June
2010
Hazardous
Fluid Piping
Design
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Pressure
Vessel
Design
Bridging the gap between
users' and manufacturers'
responsibilities for the
ASME pressure vessel code
Keith Kachelhofer,
Jedson Engineering Inc.
T
racing its origins to 1915, the
American Society of Mechanical
Engineers (ASME) Boiler and
Pressure Vessel Code (the code)
[1] has become the established safety
standard governing the design, fabrication and inspection of boilers and
pressure vessels, as well as nuclear
power plant components during construction. Section VIII, Division I of
the code addresses pressure vessels
operating at either internal or external pressures exceeding 15 psig.
Despite the prevalence of pressure
vessels in the chemical process industries (CPI), a clear understanding
of the basis-of-design responsibilities
involved in designing, fabricating and
repairing such a device remains elusive. Vessel users are responsible for
providing all necessary data to ensure
the manufacturer can design and fabricate a pressure vessel in full compliance with the code.
The lack of clear understanding can
result in a disconnect between users
and manufacturers during pressure vessel specification. The disconnect is often
magnified because, although Section
U-2(a) of the ASME Code clearly defines
the responsibilities for establishing the
28
basis of design, users often lack access
to the code language and its associated
interpretations. Basis of design refers
to well-defined information that could
form the foundation for inspection and
test acceptance criteria.
While engineering specifications
often provide sufficient data for a manufacturer in certain basic areas — such
as internal and external pressure, temperature, vessel orientation, material
of construction, corrosion allowance
and vessel contents — pressure vessel
fabricators usually receive insufficient
information from users in areas such
as wind, seismic and external loadings.
The incomplete specification information makes a proper and complete
vessel design difficult and can lead
to inaccurate price quotes. Providing
complete information will help avoid
cost overruns and change-orders.
The intent of this article is to clarify
those areas of pressure vessel specification where information is commonly
omitted and areas where further clarification is required. Further, this article
is intended to improve understanding
of which responsibilities are shouldered by vessel users and which by
manufacturers. By providing a more
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010
comprehensive basis of design for a
vessel, users and manufacturers can
save money and formulate specifications with public safety in mind.
All 50 U.S. states, all Canadian
provinces and many local jurisdictions
and territories have formally adopted
the ASME Code as a safety standard
for boilers and pressure vessels. Each
jurisdiction employs a chief inspector
who is a member of the National Board
of Boiler and Pressure Vessel Inspectors. Meanwhile, the code is frequently
a prevailing basis in other countries
throughout the world.
VESSEL DESIGN
Design versus operating T and P
In engineering specifications, often no
distinction is made between the design
pressure and operating pressure. Section UG-21 of the code recommends a
suitable design pressure above the operating pressure of the vessel at which
the vessel will normally operate. The
operating pressure should represent
the most severe exposure of pressure
and temperature the vessel is expected
to experience under normal operating
conditions, whereas the design pressure should allow for potential pres-
I •• B
TABLE 1 . SURFACE FINISH COMPARISON
Grit
finish
Ra
(microinch)
RMS
(Mm)
36
142
4.06
60
87
2.49
80
71
2.03
120
52
1.47
that is the case, clearly indicate
it in the specification.
A material specification for
42
150
1.20
appurtenances
is commonly
180
30
0.86
not given. Items such as lift220
19
0.53
ing lugs, support lugs, skirts
15
240
0.43
and support legs can often be
specified with a different grade
12
0.36
320
material than that of pressure
400
9
FIGURE 1. The pickling process re0.25
retaining
items. The user may
moves the heat tint produced during
Mirror
+/-4
0.13
have a vessel where all of the
welding (left = before; right = after)
pressure-retaining items and
sure surges up to the setting of the strictions for material-grade, post-weld wetted surfaces are 316L stainless
pressure-relief device. The design tem- heat treatment, and allowable hard- steel, but the lifting lugs and support
perature should account for the lowest ness of the weld and heat-affected zone; ring may be fabricated from 304 stainand highest operating temperature, in all of which will impact the manufac- less steel. Depending on the user and
addition to operational upsets, atmo- turer's cost for fabrication. The user the service, a stainless-steel vessel
spheric temperature and other sources should identify dangerous compounds with carbon-steel legs might be acceptof cooling. The design and operating in its process and address the dangers able, provided there is a "poison pad"
conditions should be established in a in a process safety review meeting.
