Hydrocarbon Processing - February 2022 - 51

Plant Design, Engineering, Construction and Commissioning
pipe, will cause galvanic corrosion, making carbon steel more
vulnerable to corrosion. Furthermore, dissimilar welds are generally
not allowed in wet H2
more expensive to insulate a flanged IP compared to a welded
IP due to greater surface areas to be covered by the insulation.
S service. For such cases, welded
interface is not recommended and flanged IPs should be used.
Another disadvantage is the hydrotesting of the interface
weld joint, which requires special tools and isolation for items
that have already been hydrotested.
FLANGED PIPING INTERFACES
Advantages. Flanged joints offer reduced construction lead
time onsite due to prefabricated pipe spools with flanged ends
at the IP that only require time for the mechanical fitment,
while avoiding the hot work (welding) and other quality control
processes such as post weld heat treatment and weld inspection
required for the welded interface points. In certain
conditions, executing a field weld for a welded IP will be more
time consuming and may involve a higher cost if the majority
of the welds are conducted at a shop with no or a limited number
of field welds at a given IG location.
Flanged joints also offer an opportunity to use two dissimilar
piping materials, allowing an individual LSTK contractor
to use the piping material to suit its package design
conditions, and a more economical design that uses insulating
gasket kits at the flanged IP to avoid any concerns related to
galvanic corrosion.
Similar to a welded IP, flanged joints can also be inspected
in a non-intrusive manner for corrosion on the flange faces by
existing techniques, such as phased array ultrasonic testing,
without needing to dismantle the flanged joint [as per American
Petroleum Institute (API) RP 574].
Disadvantages. The main disadvantage of a flanged joint
at a piping interface is potential leaks, which could have profound
consequences including safety hazards that can affect
plant availability.
Further, a flanged IP does not reduce the number of welds
in a given piping system. Each LSTK contractor must provide
a flanged connection where a flange is welded to the pipe
within its own boundary with the final gasket and bolts provided
by the lead contractor. For a welded IP, only one final
weld joint by the lead contractor is required; however, for a
flanged IP, each contractor must provide a pipe-to-flange
welded joint in its scope.
Flanged joints will occupy more space and will require the
staggering of flanges if two adjacent pipes are both flanged at
interface points. Additionally, the bolts and nuts on flanges
may require inspection at periodic intervals to ensure the
integrity of the flanged joints and for potential corrosion on
bolts, particularly when these flanged joints are under insulation.
Each time a flanged assembly is opened, the gasket must
be replaced as per project specifications and guidelines.
The majority of piping IPs are generally located in nonoperating/interconnecting
areas of the plant typically outside
the plant battery limit and infrequently accessed by plant operations.
This requires further inspections or walk-throughs to
ensure flanged joint integrity (e.g., flange loosening or bolt distortion
due to pipe movement, corrosion on bolts, loosening
of any insulation). Where flanged joints are insulated, it will be
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Takeaway. The individual comparison between the two types
of piping interfaces shows that welded interfaces are preferred
and should be prioritized over flanged interfaces. Flanged IPs
should be used only where welded interfaces are not desired,
such as in the case of two dissimilar or incompatible piping
materials at the IP between two LSTK contractors.
LITERATURE CITED
1
American Petroleum Institute (API) Recommended Practice 574, Section 10.3.3,
" Inspection practices for piping system components, " 4th Ed., November 2016.
ABDULAZIZ ALSUAYYID is a certified Project Engineer for
Saudi Aramco, with 7 yr of experience in project management.
He has also worked for numerous mega-programs, such as
the Shyabah NGL Program and Fadhili Gas Program. His
experience also covers piping engineering, prefabrication,
construction and commissioning. Mr. Alsuayyid holds a BS
degree in mechanical and manufacturing engineering
technology form Northern Kentucky University in the U.S.
NIKUL JADAV is a Consultant Piping Engineer for the Tanajib
Gas Plant Projects, part of Saudi Aramco's Marjan Development
Program. He has worked over the last 20 yr across diverse
projects and clients within the oil and gas sector, with a key
focus on plant layout, piping design and engineering. Mr. Jadav
holds an MS degree in project management from The University
of Liverpool, U.K.
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