Hydrocarbon Processing - September 2021 - 71

Process
Optimization
S. MAITI, Wood Group Plc., Kolkata, India
External pressure design of large-diameter piping:
Optimal analysis approach
Large-diameter piping used in the petrochemical, oil and
gas, steel plant and power industries often carry toxic fluids
and can pose significant safety hazards. Their stability and reliability
are important while they operate under external pressure
loading due to vacuum formation or any other external
load that can potentially collapse the piping system.
Major piping codes, such as the ASME B31 pressure pip,
refer pressure vessel codes for the design of piping
series1
ing subjected to external pressure. However, piping and pressure
vessel configurations are usually different, and a pressure
vessel external pressure design approach should be carefully
considered when applied to piping. In the author's experience,
a design by rule (DBR) approach as stipulated in various
pressure vessel codes is conservative in nature; those rules are
conservatively applied to a specific piping system without due
consideration of its actual geometry, pipe support location
and configuration. However, these aspects have considerable
impact on the external allowable pressure/loading capacity of
a piping system.
However, a design by analysis (DBA) approach can address
this issue effectively, considering the actual layout of the
piping and support location/configuration and even optimizing
the piping wall thickness requirement while meeting allowable
external pressure loading criteria. This article briefly
compares the various rules and approaches of the external
pressure design per various international codes and intends to
help practicing engineers to adopt a suitable approach for specific
applications to optimize design within current pressure
vessel code framework.
RULES, PROCEDURES AND PRINCIPLES
Classical approach for infinity long shell. External pressure
causes compressive stress in piping or pressure vessels,
creating stability problems and potentially leading to failure.
Large-diameter and thin pipes are more susceptible under external
pressure than thicker pipe; however, even thick piping
may also be subjected to failure where external pressure is significant,
such as in a subsea environment.
Bresse-Byran offered an allowable external pressure design
formula2 with a factor of safety (FOS) of 3 considering two
lobes for an un-stiffened portion of piping (Eq. 1):
2Es
Pallow =
3 1− vs
(
2 )
⎛
⎜⎝
t
D0
3
⎞
⎟⎠
(1)
Another commonly used formula (Eq. 2) for above-ground
steel pipe from AWWA M-113
is often used by engineers work(without
an FOS) for an
Pc =
(
1− vs
2 )
(2)
ing on a water main of steel construction. This formula is from
the original Timoshenko equation4
unreinforced, infinite length:
2Es
⎛
⎜⎝
t
D0
3
⎞
⎟⎠
DBR approach for pressure vessel codes. DBRs on external
pressure design are outlined in various international codes,
such as ASME Boiler and Pressure Vessel Code (B & PVC)
Section VIII, Div. 1, UG-28 to UG-30, and B & PVC Section
VIII, Div. 2, EN-13445/PD 5500.5,6
However, these code rules
are primarily meant for tubular structure design under only
external pressure loading, but are not applicable for combined
additional loading, such as axial compression or bending.
ASME B&PVC Code case 2286,7
first published in 1990
and reaffirmed in the 2004 edition, was intended to address
the combined loading of compression and external pressure
applied to a tubular structure. The 2286 code case is applicable
to both Div. 1 and Div. 2 vessels and was later merged into the
main code of Div. 2.
However, the methodologies and conservatism associated
with each code related to external pressure design are different.
ASME B & PVC Section VIII, Div. 2 offers both DBR and
DBA in Parts. IV and V, respectively. B & PVC Section VIII,
Div. 1 (UG-28 to UG-30) DBR is a little conservative in approach
but still preferred and widely used for piping/pressure
vessel design, as these calculations are less time-consuming
and more straightforward.
DBA approach for pressure vessel codes. In 2007, new
code rules were introduced in ASME B & PVC Section VIII,
Div. 2, and the DBA approach was included under the ASME B
& PVC rewrite program. The DBA approach offers a rigorous
method of analyzing pressure vessel/piping engineering problems
using a numerical method.
Hydrocarbon Processing | SEPTEMBER 2021 71
Hydrocarbon Processing - September 2021

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