Mixing it up

Creating aggregate – the main ingredient in concrete – involves mixing the right combination of sand and gravel to meet specific building requirements. Mined aggregate is washed, crushed and segregated according to size. These components are then recombined according to meet international specifications, creating products used in making concrete, asphalt and aggregate.

To more accurately control and monitor that process, California-based CEMEX uses electro-pneumatic (E/P) positioners to open and close the gates that discharge material on conveyor belts, which then transport the material to predefined locations within the plants. Any combination of up to four different gates can be opened to blend ingredients depending on the material being produced.

The E/P positioners allow CEMEX to control gate positions and protect the mixing lines at its Pleasanton and Redwood City plants from overfeeding in the case of a power failure or equipment malfunction. The right mix is important as CEMEX markets the aggregate to manufacturers of concrete block, interlocking pavers and lightweight roofing tiles.

At CEMEX’s Harbor Sand and Gravel plant, the E/P positioner opens and closes a gate to control the amount of material released onto a conveyor to maximize the tonnage being transported without overloading the system. Feedback from a belt scale downstream opens or closes the gate controlled by the E/P positioner to maintain a tons-per-hour flow set by the operator. Once the E/P positioner is given a setpoint by a PLC, it moves the gate to the given position and then monitors that position, making adjustments as necessary. Feedback from the E/P positioner is monitored by the PLC, which alerts the operator in the event of a malfunction.

The E/P positioners replaced existing air cylinders at each aggregate mixing location, making installation easier and less expensive since air was already available. A push-button station was already being used to interface with the existing air cylinders, so it was relatively simple to integrate the new cylinders into the system while the plant maintained normal day-to-day operations. This is the one of the first cases where this type of automation has been used in aggregate mixing.

One challenge faced by the project team was the need for a five-inch bore that had to be 'fail-safe’ in the extended position. CEMEX couldn’t risk leaving the chute gate even partially open, as the silo could completely empty on a stalled conveyor and create a costly clean-up operation.

By developing a fail-safe strategy, the cylinders are set up to fully extend in the event of power failure. In that case, any residual air pressure in the lines is used to extend any open cylinders to their fully extended position, thus closing any open gates and eliminating the associated spills and cleanup.

Another issue was the type of power failures that often plague California. Each time the power failed, the cylinders remained in their last position because the solenoids were electrically actuated. This allowed the contents of the bunker to spill onto the stopped conveyor belt, burying the belt and requiring several hours of cleanup before the belt could be restarted.

By reversing signal wires and plumbing lines, adjusting command signals accordingly and adding a normally-open 3/2 valve, which overrides the pilot pressure to the directional valve that extends the cylinder, the team was able to achieve the desired effect. The system is also equipped with an air reservoir to allow cylinder actuation even after a power failure stops the air compressors.

Quality control samples are taken daily, and the results are used to fine-tune positioner settings. These are then periodically updated to make a more consistent blend with fewer operator adjustments. Also, less 'out-of-spec’ material means less rework and less repurchasing of unsatisfactory material.

Tony Fuentes of CEMEX’s quality control department said the automated positioners have improved the reliability of the aggregate products made through the load-out system.

“We have noticed a marked improvement in our operating band, and we have tightened our operating ranges on several products,” said Fuentes. “The new system has taken a lot of the guesswork out of the gate positions, and one of the biggest benefits is that we can make changes with little or no effect to our product or operating parameters. We can also offer custom blends with more accuracy, which is a huge benefit for special projects.”

Drew Hinds of Bosch-Rexroth Pneumatics, which helped oversee the CEMEX installation, said the E/P technology is applicable to other industries “where you want to provide intermediate pressure to control something besides material flow. For example, it is used to control web tension for the printing industry, where paper is running through a printing press. The positioner helps keep the paper properly tensioned to ensure quality printing.

“The E/P positioner could be used in any situation where you are controlling the flow of bulk material, solids or liquids,” Hinds said. “For example in food manufacturing where you are controlling product flow such as rice, powders or produce; or in agriculture where you are controlling grain or materials in a feed mill.”

When evaluating this kind of problem, there are a number of things plants should insist on, according to Hinds. Determine the amount of force required for the application and the available supply pressure – 100 PSI nominal or 125 PSI maximum at 5-micron filtration is recommended. Also note the requirements for length of the stroke, accuracy and stroking speed. Finally, determine if a meter drive or 0-20 mA output is desired.

Without the positioner, additional I/O slots from the PLC would have been required to open and close solenoids on the cylinder to maintain the set point. The size of the cylinders is big enough to stop the flow of material when closing, move the gates and control the flow of material. Typically, the actuators default to the retracted position upon signal failure, or stay where last commanded upon total loss of power.

The E/P positioner was selected because it could provide feedback on its actual position after it had been told to go to a particular position. In this way, a signal can be sent to the positioner giving it the position it should go to, and then look at the return data to verify that the gate in fact moved and was in the correct position.

The unit’s feedback device is an internally mounted linear potentiometer for applications in which infinite positioning requirements allow electrical analog control signals. At the CEMEX plants, the positioner interfaces with a PLC, and comprises a cylinder with an integral sensor, optimized valving and an electronic controller. The basic concept involves a cylinder with integral feedback potentiometer in conjunction with a controller and matched solenoid valves.

The Pleasanton plant recently introduced the positioners in an aggregate blending application where units are used to divert the effluent from a rock crusher onto two conveyors.

Since then, the pneumatic system has been bombarded with gravel, sand, dirt and water, yet the unit has functioned daily with only minimal maintenance required.

“Ultimately, the positioners have helped improve the quality and reduce the cost of material being shipped to our customer, which allows CEMEX to remain very competitive in the marketplace.” Fuentes said.

This article was compiled from materials contributed by Bosch Rexroth.