Chemical Engineering April 2019 - 40

Cover Story
Emerson Automation Solutions
over-pressure scenarios and improve reaction
time. Speed is critical because the OPS
must respond immediately to initiate the
pressure protection system and protect the
downstream equipment.
Advanced transmitter capabilities can include
functions such as plugged impulse-line
detection, which can be communicated to
the logic solver via a wired or wireless digital
communications protocol. This valuable information
can be passed on to the larger process-control
and maintenance systems without
interfering with the basic safety function.
While one transmitter may be sufficient
FIGURE 3. This 12-in. valve
is typical of a shut-down
valve for OPS service. It is a
trunnion-mounted ball valve
assembly with a scotch yoke
pneumatic spring return
actuator and digital valve
controller
will not provide any diagnostic information,
and won't have any capability to minimize false
trips. One shutdown caused by a false trip or
pressure switch failure will likely cost more than
installing a more sophisticated system.
Implementing an improved OPS
The same sophisticated technologies that
make instrumentation smart and control
systems more effective can also apply to
SIFs. Let's look at how they might be applied
in this situation.
The sensor can be a pressure transmitter
FIGURE 4. This typical OPS
monitors the feed to a reactor
or other part of a process unit
for high pressure
(Figure 2), which is safety-certified and able
to provide diagnostic information in addition
to a basic pressure measurement. A pressure
transmitter with advanced capabilities
can evaluate the complete measurement
system, extending diagnostics to capture
Emerson Automation Solutions
40
to provide the safety function, highly critical
systems where shutdowns are exceptionally
costly may call for three pressure transmitters
arranged in a two-out-of-three (2oo3) voting
scheme to reduce the potential for a false
alarm caused by a transmitter malfunction.
Using this approach, at least two transmitters
must report an overpressure incident to trip
the system and cause a valve closure. This
also provides a backup if one or even two
transmitters are damaged or malfunction.
We'll come back to the logic solver in a
moment, but let's first look at the final element.
The ultimate purpose of an OPS is to
close a valve to cut off the source of pressure.
It may be a single valve, but as the diagram
shows, it may also be two valves in series
so that either one can close and isolate
the process. Having two valves eliminates a
valve malfunction as a single point of failure.
Valves in this service should be safety-certified,
and must fail closed (Figure 3). If there
is a loss of any or all plant utilities, such as
compressed air to a valve, or if communication
is cut off to the logic solver, the valve
must close itself and stay shut. To handle
these and other similar situations, fail-closed
valves are typically spring loaded and have to
be held open by the instrument supply medium,
such as compressed air. But as mechanical
devices, a potential failure mode is
becoming stuck in one position, leaving them
unable to do their job when called upon.
This is particularly important for SIFs, since
these valves don't normally change position
very often. Fortunately, the actuators
that move these valves can include a smart
digital valve controller (DVC). It can perform
self-diagnostics and determine if corrosion
or some other condition is causing a valve to
require more force to move and can detect
when there is a change in the compressed
air supply, power quality or whatever other
motive force they may depend upon. The
DVC performs operations that can assist
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Chemical Engineering April 2019

Table of Contents for the Digital Edition of Chemical Engineering April 2019
