SMART ALTERNATIVE TO VIBRATING FORK LEVEL SWITCHES

Phil Black - PII Editor 09/02/2011

265 5 minutes read

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By – Mikael Leth Jensen – Product Manager at Baumer Group

The process of level measuring and monitoring can be based on various technologies. Vibration level switches are a widely used method. However, they do not work equally well in all media. The recently developed smart alternative based on the frequency sweep technology: the LBFS level switch always performs accurately and reliably in almost every medium.

Point level detection is one of the most common applications in the process industry. It detects the level at a specific point (minimum/maximum) in order to avoid overfills and dry runs. There is a series of limit switches based on various technologies, their suitability depending on the application and therefore the customers” requirements. Application-specific factors like foaming, highly aggressive or inflammable substances as well as narrow surroundings have to be considered. Further important criteria for choosing the right measuring device include the speed of the filling process and the required degree of accuracy.

Disadvantages of vibrating fork level switches

The range of possible applications for level switches is often limited by the fact that they can only be mounted inside tanks in specific positions. If their sensitivity is too high or too low, this may cause incorrect results. The mechanical qualities of a sensor, too, may cause problems. Some of the widely used vibration level switches, for instance, have rather large vibrating forks which need to be inserted far into the medium in order to detect the frequency change which activates the sensor output. This often leads to incorrect readings, as pasty media can easily stick to the forks. Coarse media such as grains can easily get trapped between the fork legs, which also causes incorrect results. As the forks are inserted five to ten centimetres into the medium, they are difficult to clean. The longer the forks, the more prone they are to mechanical damage, such as bending. Moreover, vibration level switches require a relatively large amount of space outside the tank. The specific gravity of the medium can also limit the range of applications for vibrating forks: the resistance of very light-weight media, such as ashes, to the vibrating forks can be so low that the forks are unable to detect the medium at all.

Although vibrating fork level switches can be used in quite a wide range of applications, their flexibility is limited because liquids and powders require different versions. However, other methods of measurement, including conductive ultrasonic and optical sensors, often also have a very limited range of application.

LBFS – one sensor for all applications

So far, it was hardly possible to cover all types of application with one single level switch. Baumer’s newly developed LBFS level switch fills this gap. It is based on the frequency sweep technology, which means that a frequency is swept by the transmitter and becomes subject to a phase shift depending on the medium. When in contact with the medium, which has a different dielectric constant (DK) inside the selected DK range, an electronic switch is triggered. The high sensitivity over a large sensing range from DK 1.5 up to more than 100 allows the level detection of all kinds of powders, granulates, and liquids. Even difficult substances like polyamide granulate or paper can be detected reliably. Compared to other measuring methods like vibrating forks, conductive ultrasonic, or optical sensors, this technology has further important advantages. It has no vibrating parts and is not sensitive to changes based on conductivity, temperature, or pressure. Furthermore, the sensor is suitable for all kinds of applications, for which a range devices based on different technologies would have been required up until now.

The sensor is mounted in a compact, very rugged, and corrosion-resistant stainless steel housing. It can easily be installed and sealed with e. g. a Teflon® tape. A bright 360° LED display ensures good visibility of the switch point, even if the sensor is mounted on top of a tank. The sensor can easily be configured on PC using the FlexProgrammer 9701. The device can be used within a wide operating temperature range from –40 up to 115 °C. Even adhesive media do not stick to the compact, smooth sensor head. For such difficult media, a teach-in function enables manual adjustment of the level to be detected. Moreover, the sensor head has a very low insert distance into the medium, which makes it easy to clean.

The LBFS detects high and low levels in tanks and pipelines. In storage, buffer, and filling tanks, it is usually used for detecting high limit levels. Due to its fast response time of just 0.2 seconds, the sensor enables fast filling processes, but also helps to accurately and reliably maintain a given level. The sensor is also suitable for overflow protection and for dry running protection of pumps. Dry running of pumps is a critical problem which can lead to errors in apportioning and to mechanical damage. The LBFS offers reliable dry running protection even with viscous or adhesive media. In processes where the medium passes through different phases, the LBFS helps to either detect them (phase separation with oil-water mixtures) or to exclude them from detection (e.g. layer of foam).

The device is even able to detect the level of viscous or dry substances and can be mounted in any position in tanks or pipelines, even in narrow surroundings. The sensor is unaffected by flow, turbulence, bubbles, foam, and surface contaminants. Moreover, it is absolutely soundless during operation.

The LBFS is available with G½”, G¾” und G1” process connections. Other process connections can also be supplied on request. The switching status can be chosen as “normally open” or “normally closed”. The type of output signal can be defined as PNP or NPN. The sensor complies with protection class IP67.

Wide variety of applications

As the LBFS can be used in all media and even in narrow spaces difficult to access, there are many fields of application, for instance in the following fields: drinking water, wastewater, filtration, plastic injection moulding machines, flour mills, cleaning machines for metals, HVAC, hydraulics, transmission fluid levels, oil and gas, wood pellets, railway transportation, bio energy, and pump systems.

For applications in water treatment, level measuring devices need to be extremely rugged. First of all, this concerns the materials used in cases where the measuring instruments have contact with aggressive or corrosive media, such as in desalination plants or sewage sludge processing. In addition, the devices need to be highly resistant against strong vibrations, which continually occur in applications with volumetric pumps, for example. Further challenges include foaming, surface contaminants, and condensation inside tanks. In such surroundings, the LBFS provides reliable and accurate level detection.

The hygienic version: LFFS

The LBFS can be used in all industries and applications with the exception of hygienic applications. For the food and beverage industry, a hygienic version is available – the LFFS level switch. It complies with the EHEDG demands and has 3A approval. It is suitable for high process temperatures up to 200 °C (with cooling neck). The LFFS, too, can detect the levels of liquids, viscous media, and dry substances, which makes it suitable for a large number of media.

Baumer Ltd – Process Instrumentation
Swindon
Wiltshire

Can be contacted on

Tel.: +44 (0) 1793 783839
Fax: +44 (0) 1793 783814
E-mail : sales.uk@baumer.com
Web: www.baumer.com