UG-80 – UG-83
ASME BPVC.VIII.1-2015
UG-81
TOLERANCE FOR FORMED HEADS
(a) The inner surface of a torispherical, toriconical,
hemispherical, or ellipsoidal head shall not deviate outside of the specified shape by more than 11/4% of D nor
inside the specified shape by more than 5/8% of D , where
D is the nominal inside diameter of the vessel shell at
point of attachment. Such deviations shall be measured
perpendicular to the specified shape and shall not be
abrupt. The knuckle radius shall not be less than that
specified.
(b) Hemispherical heads or any spherical portion of a
torispherical or ellipsoidal head designed for external
pressure shall, in addition to satisfying (a) above, meet
the tolerances specified for spheres in UG-80(b) using a
value of 0.5 for L / D o .
(c) Measurements for determining the deviations specified in (a) above shall be taken from the surface of the
base metal and not from welds.
(d) The skirts of heads shall be sufficiently true to
round so that the difference between the maximum and
minimum inside diameters shall not exceed 1% of the
nominal diameter.
(e) When the skirt of any unstayed formed head is machined to make a driving fit into or over a shell, the thickness shall not be reduced to less than 90% of that
required for a blank head (see UW-13) or the thickness
of the shell at the point of attachment. When so machined,
the transition from the machined thickness to the original
thickness of the head shall not be abrupt but shall be tapered for a distance of at least three times the difference
between the thicknesses.
(-c) for spheres, L is one‐half of the outside diameter Do.
(3) For cylinders and spheres, the value of t shall be
determined as follows:
(-a) For vessels with butt joints, t is the nominal
plate thickness less corrosion allowance.
(-b) For vessels with longitudinal lap joints, t is
the nominal plate thickness and the permissible deviation
is
(-c) Where the shell at any cross section is made
of plates having different thicknesses, t is the nominal
thickness of the thinnest plate less corrosion allowance.
(4) For cones and conical sections, the value of t shall
be determined as in (3) above, except that t in (3)(-a),
(3)(-b), and (3)(-c) shall be replaced by te as defined in
UG-33(b).
(5) The requirements of (b)(2) above shall be met in
any plane normal to the axis of revolution for cylinders
and cones and in the plane of any great circle for spheres.
For cones and conical sections, a check shall be made at
locations (2)(-b)(-1), (2)(-b)(-2), and (2)(-b)(-3) above
and such other locations as may be necessary to satisfy
manufacturers and inspectors that requirements are met.
(6) Measurements shall be taken on the surface of
the base metal and not on welds or other raised parts
of the material.
(7) The dimensions of a completed vessel may be
brought within the requirements of this paragraph by
any process which will not impair the strength of the
material.
(8) Sharp bends and flat spots shall not be permitted
unless provision is made for them in the design.
(9) If the nominal thickness of plate used for a cylindrical vessel exceeds the minimum thickness required by
UG-28 for the external design pressure, and if such excess
thickness is not required for corrosion allowance or loadings causing compressive forces, the maximum permissible deviation e determined for the nominal plate
thickness used may be increased by the ratio of factor B
for the nominal plate thickness used divided by factor B
for the minimum required plate thickness; and the chord
length for measuring e max shall be determined by D o /t for
the nominal plate thickness used.
(c) Vessels and components fabricated of pipe or tube
under internal or external pressure may have permissible
variations in diameter (measured outside) in accordance
with those permitted under the specification covering its
manufacture.
UG-82
LUGS AND FITTING ATTACHMENTS
All lugs, brackets, saddle type nozzles, manhole frames,
reinforcement around openings, and other appurtenances
shall be formed and fitted to conform reasonably to the
curvature of the shell or surface to which they are
attached.
(a) When pressure parts, such as saddle type nozzles,
manhole frames, and reinforcement around openings, extend over pressure retaining welds, such welds shall be
ground flush for the portion of the weld to be covered.
(b) When nonpressure parts, such as lugs, brackets, and
support legs and saddles, extend over pressure retaining
welds, such welds shall be ground flush as described in
(a) above, or such parts shall be notched or coped to clear
those welds.
UG-83
HOLES FOR SCREW STAYS
Holes for screw stays shall be drilled full size or
punched not to exceed 1/4 in. (6 mm) less than full diameter of the hole for plates over 5/16 in. (8 mm) in thickness and 1/8 in. (3 mm) less than the full diameter of the
hole for plates not exceeding 5/16 in. (8 mm) in thickness,
and then drilled or reamed to the full diameter. The holes
shall be tapped fair and true with a full thread.
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