HYDROCARBON
ENGINEERING
74
n
Minimising cabling requirements.
n
Minimising operation shutdowns during implementation.
n
Design and implementation of SIL 2 certified safety
interlock loop of overfill protection system standards.
The loop design consisted of sensors (radar level gauge
and level switch), a logic solver (ESD) and the final control
element remote operated shut-off valves (ROSOVs). The
design of a SIL 2 rated loop required critical analysis of
sensor and logic solver safety-related data.
The SIL certified radar gauges are independent and
separate from the existing tank gauging system. Major design
considerations for selection of the radar gauges included:
n
SIL 2 (or better) certification of radar gauges and its
safety function, by an accredited third party safety
certification agency, as per the latest IEC 61508 safety
standard.
n
Capability of hard wiring of safety function (relay
output) to ESD system.
n
Transmission of level measurement data over wireless
protocol to minimise signal cabling, and availability of
the level data as additional inputs in terminal
automation system.
n
Performance of radar level gauges to provide an
accurate and precise level measurement and to perform
as overfill protection device through its safety function.
n
Proven performance record of radar level gauges in class
‘A’ and class ‘B’ products.
n
Assured performance of radar level gauges in existing or
new stilling wells in floating roof tanks and man holes
on top of cone roof tanks.
n
Design and deployment of low accuracy radar level
gauges for new fire water tanks, with wireless level data
transmission capability to the control room.
Solution
HPCL chose Honeywell’s SmartRadar FlexLine for
implementing the requirements of this critical project.
SmartRadar FlexLine combines new software algorithms
with Honeywell Enraf’s planar antennae technology to
provide precision levels demanded for custody transfer
accuracy.
Honeywell’s gauging solution features
a modular design and flexible architecture
that would allow the radar level gauge to
synchronise with multiple third party
applications and environments, and also
address the specific requirements of the
safety enhancement project. It offers high
flexibility that can be used for dedicated
tank applications such as independent
overfill protection, floating roof
monitoring, rim fire detection, roof
immersion compensation, etc. The
FlexLine wireless option reduces the cost
for wiring infrastructure and increases
flexibility, as various instruments can be
added to the network without adding
cables. The radar tank gauge is also
certified for use in custody transfer
applications.
The enhanced performance signal processing enables
the radar tank gauge to provide an accurate level reading,
even in difficult applications (close to the tank shell), and
prevents the need for a costly stilling well. The FlexLine
radar tank gauge can be used in overfill protection
applications, while additional diagnostics have been
added to detect hardware failures, and a 2004D voting
algorithm detects, reports and isolates failures on the
boards without interrupting the loading or unloading of a
tank.
HPCL’s chosen solution is SIL certified by TÜV. The
safety instrumented function design for overfill protection
included the following main criteria:
n
Loop average probability of failure on demand
(PFDavg): the PFD value contributes to achieving the
target loop PFD as defined in IEC 61511, bringing down
safety loop budgeting.
n
Architectural constraints: meeting the requirements of
minimum SIL 2 per IEC 61511.
n
Proof testing requirement of primary sensor (radar) and
loop (SIF): meet or exceed loop proof test requirement
of once in 10 years.
The overall design of the ESD system, considering other
safety loops within each location, was thoroughly and
carefully scrutinised by HPCL. The entire process resulted in
a sophisticated safety system with end to end
consideration of all safety features.
Close to 600 product storage tanks, distributed in
63 project locations, are covered in HPCL’s safety
implementation project. Honeywell’s SmartRadar FlexLine
is being delivered to all locations through HPCL’s
integrators, including 23 locations on a turnkey basis.
Conclusion
The enhanced safety project has enabled HPCL to reduce
financial and environmental risks. The safety certified gauge
enhances safety by actively guarding and alarming. HPCL
has increased terminal efficiency with their
implementation, assuring onsite accuracy, which helps
optimise available storage capacity and product movement
guidance, and improves resource planning.
T&T
Figure 3.
Design of overfill prevention safety rated loop.
