Nozzle Reinforcement to counteract Pipe Loads 1

[MrOakshott]

I have a vessel which, due to location and service has very high nozzle loadings, particluarly moment loading, presenting problems with allowable stresses being exceeded.

We are limited in the space available to include repads and do not want to increase the shell/nozzle thicknesses - I understand that webs betwen the shell & nozzle can be used to isolate the bending moments but ASME VIII Para UG37 only covers pad reinforcment - how can the stresses be calculated and proven?

 

[DSB123]

MrOakshott,
I presume you have performed a Pipe stress analysis on the attached piping to come up with the nozzle loads? If so was the shell flexibility of the vessel included in the analysis. If not then this could reduce the actual loads significantly.

 

[Shofa]

Mroakshott,

I really support DSB123, The shell flexibility of the vessel should be included in the pipe stress analysis.

However, the best solution for your problem is to reduce the nozzle loading by implement some modification on the piping system itself not to increase the shell / nozzle thickness or to reinforce the nozzle.

 

[bvi]

How to calculate stresses in the vessel was the question? Here are some approaches that are used.

The old WRC 107 method (Bijlaard), which is incorporated in various computer programs such as CAESAR and CodeCalc.

FEA from scratch.

FEA using programs such as FEPipe that incorporate automatic mesh generation.

Other methods, such as the simplified approach provided in Bednar's book, Pressure Vessel Design Handbook.

Don't forget to include the pressure thrust load as part of the loads acting on the nozzle.

 

[MrOakshott]

Thanks folks, this is as I suspected. The pipe analysis has been provided by our customer for an exitsting piece of pipework to be fitted between new vessel equipment

Part of the issue is a relatively large diameter, yet short length of pipe, giving a limited degree of flexibility. It appears that this will have to be modified.

My main questions was as to whether webs can be used in place of, or additional to, repads to compensate, aleviate or distribute stresses due to nozzle moment loads. FEA would provide a perfect analysis, but can the effects of the web be calculated by hand in a similar method to the pressure area method for pressure reinforcement be used (as outlined in UG37)?

My suspicions are that they cannot, as the load is still applied to the shell and if present, reinforcement, which must still be thick enough to hold the load.

 

[prex]

In my experience the most stressed part, when dealing with high piping loads, is in the shell at the periphery of the repad: if that is your condition, gussets wouldn't help much. Moreover they are not normally a good solution especially for fatigue resistance reasons, I've seen them used only for supporting a dead weight at small nozzles.
I concur with other responders: if you can't really take those loads with a repad, you should have either the piping modified or a more in depth analysis made.

prex

http://www.xcalcs.com

Online tools for structural design

 

[JoeTank]

As prex noted, if the stress problem is at outer edge of the repad, the use of radial web-type stiffeners will not help much. I have used this appraoch quite a few times for small agitators, but typically had to made the repad much larger than one would normally expect.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[StressGuy]

If it's a short length of pipe, I would definitely see about trying to feed the customer some stiffness values to feed back into his stress analysis. Programs like Caesar love to give you massive loads in closely coupled systems with rigid anchors. It may be that it only takes .01" of deflection in your shell to make the piping loads essentially disappear.

The problem is always a bit of chicken and egg - since your customer doesn't have details on the shell flexibility to start with, he has to assume a rigid connection, which in turn leads to larger anchor loads.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[davefitz]

It sounds like the entire piping system and vessel needs to be modeled together in one global model and analyzed. With luck, the only changes needed would be a change to supports ( ie, replace anchors or solid supports with sliding supports, spring supports, or spring and snubber supports). It is also possible that piping changes would be needed,and that currently unforseen problems will be made apparent.

If you have the opportunity to modify the vessel nozzle, a thicker "nozzle reinforced" connection with a full pen weld and appropirate vessel membrane flexibility may indicate teh existing piping and supports are OK.