VEGA at HOPE Cement works
HOPE Cement works
Lafarge Cement UK’s Hope Works is one of the largest cement works in the UK. It has capacity to make up to 1.3 million tonnes of the famous Blue Circle brand cement each year.
A factory has operated on the site since 1929; the first construction was by G&T Earle. The site is ideal, sitting between reserves of limestone and shale, both essential ingredients in the cement making process.
Over the years, the plant has developed from a wet cement making process to a more fuel-efficient dry process. A further £35 million upgrade was recently completed. This was done to improve production efficiency, environmental performance, and safety and broaden the mix of products made
Lafarge focused £11 million of the investment on equipment that has significantly improved environmental performance. In one key area, the installation of bag filters has given the Works the lowest dust emissions from kiln exhausts in the country.
Maintenance & Control
At Hope, £3.5 million is spent on pro-active maintenance each year; they employ world-class methods to avoid unscheduled maintenance. Part of this is making sure that all processes on the plant are monitored and controlled using the most advanced and efficient equipment available. These are monitored from Hope’s computerised plant control room into which information from over 750 sensors around the works feed information. As is to be expected in this computerised era, the process monitoring used around the plant, is the latest technology and fully automated.
Cement
The Limestone and Shale mixture (used in a ratio of 82% Limestone and 18% Shale, in order to achieve the optimum chemical balance) are ground into a fine powder in the raw mill, known as raw meal. These mills revolve at high speeds in which steel balls pulverise the stone. Once the raw meal is milled, it is fed from storage silos to a number of FK feeders via two 40" rotary screw conveyors. For the smooth and optimum operation of the process, and therefore the works, it is vital that the screw conveyors run efficiently and are neither under or over filled; both can have a knock-on effect on the previous and subsequent process.
Before the raw meal enters the 70m long kilns, the crucial point of the cement making process, it passes down the pre-heater tower, heated by the warm kiln exhaust gases rising up the tower through a series of cyclones.
Once in the rotating kiln, which is heated by a flame fuelled with pulverised coal, the raw meal reaches the "burning zone" and is heated up to 1450° C. At this temperature it converts into "nodules" of clinker, containing hydraulic calcium silicates.
After being cooled, the clinker is transported to the cement mill, and ground up with a small amount of gypsum creating the final product; cement.
The Problem
Over or under feeding of the screw conveyors containing the ‘raw meal’ has been a problem due to the unreliable measurement equipment, with the consequences of starving material feed to the Preheater tower or wasting energy in returning unused material back to the storage silos.
In extreme circumstances this has resulted in material spillage, transport equipment failure and loss of production.
Ideally, in order to correct/adjust the amount of raw meal being fed into the screw conveyor (the feed rate of which is approximately 300 tph) from the control room, a reliable continuous level measurement solution was required.
Air pressure switches connected to a positional cylinder to pneumatically determine the product level and more recently capacitance probe switches have been tried in the conveyors. These methods were intrusive into the screw conveyor and required a section of screw flight to be removed.
The air purge pressure switch arrangement required regular maintenance and cleaning of the purge system to maintain the equipments availability to an acceptable level.
Similarly, capacitance probes proved unreliable with high maintenance requirements. Changes in the dielectric value of the product resulted in drifting switch point values. Calibration was problematic and the system was still intrusive.
Unhappy with the existing set-up, VEGA Controls Ltd were asked to propose a solution.
VEGA Controls Ltd are manufacturers and suppliers of level, pressure and density instrumentation, priding themselves in reducing error probability and increasing operational safety.
Having supplied Lafarge with reliable level instrumentation solutions on bunkers, crushers and silos for the past 4 years, they were confidant in VEGA finding an optimum solution even for an application as challenging as this one.
Installing an Ultrasonic device was considered first, but after further investigation it transpired that this would not offer Lafarge the "Fit and Forget" solution they were looking for. A substantial amount of mechanical installation as well as doubts regarding the accuracy of the measurement due to space availability, blocking distance requirements, dust and uneven material surface, VEGA considered their VEGA Ohmart Nucleonic range. Ohmart VEGA corp., a sister company in the USA - part of the VEGA group, is the worlds largest and one of the longest established nucleonic gauging companies.
The Fit and Forget Solution
A Nucleonic Continuous Level Measurement gauge from VEGA’s Ohmart range was installed on one of the conveyors along with full installation drawings, and recommendations. The device is a non-contact solution, which means that the conveyor did not have to be modified in any way and calibration was simple. The way in which it measures the level is as follows:
Lafarge chose the VEGA Ohmart nucleonic continuous conveyor-weighing gauge; which complements their philosophy of low maintenance through cutting edge technology and innovation. VEGA carried out the commissioning and critical examinations for the system installation. The accuracy of the gauge is better than +/- 1% of the span. They can also offer complete after sales support for wipe testing, storage and disposal of sources.
James Boot, Assistant Process Control Engineer at Lafarge commented, "We were particularly pleased with the ease of calibration and the non intrusive nature of the gauge, enabling us to mount the gauge without halting the process".
The success and straightforward installation of this initial application has prompted Lafarge to order 3 further nucleonic gauging systems to replace other systems on the site.