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.