Capacitance point level detection in liquid naphthalene – A point level measurement to avoid overfilling or emptying is required in almost all process containers today. In the chemical industry, sensors are usually exposed to aggressive, adhesive, or caustic materials. The measurement technology must deal with high temperatures, varying DK values, and viscosity fast and efficiently. To meet these conditions in practice, system planners are increasingly using the capacitive rod probes.

Precision & process compatibility through adaptable point level detection

A leading chemical manufacturer, mainly processing naphthalene derivatives in addition to carbon, coal tar, pitch, and special oils required reliable and robust level monitoring, which could be flexibly adapted to fluctuating ambient conditions.

As a combustion product of organic materials, naphthalene occurs in low concentrations almost everywhere in the environment and can be used as a contaminant, e.g. in carbon blacks, which are used for blackening, and in mineral oils, which are used as plasticizers. Another application is in the production of solvents, plastics, fuel additives, and insecticides.

In the liquid state, naphthalene is an aggressive substance that quickly corrodes common materials. If it becomes firmer as it cools down, it tends to become heavily attached.  Tests concluded that tanks can be constantly monitored using the capacitance measuring principle of UWT’s robust RFnivo® probe. The RF series are adaptable rod probes, which due to their design do not require any temperature-restricting components for media detection and can, therefore, be used smoothly in processes up to 500°C. They work with the capacitive measuring principle, whereby the capacitance is measured regardless of the length of the sensor of the medium between the probe tip and the container wall.

The chemical manufacturer has several storage tanks that are electrically heated to maintain the melting point of the naphthalene. There is a process pressure of approx. 0.8 bar (11.6 psi). The DK value fluctuates between 1.6 and 2.0. Each tank was equipped with a RFnivo® installed on the overflow line for full notification to ensure reliable protection against the overfilling of the containers. When the material is removed, the temperature drops rapidly causing caking on the sensor boom. Overflow line being there at a height of 1700mm, the installed RF probes work within this overflow line under constantly changing process temperatures (max. up to 80°C (176°F). The “Active Shield” technology integrated as standard in the RF probe ensures that the measurement result is not influenced in the long term by the buildup on the probe. The storage tanks could thus be provided with a reliable content monitoring system, which, due to the design, can optimally adapt to the changing environmental conditions, which meant the optimal support of a smooth and maintenance-free manufacturing process for the plant operator. Special attention was also paid to the aspect of process safety in chemical plants. A certified design in accordance with international approvals according to ATEX, IEC-Ex, FM, FMc, etc. qualifies the RFnivo® for the safety standards of this demanding industry.

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 QuickStart 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 now.

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.

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 situations.