Radar is the fastest growing level measurement technology across an expanding application base. Olle Edvardsson highlights how health and safety regulations, and the need to reduce maintenance costs and improve reliability, have resulted in the development of the first radar level instruments.
The emergence of radar has been an important advance in the level measurement field. Radar represents a cost effective, accurate solution that is immune to density and other process fluid changes as well as most vapour space conditions. Radar level measurement systems are available in contacting and non-contacting versions. Contacting is generally a good fit for small spaces, and is an easy replacement of older technology such as displacers and capacitance probes. Non-contacting is generally a better fit for dirty, viscous and corrosive applications and when agitators are present. Currently, contacting devices, called guided-wave radar (GWR), are slightly more prevalent primarily because they are capable of providing interface level measurement (for example oil and water), as well as standard direct level measurements. But both formats are now widely accepted by the process industry.
There are some important considerations when applying both types of radar technology. For example, end users working with steam applications of more than 400-500 PSI (30-35 bar) should look for GWR systems that have a dynamic vapour compensation method to ensure the accuracy of the device in such an environment. Similarly, in applications where the signal reflection is weak, it is important to select a high performance radar with innovations such as dual port or direct switch technology, which minimises the losses in the returned signal. In cases where the return signal is so weak that it occasionally disappears, the device should be able to provide an alternative measurement, such as probe end projection, where the unit uses a combination of the known length of the probe and an online measurement of dielectric of the material to determine level. The applications may include those with low dielectric fluids and turbulence due to boiling or entrained air as well as some solids applications.
While GWR works in many conditions and is not dependent on reflecting a signal off a flat surface, some precautions need to be taken with respect to probe choice. Several probe styles are available and the application, length, and mounting restrictions influence their choice. Unless a coax-style probe is used, probes should not be in direct contact with a metallic object, as that will impact the signal. Twin and coaxial probes are susceptible to clogging and build up. If the application involves liquids that tend to be dirty, sticky or can coat, then only single lead probes should be used. For such applications, devices offering signal quality diagnostics can help the user determine if the probe needs to be cleaned and allows maintenance to be scheduled only when needed. In general, GWR is not suitable for extremely viscous products where fluid flow is minimal. If GWR is used with very viscous fluids and is installed in a bypass chamber, then the chamber should be heat traced and insulated to ensure fluidity. Furthermore, the connections from the tank to the chamber and the chamber's diameter should be sufficiently large enough to allow good fluid flow. Applications such as asphalt, where heavy coating is likely, are more suitable for non-contacting radar.
Stilling well and bypass chambers
Chambers, also known as bypass pipes, provide a calmer surface in case of turbulence. They offer external mounting with valves, allow for easier servicing of level devices, and enable radar measurement in tanks that only offer side-connections 'such as towers.
For pipe or stilling well applications, both non-contacting and GWR work well, although GWR is far simpler to install and maintain accuracy and sensitivity independently of the pipe. For these type of applications, dimensioning the chamber correctly and selecting the appropriate probe is essential. 75mm or 100mm chambers are recommended. Any less and build up and flow-through may be a problem, and the chances of the probe contacting the chamber wall are increased.
When implementing radar within a chamber installation, a single probe is recommended as they are less susceptible to build up. In most cases rigid probes are applied, but flexible probes may also be used. Care must be taken to ensure that the probe is suspended vertically and does not touch the pipe wall. A centering disk is recommended as this keeps the probe centred in the chamber and away from the walls. For shorter pipes, GWR with rigid probes are recommended.
High temperature and pressure applications
In applications with extreme temperature and pressure conditions it is important to select a heavy-duty process seal with multiple layers of protection and a flexible assembly to handle the stresses and the forces induced. This is to prevent leakages and ensure the safety and efficiency of your plant.
When measuring liquids at very high temperatures in a chamber it is important to insulate and heat trace the chamber. Fluctuations in temperatures alter the density and volume of the product which then affects the level in the chamber. Maintaining the temperature of dirty liquids such as heavy oil also helps to avoid clogging and sticking within the chamber and enables adequate flow-through.
