister
Houston, Texas

 

[MJCronin]

Sharik...

I think that NozzleTwister is correct....there are advantages and disadvantages to either configuration.

There is no ASME code paragraph that either bans or endorses either configuration....

There is a research paper entitled " An Experimental Study of the Collapse of Horizontal Saddle-Supported Storage Vessels" which evaluates both types of construction.

http://www.ingentaconnect.com/content/pep/jpm/1998/00000212/00000003/art00005

There have also been studies done on optimum saddle support configurations:

"Ong, L.S. and Lu, G. 1993. Optimal Support Radius of a Loose-Fitting Saddle Support Int. J. Pressure Vessels & Piping (Elsevier), Vol. 54 (3), UK, 465-480"


You could also check the "Koran" of Pressure Vessel Design....Brownell and Young..."PROCESS EQUIPMENT & VESSEL DESIGN STORAGE TANK SHELLS"

My opinion only

-MJC

 

[vesselguy]

Saddles are generally fillet welded to the shell, via repad, because it makes sense from shipping point of view. Imagine you are shipping with the saddle free from vessel. What would happen?

The original question has been answered which is "no" there is no Code paragraph that tells you to weld the saddle to the vessel. However, the issue of corrosion between the saddle and the shell is not addressed.

Not enough information is provided in the original posting. Hence, I have to make an Ass Out of You and Me. I assume there are repads to reinforce the shell. If so, then the corrosion is between the repad and the saddle web. If so, then lift the vessel and grind out the corrosion and, if required, weld fill the repair. Repair on the surface of the repad is , as I recall, NOT considered to be the shell and hence do not require rehydrotest after welding. Repair any painting in the localized area and your good as new.

Any comments on this from anyone?

 

[NozzleTwister]

I would imagine that the free saddles would double as shipping supports.

I agree that generally, saddles are welded to the vessel shell. But I have seen just as Sharik has, saddles that are not. The vessels I saw with this design were about 80 feet long and in similar if not the same service.

Corrosion prevention keeps being brought up as a reason to weld Sharik's saddles on but it has not been determined if there is a corrosion problem with the saddles. If there is a corrosion problem, other alternatives that address the problem should also be reviewed.

As I mentioned earlier in this thread, welding the saddles on potentially creates a whole new set of problems that didn't exist before. We can talk about pros and cons all day but the number one question that needs to be answered is......

Are the subject vessels experiencing a corrosion problem between the shell and saddles and does the client care to fix it if they are?

That's my 2¢,

NozzleTwister
Houston, Texas

 

[Sharik]

There is corrosion occurring between the saddle repads and the vessel shell. I've recommended they blow out the area between the shell and the repad with dry compressed air and then seal the repads to the shell with an applicable caulking to prevent moisture ingress.

These are not vessels subject to a significant amount of expansion and contraction due to process.