Hydrocarbon Processing - February 2022 - 50

Plant Design, Engineering, Construction and Commissioning
any operating scenario throughout the life of the plant,
typically noted in the line designation list.
Flanged interface. An interface connection between two
contractors at the predefined IP location is achieved via a
flanged joint between two pipes, as depicted in FIG. 2. There
may or may not be a valve at the interface point, as agreed
between both parties.
The appointed lead contractor will be responsible for the
supply and installation of gaskets and bolts and for the final
system test before hand-over activity, as specified in the
project contract.
The flanged IP refers to the individual contractor supplying
the pipe spool with a flange at the end of pipe, or a valve
with a flanged end if a valve is required. Similar to the welded
interface point previously mentioned, both contractors must
exchange, agree on and finalize the relevant piping and process
related interface information before executing the work.
Additionally, for the flanged interface point, both contractors
must agree on the flange type, flange full description, flange
facing and finish, gasket and bolts to ensure both flanges within
the individual contractor boundary are compatible to make
the final connection. If a valve is required at the IP, then all
relevant information must be agreed beforehand. The flange
face will generally be a flat face, raised face or a ring-type joint,
as per relevant project specifications and guidelines.
Review of limitations. Limitations observed from project
specifications, guidelines and American Society of Mechanical
Engineers (ASME) B31.3 code1
are summarized here.
Welded interface. Generally, project standards and specifications
may not have limitations for welded piping joints,
other than to comply with various requirements stipulated in
ASME B31.3 and other such standards, including the use of
approved welding procedures. When a flanged connection is
preferred over a welded connection, exceptions include:
* When a dissimilar metal joint is involved
to avoid galvanic corrosion or wet hydrogen
sulfide (H2
S) damage
* A metallic-to-nonmetallic piping connection
* An underground to above-ground piping connection.
Flanged interface. Generally, project standards and specifications
may show the following limitations for flanged joints:
* The limitations of using different flange types
(e.g., slip-on, lap joint and compact flanges).
* The use of flanged connections may be prohibited
when butt-welded joints can be used (e.g., cyclic or
vibration services, potentially toxic material, steam
of ≥ 900# pressure rating, piping subject to
large bending or external loads)
* When project specifications recommend minimizing
flanged connections in internally fusion bond epoxy
(FBE)-coated piping systems
* Limitations of the allowable misalignment of
flanged joints/bolt holes
* Project specifications, guidelines and ASME B31.3
provide guidance for pipe spool fabrication/erection
about bolt straddling, offset of flange bolt-holes and
permitted misalignment of flanged connections
* Project specifications provide bolt-tightening
requirements.
WELDED PIPING INTERFACES
Advantages. Welded joints are considered technically superior
to flanged joints due to better joint integrity and/or leak
tightness. With welded joints, no special precautions and limitations
are required as applicable to flanged connections, such
as flange misalignment, concerns with overstressing or distortion
of the bolts, correct selection of gaskets and bolts, etc.
Welded joints also have an advantage over flanged joints
since after the welding and any subsequent post weld heat
treatment (stress relief), as required, the piping system is considered
one continuous homogeneous system. This not only
offers improved thermal integrity of a joint when exposed to
an external source of heating or fire, it offers improved stress
distribution for a piping system under internal pressure and
expansion or contraction, as well as under different temperature
conditions the pipe may endure during its design life.
Additionally, welded piping interfaces are considered more
beneficial from cost and schedule perspectives for the following
reasons:
* Capital cost savings due to the use of two flanges
and associated gasket and bolts saved per piping IP.
For larger pipe sizes and ratings, the savings can be
substantial. This also reduces the dead weight of the
piping system.
* Since the majority of the piping components within
the piping system (pipes, fittings, flanges, etc.) are
joined via butt-welds, adding an extra butt-welded
joint executed at the worksite at the IP limit can be
considered an incremental cost only where it will
replace the welded joint between the pipe and flange
used for a flanged interface.
* A savings in engineering work hours to analyze the
flanged joints for the leakage checks, as required by
project specifications. Since this check is conducted
at IPs, it will require further exchange and agreement
of information between two LSTK contractors before
the execution of an IP.
FIG. 2. Flanged interface.
50 FEBRUARY 2022 | HydrocarbonProcessing.com
Disadvantages. The main disadvantage of welded joints in
the context of this discussion is that they do not offer electrical
discontinuity desired for certain cases, such as welding between
two dissimilar piping materials. A welded piping interface with
two dissimilar materials, such as carbon steel and stainless steel
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