between the two materials such that
process safety review meeting within
The user should provide the specific the pressure-retaining items are not
the user's organization.
gravity of the process fluid, since the at risk of carbon contamination. Often
Based upon the material of construc- manufacturer must account for the ad- manufacturers will specify an altertion, the nominal plate thickness and ditional static pressure due to the static nate material grade for appurtenances
the minimum design temperature, the head of the liquid, per section UG-22(b). in order to minimize fabrication cost. If
manufacturer will have to determine
alternate material grades are unacceptthe requirements for welding. For car- Materials of construction
able for appurtenances, that should be
bon steel and low-alloy vessels, the re- While the material of construction is stated in the specification, particularly
quirement for Charpy impact testing commonly included in equipment speci- when manufacturers are competitively
can be determined in Section UCS-66 fications, clarification is often required bidding for the contract.
of the code. For high-alloy vessels, as to the impact the material specifisuch as those fabricated of austenitic cation has on the fabrication and the Stainless-steel surface finish
stainless steel, the manufacturer will quote. Information in the specification For users in the food-and-beverage and
refer to Section UHA-51 of the code. should allow the manufacturer to deter- pharmaceutical industries, there are
The manufacturer will determine if mine whether or not its qualified weld often requirements for special interimpact testing is required and if the procedures and qualified welders are nal- and external-surface finishes. Amshop has a qualified weld procedure to sufficient for the alloy specified. In cases biguity and different interpretations
meet the requirements of the Code.
where the manufacturer has to qualify about user expectations and manufaca procedure for an alloy not commonly turer capabilities can arise when the
Vessel contents
welded in the shop, the cost impact mechanical finish is specified as one
With regard to vessel contents, the should be evaluated.
of the following: satin, polished, bright,
key phrase for the fabricator is "lethal
The material specification and the dull or mirror. Parameters such as conservice." Vessels are considered lethal grade designation should also be pro- tact time, material feedrate, abrasive
service if the contents, whether mixed vided to the manufacturer. For ex- pressure and application of lubrication
with air or alone, are dangerous to life ample, if the vessel is fabricated from will have an impact on the finished
when inhaled. Lethal service imposes austenitic stainless plate, then indi- product. Special finishes supplied by
mandatory Code-compliance require- cate the ASTM International (ASTM) the manufacturer are not published by
ments on the manufacturer, such as specification and grade, such as: A240- ASTM International.
100% radiography of all welds. These 316L. For the nozzles the specificaPolishing and grinding involve the
requirements can substantially in- tion and grade will be A312-316L. If removal of metal from a surface with
crease the vessel's fabrication cost.
the process safety review determines an abrasive, resulting in surface direcIf the process involves hydrogen that seamless pipe is required for the tional marks. There is no definition of
sulfide, where there is a risk of sulfide nozzles, then this should be clarified, an abrasive grit size that differentiates
stress cracking, then the manufacturer since it will impact the vessel's cost. grinding from polishing. As a guide,
needs to be advised of the require- For flanges, the specification and grade however, grit sizes of 80 and coarser can
ments of the National Association of will be A182F-316L. Depending on the be associated with grinding, whereas
Corrosion Engineers (NACE) standard process, some users may prefer using a grit sizes of 120 and finer can be asRP0472 and NACE publication 8X194.
carbon-stoel backing flange in conjunc- sociated with polishing. Nevertheless,
Hydrogen sulfide service will have re- tion with a stainless-steel stub-end. If simply specifying the grit size cannot
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010
29
TABLE 2. GUIDELINE FOR HEADTHINNING 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.
Allowable thinning during
forming
0.032 in.
