Chemical Engineering June 2010 - 32

Table 3. examples of Combined loadings
Cover Story
inspection openings on the vessel
prior to design and fabrication.
Overpressure protection
All vessels are required to have overpressure
protection in accordance with
Section UG-125 of the code. The relief
device can be located directly on the
vessel or installed within a process or
utility pipeline connected to the vessel.
In either case, authorized inspectors
may require identification of the
nozzle that will be connected to the
safety relief device. The identification
of the nozzle for safety relief is the responsibility
of the user and should be
discussed internally during the user's
process safety review.
Stainless-steel surface treatment
Users requiring a stainless-steel vessel
often do not provide specifications for
cleaning the vessel prior to shipment.
Stainless-steel surfaces and welds require
special surface treatments in
order to remove light surface contamination.
During fabrication, a vessel may
be exposed to shop dirt, carbon-steel
particles, permanent marker, crayon
marker, oil and grease - all contaminants
that can accelerate corrosion.
Carbon steel particles and iron can become
embedded in the plate and heads
due to routine shop handling, forming
rolls, layout tables, cutting tables and
carbon-steel grinding operations.
The most complete resource for
cleaning stainless steel is ASTM
International A380-06 (Standard
Practice for Cleaning, Descaling and
Passivation of Stainless Steel Parts,
Equipment and Systems) [2]. The
standard recommends the user precisely
define the intended meaning
of passivation since there are several
distinct operations.
The first of these operations is defined
in Paragraph 1.1.1 of ASTM
Standard 380, where passivation is
a process by which a stainless-steel
surface, when exposed to air or other
oxygen-containing environments, will
spontaneously produce a chemically
inactive surface. It is now accepted
that this film will develop in an oxygen-containing
environment provided
the surface has been thoroughly
cleaned and descaled.
For most users, passivation is the
Design Pressure
in Inner Vessel,
psig
+250
-15
Design Pressure
in Annulus, psig
+50
+200
+100 and -15 +150
Pressure Used for Design
of Inner Vessel,
psig
+250 and −50
−215
+100 and−165
removal of iron compounds from the
surface of stainless steel by means
of a chemical dissolution. This is accomplished
with an acid solution that
will not etch the surface or have significant
effects on the material. Some
methods involve cleaning the vessel
with a 20-25 vol.% nitric acid solution
at 120°F for 30 min. The nitric acid
solution removes contaminants and
oxidizes nickel on the surface to form
a chromium-oxide film on the surface
and thus prevent further corrosion
and oxidation.
Citric-acid treatment is the least
hazardous and most environmentally
safe method for removal of free
iron and other metal and light surface
contamination. Citric acid is
preferred with most manufacturers,
since no special handling equipment
or safety devices are required; no
NOx fumes are released and no corrosion
occurs in nearby equipment that
might come in contact with the solution.
Typical citric acid solutions are
4-10 wt.%. Spraying the solution and
lightly scrubbing the surface with a
soft brush is the preferred method for
cleaning large vessels.
The user must be aware that a passivation
treatment includes degreasing,
immersion and rinsing. The degreasing
process is crucial since air
cannot form a protective film when
grease or oil is present on the surface.
Therefore, it is important that the
manufacturer clean the vessel with a
commercial-grade degreaser prior to
passivation. After treating the vessel
with nitric or citric acid, a thorough
rinse with clean water should follow
without allowing the surface to dry
between steps.
The pickling process removes the
heat tint produced during welding
operations (Figure 1). Since nitric
and citric acids do not remove surface
layers, pickling removes the protective
oxide layer between 0.001 and
0.0015 in. of the substrate layer. Pickling
paste, nitric-hydroflouric acid
(HNO3-HF), is typically applied with
a nylon brush and can only be left in
contact for 15-30 min before excessive
corrosion is initiated. Similar
32 ChemiCal engineering www.Che.Com June 2010
Pressure Used for
Design of Jacket,
psig
+50
+200
+150
to the passivation treatments with
nitric acid, shop personnel will need
to wear the proper protective clothing
and receive proper training for handling
the product.
ASTM standard A380 also addresses
mechanical cleaning, including
such processes as power brushing,
sanding, chipping, grinding and
abrasive blasting. For removal of
localized areas of scale, grinding is
typically the most effective. To avoid
the risk of contaminating the stainless
steel, grinding operations have to
be carefully monitored to ensure the
grinding wheels being used have not
been previously used on carbon steel
plate. However, the standard does not
recommend abrasive blasting with
silica, since it is nearly impossible to
remove the embedded silica from the
surface of the material. Walnut shells
or glass beads are the preferred media
for abrasive blasting.
ExtErnal loadingS
Section UG-22 of the code provides a
short list of various external loading
conditions that need consideration,
including the following: wind, snow,
seismic loadings, as well as superimposed
static and dynamic reactions
from attached equipment, such
as machinery, piping and insulation.
While most specifications issued to
fabricators cover the bare necessities
for sizing a new vessel, many exclude
external loadings. Some specification
sheets are incomplete, such as those
requesting consideration for wind
and seismic loadings.
Specifications typically will reference
the required code for wind and
seismic loadings, such as American
Society of Civil Engineers (ASCE)
Standard 7-05 Minimum Design Loads
for Buildings and Other Structures
[3]. However, they often do not provide
specific information on the vessel's geographical
location, the wind exposure
category, the elevation of the vessel
from grade or the importance factor.
Without these details, the wind and
seismic loadings provide an inaccurate
picture as to what the vessel might
see in an upset condition. The ASME
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