UWT application of the month
Single-rod probes using the capacitive measuring method are in many cases a clever solution when it comes to level detection. They can be found in overfill protection and as demand or empty detectors in process or storage containers. These limit switches are suitable for almost all bulk materials from different industries, due to high sensitivity, high mechanical load and the variety of probe extensions. But these qualities also make them a suitable solution in the fascinating world of varied liquid applications, be they in food, wood, rubber, plastics, environmental and power plants, building materials and chemical industries. The use of the RFnivo® capactive rod is fully explained here within the textile production process featuring the aggressive substance of acetone. The Rotonivo® rotary paddle switch having provided the solution for many years for the plant operator, this time the application required an alternative answer from the UWT team.
Satisfied by a classic solution – The Rotary Paddle Level Switch
A synthetic fibre manufacturer in Belarus has been working for many years with the UWT rotary paddle detector, a veteran of the level measurement systems. Since 1964 the company has been producing a wide range of polyester textile yarns, geotextiles and carbon and heat resistant polypropylene materials, for products such as special protective clothing or packaging solutions in agriculture and construction.
The Rotonivo® devices for different powders and granules were installed in several process vessels. Depending on the requirements, they function as full, demand or empty detectors and, thanks to their simple electromechanical measuring principle, can also be adapted to extreme process conditions, such as high temperatures up to +1,100°C. The RN rotary paddle is mainly used for storage and drying of plastic particles as a full detector as well as in dosing tanks and extruders for demand reporting. During conveying processes, it works as an overfill detector. For protection against temperatures of below -20°C, a heating function was integrated in the electronics, which has proved quite useful in the winter months of Belarus.
Capacitance solution in tanks with clean and contaminated acetone
for overfill protection and empty detection
As the synthetic fibre manufacturer was very satisfied with their chosen rotary paddle detectors for many years, they also required a measuring solution from UWT for their acetone containers. So the company conducted preliminary tests in the solvent tanks. During synthetic fibre manufacturing, acetone is used to impregnate the fabric. Each fabric to be produced is passed through a bath of clean, pure acetone, where amongst others excess epoxy resin, which is used for bonding the fibres, is washed off. The majority of the resin is usually cured after the heat treatment, non-hardened epoxy resin can be removed during this process step. Thereafter, the acetone is considered polluted and enters a separate tank, where it is cleaned in a special mechanical process. Subsequently, the purified solvent is filled in the tank container with so-called regenerated acetone.
Acetone is an extremely aggressive substance that secretes corrosive vapours and quickly corrodes ordinary materials. For the individual tanks with clean, contaminated as well as regenerated acetone the synthetic fibre manufacturer needed a reliable, and at the same time robust, level monitoring system. Tests carried out with a variety of sensors made it clear that the robust RFnivo® probe was the most favourable solution. The RF series consists of single-rod probes that require, due to their design, no temperature-limiting components for media detection and can therefore be used in processes of up to 500°C. They work with the capacitive measuring principle, whereby the capacitance is detected independently of the length of the sensor of the medium between the probe and the container wall.
The customer has different tanks measuring from 1.2 to 1.6 meters in diameter. Apart from a reliable measurement, even with frequently occurring pressure fluctuations, it was also essential that the sensor carried a corresponding certificate according to gas explosion protection (Zone1). Firstly, each tank was equipped with an RF probe (standard version) for full signal, that ensured reliable protection against overfilling of the acetone containers. Secondly, an empty detection system was installed in each container, that switches on time to prevent dry running.
As it is common that the sensor probe is exposed to the acetone and the aggressive molecules of the acetone vapour for longer periods of time, especially during the empty detection process - it was realised that an extra robust version of the device was required. So the capacitve single rod was thus refined - 1.4404 (316L) stainless steel and PPS isolators were used with a FFKM seal in the product-contacting probe. The FFKM seal (perfluoroelastomer / perfluoro rubber) has a particularly high chemical resistance against aggressive materials. Perfluoro rubber is considered as the new generation plastic. It is a true high-tech elastomer, which is always used when special demands are placed on resistance to chemicals, temperature and form. The equipping of the probe seals of the capacitance level indicator with the highly robust polymer made them absolutely resistant to the acetone. Clean and regenerated as well as contaminated acetone tanks can be similarly monitored using the capacitive measuring principle. The Active Shield Technology, which is integrated in the RF probe as standard, also ensures that the measurement result will not be affected by any buildup on the sensor probe. A total of 16 acetone tanks could be fitted with a reliable content monitoring, which meant the optimal support of a smooth and maintenance-free manufacturing process for the synthetic fibre producer.
What is important to the operator?
The RFnivo® calibrated itself after mounting and wiring in the box after power was applied. The operator had to do nothing further but set the signal output in the Qickstart menu after its application. If an RFnivo® is used in an application to which the supply voltage has already been applied, only the calibration button needs to be pressed after installation in the container. The probe automatically adjusts to the new application. Thus, the calibration of the RFnivo® is easily performed. For “diehard” RF probe operators, there is also a powerful manual and menu-driven calibration. This ultimately leads to the same result as the automatic calibration.
The universal voltage electronics are clearly equipped with a display and 3 control buttons and provide a DPDT signal output for signal evaluation. During operation, the display will always show the capacity measured at the moment. Parameters such as FSH (fail save high) and FSL (fail save low) as well as reaction time of the signal output can be clearly arranged in a quick start menu. A test button allows the manual start of a self-diagnosis of the sensor. If required, an automatic self-diagnosis can be set, which then regularly checks the correct function of the RFnivo® and activates the failsafe in case of error. This self-diagnostic runs in the background without affecting the function of the probe in the application. So a simple operating system was integrated into the capacitance limit switch, which can be easily operated.
“The capacitive sensor has been operating smoothly in the acetone tanks for some time now and reliably detects the level. It was an exciting project, where we were once again delighted by the quality and reliability of the UWT equipment.”
More solutions with the capacitance series for level measurement in liquids, pastes and foams
After gaining valuable experience within liquids applications, UWT has expanded its portfolio to include compatible solutions for all liquids, pastes and foams. The RFnivo® 8000 series was included in the RFnivo® level switch series, which is especially suitable for the detection of pasty, adhesive and corrosive liquids as well as for foam detection. It can also be used for interface measurement of different liquids or sludges and is with WHG and according to Lloyd's Register certified. The compressive strength of this RF version is up to 35bar. The optionally available PFA coating also provides improved chemical resistance. The devices are equipped with encapsulated electronics and work with the innovative Inverse Frequency Shift Technology, which allows a very precise sensitivity adjustment to the medium. Due to the variation of versions with rod and tube extensions, the sensors can be easily adjusted to the respective container conditions and installation situation.