Chemical Engineering June 2010 - 31

Inside dish radius
Straight
flange
Tangent
line
Material thickness
Outside
diameter (O.D.)
Inside dish radius
80% (O.D.)
Straight
flange
Tangent
line
Material thickness
Outside
diameter (O.D.)
Knuckle radius
10% (O.D.)
Inside dish radius
90% (I.D.)
Inside knuckle radius
17.3% (I.D.)
Tangent
line
Material thickness
Inside
diameter (I.D.)
Figure 2. The geometry of an ASME 80-10 torispherical head is such that the dish
radius is 80% of the head diameter and the knuckle radius is 10% of the head diameter,
while an ellipsoidal head has a dish radius that is 90% of the inside head diameter
and a knuckle 17.3% of the inside head diameter
Type A or B stubs ends. For the nozzle
necks and any internal piping, specify
electric-resistance-welded (ERW) pipe
or seamless pipe (SMLS). Seamless
pipe cost is considerably higher and
will increase the vessel's cost. The
manufacturer is responsible for determining
if the nozzle requires a reinforcement
pad.
Users should also define the nozzles
used for inspection and overpressure
protection. If there is not a safety relief
device attached directly to the vessel,
most users will not identify an overpressure
protection nozzle. If the safety
relief device is not directly attached
to the vessel, identify the nozzle connected
to the piping system containing
the safety relief device. If the vessel
has a manway opening then clarify
whether or not a hinge or davit arm is
required. If the vessel requires a stud
pad or sight glass, then specify it.
A manufacturer and model number
for the sight glass should be provided
with the vessel specification so the
manufacturer can obtain a quote. For
stud pads, be sure to specify the size
and flange rating for the pad.
Safety and teSting
Corrosion allowance
The user should also specify a corrosion
allowance for the vessel according
to Section UG-25 of the pressure
vessel code. The only situation for
which a corrosion allowance is not required
in the specification is when experience
in " like service " has proven
corrosion did not occur or the corrosion
is superficial. Both the internal
and external surfaces of the vessel
should be considered.
If the vessel is subject to internal
corrosion, then the design should incorporate
a drain nozzle at the lowest
point or a pipe extending into the vessel
from any other location to within
0.25 in. of the lowest point.
Depending on the service, the user
may elect to have telltale holes drilled
part of the way into the pressure retaining
items. The code requires the
holes to be 1/16-in. to 3/16-in. dia. The
holes' depth must be greater than 80%
of the thickness required for a seamless
shell of like dimensions. The holes
should be on the surface opposite
where the deterioration is expected.
Straight
flange
Non-destructive examination
A common oversight is the specification
of the degree of non-destructive
examination (NDE) testing that is required.
Radiographic examination is
the most common method of NDE and is
incorporated into the code to establish
joint efficiencies for the weld seams.
All radiographic examination should
be in accordance with Section VIII,
Division I, UW-51 and with Article 2,
Section V of the code. Section UW-52
provides the minimum extent of spot
radiography, as well as procedural
standards, evaluation and retesting.
When specifying NDE, be sure to clarify
what level is required. Radiography
will increase the cost of the vessel.
Inspection openings
Inspection openings are important for
routine inspections of the vessel for
safety and life expectancy. Elliptical
manhole openings are permitted by
the code provided the opening is not
less than 11 in. by 15 in. or 10 in. by 16
in. and a circular manhole is not less
than 15-in. inside diameter (ID).
For hand-holes, the minimum size
restriction is 2 in. by 3 in., except
for vessels over 36 in. dia. where the
minimum size handhole is 4 in. by 6
in. and is used in place of a manhole.
If the vessel is less than 18 in. ID but
over 12 in. ID, the code requires the
vessel to have at least two handholes,
or two plugged and threaded inspection
openings, no smaller than 1.5 in.
nominal pipe size (NPS).
For vessels with ID between 18 and
36 in., the code requires either a manway,
two handholes or two plugged,
threaded inspection openings not less
than 2 in. NPS. For vessels with IDs in
excess of 36 in., the code requires one
manway opening, with the exception
that two 4- by 6-in. handholes can be
used if the vessel geometry does not
permit a manway.
Nozzles attached to piping or instrumentation
can be used for inspection
openings,
provided the
openings meet the required size and
are located to afford an equal view
of the interior of the vessel. It is the
user's responsibility to identify the
ChemiCal engineering www.Che.Com June 2010 31
Inside knuckle radius
Telltale holes are not permitted in vessels
intended for lethal service.
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Chemical Engineering June 2010

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