Nozzle Thrust Load

[mechengineer]

Nozzle thrust load is an old topic. I think that thrust load cannot exist independently without internal pressure. The connection of the nozzle to the shell has been evaluated based on the internal pressure and reinforcement. In other words, the so-called "thrust load" has been considered in the internal pressure analysis for reinforcement (opening local stress). If there is no pipeline connecting the nozzle and the nozzle with blind flange, the thrust load still exists, do you need to check the thrust load?
Therefore, if it is 1, the nozzle meets the reinforcement requirements under the internal pressure. 2. the external load of the nozzle from the pipeline meets the requirements of WRC 297, the local stress at the connection between the nozzle and the shell should be no issue.
For high pressure and thicker wall, the local stress at the nozzle to the shell joint will be controlled by the internal pressure and the local stress caused by the external load of the nozzle will be at a lower level.

 

[BJI]

If your nozzle is venting to atmosphere, then you have no 'thrust' stress in the nozzle neck, but under normal circumstances when connected to piping or a blind, there will be thrust loads when the system is in equilibrium. Of course it is more complicated than this when you consider piping system flexibility and support arrangements, but it would be incorrect to assume no thrust loads. Therefore, in the nozzle this is just the longitudinal stress from pressure, the same as you would calculate for your main vessel shell. If you don't need to assess external nozzle loads, then your opening reinforcing assessment would be sufficient and no additional thrust calculations are required for the junction.

If you are assessing external loads, then it is pressure and external loads that need to be included in the assessment. Refer to division 2 table 5.6 and part 5.6 for stress classification in nozzle necks, specifically within and outside the limits of reinforcement. WRC 297 may be conservative for stresses in nozzle necks, but the pressure still needs to be included. How are you allowing for the pressure stresses at the opening with the WRC 297 method?

 

[BJI]

Here is a relevant discussion on thrust loads:

https://www.eng-tips.com/viewthread.cfm?qid=454328

 

[TGS4]

mechengineer - as you can see from the previous discussion that BJI linked to, this is a matter that a number of engineers find confusing.

Yes, there is a pressure thrust. Yes, the effects of the pressure thrust are included in non-FEA methods, such as WRC 297. There is no need to include the pressure thrust load as an additional external load in these non-FEA methods.

 

[mechengineer]

TGS4 - Thanks for your valued comments.
Two kind of loads generate two kind of stresses and calculate by different ways. And then combine the stresses together to evaluate the nozzle-shell junction stress level. The stresses subjected to internal pressure + the stresses subjected to the external loads.
The thrust pressure is within internal pressure. The WRC368 (thrust load) if for intenal pressure load only.
If pick out the thrust load from internal pressure and put it into the external load to check the local stress, so, in opposite, is it necessary to add the influence of external load to the internal pressure to check the local primary membrane stress? Hope the discussion here may help those engineers confusing with the thrust load to understand how to use the thrust load.
However, the thrust load as an option provided by PVElite for WRC297. I think it is better to put the option of WRC368 or thrust load into nozzle reinforcement under internal pressure section to avoid confusing.

 

[TGS4]

You need to go to the actual documents themselves (particularly WRC 297) to better understand how they handle the stresses due to internal pressure (pressure thrust included). Yes, both the stresses due to internal pressure plus external loads is combined by linear superposition. The limits are per Part 5.2.2.

 

[Some Curious Guy]

Thanks to the experts here for bringing clarity to the issue of pressure thrust.
I just have some doubts in the the below statement of the original post of mechengineer

Quote

The connection of the nozzle to the shell has been evaluated based on the internal pressure and reinforcement. In other words, the so-called "thrust load" has been considered in the internal pressure analysis for reinforcement (opening local stress)

Unquote.

I think the reinforcements would just take care of circumferential hoop stresses and the longitudinal force/stress acting axially along the shell. The reinforcement would not take care on any lateral force perpendicular to long axis of the shell. Hence for a nozzle with blind flange it is incorrect to assume that area replacement reinforcement method would take care of the lateral thrust load acting at opening junction.

Please correct me if I am missing something here.

 

[mskds545]

Some Curious Guy,

Area replacement is more tried-and-true than rigorous, but it does take care of the pressure thrust load. If you draw a circle on the side of the vessel, there is a radial thrust load inside the circle and Pr/t takes care of that. Cutting in a nozzle adds a stress concentration, but the overall force balance in the radial direction remains the same. If a blind nozzle is reinforced to code, there is no need to separately consider the effects of the radial load and this has been proven a million times over in industry.

There is a lot of concern about nozzles with pressure thrust load, but a more complicated and difficult-to-analyze situation arises if you have one of the rare situations without a pressure thrust load, such as an untied expansion joint. In this case, the vessel no longer has a uniform outward radial load over its internal surface. This causes the nozzle to effectively push inward and creates significant bending stresses around the nozzle. The unbalanced load is carried at the vessel supports, or in other piping if it is stiff enough.

-mskds545

 

[TGS4]

Some_Curious_Guy - interesting suppositions there. Care to back tat up with anything such as calculations, maybe a Free body Diagram? These methods have been around for a very long time and have been examined by some of the best in the business. More than likely, you are missing something. But that's for you to do your own research instead of us pointing out to you where you might possibly be wrong (without knowing how you arrived at your suppositions).

 

[Some Curious Guy]

@ mskds545
Thanks for clarifying. This is what I infer from your post. The radial thrust load due to pressure at the blinded nozzle in most cases is balanced by radial thrust load at the end directly opposite to that of the blinded nozzle. So these forces cancel each other and no need to consider an unbalanced force except under some special circumstances.

@TSG$
I had developed my supposition based on a derivation of the Pressure Area method of Nozzle reinforcement. To describe it very simply the derivation drew the Free body diagram by cutting the shell at the opening in half along the long axis. Kind of like a sectional Front view/ Elevation View of the shell along with the nozzle. The forces due to pressure ( Pressure x Area on which pressure is acting ) trying to separate the these two halves were then balanced by the area resisting these forces by restoring stresses. This derivation did not consider the radial force ( thrust load at the nozzle ). Area replacement is just some tried and tested thumb rules of Pressure Area method. Hence I developed an impression that the reinforcement would not consider thrust load. Thanks to you and @mskds545 my this supposition is now corrected.
Experts like you, mskds545 and others in the forum, who knows the basics as well as the advanced concepts are very very rare to find. I guess this is the prize we all young engineers paid for over dependence on softwares .Most of us young engineers taught ourselves with their own self study and research. We would always welcome some basic guidance and direction in our self study and correction. Thanks once again for your guidance and correction.

 

[mechengineer]

The pressure thrust load is for bellow design in piping line to get the axis force of the bellow.
For a pressure vessel or a nozzle on pressure vessel, the pressure thrust load is balanced with the longitudinal stress. In another word, the thrust load due to internal pressure which will generate a primary general membrane stress, local membrane stress due to discontinuity/opening & sencodary bending stress due to discontinuity/opening. Thus, the pressure thrust load should not be considered repeatedly as another external force (like nozzle loading) applied on the nozzle again to generate an 'additional' primary membrane local and bending stress.