Chemical Engineering April 2013 - 65

Integration options:
* Modbus serial
* Native interface
* Modbus/OPC
Modbus serial
Hart
data
4-20
Only
FIGURE 6. WirelessHART uses a self-organizing mesh. Data packets will
take whatever path is necessary to get back to the host, providing reliability
comparable to wired communications
efficiency communicate wirelessly to
both the basic process control system
and the AMS, and the user can choose
to compare these new wireless transmitters
to the wired safety-system
transmitters to improve diagnostic coverage.
Installation costs are minimized
when the devices are battery powered,
though the need to install and maintain
devices in hazardous areas limits
battery capacity. Although devices can
be configured to communicate every
second, with current battery and radio
technology users are achieving 3- to 10year
battery lives with 4- to 30-second
update rates.
Security is important because wireless
data and devices can be accessed
from outside the plant fence, bypassing
the usual plant security. All data
should be sent with encryption, so
someone listening in will not be able
to decode the message and steal the
References
1. Brown, S., Menezes, M., Design Safety Instrumented
Systems with Relevant Data,Chem.
Eng., pp. 54-58, July 2003.
2. Brown, S., Menezes, M., Measurement Best
Practices for Safety Instrumented Systems,
Chem. Eng., pp. 66-72, Sept 2006.
3. Menezes, M., When Your Plus and Minus
Doesn't Add Up, Flow Control, January
2005.
4. Pollack, A., Reactor Accident in Japan Imperils
Nuclear Program, Feb 24, 1996, New York
Times, p. A-1.
5. Menezes, M., Improve Compressor Safety &
Efficiency with the Right Pressure Transmitters,
Control Solutions, November 2001.
6. Fox, A. and Patterson, D., Self-Repairing
Computers, Sci. Am., June 2003.
7. Liptak, B., " Process Measurement and Analysis, "
CRC Press, 2003.
8. Menezes, M., Improve Plant Safety Through
Advanced Measurement Diagnostics, Chem.
Eng., October 2000.
9. Welander, P., Safety on Fieldbus, Control
Eng., June 2009.
10. " 2010 Pressure Transmitter Worldwide Outlook
Study, " ARC Advisory Group, USA.
11. Boyes, Walt, All Quiet on the Wireless Front,
Control, August 2011, p. 30.
FIGURE 7. Process variables are communicated
using traditional 4-20-mA connections to the logic
solver, while diagnostic information travels via the
WirelessHART to the maintenance terminal
data. Related is authentication/verification
- only valid devices, and not
hackers, can gain access to the system.
Finally, even the most secure design
can be defeated by poor password/
code management - human error.
To add a new device to the network,
the user manually inputs the network
name and " join key " using the
familiar HART handheld, but only the
system-generated, encrypted rotating
key is broadcast over the network.
Wrapping up
The safety system designer's first approach
to minimize risk should be to improve
strength through best practices.
What cannot be eliminated should be
avoided via a diverse technology that
is resistant to common cause failures.
What remains should be diagnosed,
using a combination of new wired and
wireless measurements, and device
diagnostics. Diagnostics are available
to detect problems in the device, wiring,
process connections and in the
process itself. For maximum benefit,
information should reach operators
and maintenance personnel in realtime,
with context-sensitive remedial
action. Logging all diagnostics and
maintenance action from a central
asset management system will ensure
consistency, and simplify collection of
failure statistics. Historically, users
used multiplexers to obtain this diagnostic
information. Open protocols
such as WirelessHART now provide
the same benefits at a much lower
installed cost.
■
Edited by Suzanne Shelley
Authors
Stephen Brown is a senior
safety and control systems engineer
with E.I. du Pont Nemours
& Co. (Phone: 304-8634727;
Email: stephen.r.brown@
dupont.com). He has 23 years
of experience in control and
safety systems and has been
involved with the ISA84 Committee.
Brown holds a degree
in electrical engineering from
Carnegie Mellon University.
Mark Menezes manages Emerson's
measurement business
in Canada (1421 Samuelson
Circle, Mississauga Ont., Canada
L5N 7Z2; Phone: 416-4595935;
Email: mark.menezes@
Emerson.com.). He holds a
B.S.Ch.E. from the University
of Toronto, and an MBA from
York-Schulich University in
Toronto. Menezes has 22 years
of experience in industrial
automation, specializing in
control systems, loop controllers and flow measurement.
CHEMICAL
ENGINEERING WWW.CHE.COM APRIL 2013 63
Native interface
Modbus/OPC
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