Hydrocarbon Processing - November 2021 - 23

Special Focus Process Control, Instrumentation
and Automation
M. A. BALIGA, Reliance Industries Ltd., Mumbai, India
Manage integrated operating limits
For safe and smooth operations of petrochemical
and refining facilities, it is vital
that they are continuously run within
defined operating limits, ensuring high
reliability of the plant and equipment and
delivering per rated capacities throughout
their lifecycle. However, operating limits
are not cast in stone and must be constantly
reviewed to derive maximum benefit with
respect to production, quality and cost.
Operating limits are not a single set of
limits, but are layered and vary depending
on product grades and modes of operations,
including startup and shutdowns.
Often, undesirable events [e.g., pump seal
leaks, overflow of tanks, over-pressurizing
of vessels, popping of pressure safety valves
(PSVs)] occur during shutdown and startup
because the safe operating limits for
these modes of operations were either not
defined or ignored. Based on reactor performance
as well as economics-depending
on the aging of the catalysts-operating
limits around the reactor and catalyst
should be revised between turnarounds.
This article discusses the development
and management of operating limits with
an integrated approach between functions
that include operations, engineering, technical
services and quality assurance (QA).
Aspects of monitoring and control
parameters, limits. Plant design
documents explain the parameters to
be checked and controlled. Key design
documents that provide this information
include process flow diagrams (PFDs),
piping and instrumentation diagrams
(P&IDs) and equipment data sheets. The
licensor or vendor manuals or the trips
and alarm schedule supply the recommended
operating limits.
The required details are available in assorted
documents and come from different
sources, and a change in a parameter
in one document may have an impact on
another parameter in another document.
Therefore, it is important for the operator
(operating company) to combine this
information into a single master data set,
which should be considered as a control
document. The plant manager must ensure
that this master data set is always kept
current and valid. The technical services,
reliability and engineering groups must
confirm that all relevant information is correct
and maintain an audit trail of changes.
During the lifecycle of the plant, this
master data set should be periodically reviewed
and updated based on risk analyses,
the outcome of failure investigations,
regulatory changes, upgrades in engineering
standards and customer requirements.
Recommendations from process hazard
analysis (PHA) studies [hazard and operability
(HAZOP), layers of protection
analysis (LOPA), etc.] and reliability
studies should be considered for updating
the master data. The master data must
be reviewed after every turnaround and
catalyst replacement. All changes must be
conducted through the company's change
management process.
The master data of the plant's parameters
and their respective limits become especially
stringent as the plant ages. Based
on analysis of failures and inspection data,
new parameters may be added or limits
may become tighter. Certain parameters
appear only for certain operating modes
and may not be relevant for other modes.
The master data set should contain all
relevant information needed to run the
plant in a safe manner. All parameter limits
must be set as wide as possible-any
operation outside of its limits for a reasonably
long duration can lead to degradation
of the equipment, piping or the product
itself. Limits are set in many cases where
trade-offs exist [e.g., lower hydrogen (H2
)
quench in the hydroprocessing reactor],
helping increase production rates and
lower costs, and impacting catalyst life
and product quality. A similar example
is the reflux rate in distillation columns,
which affect throughput and operating
costs, and improve product quality. Tightening
limits on one factor may lead to
giveaways or losses in other aspects.
Aspects of a master list. A typical
master list should contain the following
information:
1. Tag ID: Taken from a P&ID, every
tag that is shown in a P&ID should
be in the master list; in turn,
every tag that appears in a master
list should appear within the
distributed control system (DCS).
2. Tag description: The unique
name of the measurement point.
3. Associated equipment ID: Every
measurement point or tag must be
associated with some equipment
or other; association may be done
either based on equipment that
influences the parameter most,
or equipment that is mostly
impacted by the process value.
4. Name of the associated
equipment; the unique
name of the equipment.
5. Location of measurement;
the description of the location
of measurement-in the
field and in the panel.
6. P&ID number: The
P&ID number where the
tag can be found.
7. Boundary definition: Is
the purpose of the tag for
monitoring or control? If the
purpose is control, then,
a. Boundary types: Whether
the tag influences reliability,
integrity, environment, energy,
production and quality.
These boundaries are not
Hydrocarbon Processing | NOVEMBER 2021 23
Hydrocarbon Processing - November 2021

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