Minimum thickness in taper zone of hemipsherical head?

[barrind]

We have a vessel designed to PD5500. It has hempisherical ends which are connected to the main cylinder in a fashion identical to the bottom picture on page 3/210 (page 268 in the pdf version) of PD5500:2009, i.e. the hemispherical end is thinner than the cylinder and with the median lines of both in line with each other, and the transition between them formed by a taper or "cone" section, with a butt weld end the thin end of the taper. My question is what minimum thickness applies throughout the taper section? Common sense tells me this should be the same as the minimum required for the hemispherical end, i.e. Eqn 3.5.1-3 on page 3/8 (page 66 in pdf), but is it as simple as this? Does the minimum thickness ramp up through the taper as it nears the cylinder? The reason I am asking this is there is pitting corrosion in this region and we are wanting to carry out a fitness for service assessment on the vessel which requires the minimum thickness as a starting point. If the pits do not go beyond the minimum thickness then the assessment does not have to be carried out as such.

On a related note we are going to do a thorough thickness scan of this area from the outside,I guess this will tell us the thickness perpendicular to the outside taper face, and not the actual cross sectional thickness if you were to cut a vertical plane through the taper, is this relevant and how would we deal with this?

 

[prex]

The taper is formally part of the spherical head, so the minimum thickness there is the same as for the head.
However a transition with a maximum slope is also mandatory, hence the minimum thickness in the transition is given by the taper with the maximum slope connecting the minimum thickness of the head with the minimum thickness of the shell.
Concerning the measurement, I think you can go by locally measuring the thicknesses of the sphere and of the cylinder, then determining the thickness in the taper with a ruler laid down onto the faces of the taper.

prex

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[barrind]

Thank you. Could you point out a reference in PD5500 or other standard where it says the taper is considered part of the head, just so I can refer to this?

The image on the page I refer to has the maximum slope defined and the drawing of the vessel appears to comply with this, so I can assume that from the point where the tape starts on the cylinder then minimum thickness is that of the head via its equation.

As for the measurement, my concern is that it is the remaining thickness that is important for our FFS assessment, i.e. the depth remaining under individual pits in this tapered zone. We are using an automatic scanning system which traverese across a surface on guides to build a 3d map of the thickness. I guess what I am interested in is the distance from the base of a pit to the nearest outside surface and take that as the actual thickness in this region.

 

[Duwe6]

In a Strength Analysis, pits that are scattered [good distance between pits] are insignificant.

Where pitting matters is if the average, nominal thickness has been reduced. API-510 and 653 both have methods of averaging that are based on sound engineering principles, and are proven to work. You should use them, and may be required to by USA statute.

Pitting progression -- growth of depth per year -- also needs to be calculated. Gives estimate as to when the leaks will start. Again, see API-510/653

 

[barrind]

I will be using API 579 for the FFS assessment. This document covers pitting corrosion but in terms of depth, radius and distance between pits, sounds similar to the statistical methods you refer to in 510 and 653. This will end up being a level 3 assessment however if the assessment is required due to the proximity of nozzles to the affected area. However if the depth of all pits does not breach the minimum allowable thickness plus future corrosion allowance then no assessment is needed according to API 579, thus me trying to confirm how the tapered area is treated as regards its minimum allowable.

 

[prex]

The fact that the taper is in the head is simply because it's beyond the tangent line, so it's in the head. If it was considered to be in the cylinder, of course a thickness below the minimum for the cylinder couldn't be accepted.
And I'm not saying that from the point where the tape starts on the cylinder then minimum thickness is that of the head via its equation.
The minimum thickness at each position in the taper is the thickness resulting from a 'minimum taper'. If, for example, your taper was longer than what results from the code, you would have some extra thickness in the taper. But if both thicknesses of head and cylinder are equal to the minimum required, and the taper conforms exactly to the minimum requirements per code, then you have no extra thickness anywhere.

prex

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