pressure thrust at nozzle-vessel connection

Do we really have to consider the so called pressure thrust as an external force that tends to push the nozzle away from the vessel & include it in WRC 107 calculation ?

With a closed system that is thick enough to withstand the longitudinal & hoop pressure stresses, I cant imagine how a steady state internal pressure alone can cause such an external force the same as radial load "P".

Regards,

Hi there hch,

So far it's interesting for me to see people around the world participating in this forum sharing ideas, mentor other guys, etc. For me, yes you really need to. Charles Becht in his book Process Piping: The Complete Guide to ASME B31.3 has this emphasized. In my case, I always remind mysel that stress analyis should be as accurate as it should be and we all knows the underlying philophy on that. Pressure could be very high. A very common coling water system is about 125 psig static pressure and in an 8" line, this gives you 6200+ lbs. of thrust to your elbow - and as you said, a kind of an external force that tends to pulll the nozzle away from the vessel's shell. Double the pressure and the thrust also doubles.For me this is a high figure to neglect with no matter how thick the vessel shell is. For me document properly everything regardless of the outcome.

Regards,

When I was younger, I too tried including the pressure thrust in WRC-107 calcs. After all, the check box is there. Everyone fails. In fact, they fail under pressure thrust alone, every time. Yet, these nozzles work just fine.

I can't explain the why anymore, although it was explained to me to my satisfaction previously, but the pressure thrust option should not be turned on for WRC-107 local stress checks.

Hi everybody!

Mr. Klein is right. Generally, when the pressure thrust force is employed, the strength criteria fail.

In my opinion, the most suitable approach for WRC 107/297 method is to employ "Include Pressure Stress Indices (Div 2)" option and TO IGNORE "Include Pressure Thrust" option.

The reason is that ASME section VII Div. 2 Pressure Vessel Code provides the Pressure Stress Indices values (at Article AD-560.7 and Table AD-560.7, or Article 4-6, Par. 4-612 - I have an older edition, I appologize if the reference paragraph numbers have been changed) in order to assess the highest peak pressure stresses at the "nozzle-shell" junction.
The highest peak pressure stresses are obtained by multiplying the nominal vessel hoop stress by those indices values. NO PRESSURE THRUST FORCE is employed.

So, since ASME VIII-Div.2 uses those pressure stress indices without any pressure thrust force, in my opinion this approach means that those indices include the pressure thrust force effect. This is quite natural since in the most of the practical situations, the nozzles are "closed" either by a blind flange or by a connected pipe with a direction changing (bend/elbow) and consequently the peak pressure stresses are computed by including the pressure thrust contribution.

Regards,

Dorin Popescu

You may try reading the content of this. It might help. I've been thinking more on this too. Not so veratile yet in pipe stress analysis but trying..

http://www.carmagen.com/newsletter/news22.htm

Regards,