April 2011 - Control Global

O N T H E B U S
Attack of the Mutant Chicken Foot!
A few months after start-up, our facility needed to add an alternative purge medium
for a process vessel. Along with a flow loop, we needed a couple trios of new doubleblock-and-bleed
actuated valves in a spot about 90 feet from grade. So did we need
to run nine twisted pairs (or more—up to 20 for conventional on-off valves) all the way
john Rezabek
contributing Editor
jrezabek@ispcorp.com
down to grade? We found we could just extend
an existing fieldbus segment from an instrument
at that level—so maybe 25 feet of light
conduit on a platform were needed instead of
100 feet of 2-in. down the side of a structure.
Why hadn’t we done this from the start? Our
original “chicken foot” network was grafting
eight toes on one of its toes.
In recent issues, we’ve explored how bus
technologies create new challenges for end users’
Engineer, Procure, Construct (EPC) consultants
and systems integrators. Bus technologies
provide a standardized infrastructure,
allowing smart devices to integrate happily
with the host DCS and one another. End users
want this because they see a path to improved
control, process availability and risk/asset management.
But for the EPCs, it’s disruptive.
Bus topology—the ability of multiple devices
to share the same two conductors on a network
or “segment”—is one of the disruptive forces.
We have deployed networks elsewhere, so why
would fieldbus cause consternation?
The challenge is this: process plants have
hundreds or thousands of devices distributed
to scores or hundreds (or more) segments. In
the point-to-point world, there is a one-to-one
relationship between device, single-pair cable,
multi-pair cable “pair,” field JB terminals, marshalling
terminals, scatter wire, all the way to
the I/O processor terminations, so much so that
the whole process can be largely automated.
Designers and engineers work from a database,
whose fields then populate loop drawing templates,
junction box drawings and termination
schedules.
With fieldbus, all the one-to-one relations of
the old world are scrambled: devices can have
multiple roles; home-run cables can serve as
few as two devices or as many as 32; and the
rules for grouping devices are anything but random
and can be crucial to a successful project.
Tools do exist, such as Intergraph’s SmartPlant
INTools CAD products, but they require a new
level of training and effort that was previously
unimportant or automated. So have some compassion
for your EPC and simplify.
For your first fieldbus job, consider a simple
“chicken foot” or “star” topology. By standardizing
on six segments (~72 devices) per field
junction box, your install will look and feel
close to conventional, but you’ll be running
some bigger conduits and raceways. It will look
just like conventional point-to-point from the
JB to the field. If a certain device needs to move
from segment “A” to segment “B” late in the
job, there’s an increased chance they’ll be in
the same JB. While less optimal for the project
in the CAPEX phase, the strategy lends itself
well to future expansion. In this scenario,
the client can still add to the installed “chicken
foot” as every fieldbus node can become an “expansion
point” for future additions.
Users with great confidence in their segment
loading and their team’s fieldbus savvy may
choose what many consider an “optimum” topology,
what I call the “mutant” chicken foot.
Spurs—“toes” if you will—grow on the trunk
wherever needed. The trunk strings these clusters
together as a single pair, so conduit size is
minimized. If local codes allow, using “basket
trays” can be more forgiving for late changes
and future modifications. Physical layer suppliers
such as Pepperl+Fuchs, Relcom, Turck
and Phoenix Contact have some nice modular
hardware to accommodate these multi-drop hybrid
schemes. Every project will have its unique
mix of experience and confidence on both the
client and consultant sides. Making discrete topology
choices in light of your team’s strengths
and challenges can help ensure a smoother and
less stressful journey.
We have deployed
networks elsewhere—so
why
would fieldbus
cause
consternation?
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