Although radar technology is not affected by density changes, dielectric changes can have an impact. For boiler and feed water systems, where boiling water and high pressure saturated steam vapours are present, the returned signal from the surface becomes weaker as water temperature increases.
Level and interface measurement has been successful with GWR but presents a separate set of challenges. The fluid with the lower dielectric must always be on top. The two liquids must have a dielectric difference of around 10, and the upper layer dielectric value must be known. Certain thicknesses of layers are also required for effective measurement to take place. Typical successful applications have a hydrocarbon-based fluid with a dielectric around 2 on the top layer and water based fluid with a dielectric over 40 at the bottom. Interface measurement applications where the density of the two fluids is very close, or where emulsifier chemicals are used, can produce fairly large emulsion between the products. This may make the interface indistinct. Heavy and thick emulsion layers or liquid layers with similar dielectrics can pose a problem for GWR as the technology requires a distinct dielectric difference to detect the interface.
In applications with large emulsions, a displacer device, which relies on a buoyancy effect rather than any dielectric value, tracks the midpoint of an emulsion layer and may provide a better solution. However, the great disadvantage of this technology is that it has moving parts that require frequent cleaning and replacement, thereby reducing the reliability of the measurement and incurring greater maintenance costs.
Open air and non-metal tank applications
Radar often works well in open air or non metallic tank installations. However, in some cases outside disturbances may interfere with the radar signal. Here it is important to select a radar device with high resistance to EMI such as a GWR with a smart galvanic interface. For most open sump and well installations, an ultrasonic meter is a more cost effective solution. However, should vapours be present, then a low frequency radar device is the preferred solution.
In critical level applications it is necessary to use a minimum of two level technologies or devices and if the same technology is used, employ a voting scheme. Using technologies less influenced by process conditions, such as radar in combination with vibrating fork switches, is a good step to more accurate and reliable measurements. For radar, most failure modes relate to a loss of signal. High sensitivity normally results in high availabilities. High sensitivity is achieved by increasing the signal to noise ratio using technologies such as dual port and direct switch technology. Enhanced echologics - the ability to ignore false echoes - and smart software functions also improve the performance of the radar.
Advanced diagnostics is another step in the right direction for safe measurement. For example, in some GWR devices signal quality metrics informs the user in real-time if the probe gets coated. This provides the opportunity to schedule proactive maintenance.
Today there are vibrating fork switches that continuously monitor corrosion of the forks, external damage to the sensor, internal wire disconnect or breakage, and over-temperature. This results in a fault indication and gives a fail safe operation of the switch output.
A good installation is key to success with radar. When installing a new radar device it is usually on an existing nozzle. This nozzle can sometimes be too tall or narrow for the instrument. It is recommended that users try to minimise the height of the nozzle used. Ideally nozzles should be at least two inches, but no more than six inches high for GWR. For non-contacting radar, it is preferable that the end of the antenna extends slightly beyond the nozzle. Longer nozzles can be used with high frequency non-contact radar, but they need to be clear of obstructions and smooth.
Another installation problem is when the nozzle is positioned directly over a pipe, baffle or some other obstruction. The obstruction interferes with the radar beam and becomes the level measurement rather than the process medium in the vessel. Similarly if the fluid stream coming into the tank falls into the path of the radar beam or on the probe, then this will impact the reliability of the measurement.
As with all instrumentation, radar devices must be configured correctly in accordance with application needs. Special care must be taken when inputting thresholds for the radar signal. These will change depending on the medium being measured. For example, oil appears very different to a radar device than water and therefore requires very different threshold settings. However, today there are good set-up guides and functions such as Measure and Learn, so in most cases configuration is easily achieved in just a few steps.
The correct selection and implementation of process level solution is crucial to obtain accurate and reliable measurements.
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