Taper between vessel wall and hemi head ASME VIII

[Henderson17]

Hi all,

We are subcontracting design and manufacture of a pressure vessel and the initial drawings from our subcontractor are showing the hemi-head butting directly onto the tan line of the vessel.

Question 1: Is taper allowed to be constructed of weld build up?

We questioned this, as it was the first time we had seen such a construction, and our vessel fabricator directed us to UW-13 (3)]. Our client has subsequently said they do not think this conforms.

Question 2: If the taper constructed in this matter is it not standard to have this starting beyond the Tan line?

many thanks Scott.

 

[gr2vessels]

Hi Scott,
Hope the lines below will help..

Question 1: Yes, taper can be constructed by welding build-up.

ASME VIII Div 1, UW-13b(3)..."When a taper is
required on any formed head thicker than the shell and
intended for butt welded attachment [Fig. UW-13.1
sketches and (o)], the skirt shall be long enough so
that the required length of taper does not extend beyond
the tangent line. When the transition is formed by removing
material from the thicker section, the minimum thickness
of that section, after the material is removed, shall
not be less than that required by UG-23(c). When the
transition is formed by adding additional weld metal
beyond what would otherwise be the edge of the weld,
such additional weld metal buildup shall be subject to
the requirements of UW-42. The center line misalignment
between shell and head shall be no greater than one-half
the difference between the actual shell and head thickness,
as illustrated in Fig. UW-13.1 sketches (l), (m), ,
and (o)."

Question 2: Fig UW-13.1 The last note below the sketches and (o) states that the "Length of required taper l may include the width of the weld."

Cheers,
gr2vessels

 

[roca]

It seems like your fabricator is trying to save on shell material.

We normally ask for the centre line of the head and shell plate thickness to be aligned – e.g no offset between the plates, and for the shell to be extended to the point where the inside shell and head surfaces meet – (as in fig UW-13.1 (l) of ASME VIII Div.1).

The idea of a transition taper between components of different thicknesses is to avoid excessive local stress.
In my opinion by welding the hemispherical head directly onto to vessel tan line and then applying several layers of weld build up in order to produce a satisfactory taper seems to defeat the object trying to avoid excessive local stresses.

Even though the ASME Code allows a weld build up for the taper transition between the hemispherical head and the shell I have never seen it done in this way in Europe and would not accept it.

Of course you will have your own or Client specifications stating this otherwise it seems like you are stuck with the detail.

 

[vesselfab]

normally, the head is formed to a larger radius than the shell to line up the mean centerline of the head and shell. The shell is extended past the tangent line and head us cut back. The shell is manually tapered on the outside to meet the outside of the head. In my experience, it is a whole lot easier to remove excess shell material than to add weld metal build ups on both inside and outside of head to achieve ASME geometry for the joint.

 

[waskillywabbit]

If it does not conform to the client's specifications or yours then what the vessel contractor chooses to do relative to minimum Code requirements is mute.

Brian

 

[MJCronin]

Henderson:

See:

https://www.trinityheads.com/templatepage.asp?pageno=3

Pressure vessel heads are typically ordered with a straight flange and the desired internal or external taper.

"Buttering up" the joint and forming a taper to match thicknesses in not typically done in vessel fabrication. It is taken care of when the head is ordered.

Is this the first vessel that your subcontractor is attempting ?

-MJC

 

[gr2vessels]

I have had a similar experience some years back, when I assumed more than I should have and not clearly specified in the spec that the head should come with straight flange, min 2" long.
My vessel fabricator bought a second hand, cheap head with almost no straight flange and demonstrated to me with the help of ASME VIII that the head and the assembly are acceptable to the Code. Then I started a diary called lessons learned....
Cheers,
gr2vessels