Limit switches – or point level sensors – are based on old but reliable technology. Vega has re-engineered this technology, bringing it up to date and developing a sensor for extreme situations – the Vegaswing 66 vibrating level switch. Holger Sack, head of product management at Vega, explains why these new sensors can handle temperatures up to 450°C and pressures up to 160 bar.
The term ‘Grenzschalter’ (limit switch in German) appears rather infrequently in technical texts. Why?
Holger Sack: Today the old, tried and tested analogue limit switches are gradually being replaced by digital technology. That’s why these devices are now more commonly called point level sensors. Even in the English language, you’ll more often hear people talking about point level measurement, ie, detection of a substance at a specific point. I suppose ‘point level’ has prevailed over ‘limit level’ for that reason. We’ve found that point level measurement is indeed very closely related to level measurement.
That is very likely the reason why ‘level’ is used in some descriptions as a synonym for ‘point level.’ To what extent are the two technologies different, and what are their similarities?
Holger Sack: Level measurement is used to describe continuous measurement of a changing level, whereas point level is used to indicate a discrete condition, i.e. the existence of the level at a certain point. To clarify, ‘level’ means the continuous measurement of contents from empty to full, with the values being output in percent, volume or other units of contents. ‘Point level’, on the other hand, means a discrete on/off signal given when a product has reached a significant level in a vessel.
Continuous or discrete – can the two areas of applications be clearly separated from one another?
Holger Sack: No, they can’t. You can find both level and point level sensors everywhere, even combined in one application, depending on the customer requirement. Usually when both sensor types are installed it is to increase safety, for example in the chemical industry.
Does the simpler technology behind point level switches mean that they are cheaper than continuous level sensors?
Holger Sack: Roughly speaking, yes – because continuous level sensors are more complex in structure and sometimes also in terms of technology. But this need not always be the case. With regard to the process, point level sensors have to meet the same requirements as continuous level sensors. Level switches just carry out a simpler evaluation of level data. That’s why the price of a level switch, or point level sensor, is usually less than that of a continuous level sensor.
In point level detection, a switching command starts or stops the filling equipment. How do you monitor the process, ie, ensure that the sensing element and the electronics are working properly?
Holger Sack: On the one hand the individual sensor has to be considered, and on the other the entire measurement chain. In modern sensors, microprocessor technology enables numerous functions that monitor the electronics as well as the sensing element during operation. A high percentage of faults in the electronics as well as in the sensing element, such as build-up or corrosion, can be detected and reported. Looking at the entire measurement chain, we see that information on tank contents is forwarded to the control system or special actuators through cables or bus systems. These systems are responsible for ensuring that valves, pumps, etc. operate at the right time. Until now, all devices in a system were analysed individually from a safety-engineering standpoint; but this has changed. Today, engineers look at the entire measurement chain, from the sensor to the transmission of measured values to the actuating components (valves, pumps, etc). This ultimately ensures that the switch-off mechanism in its entirety works, preventing overfilling or dry running of pumps.
When it comes to safety, a lot has been done in recent years. Has level switch technology also been made better, safer?
Holger Sack: The basic technology is very old. People built level switches long before they started building continuous level measuring instruments. For that reason there are still a lot of old but proven technologies in service, such as floats or paddle switches. The capacitive measuring principle is also a very old, tried-and-trusted measuring method, albeit with a few limitations when compared to the vibration principle. Next to microwave or radar, vibration is currently the most universal measuring principle that we offer. We at Vega have been focusing on electronic measuring systems for years because they have significant advantages in terms of maintenance and means life cycle costs. That’s why, although they are also a little more expensive to produce and to buy, we believe that this extra outlay pays off over a service life of 15 years or more.
Does this mean that your new vibrating level switch Vegaswing 66 is just old, well-known technology in a new guise?
Holger Sack: No, not in this regard. Here we are breaking new ground with our new, patented technology. The inside story is that this instrument can be used in temperature and pressure ranges where previously only a few technologies could be deployed, and certainly not the tuning fork measuring principle. The basis of this technology is a tuning fork that is electrically excited and made to vibrate over a range of a few micrometres. Until now, it was not possible to use vibration technology in temperatures above 250°C. With our Vegaswing 66, applications up to 450°C are now possible. It is also capable of withstanding temperatures as low as -195°C. We are the only company that can offer this technology for use in such temperatures and in pressures from -1 to 160 bar.
How did you make it possible to use the switch in the extreme temperatures and pressures commonly found in the process industry?
Holger Sack: For one thing, we replaced the previously used piezoelectric drive with a special solenoid that we developed ourselves. This solenoid now drives the tuning fork and is able to withstand the high temperatures. Another point is that we now use ceramic materials and have designed the electrical connections to be so secure that they operate reliably, even at 450°C and last but not least, we achieved the high pressure resistance through the mechanical stability of robust materials that withstand pressures up to 160 bar.
For point level detection, the user can choose between different measuring principles. How can the customer be sure he gets the right one, the one he really needs for his application?
Holger Sack: It is always important that the customer tells us in advance what his requirements and application conditions are. With this information we can recommend the right measuring principle. However, we are confronted again and again with new challenges that push the limits of our technology because our customers are not standing still, they’re continuously developing their processes. In most cases it’s about new combinations of pressure, temperature and chemicals. That’s why we’re constantly developing and improving our products and measuring principles, optimising them to meet the latest process requirements.
Current problems are, for example, difficult product properties or foaming. To what extent do these factors influence the quality of the measuring results?
Holger Sack: On Vegaswing 66, for example, we can detect build-up, and we can also detect whether the tuning fork is corroded or broken. This is possible due to the measuring principle itself, as well as monitoring of the natural resonant frequency. Build-up changes the amplitude of the oscillation, which allows us to use the available processor technology to electronically evaluate this change and notify the customer that a problem exists. Such anticipatory diagnostic capabilities are being demanded by customers more and more.
In addition to diagnostic capabilities and safety, users nowadays are calling for the simplest possible instrument handling. Do your level switches also follow the motto of the plics platform, ‘simpler is better’?
Holger Sack: Absolutely. The whole idea of the plics concept is to make level and pressure measurement as simple as possible for each and every customer. The customer doesn’t have to be an instrument engineer to be able to use our instruments. His job is to control the process and keep it running smoothly. By asking questions about his process and its requirements, we are able to recommend the instrument that is best suited for the application. Vegaswing 66 is also designed according to the modular instrument platform, plics. This means that the customer can combine different components as required. But plics is more than that – it’s a system designed to make handling easier for the customer throughout the lifecycle of the instrument.
Another important point is delivery time. We deliver 80% of our products within two to five working days where previously it took six to ten weeks. Installation and setup are also greatly simplified by the modular system because if the customer already knows how to operate one Vega instrument he can operate all the others just as well. Installation, adjustment and connection are completely standardised. This applies to 80% of our instruments. If servicing is required, our employees look after customers personally and make sure the job gets done quickly and without any fuss – because every servicing event also provides us with feedback on the product and a chance to improve it and ourselves.
For more information contact Chantal Groom, Vega Instruments SA, +27 (0)11 795 3249, [email protected], www.vega.com
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| Email: | [email protected] |
| www: | www.vega.com/en/home_za |
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