12-gauge, up to and including 0.25-in. plate
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%
Head thickness range
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 reVessel heads
sults in a smaller blank size and reSome of the most common heads in duced labor cost.
service are as follows: ASME flanged
A third option for a torispherical head
and dished (torispherical), 2:1 elliptical is an ASME high-crown head, where the
flanged and dished (ellipsoidal), conical, dish radius is 80% of the head diameter
toriconical, hemispherical and flat.
and the knuckle radius is a minimum of
Heads are formed based upon out- 6% of the head diameter.
side vessel diameter, with the excep- Ellipsoidal (2:1) head. A 2:1 elliptical
tion of elliptical and hemispherical flanged and dished head provides a dish
heads, which are formed to the inside radius that is approximately 90% of the
diameter. When ordering the head, the inside head diameter and a knuckle
vessel manufacturer will provide the that is approximately 17.3% of the inhead manufacturer with the minimum side head diameter. The geometry of the
permitted thickness that is required ellipsoidal head is provided in Section
based upon the calculations. Thinning UG-32(d) of the ASME Code.
of the vessel head takes place primar- The decision of whether to specify and
ily at the knuckle regions and the cen- use a torispherical head versus an elter of the dish (Table 2).
lipsoidal head is mainly an issue of
Torispherical heads. Torispherical head clearance. Users should decide
heads have dish radii equal to the di- which head better suits their needs.
ameter of the head or vessel shell, and
If a dished head requires a boltthe knuckle is 6% of the head inside- ing flange, then the manufacturer
crown radius as required by Section must design the head and flange in
UG-32(e) of the ASME Code (Figure accordance with the code's Appendix
2). The straight flange (skirt) is a 1 (1-6). The cost of adding a bolting
standard 1.5 in. for heads formed from flange is significant.
3/16-in. plate and heavier. Straight Toriconical heads. The transition
flanges up to 2 in. can be provided geometry of a toriconical head is typiupon request. For some head manu- cally limited to a maximum half-apex
facturers, a 3-in. straight flange can angle of 30 deg (Figure 3). The knuckle
be provided for head diameters rang- cannot be less than 6% of the outside
ing from 36 to 54 in. as long as there diameter of the head skirt or lees than
is a minimum plate thickness of 3/16 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 l-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
Straight
flange
Telltale holes are not permitted in vessels intended for lethal service.
Outside
diameter (O.D.)
Knuckle radius
10% (O.D.)—'i
Outside
diameter (O.D.)
Inside knuckle radius
173% (I.D.)
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.
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
TABLE 3. EXAMPLES OF COMBINED LOADINGS
1 Cover Story
Design Pressure
in Annulus, psig
Pressure Used for Design of Inner Vessel,
psig
Pressure Used for
Design of Jacket,
psig
+50
+250 and -50
+50
-15
+200
-215
+200
+100 and -15
+150
+100and-165
+150
iLTilittMll
psig
+250
inspection openings on the vessel
prior to design and fabrication.
Overpressure protection
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 environmenStainless-steel surface treatment tally safe method for removal of free
Users requiring a stainless-steel vessel iron and other metal and light suroften do not provide specifications for face contamination. Citric acid is
cleaning the vessel prior to shipment. preferred with most manufacturers,
Stainless-steel surfaces and welds re- since no special handling equipment
quire special surface treatments in or safety devices are required; no
order to remove light surface contami- NOx fumes are released and no corronation. During fabrication, a vessel may sion occurs in nearby equipment that
be exposed to shop dirt, carbon-steel might come in contact with the soluparticles, permanent marker, crayon tion. Typical citric acid solutions are
marker, oil and grease — all contami- 4-10 wt.%. Spraying the solution and
nants that can accelerate corrosion. lightly scrubbing the surface with a
Carbon steel particles and iron can be- soft brush is the preferred method for
come embedded in the plate and heads cleaning large vessels.
due to routine shop handling, forming
The user must be aware that a pasrolls, layout tables, cutting tables and sivation treatment includes degreascarbon-steel grinding operations.
ing, immersion and rinsing. The deThe most complete resource for greasing process is crucial since air
cleaning stainless steel is ASTM cannot form a protective film when
International A380-06 (Standard grease or oil is present on the surface.
