Hydrocarbon Processing - December 2020 - 48

Plant Design, Engineering and Construction
ing feature between Division 1, Division 2 Class 1, and Division 2 Class 2 is their values of maximum permissible stresses.

Division 2 prior to the 2007 edition of the standard, when it was
improved to 2.4. However, in the 2017 edition, it was split into
2.4 and 3 to accommodate the two vessel classes.
Assuming y(T)/1.5 is not governing either for the
The availability of more shops to fabricate
material or at a temperature T, then it is the value of
the design margin on tensile strength that governs the
Class 1 vessels means the availability of more
allowable stress for that material at that temperature.
suppliers locally and internationally, with the
To understand how these design margins impact the
allowable stresses that impact the required wall thickpossibility of obtaining more competitive pricing.
ness, the plots of y(T), UTS(T), UTSmin and allowable
stresses, percent reduction in wall thickness between
Maximum permissible stress is defined as the maximum
Class 1 and Division 1 for commonly used carbon steel, chrounit stress permitted in materials used in the construction of
mium molybdenum (Cr-Mo) and chromium molybdenum vapressure vessels, and this can be expressed in its most simplinadium (Cr-Mo-V) pressure vessel plate materials are provided
fied form (Eq. 1):
in FIGS. 1-4. The plate material is selected for the plots, as they
contribute to the bulk of the vessel weight as fabricated. As the
y (T ) UTS (T )
(1)
thickness is proportional to vessel diameter and internal pressure,
S = min
,

DM
1.5
the plotted reduction in thickness holds true for a range of vessel
dimensions and internal pressures.
where:
From the plots in FIGS. 1-4, it is evident that the allowable
y(T) = Yield strength of materials at temperature (T)
UTS(T) = Tensile strength of materials at temperature (T).
stresses of Class 1 are higher than Division 1 over a reasonable
Note: UTS(T) = UTSmin for Division 2 Class 2 where UTSmin
temperature range or until the yield starts governing. With everything else the same, it is these higher allowable stresses that
is the minimum specified tensile strength, which is a constant
result in reduced wall thickness. As seen in the plots, a 5%-15%
value.
reduction in wall thickness could be expected in Class 1 vessels
DM = The design margin (3.5 for Division 1, 3 for Division 2
vs. Division 1. However, it may still be beneficial to design critical
Class 1, and 2.4 for Division 2 Class 2).
equipment (e.g., hydroprocessing reactors) to Division 2 Class 2,
It is interesting to note that the design margin used to be 3 for
as they are normally built to higher grades of low-alloy steels and
their allowable stresses are highest among the three categories.
Takeaway. With the introduction of the Class 1 vessel category

in Division 2, ASME Section VIII now provides an additional
option to the vessel engineer to design vessels. In the past, vessels had to be designed to either Division 1 or Division 2, which
is now Class 2.
If properly evaluated, Class 1 vessels could provide an overall advantage vs. Division 1, especially to an engineering, procurement and construction (EPC) contractor. Class 1 vessels
have higher allowable stresses, which mean reduced wall thickness-and, hence, lighter vessels. Lighter vessels tend to be less
capital-intensive, and require lighter foundation or structure,
along with less transportation and erection costs. Furthermore,
in some cases, the exemption of a UDS certification means that
a vessel engineer requires less effort to issue a UDS. Lastly, the
availability of more shops to fabricate Class 1 vessels means
availability of more suppliers locally and internationally, with
the possibility of obtaining more competitive pricing.
It now makes sense for EPC contractors to perform a preliminary investigation during the design phase of a project (i.e.,
front-end engineering design) about the potential benefits of
Class 1, and to define their purchase specifications accordingly.
Alternatively, vessel suppliers could perform similar activities
and make suitable proposals to the purchasers, as well.
SURESH NAWANDAR is a Senior Mechanical Engineer with
McDermott International Inc. He specializes in the engineering
of static equipment and has a broad range of relevant experience
in vessel fabrication, vessel design approvals and certification.
He earned a BCh degree in mechanical engineering in India.
He is a Chartered Engineer and regularly authors papers.

DECEMBER 2020 | HydrocarbonProcessing.com

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Hydrocarbon Processing - December 2020

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