The Island Water Association (IWA) of Sanibel, Florida, supplies potable water for Sanibel and Captiva Islands. Using a reverse osmosis (R.O.) process of purification, water is obtained from wells in two deep artesian aquifers (Suwannee and Hawthorne) beneath the surface of Sanibel. Ten of the sixteen wells actively providing feed water to the R.O. plant have now had their conventional copper wiring and hardwired relays replaced with a fieldbus system. Profibus was chosen to implement an architecture with remote I/O and fiber optic cabling to reduce nuisance trips and outages.
Nuisance Trips and Outages
Each of the wells is equipped with a submersible pump. The problems that existed before the recent modifications were caused by induced voltages and transients that were entering the miles and miles of underground cable, especially during electrical storms. The events could create interruptions and/or unexpected starting of the well field pump motors. Speed and intelligence had become a critical issue for the old system because if a pump started and a valve was not reliably in the proper position, it would occasionally blow fiber glass inlet pipes and could even blow the well head itself. Downtime and parts replacements were a growing problem as the system aged. Resetting a tripped relay would require physically driving to the well site and doing so manually.
Pressures at the Membrane Walls
Another critical issue at the R.O. plant concerned the ability to maintain the 200p;250 psi needed for the feed water that flows into the R.O. trains that contain the membranes for separating fresh water from salt. There can be conditions where the pressure is either too low or too high. If the pressure becomes too high, the manifold gaskets at the feed pumps can blow, flooding the R.O. room. One such scenario could be the result of a pump starting somewhere at a well site when the valve is in the wrong position. Another scenario could be from a voluntary shutdown such as cooperating with the local electrical company during peak demand times. If a pump were to start during the shutdown, the water would have to go somewhere. If the system has valves set in the wrong position, a gasket anywhere, including the feed water manifolds located miles away at the R.O. plant, can blow.
Post Treatment Issues
Post treatment of the filtered water includes the use of chlorine and sodium hydroxide. These two chemicals are maintained in a separate building from the R.O. plant. The chlorine is injected into the permeate stream and mixed. Enough chlorine is injected to also leave a small residual amount for disinfecting purposes. Sodium hydroxide (caustic soda) is used to neutralize the acids and increase the pH to about 7.5.
Both of these chemicals have to be carefully monitored. When the chlorine and pH levels are not maintained within certain acceptable parameters, the system must be shut down. Prior to recent modifications, adjustments for dispensing these chemicals and analysis of the treated water were heavily dependent on manual operations.
The Profibus Solution
To address its challenging logistical requirements, IWA selected a system employing a Profibus digital fieldbus system connecting the R.O. plant to the pumping stations via fiber optic cable.
In the application, one single cable connects decentralized Siemens ET200 B Profibus I/O blocks in the field to a Siemens SIMATIC 555 PLC located at the central control station. The sensors and valve actuators of the individual pumps are hardwired to the I/O blocks with traditional wiring.
System performance using this fieldbus has increased dramatically. With transmission rates up to 12 Mbaud, Profibus provides a responsiveness more than needed at the pump stations.
Fiber optic cabling allowed replacement of much of the underground cabling for the pump system. Approximately 21¦2 miles of fiber optic cable was installed, affecting nine of the 16 pump locations that actively provide feed water. The fiber optic solution also results in a much more simplified configuration than with the bundled copper cable formerly used
- Necessary grounding and lightning concerns did not need to be addressed.
- Tests can be built into the programming to detect nuisance trips accurately.
- Resets can be accomplished remotely over several hundred meters with the greatest confidence.
A much more accurate alarm and warning system is now in place at the central station due to the increased speed of the remote I/O. Nevertheless, most of the data being reported on the screen monitors is also being reported via meters and indicator lights mounted in a mimic board that is in the central control room. The mimic board has been in place long before the Profibus I/O and fiber optics were put into place. Although there is duplication in having the on-screen information via the computers and the information via the mimic board, there is an element of safety felt in having the two co-exist for the time being. Eventually, when the personnel become more accustomed to the screen information and begin to rely less on the older mimic board, the board will be phased out.
At the post-treatment building, another PLC was installed to monitor and adjust chemical dispensing for the chlorine and sodium hydroxide. This data also is available at the central base station. There are some automatic operations built into its stored program and adjustments can also be made interactively from the central control room. One very critical parameter being monitored is the chlorine gas leak detection. Sensors send data to the PLC where emergency action is automatically handled.
The first modifications were made in April of 1997. Since that time there have been no outages due to nuisance trips. There have been no over-pressure problems at the membrane walls that have resulted from accidental starting of pumps or valves being in the wrong position. Chlorine and pH levels have been more accurately maintained than ever before. More accurate logging of events now is available. Recent Florida regulations have made accurate measurements (such as daily flow data) essential. This logging capability enables the plant to do a much more thorough and accurate job of reporting.