Elliptical Manway in Ellipsoidal Head

[DMay121]

Hey all,

I am trying to determine which diameter to use in reinforcement calculations for a 12"x16" manway that is inserted into an ellipsoidal head. I know on the shell there is the F factor to take into account the difference between longitudinal and circumferential stresses, but I am not sure how this logic applies to ellipsoidal heads.

Should I use the 12" as the nozzle ID, or 16"?

Thanks!

 

[TomBarsh]

The F factor accounts for difference in membrane stress due to pressure depending upon the angle of the plane cross-section from the longitudinal axis (remember, circ stress equals twice longitudinal stress, in general, etc.). It's used in the formula for required area, in which the required thickness is based on circ stress. So as the plane's angle varies, the required area should vary as well.

Of course, all this does not apply to nozzles located in heads and the Code states that F=1.0 for all configurations except for integrally reinforced openings in cylinders and cones...not heads.

 

[DMay121]

So is the stress in a formed head direction-dependent? Or can it be assumed to be Omni-directional? In a shell, it makes sense to orient an elliptical manway with the shortest diameter in the longitudinal direction (since that places the smallest area normal to the circumferential stress). If there are stress components in the head, then the same line of thinking would need to be applied to the head, even if no F-factor type variable is defined.

 

[TomBarsh]

Personally, I would orient the access opening such that people could squeeze through there conveniently...most of the time this might imply that the longer axis would be oriented horizontally. Of course, this would present the long axis in a favorable direction for a vertical cylinder, but in an unfavorable direction for a horizontal vessel. But that's just me and how I would myself prefer to squeeze through there.

Assuming a head as a doubly curved shell of revolution, and that is not a hemisphere (eg: it's semiellipsoidal or torispherical) there will be variations in directional stress in the Nphi and Ntheta directions. But ASME B&PV Code Section VIII Div 1 essentially disregards this for purposes of UG-36 and UG-37.

I think that most people will design elliptical openings in heads per UG-37 using the largest chord opening and F=1.0.