Practice for Cleaning, Descaling and Therefore, it is important that the
Passivation of Stainless Steel Parts, manufacturer clean the vessel with a
Equipment and Systems) [2]. The commercial-grade degreaser prior to
standard recommends the user pre- passivation. After treating the vessel
cisely define the intended meaning with nitric or citric acid, a thorough
of passivation since there are several rinse with clean water should follow
distinct operations.
without allowing the surface to dry
The first of these operations is de- between steps.
fined in Paragraph 1.1.1 of ASTM
The pickling process removes the
Standard 380, where passivation is heat tint produced during welding
a process by which a stainless-steel operations (Figure 1). Since nitric
surface, when exposed to air or other and citric acids do not remove surface
oxygen-containing environments, will layers, pickling removes the protecspontaneously produce a chemically tive oxide layer between 0.001 and
inactive surface. It is now accepted 0.0015 in. of the substrate layer. Pickthat this film will develop in an ox- ling paste, nitric-hydroflouric acid
ygen-containing environment pro- (HNOg-HF), is typically applied with
vided the surface has been thoroughly a nylon brush and can only be left in
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.
cleaned and descaled.
For most users, passivation is the
32
contact for 15—30 min before excessive corrosion is initiated. Similar
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010
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
Inside
diameter
(I.D.)
—;—
- nside
Straight
flange
Tangent -=
termining wind speed, importance factor, exposure category,
topographical factor and gust
factor. The wind speed is obtained from ASCE 7-05., which
provides a map of the U.S. with
basic wind speeds for various locations, including special wind
areas at the hurricane coastlines of the following regions:
the west coast of Mexico, the
eastern part of the Gulf of MexFIGURE 3. Toriconical heads and transition
ico
and the Southeast U.S. and
geometries are limited to a maximum half-apex
the Mid- and North Atlantic.
angle of 30 deg
It is important to understand
that it is assumed that the wind
could come from any horizontal
direction. Where there is mountainous terrain, gorges or other
K^_<£ Vessel —
special wind regions, there can
be an adjustment made to the
FIGURE 4. Requirements of un-stayed flat
values in Figure 6-1 of the code
heads and covers include bolted blind flanges,
flat plates with retaining rings and threaded cov- to account for higher local wind
speeds. This adjustment shall
ers. Here are a few examples
be based on local meteorologiBoiler and Pressure Code, Section VIII cal information.
is referenced in ASCE 7-05.
The occupancy category is provided
Seismic loadings. Section 15.7.2 (c) in Table 1-1 of ASCE 7-05. The occuof ASCE 7-05 requires hydrodynamic pancy category is based upon the navertical, lateral and hoop forces to be ture of occupancy during upset condiconsidered in cylindrical tank and ves- tions involving excessively high winds
sel walls. These forces shall be evalu- or earthquakes. The categories for ocated to determine the increase in hy- cupancy range from Occupancy Catdrostatic pressure and hoop stress.
egory I (buildings with low hazard to
In addition, Section 15.7.3 requires human life in the event of catastrophic
the evaluation of all structural com- failure) to Occupancy Category IV (esponents that are an integral part of sential facilities, such as hospitals).
the lateral support system. The evalu- Facilities that manufacture or process
ation should ensure that connections hazardous fuels, hazardous chemiand attachments for anchorage and cals, hazardous waste or explosives
other lateral-force-resisting compo- are considered to be an Occupancy
nents, as well as nozzle penetrations Category III. If the hazardous mateand openings are designed to main- rial exceeds a threshold quantity estain structural stability and integrity tablished by a local jurisdiction, then
of the shell. Vessel stiffness in rela- the vessel is classified as Occupancy
tion to the support system should be Category IV. The importance factor
used to determine the forces on the for wind loadings is provided in Table
vessel. If the vessel is oriented hori- 6-1 of ASCE 7-05 and is based upon
zontally, then analysis is required the occupancy category.
