Distributed Control System Offers Local & Central Access
A 60-mgd direct filtration plant incorporates rapid sand filtration and replaces eight existing slow sand filters
Located near Springfield, Massachusetts, the West Parish Filter Plant has a history of planned growth to meet area water supply requirements. By the late 1960s, demand projections to the year 2020 showed a need for plant capacity to increase to around 100 million gallons per day. This was met by construction of a new 60-mgd direct filtration plant that incorporates rapid sand filtration, and replacement of 8 of the 18 existing slow sand filters.
Camp Dresser & McKee Inc. of Cambridge, Massachusetts, engineered the new plant, which incorporates rapid-mix and flocculation basins, as well as dual-media, double-bay filter beds. A brochure printed in 1975 stated, "The daily operation of the plant has been automated to a sophisticated degree. Plant inflow and chemical dosages are regulated from the control room, and important operating data are monitored."
That broad description still fits the new distributed control system (DCS) installed in 1992 on the rapid sand filter portion of the plant. The degree of sophistication, however, has advanced tremendously during the intervening two decades. Today's DCS has given the plant operators added assurance that they can produce a high-quality water supply for their customers, and provide management with a variety of plant performance logs and reports.
The Process to Be Controlled
The basic rapid sand filtration process, now DCS-monitored and controlled, remains unchanged from the 1975 design. The consultant had found that sedimentation basins, which normally precede the filter section, could be eliminated-provided chemicals were used for coagulation of solids ahead of the filters. Savings estimated at the time were $2.5 million.
Raw water for the plant comes from upstream reservoirs, which hold about 42 billion gallons of collected surface water. Entering the plant through two conduits (72-in. and 36-in. diameter), the influent feeds to a settling basin before passing on to pretreatment. This section contains 14 units, each consisting of two rapid-mix basins in series followed by two flocculators also in series. A cationic polymer is added for coagulation, and gentle agitation in the flocculators encourages the formation of larger, more readily filtered flocs.
Two channels convey the water in treatment to an adjacent building containing six double-bay rapid sand filters. Each pair has 1,400 sq ft of filtration area and shares a common influent and drain gullet, but each bay has a separate effluent line. A 15-in. bottom layer of silica sand on an under-drain of Camp-nozzle tile blocks, and a 24-in. upper layer of anthracite coal, make up the filter media. Cleaning is accomplished with a complex air and water backwash schedule. The new DCS was assigned to handle this operation, which had been controlled automatically by an earlier installation.
Controlled volume metering pumps are used to feed the various chemical solutions, such as caustic soda and cationic polymer. These pumps are paced by the incoming raw water flow rate.
Filtered water passes to intermediate storage. Then, together with output from the slow sand filters, it enters 42-, 48-, and 60-in. pipes that lead downstream to the 60-million gallon Provin Mountain Reservoir. Caustic soda and chlorine are added as needed for pH and chlorine concentration control. The reservoir supplies finished water to the distribution systems of Springfield and its environs. To gauge demand, level at the dam breast in this reservoir is monitored and used to govern plant throughput accordingly.
Benefits of the New System
The West Parish Filter Plant's distributed control system, with two redundant display center terminals (DCTs) in a control center, and three local processing units (LPUs) located at strategic points in the filter plant building, is shown schematically in Figure 3. These main components are connected through a redundant Ethernet data highway.
Each LPU contains a vital part of the DCS: a distributed control unit (DCU), which has the capability of handling over 1,000 process inputs and outputs (I/Os). Each DCU is assigned a portion of the total monitoring and control task: DCU-3 for rapid sand filter operations; DCU-2 for chemical feed controls; and DCU-1 for monitoring variables (levels, flows, pH, etc) associated with both plant influent and finished water.
This DCU was specified to have full component redundancy since it carries out the important functions of monitoring and controlling the filters.
Some of the operational benefits of this DCS design are
- The system is "user friendly." As the photograph shows, an operator can bring up as many as four related graphic displays on one 21-in. color CRT screen. This is especially valuable for monitoring the rapid sand filter backwash activity.
- Each DCU has a 32-bit controller and several 32-bit intelligent I/O cards. It can continue to perform assigned functions, even if completely cut off from the control center.
- The system has an on-line spreadsheet software package that accesses both the DCUs and the historical database to generate reports. It automatically provides management and operators some 20 different reports and logs. For example, a backwash report indicates time between backwashes, turbidity and filter head loss prior to backwash, and volume of backwash water used.
