Wireless Water Applications
San Antonio Water makes full use of frequency hopping spread spectrum
- By Terry McVay
- Jan 15, 2008
Reliability is paramount to the
monitoring and control of
water/wastewater facilities. With
facilities relying on real-time
data transfer between remote
systems and plant control areas,
managers and operators cannot afford downtime
or errors in information transfer. At the
same time, these facilities must reduce engineering
and maintenance costs, maintain security,
and ensure compliance with expanding regulations.
Today’s wireless technologies strive to
meet these needs head-on.
The management team at the San Antonio
Water System (SAWS) recognized that wireless
technology held promise for many applications
at its facilities. A public utility and water/wastewater
facility owned by the city of San Antonio,
Texas, SAWS serves more than 1 million people,
businesses, and other facilities. It provides
water services to retail customers and sells wholesale
water to several smaller utility systems.
SAWS collects and treats wastewater for the
greater metro area and provides stream generation
for certain large customers in the downtown
area.
Between 2003 and 2005, SAWS implemented
a series of projects in various areas and
departments. The system uses frequency hopping
spread spectrum (FHSS) technology, an
industrial wireless input/output (I/O) interface
system, to implement solutions in areas where
wired technology once was thought to be the
only option.
FHSS has proved reliable and cost effective.
These interfaces can send analog and discrete
signals from a sensor to a PC or programmable
logic controller (PLC), or from the same PC
or PLC to a specific pump or other end device.
These devices report levels, pressure, flow, status,
and alarms to control pumps, valves, and
switches.
No more tethers
Among its user benefits, FHSS allows operators
to monitor issues and events safely, flexibly, and
efficiently, particularly when they are away from
standard monitoring devices, such as monitoring
computers or human machine interfaces.
Before wireless technology, operators had to get
information from multiple sources via scattered
I/O modules. Now, they can be as mobile as
they need to be.
For a utility, FHSS eliminates the need to
install conventional wiring, which can cost as
little as $10 to as much as $1,000 per foot.
Wireless applications for the water/wastewater
industry historically have centered on the
need to obtain signal readings and indication
notifications from remote well sites back to a
central control center. SAWS instituted multiple
monitoring applications involving remote
tanks or analyzer units in the field to fulfill the
need to send level indication, suction pressure,
well-flow information, or pump actuation status
from various remote sites back to the central
control center located outside the downtown
area. These signals typically are analog 4/20
mA or digital in nature.
An installed network link hardwired back to
the main supervisory control and data acquisition
(SCADA) network is not available in most
of these areas. Digging trenches, laying conduit,
and pulling cable made acquiring these signals
costly. These factors, along with the associated
costs of engineering and inspections and the
time needed for implementation, made these
projects cost-prohibitive.
Wireless I/O interface modules were the best
alternative for the SAWS monitoring network
application. To implement these point-to-point
applications, SAWS used wireless radio modules.
The modules are transmitter/receiver pairs
that come factory-programmed, calibrated, and
tested as sets. They can send and receive one
4-20 mA current loop and two digital status signals.
Typical range allows in-plant communication
with no line of sight, and the ability to
transmit up to 20 miles with the proper relative
line of sight and antenna equipment. A 12- to
30-volt DC current source powers the transmitter
and the receiver. At the transmitter end,
the process signals wire directly into the input
terminals, and the output connects via wires
to the receiver. The modules eliminate the need
for conduit and wire.
How FHSS works
This SAWS configuration uses FHSS technology.
With FHSS, the radio is a narrow band
fixed frequency radio—but only for an
instant—before it hops to another fixed frequency
on another channel and continues to
hop pseudo-randomly. Frequent sampling of
small data packets ensures reliability. Whereas,
traditional telemetry SCADA requires a significant
amount of information to be sent
through the air, wireless I/O requires only bytes
of information be moved. Because errors occur
when bits are received incorrectly, the smaller
packet size reduces the chance of an error. Interfering
signals can knock one packet out of an
FHSS radio’s hop pattern, but the rest of the
updates will make it through successfully.
The use of spread spectrum radios in the
water/wastewater
field is not limited
to I/O applications.
Serial data
(RS 232, RS 422,
RS 485) transmission
applications
are extremely
prevalent. Such
was the case for
the SAWS Heating
& Cooling
Plant in its downtown
facility.
SAWS maintains
a large steam plant
that originally was
built for the 1968
World’s Fair.
Steam from this
facility is sold to many of the larger hotels and
business buildings throughout downtown.
In the past, SAWS leased dial-up phone lines
to monitor customer steam usage and flow.
Operators regularly dialed up each customer site
to poll the flow processors and then logged the
information through a modem to the control
computer. The leased phone lines, however, were
expensive to rent and maintain. They went
down often, and outside contractors had to be
called in to make repairs. When this happened,
sites were monitored manually, increasing costly
man hours. After an engineering cost analysis,
team managers determined wireless technology
was an ideal solution.
San Antonio implemented the same FHSS
wireless technology it had used with its I/Obased
applications but in a form factor that communicated
between serial devices. SAWS
removed the leased phone lines at customer sites,
installed data radios, and “networked” the individual
sites back to the central control center
that monitors and records steam usage. The
wireless network automatically polls customer
sites and records steam usage in real time. A
computer at the central control center tracks
and records this information.
The modules eliminated the costs and constant
manual monitoring of the leased phone
lines while maintaining accurate monitoring
information. The Heating & Cooling Plant realized
a two-year payback on the wireless technology
by not having to pay for the leased land
phone lines any longer.
Wastewater plant applications also can benefit
from wireless technology. SAWS wastewater
treatment plant uses wireless I/O in typical and
not-so-typical ways. Due to its large size, the
SAWS facility was not wired for visual alarm
beacons and audible horns needed to alert personnel
of a high chlorine level or spill. Instead,
the management team established a wireless network
for triggering remote indicators. At the
main treatment center, alarm sensors send information
on the hazard condition to the distributed
control system (DCS). The DCS triggers
an alarm signal based on the feedback from the
sensors and feeds this information to the master
data radio. The master data radio sends out
a signal to all the remote sites around the plant,
triggering the beacons and audible horns. The
cost savings compared with trenching and running
conduit and cable to these remote sites is
considerable.
The water/wastewater industry faces numerous
economic, regulatory, and consumer
demands, and the aging of water/wastewater
infrastructures will be a growing issue in the
future. As managers seek innovative techniques
to keep their facilities ahead of the game, wireless
technology will play an increasing role by
providing real-time data communications.
References
D. Mathews, “Radio Waves,” ISA InTech, 2003.
W. Iversen, “No Wires,” Automation World,
December 2005.
About the Author
Terry McVay is a regional sales manager - industrial controls solutions for Phoenix
Contact Inc USA. His position as industrial sales engineer in the Central Texas territory
from 2001 through 2005 enabled him to get involved with various wireless technology
projects and applications in the water/ wastewater field. To contact McVay, call 800.888.7388.