Chemical Engineering June 2010 - 28

Cover story
Decoding
H
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
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
28 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 presPressure
Vessel
Design
Bridging
the gap between
users' and manufacturers'
responsibilities for the
ASME pressure vessel code
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Chemical Engineering June 2010

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