Pressure Thrust 1

[B.L.Smith]

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,

 

[TGS4]

No.

 

[B.L.Smith]

Why? do u have any reference for this? Does ASME sec VIII mention this? which paragraph?

 

[msquared48]

Thrust implies a force parallel to the axis of the pipe, not normal to it. Affects the number of threads needed to be engaged.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jtseng123]

Your case is exactly the same as manway or nozzle with blind. I had the same question as you have 20 years ago, and I curiously applied pressure thrust to manway for WRC107. Guess what ? all manways failed. So I learned you don't apply pressure thrust in that situation.
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 ?

 

[brkmech1234]

jtseng123,

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?

 

[prex]

Pressure thrust is clearly acting on the nozzle, whatever the geometry of the connected pipe (or blind flange like in a manway).
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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[B.L.Smith]

Thanks friends. I'm agree with u that always nozzle can withstand pressure thrust as jtseng123 said. And I checked it by PVELITE.

 

[jtseng123]

Bradsmith, you misunderstood my point. I never say nozzle or its reinforcement can withstand pressure thrust. I said your case is just like a manway, and if it is like a manway, forget about pressure thrust, forget about WRC107, because if you run WRC107 with pressure thrust just like I experimented 20 years ago, it most likely will fail. And since on one in the world doing local stress analysis for a blinded nozzles such as manway, you don't do it. Otherwise, you will put unneccessary reinforcement beyond code "area" requirement due to pressure thrust. I did say " do not apply pressure thrust in this situation" (means not to apply pressure thrust to nozzle with cap or blind, manway.)

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 ?

 

[prex]

jtseng123, pressure thrust is always there, for blinded manway just like for a connected pipe. The difference is in the calculation. ASME VIII Div.1 is satisfied by checking nozzle reinforcement only (when pressure is the only loading): this is because pressure thrust transmitted by the nozzle neck to the border of the nozzle hole causes only secondary (discontinuity) stresses, and Div.1 does not generally limit secondary stresses on the following basis

It is recognized that high localized discontinuity stresses may exist in vessels designed and fabricated in accordance with these rules...

So when you calculate a manway under the pressure thrust per WRC107 you get those high discontinuity stresses that the code recognizes as possibly existing, but does not limit.
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
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[Bobfromoh]

Why stop there at the vessel/pipe interface. If a pressure thrust is added then what
about every location in a piping system where a header meets a lateral? I doubt that
additional loads are added throughout the piping system?

 

[jtseng123]

Prex, interesting ! If your quote from UG-23 is the correct interpretation, that imply no need to worry about pressure thrust. In other words, anyone shall not add pressure thrust to local stress analysis as long as nozzle satisfy code area replacement requirements, regardless it is manway or regular process nozzle.
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.

 

[prex]

jtseng123, you didn't fully read my post. I stated that you are not required to calculate per WRC107 only if pressure is the only loading acting on the nozzle, as in that case the rules of Div.1 are sufficient and autonomous (except for special details per U-2(g)). Otherwise you are required (UG-22) to analyse the supplemental loadings with any suitable and accepted calculation method.
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.

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[jtseng123]

Prex, I am too low tech to understand what you are talking about. I will go on my practice that is bullet-proof.

 

[TGS4]

Sorry for the delays in responding. I think, however, that this should end this discussion. All of this is talking about WRC107. Please note that WRC107 has been superseded and is now WRC537 - minor point, however.

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.