Hydrocarbon Processing - December 2020 - 46

Special Focus

Plant Design, Engineering and Construction
S. NAWANDAR, McDermott, The Hague, Netherlands

Using ASME Section VIII Division 2 Class 1
to your advantage
It is globally recognized that ASME codes are widely used
international codes for the construction of mechanical equipment operating in power, refining, petrochemical and nuclear
facilities. These codes, along with other ASME standards, address every aspect of their construction, such as component
selection, ratings, materials, design, fabrication, welding, examination, inspection, testing and certification. These codes
are continually updated according to the needs of the industry.
ASME codes are published in 12 sections. Section VIII is for
the construction of pressure vessels. Pressure vessel is a generic
reference that includes simple vessels, drums, tall columns,
thick-walled vessels and reactors. Section VIII is further divided into divisions-Divisions 1 and 2 are the most widely used.
As a practice, Division 1 is selected for pressure vessels with
design conditions resulting in low-to-moderate wall thickness (i.e., up to 50 mm). Division 2 is used for cases requiring
higher thickness, although there are other factors-such as
high pressure, cyclic service, service fluid, complexity of design, and client specifications-that can mandate a Division
2 designation. Division 2 provides detailed design procedures
(both formula-based and analytical) and higher permissible
stresses for the commonly used carbon and low-alloy steel materials, of which most pressure vessels are fabricated. Therefore, vessels requiring a higher wall thickness often benefit
from adhering to Division 2 due to the reduced required wall
thickness, which reduces the vessel's weight. Other indirect
benefits of lighter vessels are ease of handling at fabrication
shops, less welding, less transportation and erection efforts,
and a lighter foundation.

However, the advantages are not always obvious. Complying
with additional requirements put forth by Division 2 on quality systems, materials, inspections, testing and documentation
involving designers and certifying engineers have a potential
to offset the benefits it might offer otherwise. Therefore, it is
imperative to carry out a detailed cost assessment involving direct and indirect savings vs. expenses for complying with additional requirements. In the past, Division 1 was the only code of
choice when the outcome of the above assessment fell toward
the right of the breakeven point.
In the 2017 edition, Division 2 had an important addition
in it-the introduction of two vessel classes: Class 1 and Class
2. Class 1 vessels are important, as they provide an intermediate solution to the situations described previously. Class 2 vessels are vessels that are built in full compliance to Division 2 as
done prior to the 2017 edition.
ASME Section VIII Division 2: Class 1 vessels. Class 1

pressure vessels are still required to comply with all the rules
and requirements of Division 2. However, there are a few limitations and exemptions, which include the following:
* Class 1 vessels are limited only to design-by-rule
methods. Design-by-analysis methods are permitted
only for those components for which design rules
are not available. Design-by-analysis methods cannot
be used in lieu of design-by-rule methods.
* Class 1 vessels still require a user's design specification
(UDS), but it is exempted from endorsement by a
certifying engineer unless the vessel is in cyclic service

250

200

Stress, MPa

150
150
100
UTSmin/2.4
UTS/3.0
UTS/3.5
Y/1.5

100

8
6

Division 1
Division 2 Cl.1
Division 2 Cl.2
Thickness reduction

0
0

10
100

200
Temperature,°C

300

400

100

150

200
250
Temperature, °C

300

350

FIG. 1. Plot for SA-516 Gr.70 (carbon steel). (A) y(T), UTS(T) and UTSmin with design margins; (B) allowable stresses and percent reduction
in wall thickness between Division 1 and Division 2 Class 1.

DECEMBER 2020 | HydrocarbonProcessing.com

2
0
400

Thickness reduction, %

200

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Hydrocarbon Processing - December 2020

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