Conditions inside steam drums and other high-temperature and pressure applications can raise some real obstacles to true level measurement. Here, the author starts by looking at these impediments and some traditional strategies to compensate for these conditions, and then he suggests some ways guided wave radar and new compensation tactics can combine for consistently accurate readings.
Radar technology in general has been introduced to the process industry as a measurement technology that uses high-frequency electromagnetic waves that are not influenced by the gas phase they travel through or by the temperature and pressure conditions in process vessels. As processes involve more extreme temperatures and pressures, it is time to have a closer look at radar behavior in those critical applications and their solutions and also at the mechanical designs of such measurement devices.
Mechanical DesignDesigning a guided-wave radar device or free-space radar device that can withstand extreme conditions is very difficult. It requires know-how of materials and understanding radar behavior in extreme conditions. It also requires know-how to construct a safe and reliable radar that meets the highest industry standards like SIL 2 for both software/algorithm and hardware development (IEC61508) and which is actually proven in use for these extreme application (IEC 61511).
Meeting these requirements should not be taken lightly; however, having a safe mechanical design does not necessarily make a radar device safe to use. Electronics should be able to differentiate between level signal and false readings and react in a safe, predictable way. Having a redundant measurement signal evaluation in the same device (EOP evaluation) makes it even safer.
MaterialsThe speed of radar is highly influenced by the impedance of the system it has to travel through or along, so the distance of an antenna to a wall of a bypass or chamber influences the speed of a radar signal. This is true for the mechanical stability and stress of the parts that transport the radar signal from the HF module to the rod/cable or antenna coupling inside a tank or bypass.
A simple example is the ceramic used for mechanical stability and isolation material in FMP45, Levelflex-guided wave radar. When one applies 5,800 psi, at 540°F, to that piece of ceramic, the dielectric constant of that ceramic part changes. This change influences the impedance of the system and thus the noise and propagation speed of a radar signal emitted though it.
Radar SignalsAll radar technologies on the market that are used to measure level use the “time of flight” principle. This means the device measures the elapsed time between emitting and receiving a pulse consisting of a bundle of high-frequency electromagnetic waves. The frequencies of the waves vary between 1 GHz for guided wave devices and 6 to 26 GHz for free space radars.
Speed of Radar SignalsRadar signals travel at the speed of light in a vacuum. This speed varies outside a vacuum. The pressure and temperature of a specific gas phase or liquid also influences the speed of radar signals. The extent of this influence depends on how polarized the gas is - in other words, how much the dielectric constant of the gas phase varies due to temperature or pressure changes in the application. Hydrocarbon vapors show little change even under high-temperature or high-pressure process conditions, but high polar steam does. The dielectric constant of steam at 212°F is 1.005806. But at 691°F, it is already 3.086.
Steam System ApplicationsThere are several critical boiler system control points, with the primary one being the boiler level control. The second is the level control of the feed water to the boiler and the condensate return level.
Boiler level control. In a typical steam application, the level of the water in a boiler is of utmost importance. Radar measurement devices are used more and more in these critical applications. They offer advanced diagnostics and insensitivity to build-up and temperature fluctuations that bother other measurement systems, such as those that use differential pressure devices and displacers. Both use the “density” of the product to determine the level, but the density of water changes significantly enough in a boiler system to lead potentially to large measurement errors between the “real” level in the boiler and the measured value.
These errors affect the amount and quality of steam a boiler produces. Not operating a boiler at its critical point leads to higher energy costs per ton of steam and reduces the life of the boiler itself. Too high a water level produces bad quality steam that can cause droplets of water in a steam system. Droplets of water traveling at high speeds are devastating for control elements, piping, and turbines. Too low a water level can cause damage to the heating tubes in a boiler and eventually lead to overheating. This again can lead to the explosion of a boiler.
Boiler feed water control and condensate return. The surface of a receiving tank for condensate return can be turbulent. This not only makes it difficult to measure with free space radar, but it also leads to issues regarding changing radar speeds and also condensation on the free-space radar antenna. It is not simple to use differential pressure in these applications. There can be large temperature shocks, especially during the start-up phase of a boiler system. Furthermore, the density of the liquid can change over temperature, leading to large measurement errors. A much more reliable system is a guided-wave radar device with gas-phase compensation in a co-ax pipe.