PLC-controlled Hopper Gates Prevent Accumulated Sludge

The East Bay Municipal Utility District (EBMUD) sewage-treatment plant for Oakland, CA, faced a possible shipment bottleneck of sludge from its multi-step digester system and dewatering centrifuge.

The district needed to load trucks with about 50,000 pounds of sludge in about three minutes for transport to district customers. The loading had to be done within a tolerance of ±1% because the city has to charge its customers correctly and keep certified records of how much of the hazardous material went where.

Filling the trucks quickly within a ±1% accuracy regardless of large fluctuations in the sludge's flow rate was seen as the possible bottleneck. These flow fluctuations, caused by viscosity changes due to batch variations in sludge dehydration and temperature and varying material heights (that is, head pressure) in the loading hoppers, cannot be modeled.

The next most serious problem was measuring real-time load-out status because the programmable logic controller (PLC) would have no direct feedback, since:

• trucks would not be sitting on scales during the loading process;
• flow meters would not work in large hoppers with variable load-out orifices, and
• the hoppers would be refilling as they dispensed their contents.

Mechanical solution. Material Systems Engineers (MSE) (San Rafael, CA) devised a hopper slide-gate system to handle the mechanics of this dynamic-control problem.

MSE partnered with systems integrator DST Controls (Benicia, CA.) to develop the model-free adaptive control package required to track and control the accurate dispatching of sludge.

Besides overcoming potential load-impeding problems, the design team had to make the system affordable.

Basic control requirements included establishing communications between the district's Westinghouse distributed control system (DCS) and DST's PLC system and achieving the load-out setpoints by integrating measured net hopper-weight changes with inferred hopper refill rates.

The control system is designed to:

• allow operator-entered truck identification information and loading instructions;
• automatically enter truck tare weight;
• time/date stamp each load-out cycle.
• collect hopper load cell data and calculate load-out progress;
• modulate diamond slide gates to hit load-out setpoints;
• display load-out progress and customer identification information;
• monitor and regulate centrifuge out-flow valves and pumps; and
• send transaction records to the Westinghouse DCS for accounting.

Process provides uniform loads. A system of 12 valves directed by the PLC conveys partially dewatered sludge to hoppers above the truck loading area.

In automatic mode—a manual mode also is provided--the load-out operator enters, via the touchpad, truck and customer identification information and intended load weights. The touchpad inputs this information to the PLC for hydraulically operating the slide-gates of the three hoppers, which are monitored by position sensors that serve as inputs to the PLC.

The PLC takes a snapshot of the hopper weight at the beginning of the sequence. The hopper weight is continuously monitored as sludge loads into the trucks.

The PLC starts the flow at full open and switches to "intermediate" after 70% of the desired load (about 35,000 pounds) is reached (about two minutes). It then closes to the "fine" position until 100% of the load-out weight (about 50,000 pounds) is achieved, at which time the PLC closes the gate.

The Modicon PLC for its three primary control functions uses the loaded-out net weight for the:

• automatic, evenly distributed sludge load-out from the three loading hoppers;
• automatic refilling of the three hoppers; and
• transmission of transaction data.

Besides loading trucks properly, the PLC must also keep each hopper sufficiently refilled to maintain loading efficiency. It does this by using hopper net weight to actuate the 12-valve sludge-transfer system.

The hopper with the lowest weight refills first. When that hopper's net sludge weight exceeds the preset differential compared with the previous highest hopper weight, the filling sequence is redirected to the new lowest-weight hopper, and the cycle continues.

Upon load-out completion, the system prints a California Department of Transportation-required bill of lading with truck identification and load-out weight, which is handed to the driver and checked for accuracy at an independent truck scale by district personnel before leaving the plant.

To date, the agreement between the certified truck scales and the weight reported by the PLC has been consistently within 0.5%.

The previous case study was adapted from "Accurate Loading of Hazardous Sludge Prevents Environmental Contamination" by Richard Simon and Richard Wilson of MSE (415-472-8550), and Richard Batchelor and Read Hayward (707-745-5117) of DST Controls. Their report appeared in the May 1999 issue of Control Engineering.