Pressure Thrust
Pressure Thrust
(OP)
Dear Friends,
Consider a vessel with a 20inch nozzle. The nozzle is connected to a 1 meter pipe with end end cap. In calculating external loads on the nozzle for WRC107 check, shall this pressure thrust(P*A) consider as radial force?(which P is internal pressure and A is internal area of the nozzle)
Regards,
Consider a vessel with a 20inch nozzle. The nozzle is connected to a 1 meter pipe with end end cap. In calculating external loads on the nozzle for WRC107 check, shall this pressure thrust(P*A) consider as radial force?(which P is internal pressure and A is internal area of the nozzle)
Regards,





RE: Pressure Thrust
RE: Pressure Thrust
RE: Pressure Thrust
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Pressure Thrust
To simply understand it: reduce the nozzle and pipe length to almost zero, the end cap ( it can be blind or any shape) become part of the vessel wall and sees only internal pressure that generate primary membrane stress. So, there is no pressure thrust.
To know the stress mechanism behind, free body diagram, solid mechanics or mechanics of material will be able to demonstrate why. TGS4 may know. I lost what I learned in school.
The other question you will face is, if this pipe makes a turn and runs down along the vessel wall and goes into other equipment, and there is no guide at all, do you apply pressure thrust to the nozzle or not ?
RE: Pressure Thrust
As per my understanding, the thrust will always act whether upstream piping is straight or curved. The same concept you can see in appendix 2 caluclation of flange(Hydro static end force).
I also experienced the same that when pressure thrust is accounted for, most of the nozzles fail( Some clients insists)
I request more people to comment on this thread so that we all come to an understanding whether pressure thrust to be considered or not and why?
RE: Pressure Thrust
And, jtseng123, the limit case of the blind becoming part of the vessel wall is incorrect, as you only get pure membrane stresses if the wall of the fake nozzle is curved like the rest of the shell and in line with it.
A properly reinforced nozzle will not have problems as far as the general membrane stresses are concerned, as the purpose of the reinforcement is exactly to replace the material that resisted the general stress before the cut.
Now the point is: what stress limits do you apply to the stresses resulting from WRC107 method?
The membrane stresses should be classified as local membrane, and the membrane+bending stresses as secondary. So the stress limits would be 1.5S and 3S. If problems still exist at these levels, then one should worry at least a bit.
However, if the only load acting on the nozzle is the pressure (this is the case for manways), then it is not necessary to calculate the nozzle per WRC107 under VIII D.1 coverage.
A different matter is to ask whether the typical nozzle loads given by many client specifications do include or not the pressure thrust. My answer would be here: yes, unless explicitly otherwise specified.
prex
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RE: Pressure Thrust
RE: Pressure Thrust
Prex, since you admit pressure thrust is there, what is the technical reason you don't apply pressure thrust to a blinded nozzle ? where in the oode saying " area reinforcement is good for pressure thrust "?
If anyone is in an engineering firm, your pipe stress group will give you nozzle loading. Have you ever questions their loads including pressure thrust or not ? Do these pipe stress people understand what is pressure thrust other than pressure thrust from expansion joints ?
RE: Pressure Thrust
Concerning the pipe stress group, pressure thrust, just like weight, will always be present in a pipe stress analysis, so I assume it is included in standard loadings, but of course one can ask in case of doubt. And if the answer is no, they are not included, then of course you would need to add them...
prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads
RE: Pressure Thrust
about every location in a piping system where a header meets a lateral? I doubt that
additional loads are added throughout the piping system?
RE: Pressure Thrust
I do not buy that. Stress is stress, overstress is overstress regardless manway or regular nozzle, unless code clearly stating no need to worry about pressure thrust.
My common practice is, for blind nozzle, never add pressure thrust --- since on one does that, but would like to know why.
(Note, if you sit on a long blinded nozzle and run local stress, do you add pressure thrust or not ?? )
For regular process nozzle, I do add pressure thrust to run WRC107, and I do not question the loads given by pipe stress guys because 9 out of 10 of these guys, they do not understand what's included in these loads, wasting time to talk about pressure thrust with them. But in your opinion quoting from UG-23, it seems I shall stop adding pressure thrust. However, I am not planning to delete it.
RE: Pressure Thrust
Now process piping has normally supplemental loads, besides the pressure thrust that's always there. But of course when you calculate per WRC107 the nozzle for those supplemental loads, you need to include the pressure thrust too. And if you include the pressure thrust only, you are neglecting the external loads that might be relevant. Also including the pressure thrust only may be a useful exercise, but it is not required by Div.1, as stated above, as in that case pressure is the only active loading.
prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads
RE: Pressure Thrust
RE: Pressure Thrust
Your answer lies in the Bulletin itself. In Appendix A, there is an excellent discussion about the additional testing that the PVRC had performed (due to limitations and deficiencies with Bijlaard's method/results). That testing was on pressurized cylinders with cylindrical nozzles. In that testing, when external loading conditions are listed, there is no mention of the "pressure thrust" being included in the external loads - despite the fact that there is definitely internal pressure applied. However, I would encourage all to read Appendix A.3.3.4 - Direct Axial Load for some additional discussion.
In short - you have a question about WRC107. Expect to find your answer in WRC107 (now WRC537). Don't bother looking elsewhere - not VIII-1, not VIII-2, notB31.3, etc. The data from WRC107 (now WRC537) was based on empirical data. You can only get your answer from the source. The same holds true for WRC297, too, BTW. In fact, to appreciate the discontinuity stresses in the shell.nozzle due to internal pressure, I would recommend reading WRC368.