- The LPU and FCC cabinets contain local switches and controllers so that an operator can, if necessary, start or stop a pump, change a setpoint, or position a valve, for example, rather than initiate such functions from the control center. By design, local panels take precedent over the DCS.
- DCU-3 has six boards (DIOs) which provide communication with distributed I/O modules in each FCC via 375k-baud networks (Fig 2). These reduced the cost of wiring since one cable connection from each FCC to DCU-3 can handle eight I/O modules.
- One project requirement was that the new DCS had to be installed without serious interruption of filter plant operations. Only one filter could be out of service at a time. Modular design of the DCU permitted loop-by-loop addition of monitoring and control for each filter console, one at a time. Installation took much longer this way, but Springfield never was without water.
- Configuration of control and alarm monitoring strategies was carried out using standard software already in the DCUs. The system can accept add-on software that might enhance usefulness or performance.
- The redundant Ethernet highway provides reliable, high-speed (10 megabaud) access to all process data, plantwide, for all existing or add-on I/Os. This network can support up to 32 DCUs, each capable of handling more than 1,000 I/Os.
- Two display center computers for X-Windows (XDCs) in the control center have complete hardware redundancy with the two display center terminals (DCTs). If one XDC goes down, operators still have a window to the process through the other XDC and its terminal.
How the System Controls Backwash
An example of how an operator uses the DCS is seen in the control of a rapid sand filter backwash. The operator/graphic interface is either one of the two redundant DCTs in the control center. The photograph shows the lit screen of one DCT. For backwash control, the operator works from the four-window display for the filter involved. He or she starts with the upper right screen-Backwash Status, Screen B. At the bottom of this screen is a list of options that can be implemented by "hitting" the screen "button" at the left of each option (using the mouse and keyboard).
For each of the six filters, the operator can select fully automatic (Auto BW), semi-automatic, DCS manual, or FCC manual. If the choice is Auto BW, he or she can pick from backwash start options based on time, volume rate, head, or turbidity. It could occur, for example, after a certain period of time has passed since the last backwash, or when the head across the filter gets too high, or if there is excessive turbidity in the effluent.
More commonly, the system is used in the semi-automatic mode, which the duty operator calls up on the screen by selecting the second option, Start/Permit Semi-Auto Backwash, or by pressing a button on the filter console. Before the operator turns control over to the DCS, however, he or she must set the control parameters, using the lower left window (Backwash filters common parameters), then the lower right window (Screen C). From the first display the backwash pump minimum speed can be set at 35 percent, or the filter restart level limit at 7.70 feet. Using Screen C, the operator can, for example, set the parameters for low and high backwash rates, wash time limits, and air wash pressure.
Once such parameters are set up, the operator "hits" the Start/Permit button on Screen B. This lets DCU-3 take over and automatically monitor and control the entire operation. It closes the influent gate, draws the water level of the filter down to its predetermined level, starts the air blower for air wash, opens the air wash valve when the blower gets to a certain speed, and then initiates backwash. At the end of backwash, the DCS prepares the filter again for filtration. It shuts down the pumps and blowers, brings the water level up to the pre-set filtering level, and opens the influent gate and the effluent water valve.
Each of the rapid sand filters can be controlled manually, either from the DCS or the respective FCC for the filter. All the required I/Os are routed through the FCC for each filter, then to DCU-3, which also monitors and controls the flow and discharge pressure of the common air wash supply.
The system also has the capability to proceed through ten "phase logics" associated with each of the six backwash control sequences and, for each, provide corresponding operator interface messages and actions. The operator starts with Phase 1 "BW Check" and can proceed to backwash the filter at the FCC, or from the Control Center. Messages on the graphic displays guide or inform the Control Center operator.
Some idea of the complexity of the filter monitoring and control task can be seen from a review of the I/Os for each of the six FCCs
- Discrete Inputs: 39 DIs that cover, as necessary, each side of the filter (e.g., Filter 1B effluent valve open).
- Discrete Outputs: 34 DOs, associated with the same equipment as for the DIs, plus alarm points for such variables as high headloss or high turbidity on either side of the filter.
- Analog Inputs: 12 AOs for level, flow, headloss, and flow set points for both sides of the filter.
- Analog Outputs: three AOs, for effluent valve control (both sides of filter) and backwash flow set point signal.
A number of these I/Os share a common function in a measurement and/or control loop. There are some 20 such loops for each FCC, giving operators many "eyes" to keep track of each stage of backwash, as well as many "hands" to perform any action called for.
A printed report compares operating characteristics of the filter, before and after backwash, to judge the relative success of the wash, and also documents other parameters, for example filtering time and filtered water volume.