JAN-FEB 2019

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38 INTECH JANUARY/FEBRUARY 2019 WWW.ISA.ORG SPECIAL SECTION system is a network of computers and programmable logic controllers (PLCs) using the AT&T virtual private network (AVPN) as the main backbone (figure 3). The main controller (poll master) of the online system is at the Lake Merced facil - ity and consists of a hot backup PLC communicating with the other main controller and remote PLCs located at each well or treatment facility. Essential signals, such as water flows, chemical flows, and water qualities (e.g., pH, chlorine), are collected at these facilities and sent to the main controller for control decisions. The SCADA system uses Wonderware as the human-ma- chine interface (HMI) software. The PLC uses Unity Function- al Blocks with detailed annotation for each logic algorithm. SCADA system data collection Field instruments, such as flowmeters and water quality analyzers, continuously feed online data to the local PLC, which also monitors and controls motorized equipment such as pumps. Some of the most critical signals for the On- line Water Quality Monitoring and Control System are the water flow rate signals. A flowmeter in the pipeline senses flow and generates an electrical signal back to the PLC. The PLC is then calibrated to determine what the current water flow is based on the strength of the signal. Safety measures have been taken in the PLC to ensure that the information received from the equipment is correct. This is true for both discrete signals, such as pump run and pump fail, and analog signals, such as flows and water qualities (e.g., pH, conductivity, chlorine residuals). Human interfacing There are two different methods for the operators to inter- face with the SCADA system. First, a local operator interface (LOI) is a touchscreen associated with the site (figure 4). This screen shows a graphical representation of the equip - ment at the site that changes as physical inputs and outputs change. This screen also provides a location for operators to change set points and manually measured values. Any pro - cess signal associated with the online system is displayed on this LOI screen. The second method of interaction occurs through a SCA DA system with any online data displayed simultaneously on computers located at any of the four operation control cen- ters (OCCs) on the respective computer's screen. This is the "mission control" of the water system. SCADA allows for all of the same interaction and graphical elements as the LOI, but over a much larger scale. SCADA's computing power al- lows it to take historical records of data for future review. Peer-to-peer communication One of the more difficult elements of a PLC network is main- taining proper communication between each PLC. For this reason, the Online Water Quality Monitoring and Control System uses an industry standard called a poll master. The poll master acts as a leader and initiates a conversation with each of the other PLCs. The poll master becomes a central Figure 2. SFGW project wells and reservoirs based drinking water standard (secondary MCL). It has not, however, consistently been found above the second- ary MCL in the SFGW Project wells. As a result of the water quality differences described above, the SFGW project team developed an online water qual- ity and control system to carefully monitor and manage the chemical treatment processes and mixing of groundwater and surface water. The major functions of this online water quality and control system are: l monitoring individual well production rates, mechanical and electrical systems in the well facilities, and chemical treatment processes l calculating the blended water quality in real time l controlling the well production to ensure that the blended water quality of key regulated parameters (nitrate, hexava- lent chromium, and manganese) continues to surpass the state drinking water standards l collecting and archiving data needed for monthly regula- tory reporting, to demonstrate compliance with drinking water standards Distribution of the groundwater blend is based on the elevations and pressure zones of the Sunset and Sutro Reservoirs systems. Control system hardware and software The control system for the online water quality control is a subset of the city of San Francisco's existing supervisory control and data acquisition (SCADA) system. This SCADA Legend Wells Groundwater pipeline routes Lake Groundwater basin boundary (from CA Dept. of Water Resources, Bulletin 118, 2003) Source: SFPUC Final Environmental Impact Report (2013)

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