at the saddle supports per Section
Pressure vessels are placed into one
15.7.14.3. The combination of these of three exposure categories (B, C or
loads should be used to establish the D), which depend on ground surface
maximum allowable working pres- roughness. To derive the exposure catsure of the vessel as outlined in Code egory, surface roughness must first be
Interpretation VIII-1-01-03.
defined. Surface roughness is deterWind loadings. Wind loadings are mined by natural topography, vegetacovered in Section 6.5 of ASCE 7-05. tion and nearby buildings and strucThe design procedure for wind load- tures. Surface roughness category B is
ings on pressure vessels requires de- defined as suburban areas and wooded
areas with numerous, closely spaced
obstructions the size of a single-family
house or larger. Surface roughness C is
defined as open terrain with scattered
obstructions of heights less than 30 ft.
This category encompasses flat open
country, grasslands and water surface
areas in hurricane-prone regions. Surface roughness D is characterized by
flat, unobstructed areas outside of the
hurricane-prone regions. This includes
salt flats, mud flats and unbroken ice.
A vessel's surface-roughness category is then used to determine its exposure category. Exposure category B
is defined by surface roughness B with
the wind prevailing in the upwind direction for a distance of at least 2,600
ft, or 20 times the height of the building, whichever is greater.
Exposure category C is used for
cases where Exposures B or D are not
applicable. Exposure D applies when
the surface roughness is defined as surface roughness D, where the prevailing
wind direction is upwind for a distance
of 5,000 ft or greater. Exposure D can
also apply to surface roughness B or C
for a distance of 600 ft, or 20 times the
height of the structure, whichever is
greater. If the site under consideration
is located in a transition zone between
exposure categories, then the largest
wind forces apply.
Topographical effects only need to
be considered for increased wind speed
over hills and ridges. Section 6.5.7 of
ASCE 7-02 provides detailed procedures for calculating topographical
effects. The gust effect factor for rigid
structures should be 0.85, per Section
6.5.8.1, or should be calculated. If the
vessel is dynamically sensitive, then
Section 6.5.8.2 of ASCE 7-05 provides
the necessary steps to calculate the
gust effect factor.
These five constants and categories
provide sufficient data for the fabricator to properly design the vessel for
wind loadings. The ASME Code requires manufacturers to consider the
combination of the vessel design pressure along with the secondary stresses
from wind or seismic loads.
External piping loads. External
nozzle loadings are typically overlooked, especially those loadings imposed by high-temperature piping. It
is the user's responsibility to notify the
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010
33
Cover Story
V1
fabricator if the piping may impose
excessive loadings on the nozzles.
This includes excessive loadings
Nozzel identiduring normal operating conditions
and during upset conditions. Sec- fication:
Nozzel size:
tion 15.7.4 of ASCE 7-05 requires
P
(Concentrated
the analysis of the piping system
radial load):
Mi (External overconnected to the vessel during
, in.-lb
turning moment):.
Nozzel identification:
earthquake conditions.
M2 (External overNozzel size:
The piping system and supports
turning moment):.
.in.-lb
. Ib
P (Concentrated radial load): —
(Concentrated
ML (External overturning moment):
in.-lb
shall be designed such that there
Jb
shear load):
Me (External overturning moment):
in.-lb
is no excessive loading on the vesV2 (Concentrated
VL (Concentrated shear load):
Ib
sel wall. The assumption that a
.Ib
shear load):Vc (Concentrated shear load):
Ib
MT (Concentrated external
nozzle is an anchor point for piping
MT (Concentrated external
.in.-lb
torsional moment):
torsional moment):
in.-lb
is poor practice. Stresses need to be
considered in the shell/head at the
FIGURE 6. Agitators and mixers are
nozzle-to-shell juncture. Therefore FIGURE 5. It is the vessel user's responsibility to notify the fabricator as to
sources of external nozzle loading. Users
all external loads are considered to whether the piping imposes excessive
should obtain the mixer reaction loads
be acting simultaneously. All exter- loadings on the nozzles
from their maker, and relate that to the
nal loads, such as the longitudinal
vessel manufacturer
and circumferential shear loads and
moment loads, have to be considered the vessel head needs to be increased.
