Hydrocarbon Processing - May 2022 - 53

Process
Optimization
W. STIKVOORT, P3 Engineering,
Delft, the Netherlands
Assessment protocol for nozzle loads
on pressure vessels
From a historical perspective, the traditional approach to
deal with " unknown magnitude of nozzle loads " can be summarized
here.
In EN 13445-3 (EU)1
ratio Φp = 1, then the other ratios, ΦZ
, must be zero
clause 16.4.6.3 and 16.5.6.3, if the
and ΦB
for the interaction requirements to be met, although there is
some provision in equations 16.4-15 and 16.5-15 for the other
ratios to be slightly greater than zero. The equivalent clauses,
as indicated in PD 5500 (UK),2
are, respectively, G.2.8.2.4 and
G.2.8.3.4, and the equations G.2.8-13 and G.2.8-39.
This means that for the initial design, some provision must
be made for nozzle loads that are not yet known. This has always
been the case, and many companies have produced their
own data tables of nozzle loads that are used for the initial design.
The piping stress analysts then know that they must maintain
the calculated nozzle loads to less than the tabulated values
and design the piping accordingly.
In the absence of tabulated nozzle loads for a particular job,
vessel designers must use their engineering judgement and
experience to assess whether the loads are likely to be small
(moderate temperatures and well-designed piping with plenty
of flexibility), or large (high temperatures, compact layout
with little piping flexibility) and then design the nozzles for
appropriate, estimated loads. The designer may choose to use
tabulated loads from another similar project.
Some degree of overdesign always exists in any pressure vessel,
but it is not usually practical to wait until the piping design
has been completed before beginning to design the vessels. If
the vessels are designed before the nozzle loads are known, it
is often cheaper to overdesign the nozzles rather than try and
modify the design at a later stage after fabrication has begun.
Where ASME BPVC Section VIII Division 13
, 5375 and 2976
is the prevail-with
the
ing design code, the commonly used approach to assess nozzle
loading is using WRC Bulletins 1074
understanding that the internal pressure load must also be considered,
which is not an integral part of the WRC bulletins.
To support the engineer involved in the design of pressure
vessels, a protocol has been developed that enables the engineer
to gain insight into the individually permissible loads on
a flanged nozzle configuration of a pressure vessel. This protocol
can be applied to recognized pressure vessel codes and/
or standards including: EN 13445 (EU)1
, PD 5500 (UK)2
and
ASME BPVC Section VIII-Division 1(U.S.)3
. The involvement
of the piping stress analyst is limited to evaluating the
piping reactions from the pipe stress analysis to the allowable
nozzle loads determined by the vessel design engineer, considering
only internal pressure.
It has been noted that in the methodologies contained in
both EN 13445-3 (clauses 16.4.6 and 16.5.6) and PD 5500
(clauses G.2.8.2 and G.2.8.3), no external loads can be allowed
by the nozzle exerted piping reactions if ΦP
= Pdesign/MAWP = 1.
However, this implies that only external loads are allowed if ΦP
< 1, which means that a certain amount of " overdesign " must be
created compared to exclusively incorporating internal pressure
loading in the nozzle design. ANNEX V of EN 13445-3 (amendment
A8: 2019)7
states that a buffer should be considered for unknown
nozzle loads that relate to the opening design. Literature8
highlights the doubts that have arisen about this approach.
From this perspective, an alternative approach has been developed
that is based on the fact that nozzles designed for internal
pressure still have sufficient load-carrying capacity left to accommodate
the piping reactions on the nozzle. The advantage
of this approach lies in the fact that, in contrast to the traditional
approach outlined, the permitted (individual) nozzle loads can
already be determined during the initial pressure vessel design
by the vessel design engineer and can be made available to the
piping stress analyst. Obviously, this requires the necessary coordination
between the two engineering disciplines. This innovative
approach has proven itself in successful applications for
many years in predominantly statically loaded pressure vessels.
Determination of individually permissible loads. The
method described by the author9
, which is based on Dekker
and Bos10
, was used to determine the individually permissible
nozzle loads.
FIG. 1. Nozzle configurations and loadings.
Hydrocarbon Processing | MAY 2022 53
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