Hydrocarbon Processing - June 2021 - GP-13

SPECIAL FOCUS: PIPELINES AND TRANSPORT
Use a seven-step procedure
to mark piping CMLs
A. ANWER and K. ALAM, ADNOC Gas Processing, Ruwais, Abu Dhabi, UAE
Piping is considered one of the most
valuable assets in any hydrocarbon processing
facility. Many meters of piping run
through these facilities, transporting process
fluid from one unit to another. The
integrity of piping systems is as important
as that of any other asset in the facility;
however, when studying piping systems of
different sizes, metallurgy and lengths, it
is important to consider inspection plans
that are not only effective but also efficient
with respect to the effort required. Marking
more condition monitoring locations
(CMLs) than necessary wastes time and
effort, but marking far fewer CMLs than
needed puts piping integrity at risk.
Several guidelines are available to define
the CMLs on a given piping system;
however, the plant inspector has the ultimate
control over defining, inspecting
and redefining the CMLs throughout the
piping life. This is a continuous process
(FIG. 1), and balance can be achieved only
after at least one full round of design life
is finished with no loss of process containment.
Data from similar operators may
help achieve a balance to the piping inspection,
which is normally the first step
in defining CMLs for any piping system. A
stepwise approach in defining, inspecting
and redefining CMLs on any piping system
is detailed in the following sections.
Step 1: Data from similar operators.
Sharing data between industries and companies
worldwide is not uncommon, but
it can be difficult to organize. Nonetheless,
if a data-sharing arrangement can be
made, information from similar service
operation can help in many ways.
Of significant usefulness are comparable
CMLs marking data for piping systems
in similar service. These provide a quick
start for other operators to launch this
continuous process. If already normalized
for a given piping system at a hydrocarbon
processing facility, verified CMLs can save
massive effort for a new operator. However,
if such data is not available or not easily
obtained, then this step can be skipped.
Step 2: Identification of credible
damage mechanisms. Information on
credible damage mechanisms must be
obtained before starting work to define
the CMLs on any piping system. If a risk
study has already been performed for the
facility, then this document may contain
the relevant information.
Step 3: Define inspection tasks. Inspection
tasks must be defined after information
on credible damage mechanisms
is made available. These inspection tasks
can provide a way forward for marking appropriate
CMLs. In most cases, inspection
tasks for piping include measuring the
wall thickness to identify different forms
of corrosion. Most CMLs are marked to
perform ultrasonic inspection tasks.
Step 4: Corrosion rate determination.
When designing a piping system, an initial
design corrosion rate is always considered
to define the lifespan of a piping
system. This critical information defines
the corrosion allowance. For all piping
systems, the initial design corrosion rate
must be tabulated to determine the actual
corrosion rate.
To determine the actual corrosion rate,
initial marking of CMLs should be done
only on baseline thickness locations, if
available. If baseline thicknesses have not
been measured, then CMLs should be
defined at bare minimum locations to enable
the inspector to determine the actual
corrosion rate.
The initial inspection frequency can
be increased so that this exercise can be
completed. A round of three inspections
should be sufficient for accurately calculating
the corrosion rate of any piping system.
Step 5: CMLs placement. An inspector
must evaluate the placement of CMLs
based on several factors:
* Credible damage mechanism
* Inspection task
* Design corrosion rate/allowance
* Piping geometry
* Piping accessibility.
Services that are not safety-critical
upon loss of process containment should
be separated and, by default, only one
CML in each 60-m run of piping should
be defined. However, all other services
that are declared process-critical should
be examined in detail, considering the
previously listed factors.
Any damage mechanism that is general
in nature should result in fewer CMLs;
however, damage mechanisms may change
with changes in piping geometry or piping
tags, resulting in varying process conditions
and more CMLs, as a general rule.
A single run of piping with similar process
conditions throughout, with no changes
to temperature, flow or other process conditions,
does not need to be inspected at
many locations. Conversely, piping runs
with many branches bring changes to existing
process conditions, which may impact
the credible damage mechanism and
require inspection at multiple locations.
FIG. 1. General work view of a piping
inspection.
Gas Processing & LNG | MAY/JUNE 2021 13
Hydrocarbon Processing - June 2021

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