Key points
Ceramic measuring cells have long been an alternative in chemical plants due to their high level of safety, long-term stability and reliability. Vulnerable impulse lines are not necessary and the material is ideal for hydrogen applications.
When the process pressure increases or decreases abnormally at an important reactor in a chemical plant, it raises concerns in the operators’ minds:
- What does this mean for the safety of the system?
- What impact will this have on the quality of the batch?
- What is causing this change in the process?
The measured value curve is then examined from both the pressure sensors and the other measuring points: temperature, flow, fill level and more. If these are within the usual specifications, the cause may be a sensor error.
Why is pressure an important parameter for safety and product quality?
In the chemical industry, however, pressure is not only measured at reactors. Fill levels, volume flows or conditions of filters can also be derived via pressure and differential pressure measurements. It is the second most frequently measured process variable in chemistry after temperature and an important parameter for safety and product quality. But chemistry also poses special challenges for the sensors used.
Metal pressure sensors have been the backbone of industrial measurement technology in chemical processes for decades. The devices are typically made of stainless steel or other high-quality alloys and are designed for use in harsh industrial environments.
Thanks to their robust construction, they are suitable for high-pressure applications and in extreme temperatures. Because the process and installation conditions in the chemical industry are very varied, a variety of different designs and measurement techniques have been developed over the years.
In addition to a wide temperature and pressure range, the main advantage of metallic sensors is their ability to withstand extreme conditions. However, the disadvantages include their susceptibility to corrosion when the process membrane is exposed to aggressive chemicals and the aging of the material, which is reflected, among other things, in drifting of the measured values. In order to maintain the accuracy of the measurement in the long term, calibration processes are necessary.
“Although ceramic sensors can be used to measure pressure in an estimated 60 to 70% of all chemical applications and have clear advantages there, their properties are still not widely known.” Says Robin Müller, International Product Manager at VEGA.
Why are ceramic measuring cells a better choice in the chemical industry?
Since the 1990s, ceramic pressure sensors have been an interesting alternative to metal devices. These sensors are characterised by their exceptional corrosion resistance, which makes them ideal for chemical processes involving aggressive substances. In addition, ceramics offer excellent long-term stability and minimal drift, ensuring reliable measurements over a long period of time.
Although ceramic sensors can be used to measure pressure in an estimated 60 to 70% of all chemical applications and have clear advantages there, their properties are still not widely known,” explains Robin Müller. The material is often prejudiced against being fragile.
“An unjustified concern,” says Robin Müller: “Ceramic measuring cells have a significantly higher overload resistance than metallic cells. While metal membranes can deform irreversibly under very high pressure, the ceramic membrane simply rests on to the cell body and later returns to its original position.”
And there are other features that speak in favour of the use of ceramic sensors: While measuring cells made of metal work with a diaphragm seal oil as a transmission medium, that can escape into the process in the event of a membrane rupture, ceramic measuring cells are dry.
They function similarly to a capacitor; measuring electrodes embedded in the membrane and the cell body and air as a dielectric form an electric field. The pressure on the ceramic membrane deflects it minimally, which changes the capacity.
The pressure can ultimately be derived from the capacity based on the factory calibration. “In contrast to metal membranes, where the material fatigues, the ceramic always returns to its zero point and therefore has a very high long-term stability. That’s why the sensors generally don’t need to be calibrated later,” explains Müller: “And that’s particularly advantageous for applications in high vacuum.”
Which technical innovations support the performance of ceramic measuring cells?
The basis of the VEGABAR sensors is the ceramic measuring cell CERTEC®, manufactured in Schiltach, Germany. In an extremely precise manufacturing process, the membrane and cell body made of aluminium oxide ceramic are printed with gold paste under clean conditions and joined together using a glass solder in a high-temperature process to form the measuring cell.
Around ten years ago, the inventors from the Black Forest solved a disadvantage that existed in the early years of ceramic pressure sensors: the sensitivity of ceramic sensors to temperature shocks and humidity.
In addition to measuring the process temperature, which compensates for the influence of temperature on the pressure measurement value, a second temperature sensor in the glass seam behind the ceramic membrane also detects the smallest temperature changes. An algorithm implemented in the sensor electronics ultimately ensures that temperature shocks are compensated for.
“The values from this relatively sensitive temperature measurement are also available as a separate signal and can be used,” says Robin Müller, describing another useful function.
Process sensors with ceramic measuring cells
The use of ceramic sensors in comparison to oil-filled metallic measuring cells in applications with vacuum or hydrogen becomes really exciting . As is well known, the boiling point of liquids drops in a vacuum, so it can happen that the oil in the measuring cell begins to boil at temperatures below the atmospheric boiling point.
A different effect comes into play in hydrogen applications: the molecules of the smallest chemical element can penetrate metals – and the thin membrane of a metallic pressure measuring cell is no exception.
“When hydrogen diffuses into and through the membrane, it reacts with the transmitter oil behind the metal membrane. The result is hydrogen accumulation, which leads to permanent changes in the measurement performance,” says Robin Müller.
In addition, hydrogen combines with the carbon in carbon steels, gradually making the materials brittle. “This does not happen with ceramic measuring cells – and even if hydrogen were to get into the measuring cell, it would not cause any damage. Ceramic pressure sensors are therefore ideal for use in hydrogen production through electrolysis, which, in contrast to previous high-pressure applications, works at low pressures,” says Müller.
Why is the ceramic measuring cell particularly suitable for measuring aggressive and toxic gases?
Another important area of application for chemistry is the measurement of aggressive and toxic gases. The most important thing here is a high level of security. Especially for applications with phosgene, planners and operators often prefer the ceramic measuring cell and forego the oil filling.
VEGA is employed in applications with aggressive acids and alkalis, on the one hand with highly resistant plastics in front of the measuring cell and on the other hand with a “second line of defence”. The measuring cell and electronics compartment are separated by a gas-tight glass bushing.
The pressure and level specialist from the Black Forest offers this safety function for the devices in the VEGABAR 82 and VEGABAR 83 series. What is also interesting is the possibility of using the welded sensors with the electronics for relative pressure measurements.
An important application in chemical processes in flow, level and container pressure measurement is differential pressure measurement. This is used, for example, when the level in a container that is under pressure is to be measured.
But flow measurement at measuring orifices or monitoring filters or heat exchangers is also carried out by comparing the pressures before and after the orifice, filter or heat exchanger. A differential pressure sensor is usually used for this purpose and its measuring cell is connected to the process areas to be compared via pulse line.
“The impulse lines always cause problems in practice,” says Robin Müller. In winter, for example, liquids or condensate can freeze in the pipes and block them. In impulse lines filled with liquid, the measurement can become inaccurate due to gas inclusions, because unlike liquids, gases are compressible.
VEGA solves the problem by using two sensors that work separately and are electronically connected to each other. This allows differential pressure measurements to be carried out without the need for complex and vulnerable impulse lines.
Conclusion
Ceramic measuring cells are also a powerful alternative to metallic pressure sensors in chemistry. Above all, their high level of safety and reliability over long periods of use speak for their use in demanding applications.
