About closed thread "Nozzle Pressure Thrust in WRC-107 Calculations"

[robertomaierna]

thread794-102016
A lot of opinions (and confusion) on the matter.
I think the common sense will help.
Summarizing:
1)the pressure thrust exists and produces local effects on the vessel's shell in proximity of the opening even if it is balanced on the opposite wall of the vessel.
2)the above occurs, when the vessel is operating, even if the piping is not attached at the nozzle (e.g. blind flange)
3)the effects of such thrust load should have been taken into account in the original design of the vessel

It means that the allowable loads issued by the vessel designer should refer to loads which are obviously not known at that stage but which can be expected when a pipe will connect that nozzle to other components of the plant.
Therefore, when the piping stress analysis is carried out, the vessel (nozzle, shell, reinforcing pad) should have been already calculated for the simoultaneous presence of pressure thrust and other (other than pressure) loads transferred by the piping and not exceeding the "allowables".
On the other hand there is reason for suspecting that some supplier of vessel, at best, do not srictly follow this approach.
Therefore, while it is responsibility of the piping stress analyst to issue the calculated external loads (not including the pressure thrust) to be compared with te allowables, it is also strongly advisable that this is clerly indicatd in the technical exchanged documents with a note saying: "Loads due to internal pressure (Pressure Thrust) are not included".
This will (should) put on alert the supplier of that vessel whose design is poor from this point of view, and in this stage it will be possible to amend the discrepancy coming to a good design.

 

[gr2vessels]

roberto,
The vessel nozzle loads have to be included in the vessel design calcs earlier than the actual piping loads can be estimated. Hence the vessel designer uses some conservative figures for estimating the stresses due to external loads, way ahead of the pipe stress analysis.
As far as the pressure thrust concerns, keep in mind that the pressurised vessel is resisting much easier the external loads, than in depresurised condition. When is deflated like a party baloon and the hot pipes keep poking into the vessel nozzles, then the trouble starts. Perhaps your supplier might not be so bad afterall.
Cheers,
gr2vessels

 

[saplanti]

Roberto,

I totally agree with your summary. In addition to that, the company procedures should reflect all these issues to distribute the responsibilities by identifying the full expectations. In case the vessel design engineer does not include the branch pressure trust loading to local nozzle calculations, the piping engineer will always be in the trouble to suit the piping loading to the given allowables if the trust load needs to be considered due to the piping arrangement.
From my experince, the trust load is always there, the piping between vessels is not straight into the vessels due to the thermal expansion issues. So, I would include the trust load during the vessel design stage in addition to the allowables given. This solves the problem on both sides.

I agree with you, it is difficult to solve problems after they occured.

Kind regards,

Ibrahim Demir