A better way to measure bitumen?

19 December 2016

Bitumen is the glue that transforms aggregates into asphalt. It needs to be delivered in accurate doses, which is not possible with traditional mechanical flowmeters. Carl Pinches suggests a solution.

Globally, around 120 million tons of asphalt is produced every year. Today it is a dramatically superior product to previous generations of the substance. It is safer, requires less maintenance and is more energy-efficient and environmentally friendly as increasingly, more recycled asphalt and other materials are being used in its production.  

By weight, 95% of asphalt consists of stone, sand and other fillers. The remaining 5% comprises an agent that binds all of these materials together. This is usually bitumen which is 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 increasing adoption of process control and instrumentation. In particular, highly-automated 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.    

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 ensure the correct amount of bitumen is added to the aggregate mix to create asphalt 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. 

Bitumen needs to be stored at a minimum of 150°C so that it remains in a liquid state – below this temperature it will start to solidify.

Temperature compensation is difficult to address. Currently, the most commonly used solution is to use manual look-up tables to determine temperature compensation. Typically, this is carried out at the start of a batch or even the start of a shift. However, in reality, the temperature is constantly changing. And this is just one of a number of critical factors that can have a dramatic impact on the specific gravity and so can adversely affect the accuracy of flow readings. 
Traditional solution
Traditional flow measurement of bitumen is based on a volumetric principle. Mechanical flowmeters are typically used, however these have several disadvantages when deployed in a state-of-the-art asphalt plant. Specifically, these meters are prone to inaccuracy; tend to drift over time; are difficult to integrate into a modern control system; and only measure volumetric flow. 

Siemens suggests that mechanical volumetric meters be replaced with with modern mass flowmeters. The Siemens SITRANS FC430 Coriolis digital mass flowmeter, for example, can provide high accuracies (0.1%) and, due to its compact size, can be can retrofitted into existing applications. Its sensor construction filters out plant vibration, which can cause measurement inaccuracy.

In addition to improved accuracy and long-term repeatability, the SITRANS FC430 also eliminates the need to compensate for other process variables. No other flowmeter approach can measure mass flow, volumetric flow, density, pressure and temperature – in a single, self-contained, device.

With the Coriolis digital mass flowmeter, operators are able to accurately prepare the bitumen dose based on an actual measured weight rather than an indirectly compensated volume. For example, for 1000 kilograms of aggregate, the operator can simply call for 50 kilograms of bitumen to make a typical mix. All other bitumen variables then become irrelevant. The resulting specific gravity is automatically taken into account even when blending different bitumen grades.

Frequently, a SITRANS FC430 is able to replace an existing flowmeter with minimal pipeline modifications. While there are standard criteria related to pump and bend proximity that must be taken into account, owing to the compact size of the flowmeter these are not a significant issue. 

The natural properties of bitumen mean that pipe trace heating and insulation need to be reinstated over the flowmeter so that the bitumen can still freely flow. Remote electronics, mounted away from the primary sensor location, means that system commissioning and maintenance can be handled from a safe area, such as a control room or electrical cabin.

Unique support tools provide direct access to all operational and functional data, certificates and audit trails. The SITRANS FC430 is believed to be the first Coriolis flowmeters to offer hazardous zone and SIL certification options, ensuring high standards of safety and reliability. 

Carl Pinches is UK industry manager – minerals and Energy – at Siemens Industry Automation.

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