A better way to measure bitumen
14 February 2017
Bitumen is the essential glue that transforms aggregates into asphalt. But it must be delivered in accurate doses, which may not be possible if you are using traditional mechanical flowmeters. Principally, this is because they can only measure volumetric flow and they can also be prone to inaccuracies and drift over time.
The real alternative is a SITRANS FC430 Coriolis flowmeter – a superior solution involving a compact, reliable, accurate and safe mass flowmeter that’s easy to install and simple to operate.
This isn’t your grandparents’ asphalt
Covering 95 per cent of our roads, not to mention ubiquitously lining the surfaces of playgrounds, pedestrian walkways, parking lots, airport runways and much, much more – it truly is indispensable. Globally, about 120 million tons are produced every year.
Asphalt plants are generally found inside quarries, where the bulk of the needed aggregate content is obtained. But there can also be “satellite” plants in urban areas, or where the demand is great, such as on site at major road or airport projects.
Asphalt today is dramatically superior to previous generations of the substance. It is safer, requires less maintenance and is more energy-efficient and environmentally friendly as more recycled
asphalt and other materials are increasingly being used.
By weight, 95 per cent of asphalt consists of stone, sand and filler – aggregates that give asphalt its strength. The precise composition of the individual elements depends on the end use. The remaining five per cent comprises an agent that binds all of these materials together. That agent is usually bitumen derived from crude oil.
Asphalt is produced by blending the aggregates and then heating them to a temperature suitable for coating with bitumen as binding agent. With the drive toward reducing the amount of energy used in the process, there is a major shift toward more widespread adoption of process control and instrumentation. In particular, highly-auto-mated batch and drum-drier plants are becoming more common because they can deliver the consistent quality mixes needed to meet even the most technically complex end user requirements.
Bitumen must address various factors
A key challenge in this process is ensuring proper preparation of the bitumen – especially measuring an accurate dose of it – so everything is just right in binding the mixture of aggregate and recycled asphalt (RAP).
Bitumen is transported from a supplying refinery to the asphalt plant, where the substance is moved into tanks that are continuously kept at 150 to 180°C. Bitumen must be stored at a minimum of 150°C so that it remains in a liquid state – below this temperature it starts to solidify.
Bitumen also has different penetration grades, which need to be stored separately so they can be used individually or blended accurately when added to the asphalt mix.
The required asphalt ratio mix is defined by weight, so that the specific gravity (SG) of the bitumen must be a compensatory factor. However, the specific gravity of bitumen varies according to both penetration grade and temperature. As a result, to correctly add the bitumen to a mix, operators must know both the grade of the bitumen as well as its temperature at the moment of mixing.
The grade can be easily established, unless a blend is being used. However, temperature compensation is extremely difficult to address. Currently, manual look-up tables are most commonly used to determine temperature compensation. Typically, this is carried out at the start of a batch or even the start of a shift. But in reality, the temperature is constantly changing. And that is just one of a number of critical factors that can have a dramatic impact on the specific gravity and adversely affect the accuracy of flow readings.