Nozzle loadings 2

[stesw90]

Dear all,
Below highlighted is statement in the specification of a clinet, same i have seen in fourth edition of pressure vessel manual, out which first condition we are satisfying in our calculation, but i didn't exactly understood the requirement of second case mentioned in bold letters, Does it mean full vaccum condition or no internal pressure i.e atmospheric. and how can I satisfy the same in my calculation.
With regard to the radial load(F A), calculations shall be made firstly with the force acting radially outwards in conjunction with the internal pressure and then with the force acting inwards. In the second instance, the internal pressure shall not be used to oppose the compressive stresses due to the force acting radially inwards; for this load condition a null pressure condition is to be considered to exist

 

[JStephen]

I would take that to mean atmospheric pressure in the second case, zero gauge pressure.

 

[TomBarsh]

I agree with JStephen.

The tensile stresses in the shell due to internal pressure will serve to counteract some of the compressive stresses in the analysis of the nozzle (or other) loads (for example, analysis as per Welding Research Bulletin WRC-107/WRC-537).

The specification copied above indicates that no advantage is to be taken for this result. Thus they are saying that internal pressure shall be "null", indicating no pressure differential across the pressure boundary. In other words, atmospheric pressure is to be assumed.

 

[TGS4]

I agree with the above. HOWEVER, in the event that there is a vacuum condition, then in order to consider the worst possible case, the external pressure ought to be used in such a calculation.

 

[jte]

To further fine-tune, the zero pressure - or hot shut-down - load should be run at:

a) zero pressure at internal pressure design temperature
b) specified external pressure at external pressure design temperature.

Expecting a loss of internal pressure coupled with coincident external pressure developing would in most processes be a double-jeopardy event. Thus, I would not normally consider external pressure at internal design temperature as an additional load condition.

 

[TGS4]

Excellent addition jte. I completely agree.

 

[JStephen]

If you assume radial loads acting inward at the same time as a full vacuum condition, you have a situation that is not covered by the normal vacuum design. You can calculate stresses okay, but analyzing the buckling would be a problem.

 

[SNORGY]

I sort of do but sort of don't agree fully.

I would at least look into the case where liquid is being withdrawn from the tank at a negative pressure corresponding to the maximum inbreathing condition. It's not worst case vacuum - which would be double jeopardy, perhaps - but it is a valid case.

 

[stesw90]

Thanks fo everyones reply,

If I have designed vessel with stiffner ring for external pressure,
May I now how nozzle external load will effect the design for external pressure,
External load will be axial where as external pressure will be acting in radial from all direction even on nozzle, which means it not gona contribute much in increasing axial load on nozzle,
And as i have already mentioned Shell itself is efficient to resist F.V condition.

 

[TGS4]

stesw90 - do you understand that axial (with respect to the nozzle) is radial with respect to the vessel? The two will add together. And that vacuum causes not only radial loads, but also longitudinal loads on the shell? Furthermore, do you understand the applicable failure mechanism for external pressure (buckling) and how it works, and is different from the other failure mode(s)? How have you determined that the shell itself is efficient to resist full vacuum condition?

 

[stesw90]

Vessel is design with stiffner rings at proper line of support with adequate thickness for shell which means shell on it's own is capable of taking loads due to external pressure hence no buckling due to external pressure,

But as you said if a condition arises were nozzle axail load add up with load due to external pressure it will be the worst case,

But if we are providing PVSV do we have to consider this situation.