Chemical Engineering October 2014 - 41

Cover Story
Emerson Process Management
is sufficient, and can allow battery
life from months to years. In some
applications (tank-farm-level monitoring,
for example), the update rate
can be even lower under most conditions.
And for such applications
with slowly changing variables it is
possible to include the wireless link
as part of a control loop. Some wireless
devices can update at a rate of
once per second, which can be similar
to some wired devices.
Figure 6. An electronic remote sensor arrangement
replaces a single DP transmitter with two sensors,
one located at the bottom of the tank and one at
the top. This is useful for tall tanks because it eliminates
the need for long impulse lines
having to walk out to the transmitter
to check it.
WirelessHART. While the first
wireless networks in plants were
almost exclusively for asset monitoring
purposes, the advent of IEC
62591 (WirelessHART) made wireless
delivery of process-variable information
increasingly common for
monitoring and control applications.
An IEC-approved WirelessHART
system takes the form of a self-organizing
mesh network, with a wireless
gateway as its connection point
to the plant's main control system.
Each transmitter constitutes a
network node that acts as both a
source of data and as a router, so
that a message from one node will
be passed from node to node until
it reaches the gateway, and a message
from the gateway to a particular
node will similarly be passed via
multiple paths in the mesh from
node to node until it reaches its
intended recipient. The redundant
paths provide for reliable communications;
not dependent on a single
path for communication. The network
is also self-healing: if one node
is disabled, network traffic will be
automatically routed around it to
ensure delivery. The arrangement
also allows the network to extend
over a larger geographic area than
the radio range of the individual
nodes. A node does not have to be
within radio range of the gateway, it
just has to be in range of one or two
other nodes in the system.
Advantages. One of the major advantage
of wireless is that it does
not require any new I/O points.
Expanding a plant, or just adding
transmitters to an existing system,
generally requires a new I/O. A
wireless network makes it possible
to expand a plant's instrumentation
without running any wiring at all
(4-20 mA or even fieldbus cabling).
One simply connects the gateway to
the control system. The additional
wireless transmitters can be battery-powered
and exist as nodes on
the wireless mesh network adding
the required process information.
Data rates. While wired HART
operates at 1,200 bits/s, IEC 62591
(WirelessHART) operates at 250
kbits/s, although the rate at which
data are delivered over time is considerably
slower. This is because a
WirelessHART transmitter does
not send data continuously; to do so
would quickly exhaust the battery
of a battery-powered unit.
For many networks, an update
rate of several times per minute
Some final thoughts
As in any field, there are many
things to consider when it comes
to selecting a pressure transmitter.
Many of these considerations are
not covered in engineering school,
and are best learned on the job -
preferably, by learning from more
experienced engineers.
One way to get a great deal of
helpful information is to ask an
application engineer from a major
instrumentation vendor; many of
these people have upwards of 30
years of experience in the field, and
have seen and dealt with situations
beginners have not imagined. They
can often provide what amounts to
free consulting services in terms of
technology selection. We hope that
this article will help you know what
questions to ask.
■
Edited by Gerald Ondrey
References
1. HART Communication Foundation, Austin,
Tex. http://en.hartcomm.org/hcp/tech/
aboutprotocol/aboutprotocol_how.html
Author
Wally Baker is currently
a marketing manager
for the Worldwide
Pressure Product group
of Emerson Process
Management (8200 Market
Blvd., Chanhassen,
MN 55317; Phone: 952949-7242;
Fax: 952-9495068;
Email: wally.
baker@emerson.com) ,
and is responsible for
Rosemount Wireless
Pressure Products. He
has 15 years experience
in industrial process and control. While at Emerson
he has held positions as Global Wireless
O&G business development manager, sales and
marketing manager for Asia Pacific Wireless &
Plantweb, global product marketing manager
in the Rosemount Temperature Business Unit,
and spent five years in the Rosemount Pressure
Engineering & Design team as an electrical and
software engineer. Baker holds an E.E. degree
from Iowa State University and an M.B.A. from
the University of Minnesota.
ChemiCal engineering www.Chemengonline.Com oCToBer 2014 39
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Chemical Engineering October 2014

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