On the Level

Article by Patricia Mattsson

State of the art: The latest level measurement devices, such as vibrating forks, incorporate built-in diagnostics that help to ensure they perform correctly within an overfill prevention system

How advanced level measurement technology increases plant efficiency and safety

KNOWING, with great accuracy and reliability, the level and volume of liquids and solids in tanks, vessels and silos is critical to the efficient and safe operation of chemical processing plants. The widespread departure from manual measurements to automated solutions has helped to provide measurement assurance as well as significant improvements in terms of worker efficiency.

However, the latest level measurement technologies are now also contributing towards reduced process equipment maintenance, increased visibility into the process, optimised process control and significant plant and worker safety enhancements.

Challenging measurement applications

Level measurement applications can be particularly challenging within the chemical industry, due to the huge variety of liquid media measured, varying processes, vessel type and size. Blending tank and reactor vessels may present difficult environments for level technology incorporating agitation, foam and condensation, all of which affect measurement performance significantly. By applying the correct measurement technology, be that ultrasonic, differential pressure, point level switches, guided wave radar (GWR) or non-contacting radar, for the specific application, measurement performance is enhanced. For reactor, mixer and blending tank applications it is important to know exactly how much media is in the vessel to achieve the required final product quality. There are a number of measurement technologies to support this, but by using a continuous measurement provided by radars, the operator will not only know the exact level, they will also have complete control after the reaction. This is important because media can expand, and foam can be created. Non-contacting radar is the most suitable solution because it can be applied to vessels that have agitators. It must be noted that the noise created by these devices can have a significant effect on the quality and reliability of measurements, but there have been recent developments, in terms of the availability of non-contacting radars based on frequency modulated continuous wave (FMCW) technology, that help to improve the reliability and accuracy of measurements, especially when applied to demanding applications.

The latest level measurement technologies are now also contributing towards reduced process equipment maintenance, increased visibility into the process, optimised process control and significant plant and worker safety enhancements

Traditionally, non-contacting radars have been based on a pulse modulation method. Microwaves are emitted towards the surface and reflected to the sensor, with the level being directly proportional to the time from signal transmission to reception. With FMCW, the radar transmits a continuous signal sweep with a constantly-changing frequency. The difference between the frequency of the reflected signal and the frequency of the signal transmitted at that moment is proportional to the distance from the radar to the surface, which enables the level to be measured. An advantage of the FMCW technique is that the process variable information is in the frequency domain instead of the amplitude modulated or time difference domain, which allows more accurate signal conversion. Most tank noise sources, such as agitators, are in the amplitude domain, so frequency modulated signal processing can ignore them, and accuracy is not affected.

The sensitivity of transmitters based on FMCW is more than 30 times higher than the more traditional pulse technology-based non-contacting radars. This maximises signal strength, which produces greater measurement accuracy and reliability and crucially provides a much greater safety margin within demanding applications.

Although FMCW technology is not new, it has always been considered power hungry, and has typically been deployed only within four-wire devices. Installing such devices often necessitates putting more cable infrastructure in place, which is costly and time-consuming. This has led to users sacrificing the additional accuracy and reliability of FMCW devices and installing two-wire transmitters based on pulse technology instead. The latest FMCW devices have overcome the power issue and are available as two-wire devices, enabling widespread deployment without costly infrastructure requirements.

Article by Patricia Mattsson

Rosemount Level Marketing Engineer, Emerson Automation Solutions

Recent Editions

Catch up on the latest news, views and jobs from The Chemical Engineer. Below are the four latest issues. View a wider selection of the archive from within the Magazine section of this site.