By Robin Shepherdson
Nestling in the hills overlooking San Diego is the San Vicente Dam, one of San Diego’s main sources of domestic water. The San Diego County Water Authority is overseeing the construction of the $1 billion San Vicente Dam Raise Project, which began in early 2009 and will take about four years. Refilling the reservoir after construction will take another two to five years.
This is part of the Water Authority’s more than $1 billion Emergency Storage Project, which is creating new emergency water storage and pipeline connections to deliver water throughout the region if the imported water supply is disrupted.
When complete the reservoir will be able to store an additional 152,000 acre-feet of water supply. The construction project was awarded to a joint venture partnership of Shimmick Construction Co. Inc. and Obayashi Corp., and the design and supply of the concrete production plant was awarded to Plant Architects/Plant Outfitters of San Antonio.
Plant Architects chose to convert an existing Con-E-Co plant for the aggregate batching section, and ordered a new section from Ocmer Impianti of Italy housing two aggregate holding hoppers, cement, fly ash and water scales, two twin-shaft mixers and two mixed RCC concrete holding hoppers feeding either of two take-away belts. One belt is short for local delivery and the other feeds directly to the dam, where rollers compact the concrete to form a stairway-shaped addition to the exposed front of the dam and continuing another 117 feet in height.
Ocmer Impianti represented by ConcreteMixers.Biz and Scale-Tron in North America, specializes in high-quality batch plants of the low-profile design rather than the tower type, with fully galvanized structure incorporating a number of features new to North America. The huge volume of RCC concrete requires a plant with output of 600-yard per hour, which is easy going for the two 5.3-yard mixers. Aggregates are batched from storage bins, replenished by inclined belts from a dual wet-belt cooling system and a drum-type sand cooler supplied by Coldcrete. Cement and fly ash are supplied from two 1,000-ton silos.
The control system is a Scale-Tron BatchTron III PLC and touch screen-based system, chosen for its reliability in high-speed production as well as its user-friendly graphic touch screen operation. In operation, it replenishes the twin holding hoppers underneath the mixers, based on level sensors, and sequences weigh scales, holding hoppers and mixers in a continuous duet that can refill each mixer the instant it is empty to maximize throughput. Peak output is calculated at 740-yard per hour, although the aggregate supply could not keep up with this pace for long.
To produce concrete at a maximum temperature of 55°F at the delivery point, aggregates and water must be cooled to about 40°F and sand to 50°F. A storage pond, insulated by a floating cushion, is cooled to 40°F by the Coldcrete system and used to spray water over the aggregates on the two wet belts, as well as supplying the water used in the concrete.
Because the aggregates are thoroughly saturated by this process and the product is a zero-slump mix, water required in the batching process is low by ready-mix standards, but still works out to more than 15,000 gallons per hour at full production. Fortunately, a ready supply is available from the existing reservoir.
A local quarry supplies aggregates that are crushed, screened and sent to the two separate wet belts. Approximately 150 feet long, these belts are of perforated rubber with spray nozzles along their length, covered by a vinyl-fabric canopy. Run-off water is collected in channels under the belts and returned to the cooling pond for reuse.
Coldcrete’s Mike Lee says that this method of cooling is more efficient and more reliable than an ice plant, giving less downtime and lower energy costs. He is currently supplying many of the world’s dam contractors with similar equipment, together with Scale-Tron’s aggregate control systems.
The two factors that persuaded Plant Architects to go with the Ocmer plant were Ocmer’s European-style design and galvanized “assembly-kit” structure. Ocmer precision fabricates all its structural frames and pre-assembles them at their plant to guarantee that when they arrive onsite that they can be erected easily with negligible delay due to ill-fitting parts.
This was borne out in practice; from delivery at the site to an erected plant took 14 working days. Electrical, plumbing and feeds from cement silos and aggregate batcher took an additional four weeks, with the first batch being poured six weeks following delivery to the site. A 1,000-yard test pour was witnessed by the U.S. Army Corps Of Engineers during March, about 14 weeks after plant equipment delivery.
Innovative plant features include:
• Aggregate Refill Control: The aggregate wet belt and sand drum cooling systems need to be run continuously, preferably at a constant speed, to optimize the cooling. The feed bins for the batch plant need refilling as batching proceeds but because production can speed up or slow down, the bins will require filling at rates that constantly change.
A separate PLC and touch screen control system uses mid-level sensors in the bins to control the average level, speeding up or slowing down the feed rate onto the wet belts as necessary. Additional sensors either stop the filling process if the high level is reached or stop the batching process if the low level is reached.
These levels should never be reached during normal operation, but are a safeguard against plant equipment breakdowns. The aggregate control system eliminates the need for an operator to monitor and control the refill process. With the addition of temperature sensors, it can also optimize the cooling process, saving further energy.
• Mixer Feed System: Ocmer’s aggregate holding hopper is a complete, enclosed assembly with scales for cement, fly ash and water distributed round the outside.
The whole assembly sits on the mixer with access to the upper level by an integral ladder and catwalk, saving structural elements and making assembly very speedy. In colder climes, cladding is added to the outside to make it weatherproof.
• Rotary Jet Mixer Cleanout: Using high pressure water through three water-powered rotating heads in each mixer, the twin shaft mixers are thoroughly cleaned twice each shift with very little in the way of extra manual cleanup.
The Italian-made heads have four jets that rotate like pinwheels around a horizontal axis and also rotate around a vertical axis, giving an omnidirectional spray pattern that is ideal for large mixers.
• Dust Recirculation: Air displaced from the mixer during charging with aggregates is recirculated into the top of the aggregate holding hopper, replacing the volume evacuated by the material as it discharges.
This would form a closed circuit if the hopper was fully enclosed. Since it is not, a small dust collector cleans up any dust around the top of the hoppers before it escapes.
READ MORE: concretemixers.biz
Robin Shepherdson is an engineer with more than 40 years experience in the concrete industry. He has recently started ConcreteMixers.Biz, which specializes in high-quality European plants, mixers and related equipment. CM
