Control monitoring system automates quarrying

Hanson Aggregates has significantly increased production at its Wykeham UK quarry, without additional staff, through the installation of a CitectScada control monitoring system.

Hanson Aggregates, the largest producer of aggregates in the world, significantly increased production at its Wykeham UK quarry, without additional staffing, through the installation of a CitectScada control monitoring system.

On an annual basis, production has been increased with little or no downtime during production shifts.

The Wykeham Quarry produces approximately 250,000 tonnes of sand and gravel annually, and has been in operation for approximately 18 years.

Its existing control systems were inadequate to meet market demands for increased productivity.

Since its inception, the quarry had also controlled various plant operations with a hard-wired relay logic system, which had been extensively modified over the years.

There was no automation and all startup and shutdown procedures were manual.

To increase production required substantially increasing staffing levels or a further investment in automation.

The plant operates by recovering the sand/ gravel/stone mixture from the quarry diggings using a mechanical dragline.

The actual workings are under water at all times and the gravel bed itself is under approximately 5m of overburden and at a depth of about 15m.

The recovered material is carried by conveyor a distance of approximately 800m to a "surge heap", a local stockpile of several thousands of tonnes of material.

This is then fed onto another conveyor that delivers the material to the processing plant where the mixture is separated into the individual saleable products.

Sand is screened off early in the process, then the rock material is separated into two sizes - smaller or greater than 25mm.

The smaller size, which is effectively gravel, is then further sorted into the grades and sizes used for premix concrete and other products.

The larger stones are passed through crushers to reduce to a size used for sub-base.

There are a number of control loops employed to ensure a constant flow of material at an ideal production rate of 170 tonnes per hour.

This includes a feedback loop to control the reintroduction of crushed stone into the gravel separation procedure and also a second stage wash to recover further sand.

All the output weights are fed back to the primary feed conveyor to maintain the input feed rate at nominal.

The need to maintain the feed rate is at the heart of the original decision to upgrade the existing manual control systems to a PLC based Scada system.

Under the manual system, there was the need for staff to be constantly on hand to tune and trim the various conveyor systems to deal with constantly changing conditions.

In practice, this was difficult to maintain and as a result, the feed rate was rarely optimised and the net effect was reduced output.

Since the plant operates 12 hours production shifts three days a week, with two days for maintenance, the process is also arduous and particularly tough on equipment.

A functional specification for automating the plant was developed.

Following a competitive tender from four systems integrators, the contract was awarded to Merlin Systems, a company that had worked with Hanson on a number of other projects.

Hanson Aggregates had not specified a particular method of automation, but Merlin recommended using a CitectScada system.

The integrator had good experience using the software and knew that Hanson's team would be comfortable with it.

CitectScada has also been used successfully in many previous quarry projects.

The system was installed starting with weighing controls being added to the conveyors, with Allen-Bradley PLC and CitectScada software being implemented for control and visualisation.

The Scada software is installed on a single node server/client PC located in the weighbridge office.

This forms the control nerve centre of the plant.

From here, staff control and monitor the plant operations and all control commands are routed through the Scada system.

Apart from a mix of graphical and text based visualisations, CitectScada software is also used to generate daily systems and production reports which are logged for historical and trend use as required.

Maintenance support facilities are also provided in the form of equipment running times and numbers of operations.

This aids in scheduling the planned maintenance routines.

Plant alarms are also generated and displayed by the CitectScada system so that plant staff can take appropriate actions.

Merlin provides technical support for the CitectScada software and has dial up access to the system via a modem link.

This facility also enables software upgrades and system modifications to be made via modem rather than a site visit.

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