Chemical Engineering June 2010 - 30

Table 2. guideline for head Thinning during forming
Cover Story
be equated to a specific surface finish.
Buffing is not intended to remove
metal from the surface. It is intended
to brighten and smooth the existing
surface with cotton- or felt-based
media and with the application of lubricants
to the buffing wheel.
For precise and consistent results, it
is recommended that the surface finish
be specified in a range of minimum and
maximum level of roughness average
(Ra). This can be expressed in microinches
or micrometers (Table 1).
The Specialty Steel Industry of
North America (SSINA) publishes a
designer handbook of specialty finishes
for stainless steel, which provides
detailed descriptions and sample
photographs. The handbook can
be downloaded free of charge at www.
ssina.com. Photographs for comparison
of certain standard finishes (Nos.
1, 2B, 2D, 2BA, 3, 4, 6, 7 and 8) for
sheets or various nominal thicknesses
can also be found at the website.
Vessel heads
Some of the most common heads in
service are as follows: ASME flanged
and dished (torispherical), 2:1 elliptical
flanged and dished (ellipsoidal), conical,
toriconical, hemispherical and flat.
Heads are formed based upon outside
vessel diameter, with the exception
of elliptical and hemispherical
heads, which are formed to the inside
diameter. When ordering the head, the
vessel manufacturer will provide the
head manufacturer with the minimum
permitted thickness that is required
based upon the calculations. Thinning
of the vessel head takes place primarily
at the knuckle regions and the center
of the dish (Table 2).
Torispherical heads. Torispherical
heads have dish radii equal to the diameter
of the head or vessel shell, and
the knuckle is 6% of the head insidecrown
radius as required by Section
UG-32(e) of the ASME Code (Figure
2). The straight flange (skirt) is a
standard 1.5 in. for heads formed from
3/16-in. plate and heavier. Straight
flanges up to 2 in. can be provided
upon request. For some head manufacturers,
a 3-in. straight flange can
be provided for head diameters ranging
from 36 to 54 in. as long as there
is a minimum plate thickness of 3/16
Head thickness range
12-gauge, up to and including 0.25-in. plate
Allowable thinning
during
forming
0.032 in.
5/16-in. nominal thicknesses up to and including 0.5-in. plate 0.062 in.
9/16-in nominal thickness up to and including 1.0-in. plate 15%
in. For heads 54 in. and larger, a 3-in.
straight flange can be provided with a
minimum plate thickness of 0.25 in.
When specifying a torispherical
head for a pressure vessel, it is important
for the user to clearly define
an ASME flanged and dished (torispherical)
head. Standard flanged and
dished heads are manufactured, but
do not meet the code requirement of
a minimum 6% inside-crown radius
for the knuckle region. As a result, the
standard flanged and dished heads
provide a higher stress concentration
factor and discontinuity in the
knuckle region. Some manufacturers
offer an ASME 80-10 head where the
dish radius is 80% of the head diameter
and the knuckle radius is 10%
of the head diameter. The advantage
of an ASME 80-10 head is that it is
thinner (~66% of the thickness of an
ASME torispherical head), which results
in a smaller blank size and reduced
labor cost.
A third option for a torispherical head
is an ASME high-crown head, where the
dish radius is 80% of the head diameter
and the knuckle radius is a minimum of
6% of the head diameter.
Ellipsoidal (2:1) head. A 2:1 elliptical
flanged and dished head provides a dish
radius that is approximately 90% of the
inside head diameter and a knuckle
that is approximately 17.3% of the inside
head diameter. The geometry of the
ellipsoidal head is provided in Section
UG-32(d) of the ASME Code.
The decision of whether to specify and
use a torispherical head versus an ellipsoidal
head is mainly an issue of
head clearance. Users should decide
which head better suits their needs.
If a dished head requires a bolting
flange, then the manufacturer
must design the head and flange in
accordance with the code's Appendix
1 (1-6). The cost of adding a bolting
flange is significant.
Toriconical heads. The transition
geometry of a toriconical head is typically
limited to a maximum half-apex
angle of 30 deg (Figure 3). The knuckle
cannot be less than 6% of the outside
diameter of the head skirt or less than
three times the calculated knuckle
30 ChemiCal engineering www.Che.Com June 2010
thickness as outlined in UG-32(h).
Toriconical heads or transitions
may be used when the half-apex angle
is greater than 30 deg and further requires
the design to be in compliance
with the mandatory Appendix 1 of
the code. A conical head or transition
does not have a knuckle. Therefore a
reinforcing ring is required by Appendix
1-5(d) and (e). Half-apex angles
greater than 30 deg for conical heads
and transitions shall be in accordance
with Appendix 1-5(g) of the code.
Un-stayed flat heads. These can
be incorporated into the design, but
have limitations in pressure and
temperature due to their geometry
(Figure 4). Section UG-34 of the code
provides the design requirements
for un-stayed flat heads and covers.
This includes bolted blind flanges,
flat plates with retaining rings, and
threaded covers. The section provides
nineteen examples of un-stayed flat
heads that can be used, but clarifies
that other designs, which meet the requirements
of UG-34, are acceptable.
The user may have an un-stayed flat
head design that is to be incorporated.
If so, users should provide a sketch of
what is desired and allow the manufacturer
to bring the proposed design
into compliance with the code.
Details are needed when specifying
closure heads on a pressure vessel.
When specifying the vessel shell
length, reference it from the tangent
line of one head to the tangent line of
the opposite head. The tangent line is
an accepted datum for most shops.
Nozzle schedule
Most users generally provide a nozzle
schedule, but significant information
is inherently omitted. When providing
a nozzle schedule, the manufacturer
is focused on size, type and quantity.
The physical placement of the nozzles,
and their projections can be addressed
during the drawing review process
(Figures 5 and 6). The user needs to be
clear on the types of flanges required
- raised-face slip-on flanges, raisedface
weld-neck flanges or lap-joint
flanges with stub-ends. When stubends
are considered, be sure to clarify
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Chemical Engineering June 2010

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