entirely to cylindrical shell
with the radial and torsional loads in Some users have invested significant • Type 2 - Jacket covering part of the
conjunction with the design pressure of expense to replace wrecked agitators
cylindrical shell and one head
the vessel.
due to flexing nozzles. Calculating the • Type 3 - Jacket covering any portion
These loads can be analyzed per the stress on the vessel shell and nozzle is
of the head
Welding Research Council (WRC) Bul- the same as those calculations for pip- • Type 4 - Jacketed with an added
letin 107 and its supplement (Bulletin ing nozzles using the WRC 107 / WRC
stay or equalizer rings to the cy297 — for cases where the stress is 297 procedures.
lindrical shell portion to reduce
evaluated in the shell only) [4,5]. Calthe effective length
culating loads with WRC 107 and 297 JACKETED VESSELS
• Type 5 - Jacket covering the cylinis time-consuming if performed manu- Jacketed vessels are addressed in Apdrical shell and any portion of eially, because numerous non-dimen- pendix 9 of the ASME Code. Appendix
ther head
sional geometric parameters have to 9 applies to the jacketed portion of the Half-pipe jackets are covered in a nonbe interpolated from multiple charts. vessel, which includes the wall of the mandatory appendix of the code, ApComputer software programs are avail- inner vessel, the wall of the jacket, pendix EE. The calculation procedure
able to aid calculations.
and the closure between the inner ves- in the code is for the conditions where
sel and the jacket. The manufacturer there is positive pressure in the vesExternal equipment loads
shall consider the combined loading sel shell or head and positive pressure
Agitators and mixers are another of the vacuum/pressure on the jacket in the half pipe jacket. The code fursource of external nozzle loadings. Ob- wall along with the pressure/vacuum ther provides restrictions to half pipe
tain the mixer reaction loads from the within the inner vessel wall and deter- jacket sizes of NFS 2, 3 and 4 with
equipment manufacturer and relate mine which of these is greater than the vessel diameters ranging from 30 to
these loads to the vessel manufac- individual loading (Table 3).
170 in. The code does permit jackets of
turer. The mixer reaction loads have
The code categorizes jacketed ves- other geometries, such as circular segan impact on the cost of the vessel if sels, which provides a convenient ments, channels or angles.
a reinforcement pad, or gussets are method to assign closures.
The vessel manufacturer should
required, or if the plate thickness on • Type 1 - Jacket (any length) confined consider other combinations of pres-
References
1. American Society of Mechanical Engineers
(ASME). "ASME Boiler & Pressure Vessel
Code, Section VIII, Division I," 2007 Edition,
Addenda, 2007.
2. ASTM International. "Designation: A380-06,
Standard Practice for Cleaning, Descaling,
and Passivation of Stainless Steel Parts,
Equipment, and Systems."
3. American Society of Civil Engineers. "ASCE
7-O5: Minimum Design Loads for Buildings
and Other Structures."
34
CHEMICAL ENGINEERING WWW.CHE.COM
4. Wichman, K.R., Hopper, A.G., Mershon, J.L.,
"WRC Bulletin 107 / August 1965: Local
Stresses in Spherical and Cylindrical Shells
due to External Loadings", Welding Research Council, 1965.
5. Mershon, J. L. and others. "WRC Revised Bulletin 297: Local Stresses in Cylindrical Shells
due to External Loadings on Nozzles - Supplement to WRC Bulletin No. 107 - (Revision
1)", Welding Research Council, 1987.
6. Tuthill, A. H. and Avery, R. E. "Specifying
Stainless Steel Surface Treatments." Nickel
JUNE 2010
Institute, http://www.nickelinstitute.org.
