Chemical Engineering April 2019 - 41

with proof testing as well as partialstroke
tests, gathering and evaluating
performance data. Proof and partialstroke
tests are important periodic tests
that confirm operability in accordance
with IEC 61511. Data gathered can be
passed to the logic solver as well as
asset management tools, which can
also be utilized by operations and maintenance
personnel to make predictive
maintenance decisions.
Depending on the nature of the process,
the valves used with an OPS
can be quite large, yet they must close
quickly and positively in a controlled
manner. Large actuators sometimes
need to be powered by hydraulic systems
to have sufficient force to close.
Even large valves must have actuators
including stored energy systems, enabling
them to fail closed. If driven by
stored hydraulic pressure, this pressure
must be monitored to ensure it is available
when called upon.
The logic solver ties together the OPS.
In the most basic form, it might be little
more than a glorified relay, responding
to a binary switching signal from a digital
sensor and telling its valve to close,
but it can also provide insight into the
safety function, and even the process,
if provided with the sophisticated functionality
available today.
A smart logic solver has the capability
to work with three pressure transmitters
in a 2oo3 voting scheme, and it can
tell if a transmitter is malfunctioning by
reading the diagnostic information. It
monitors all functions continuously, and
if there is a problem, it can compensate
while warning operators of the situation.
The same applies to valve actuators,
since they can receive, interpret and act
upon diagnostic information from smart
transmitters and actuators.
Putting it all together
A typical system could use three pressure
transmitters, each sending measurement
and diagnostic data to a logic
solver (Figure 4). Using a 2oo3 voting
scheme, the system will trip if line pressure
exceeds the setpoint and it is detected
by at least two of the transmitters.
The shutoff valves close by spring
actuation and are held open by a regulated
compressed-air supply.
If the system trips or there is a larger
failure in the unit, the solenoid valve
or digital valve controller cuts off the
compressed-air supply and dumps it
from the actuators, causing the valves
to close. The DVCs monitor air-line
pressure and valve function. They can
also perform partial-stroke valve tests
automatically to determine if the valve
condition is changing. All of this is monitored
by the logic solver using analysis
tools, which can warn operators and
maintenance personnel if there are any
changes in equipment or performance.
Creating an effective OPS requires selecting
and integrating the right components
following analysis in accordance
with IEC 61511. Once designed, it must
be evaluated as a system, usually by a
third-party, to ensure it can perform as
expected under the appropriate SIL requirements.
It can be installed with less
process downtime compared to the cost
and complexity of a flare system expansion
project. An OPS provides a variety
of cost benefits, including the following:
* Reduces or eliminates the need for
resizing the flare or relief manifold
* Reduced process downtime
* Reduced number of flaring events
and volume released
* Reduced manual system testing
As environmental regulations continue
to become more stringent and the
ability to keep qualified operators grows
more difficult, these systems will be increasingly
considered for both operational
and financial reasons.
n
Edited by Gerald Ondrey
Authors
Erik Mathiason is a global pressure
product manager for Emerson Automation
Solutions (6201 Innovation Blvd.,
Shakopee, MN 55379. Phone: 952204-4436.
Email:
erik.mathiason@
emerson.com).
He is responsible for
Rosemount Pressure Transmitters. He
has worked in various roles within Emerson,
primarily focusing on pressure
and pressure SIS solutions. He has a
B.S. in mechanical engineering from North Dakota State University
and an M.B.A. from the University of Minnesota, Carlson
School of Management.
Afton Coleman is the senior marketing
manager at Emerson Automation Solutions
(205 S. Center Street, Marshalltown,
IA 50158. Phone 641-7513439.
Email:
afton.coleman@
emerson.com).
She is responsible for
Fisher Digital Isolation Solutions used
in safety instrumented systems. She
has worked with Emerson since 2005
and is a Certified Functional Safety
Professional (CFSP). She has a B.S.E. in chemical engineering
from the University of Iowa and an M.B.A. from Iowa
State University, Ivy College of Business.
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