Accurate measurement in high temp and pressure applications
March 5, 2018
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By Rob Vermeulen, Level Product Manager – Endress+Hauser
Radar technology in general has been introduced to the process industry as being a measurement technology using high frequency electromagnetic waves that are not being influenced by the gas phase it travels through and the temperature and pressure conditions in process vessels. As processes get more extreme in temperature and pressure it is time to have a closer look at radar behaviour in those critical applications and the solutions available on the market that overcome the obstacles.
All radar technologies available on the market that are used to measure level use the “Time-of -Flight” principle. This means that the radar measurement device measures the elapsed time between emitting and receiving of a pulse consisting of a bundle of high frequency electromagnetic waves. The frequency of the waves varies between 1 GHz for guided wave devices and 6 and 26 GHz for free space radars.
Speed of radar signals
Radar signals travel at the speed of light when traveling through a vacuum. This speed however can be influenced when not traveling through a vacuum. Pressure and temperature of a specific gas phase or liquid has some influence on the speed of radar signals. The amount of influence depends on how polarized those gasses are – in other words, how much the dielectric constant changes. Hydrocarbon vapors show little effect even under high temperature and or high-pressure process conditions.
But a high polar steam does. The Dc (Dielectric constant) of steam at 212°F is 1.005806. But at 691°F it is already 3.086.
The change in speed of radar signals traveling through steam
in a typical steam application, the level of the 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 that produce significant errors due to a change in the density of water and evaporation in the compensating leg.
Steam is a highly polar gas, which means that the speed of radar signals in high pressure and temperature steam applications are subject to a reduction in speed. In a boiler for instance, this leads to a lower water level reading than there actually is. This can be dangerous and influences the performance of boilers and causes a reduction in the quality of steam. The error can easily be as large as 30-40 per cent depending on the pressure and temperature of the steam and distance from the launch of the signal to the actual water level.
Overcoming the effects of process changes on radar speed
The simplest (but not the best) ways 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 over compensating. One could also program a compensation table in a DCS or PLC and connect this to a pressure or temperature transmitter.
The right way – “built-in” dynamic compensation
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.
The use of radar signals in high temperature and high-pressure applications in especially 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 unique solution to compensate for changing radar signal speeds, offering peace of mind and confidence in the accuracy of your level process measurement.
See the article in our online magazine by clicking here.