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VEGABAR measures reliably even under extreme pressures and temperatures

Manufacturing facility for specialty chemicals

Double is better – this saying is not only used a lot in colloquial speech, it also applies to difficult pressure measurements in production facilities for specialty chemicals. Two VEGABAR sensors ensure reliable measured values and thus a reliable process.
Specialty chemicals usually have exacting specifications and are adapted in each case to a particular application. The associated processes are therefore also subject to very exacting specifications that have to be carefully controlled. Measurement technology is effectively the first link in a precisely coordinated process chain. In other words: If a particular measured value is not correct, the entire process may not necessarily break down, but it might consume too much energy or the raw materials or the quality of the end product might lie outside the specification limits.
This was the case with a critical pressure measurement in a petrochemical process. The measuring point is located in a potentially explosive area and the temperatures are between +20 and +200 °C, and the pressures between 1.5 and 200 kPa absolute. The pressure measurement at this point represents a decisive control variable in a fully automated process. This is because the efficiency of the process, and ultimately the product quality, depend on this variable. Even small deviations lead to instabilities in process control.
The pressure was previously measured by a standard differential pressure measuring system. The problem with this: Such measurement setups use an oil-filled capillary tube mounted on the outside of the 20 m-high column. Fluctuating temperatures (e.g. in countries with high temperatures during the day and cold winds at night) cause a drift in the measured values. These incorrect values cannot be corrected mathematically. Normally, this problem is solved by using larger process fittings for the pressure transmitter, but this was not possible for structural reasons. At the same time there is another problem: the boiling point of the oil in the capillary tube drops due to the high temperatures and vacuum, causing the oil to outgas. As a result, air bubbles form in the line, which also leads to unstable measurement. In other words: This measuring point delivered inaccurate measured values on a regular basis.
 

VEGA pressure transmitters offer advantages under extreme conditions

Handles temperatures up to + 200 °C: METEC® measuring cell with alloy diaphragm.

For such difficult situations, an electronic differential pressure measuring system with two pressure transmitters from VEGABAR Series 80 is recommended. These pressure transmitters are used in all areas of industry, but offer special advantages in applications with extremely high pressures. The measuring range of the instruments begins already with tiny pressures as low as 0 to 25 mbar and extends to extreme pressures up to 1,000 bar at process temperatures from -20 to +400 °C.  This wide range is achieved by using different measuring cells – in each case depending on the application. In this application, the interesting fact was that differential pressure could be measured electronically by simply connecting VEGABAR series 80 sensors together. 

The solution for strongly fluctuating ambient temperatures: Electronic differential pressure measurement with two pressure transmitters of the VEGABAR 80 series.

For this purpose, two VEGABAR 83 sensors are combined via an innovative software and hardware concept.
To cope with the high temperatures up to +200 °C, the responsible engineers selected the METEC® measuring cell, which is supplied with an alloy diaphragm and self-compensating temperature behaviour. This cell is elastomer free, absolutely vacuum proof and has good thermal shock behaviour.  The thermal coefficients of the materials in the metallic measuring cell were matched to each other in such a way that temperature can no longer influence the measurement. When the oil expands during heating, the alloy and stainless steel free up just enough space for the oil to expand into. However, if the temperature drops very low, this effect works exactly in reverse. The best part is that the process fitting for the pressure transmitter can be kept very small: just "1½". The pressure transmitter normally required for stable pressure measurement in the process at the petrochemical company would have had 3" (DN 80) fittings. VEGA's solution therefore not only resulted in better measured values but also in the use of a smaller design.
 
The engineering team was more than satisfied with the solution using an electronic differential pressure system with VEGABAR sensors. The measuring points that were previously equipped with a standard differential pressure measuring system are thus to be gradually retrofitted with the solution based on electronic differential pressure.
 

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