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Out of Sight, But Not Out of Mind

Non-contact level sensors ensure greater safety in environments endangered by hazardous substances

Chemicals are everywhere – but mostly in places where we don’t see them: from the furniture that we relax on in the evening to the packaging of our favourite sushis. What these everyday items all have in common is that sooner or later they are no longer needed. Our household waste is then joined by the waste materials created in the manufacturing processes that produce those very items. The long list ranges from tank sludge, oily mixtures and solvents to paint, varnish remover and chlorinated emulsions. Such material fractions require specialized, well thought-out disposal solutions that are in accordance with prevailing standards and ecological responsibility.

Chemicals for neutralization
If the degree of contamination of the materials is not too high, incineration can be the most sustainable option; in the EU Waste Directive, this disposal method ranks below recovery. It has two main goals: reducing the volume of waste and destroying potentially hazardous substances. This poses considerable challenges for waste disposal companies: from the bulk handling of incoming waste to the careful monitoring and control of substances re-entering the environment. Still more chemicals are used here, but their task is to purify, optimize and neutralize the process streams and residual materials. They are stored in large storage tanks at strategic points in the respective waste incineration plants.

Reliable measurement makes the difference
The British town of Belvedere, near London, is the site of such a waste incineration plant. Four polypropylene tanks filled with sodium hydroxide (caustic soda 32% and hydrochloric acid 32%) are located on site. The medium is stored here in a large container for long-term supply and in a smaller day container used for process-critical dosing and neutralization. All tanks were originally supplied with a low-cost bubbler level measuring system, which eventually failed due to corrosion and buildup. Vapours and gases escaped through the housing. Moreover, the systems were unreliable, inaccurate and very unsafe.

As little contact as possible
To ensure a lasting function, the sensor must be made of the right materials, i.e. expensive alloys, and equipped with special elastomer seals. During installation and deinstallation, the workforce has to wear extensive protective equipment in accordance with COSSH (Control of Substances Hazardous to Health) regulations. When equipment is installed or removed from the tank, the surrounding area must be closed off. These measures are necessary and involve high costs. They also pose a security risk.

The stored chemicals, especially those that are highly acidic or alkaline, are usually highly toxic, corrosive substances that can leave residues and easily outgas. Any contact with or release of such substances into the atmosphere can quickly become dangerous for personnel. At the Belvedere plant, these chemical substances are carefully monitored. Accurate level measurement helps to avoid overfilling and ensures that there are enough raw materials for the process.

“The previous level measuring system caused a lot of problems. These have been completely eliminated by the new sensors,” explains the responsible engineer. VEGA suggested using non-contact radar technology, i.e. externally mounted sensors that “see” right through the top of the tank, to measure the level inside.

The advantages of non-contact level measurement
When measuring the level of liquid chemicals, a sensor always requires a process fitting on the vessel in order to bring its “sensing element” – whether rod, cable or diaphragm – into contact with the process, even if the sensor is a “non-contact” instrument mounted on top. Regardless of the mounting method, being able to measure all process variables 100% contactlessly offers enormous advantages: longer service life of the sensor, protection against chemicals and the process as well as increased safety through reduction or complete avoidance of exposure to hazardous substances. Radar has the ability to measure the level of a liquid from the outside, e.g. through the completely opaque top of a plastic container or through a sight glass mounted in a port.

Plastic containers in use
Due to new materials and manufacturing techniques, more and more companies are using plastic containers and tanks for chemical and liquid bulk storage. These can be produced faster, are usually cheaper, have good chemical resistance and are easier to move during installation. They have a longer service life and require less maintenance than equivalent steel tanks that are painted, lined or coated, for example. Many are even supplied with a double tank wall that provides additional security. Plastic IBCs are also the most common containers for transporting medium-sized quantities of liquids. Radar technology can be used to measure the level of a liquid right through the container top.

How is radar able to see through plastic containers?
Radar is based on microwave technology. Practically every day we experience how a microwave oven works: the microwaves penetrate plastic containers and other non-metallic, non-conductive materials, heating only the conductive, water-containing food inside (a dry powder, for example, cannot be heated in a microwave oven). A radar sensor has basically the same capability. It can transmit signals through plastic and other non-conductive materials such as glass and ceramics. The radar signals are then reflected by any liquids on the other side. And if the sensor has a good dynamic range (sensitivity), even condensate or deposits on the inside of the container top are no problem.

Why 80 GHz?
80 GHz offers several advantages: 1) the improved focusing results in good penetration of the plastic container top, 2) different installation points are possible (e.g. mounting position closer to vessel wall or ports) and 3) the probability of unwanted interference signals being received is lower. This high frequency also enables a very large measuring range, suitable for very small or very large containers (up to 30 meters high) or anything in between. Another advantage is the high dynamic sensitivity. In some of these applications, even hydrocarbons with low reflectivity can be measured.

Best installation practice
Installation of the radar sensor on a suitable support above the tank, perpendicular to the liquid surface, is an ideal method and allows for a small gap between the sensor and the tank top. A lightly sloping tank top is just right for a microwave sensor because any unwanted signals reflected from the top are thus deflected away from the radar antenna. If the top is flat (i.e. horizontal) like on most IBC or small day tanks, this is no problem, since disturbing signals can be suppressed. In the case of an outdoor tank, a cover should be applied to prevent snow, for example, from collecting on the tank directly below the sensor. Also, a position should be chosen that reduces the likelihood of water puddles forming underneath, as radar sensors cannot “see” through water. Rainfall on an outdoor tank with a sloping top, however, does not affect sensor performance.

Safer measurement and control
The first radar sensor at the waste incineration plant in Belvedere is now measuring successfully through the top of a chemical tank. It is mounted on a simple frame with a holder supplied by VEGA. The sensor required only a basic setting for the minimum and maximum levels. The software and the high sensitivity of the VEGAPULS 64 sensor did the rest. “The instruments are easy to operate and Bluetooth communication for setup via the VEGA tools App or a PC/tablet with PACTware makes everything so much easier,” adds the engineer, obviously quite pleased with these features.

Using radar technology to measure through plastic tanks and even glass windows in reactors offers great advantages: increased safety and reliability, chemical compatibility and time-saving installation. Not to mention savings in sensor data. The operators of the Belvedere plant summed it up this way: “VEGAPULS 64 radar level measuring instruments give us real flexibility and security in operating our chemical storage tanks and controlling processes on site.”

Level measurement through glass and plastic walls

What advantages does the radar level sensor VEGAPULS 64 offer the user?

  • Radar technology allows measurement of the level without direct contact with the medium.
  • Radar signals can penetrate non-conductive materials such as plastics, glass and ceramics. This means that the sensors can even be mounted completely outside of containers.
  • With VEGAPULS 64, several factors contribute to making measurement through windows or plastic containers easier. Thanks to the high signal frequency of 80 GHz, interfering signals are deflected to the side if the container top has even a slight inclination. This makes the measurement more accurate and reliable.
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