Farr, J.R. and Jawad, M.H., "Guidebook for
the Design of ASME Section VIII Pressure
Vessels," ASME. 1998.
Chuse, R., and others "Pressure Vessels:
the ASME code simplified," 7th Edition,
McGraw-Hill, Inc., 1993.
Powell, C. and Jordan, D. "Fabricating Stainless Steels for the Water Industry," Nickel
Institute, Reference Book Series No. 11 026,
October 2005.
sure loadings outside of what is provided in the rules of Appendix EE.
These include the following:
• Negative pressure inside the vessel
Dry Screening Reaches New Heights
and inside the jacket
The APEX1" Screener from ROTEX Global, LLC
• Negative pressure inside the shell and
is the smart solution for dry screening. The
presents
positive pressure inside the jacket
APEX delivers high productivity and low
• Positive pressure inside the vessel and
operating costs with the same efficiency
and gyratory-reciprocating motion as
negative pressure inside the jacket
the ROTEX®Screener. Economically f
• External nozzle loadings from piping
designed to increase uptime, the
connections
APEX features side access
• Cyclic loadings to any of the above
doors that enable quick screen
combinations
changes and maintenance by
When specifying the half pipe jacket be
one person.
sure to provide the manufacturer with
To find out how the APEX™ Screener can
the pipe size required, the pitch of the
increase your productivity, go to
coils, the design temperature and the
rotex.com/apex, or call 1-800-453-2321.
design pressure. An approximate location of the half-pipe inlet and outlet
IB & c
nozzles is beneficial for the manufacturer to determine potential interferences with other nozzles on the vessel.
Dimpled and embossed jackets are
another form of jacket assemblies
and are addressed by Appendix 17, a
©2010 ROTEX Global. LLC
mandatory appendix of the Code. If
Circle 17 on p. 62 or go to adlinks.che.com/29251-17
the manufacturer is using a plate that
has been dimpled or embossed prior
Solid-Liquid Separation Solutions
to welding, then a proof test shall be
sub-micron to macro-molecules
performed in accordance with Section
UG-101 of the code. The proof test
requires the use of a representative
•^ VertkaI:::F.i>teri!ig::gfntnfygg§ - standard & custom
panel, which is rectangular with at
cGMP designs are economical and high capacity
4,
least five pitches in each direction and
(up to 35 ft') with small footprint
not less than 24 in. in either direction.
A proof test can add additional cost to
the fabrication of a vessel.
»• .High-Speed Tubular
It is strongly recommended that if a
up to 20.000 G-force for
vessel requires any type of jacket, the
separating sub-micron
user should provide a supplemental
partides from liquids and ,1?
specification sheet with a drawing of
liqutd-lsqutd separations
the vessel and the required location
of the jacket(s). The required surface
area and heat transfer calculations
are the responsibility of the user.
•
Edited by Scott Jenkins V Horizontal Peeler & Inyerting-Filter Centrifuges
designed for very fine soWs, thin-cakes and other
HE IX
Author
Keith Kachelhofer is the
corporate mechanical group
leader for Jedson Engineering Inc. (Park 50 Technecenter, 5300 Dupont Circle,
Milford, OH 45150; Phone:
912-330-7777; E-mail: keith.
kachelhofer@jedson.com)
at
its Georgia office. He holds
a degree in mechanical engineering technology from
Southern Polytechnic University in Marietta, Georgia. Kachelhofer has
over fifteen years experience with ASME pressure vessels and is a licensed professional engineer (FB; In Georgia, North Carolina, Delaware,
Maine, New York, Ohio and Utah.
drfficua applications, Superior sanitation, cGMP design
and tow partfcte breakage.
> Rebuilding & Refurbishing Services
> Large Parts inventory
>• Fully Staffed Field Service Dept.
r
The Western States Machine Co.
phone S13J63.4758 - www.wtsternstates.com
Circle 23 on p. 62 or go to adlinks.che.com/29251-23
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2010
35