Radar level measurement in high temperature and pressure work
Radar technology in general has been introduced to the process industry as being a measurement technology using high frequency electromagnetic waves that are not influenced by the gas phase they travel through.
Furthermore, the high frequency electromagnetic wavers are not influenced by temperature and pressure conditions in process vessels.
Radar signals
Most new radar technologies available on the market used to measure level are based on the ‘time-of-flight’ principle. This means that the radar measurement device measures the elapsed time between emitting and receiving a pulse, consisting of a bundle of high frequency electromagnetic waves. The frequency of the waves varies from 1GHz for guided wave devices to 6 and 26GHz for free space radars.
Radar signals travel at the speed of light when travelling through a vacuum. This speed however can be influenced when radar signals are not travelling through a vacuum. Pressure and temperature of a specific gas phase or liquid have some influence on the speed of radar signals. The amount of influence depends on how polarised those gasses are – in other words, how much the dielectric constant changes. Hydrocarbon vapours show little effect even under high temperature and/or high pressure process conditions. However, a high polar steam is effected. The dielectric constant of steam at 100°C (212°F) is 1.005806. But at 366°C (691°F) it is already 3.086.
In a typical steam application, the level of water in a condenser or boiler is of utmost importance. Radar measurement devices are used more and more in these critical applications. They offer a great alternative with advanced diagnostics and insensitivity to build up and temperature fluctuations. Radar devices also help overcome significant errors that can occur due to a change in the density of water.
Steam is a highly polar gas, which means that radar signals in high pressure and high temperature steam applications are subject to a reduction in speed. In a boiler for instance, this leads to a lower water level reading then there actually is. This can be dangerous, influencing the performance of boilers and causing a reduction in the quality of steam. The error can easily be as large as 30–40percent depending on the pressure and temperature of the steam and distance from the launch of the signal to the actual water level.
The simplest (but not the best) way to overcome this problem is to put a fixed offset in the measurement device by simply inputting the temperature or pressure and having the radar unit calculate the ‘offset’. The problem with doing that is there will be rather big ‘errors’ during the start-up of an installation. The normal operating conditions have not yet been met and thus the unit will be overcompensating. One could also program a compensation table in a DCS or PLC and connect this to a pressure or temperature transmitter.
The most accurate method is through the dynamic compensation circuit on a Levelflex guided wave radar. A reference signal at a known distance is used to compensate for the delay in speed of the radar signal measuring the water level. This is done dynamically – for example, when the reference pulse signal shows a small shift in time, the level signal will be compensated for this small shift. In converse, if the reference signal shows a large shift, then the level signal will be compensated for this large shift. Thus, Levelflex sees the error in the application and updates the output reading to the highest accuracy every second.
Conclusion
The use of radar signals in high temperature and high pressure applications, especially in polar gasses, is not as simple as it sounds. Under these conditions, the speed of radar signals can change causing large measuring errors. An Endress+Hauser Levelflex guided wave radar offers a solution to compensate for changing radar signal speeds, offering peace of mind and confidence in the accuracy of your level process measurement.
Uwe Wagner is Industry Manager Power and Energy, and Rob Vermeulen is Product Manager Level, Endress+Hauser Instruments International AG, Reinach, Switzerland. www.power.endress.com
