Editor’s choice Insurance against contamination

They control process pressure, serve as leak detection, they are used for the control of pumps, filters and cleaning processes and also for the monitoring of filling levels: pressure measuring instruments in pharmaceutical processes must fulfil the widest variety of tasks. At the same time, they are subject, like any other instrumentation, to the highest safety criterion: For the protection of consumers, there must be no contamination risk to the end product arising from the measuring instruments and assemblies. As a result, the sterile boundary must be maintained within every phase of the process, as Joachim Zipp, global market segment manager Food and Pharma at WIKA, explains.


n the sensitive processes of the pharmaceutical industry the human risk factor must be reduced to a minimum. The plants operate almost fully automatically, with the control of the processes based on electronic instruments and systems. In the case of pressure measurement, we are talking about transmitters, programmable transmitters and switches. In addition to reliable measured value

registration and processing, the measuring instruments used should be robust, low- maintenance and easy to operate – not only for economic reasons. These characteristics also support the process safety in terms of the end product. Investigations have shown that the majority of damage to instruments that might lead to product contamination are due to incorrect handling. Other reasons are due to damage caused by corrosion or unforeseeable events such as an excessive pressure surge. For applications in the pharmaceutical industry,

in many respects, diaphragm seal solutions are used. The diaphragm seal system consists of a diaphragm seal made from stainless steel or other special material, the measuring instrument itself and the system fill fluid – glycerine or paraffin oil which conforms to the GMP guidelines. The diaphragm takes the pressure and transmits it hydraulically to the transmitter, programmable transmitter or switch. Thus the measuring instrument is permanently separated from the process and delivers an accurate measuring result, protected from influences. Diaphragm seals offer a high operational flexibility. Using them, almost all instruments can be connected hygienically to a process. Thus the instrumentation is accurately custom-sized for the corresponding requirement. This is an advantage over measuring instruments with ceramic sensors, which, for example, are often only available with high-value transmitters that are therefore not suitable for all applications. The primary reason for using ceramic sensors in sanitary applications is their high accuracy. The pressure measurement is made directly with a flush-mounted capacitive sensor without any system fill fluid which might, in the event of any


damage, find its way into the process. In comparison to the metallic diaphragms of diaphragm seals, ceramic sensors are considered to have a higher resistance to shear forces or corrosion in the long term. Failure of a ceramic sensor would immediately be recognised to the operator by a loss of the signal. With a diaphragm seal system which has a damaged diaphragm, the measuring process still functions. The damaged seal goes undetected until a visual inspection is completed. But a ceramic sensor is not immune to damage

either. This can occur through hard water and steam hammer in the process or by an abrupt thermal cycling, such as during a sterilisation process. If the ceramic shatters, air and foreign matter from outside can enter the process via the ventilation and contaminate the sensor. By using diaphragm seals the process always remains sealed, even when a diaphragm is compromised, and thus the sterile boundary is maintained. However, it can happen that products are contaminated even with a sealed process. Due to the measurement accuracy, the thickness of the seal’s diaphragm is limited. Therefore, long- term overloading in the process can lead to deformations and, with that, to dead spaces. At these points, product particles can settle and, in turn, microbes can form. A similar effect can also not be excluded with ceramic sensors. The weak point here is the sealing between the sensor and the metallic process connection: In the first place, substances can diffuse into the seal, and can be released once again with the next process stage. Secondly, the seal can be overstrained with high temperature dynamics, and thus a gap can be opened up at the process connection – a point for potential deposits. If these remain undetected, they can be transferred to subsequent processes. This undetected cross-contamination usually has serious regulatory and economic consequences with costs in the millions. Therefore, pharmaceutical companies strive to

prevent such damage. In highly sensitive processes, for example, pressure measuring instruments are removed after each batch and

Programmable process transmitter with diaphragm monitoring

checked for damage. This adds costs to the manufacturing process. WIKA has developed a special solution which addresses these issues: A diaphragm seal system with a dual diaphragm and diaphragm monitoring removes the risks and the consequential damages as described above. Using this patented system, the space between the two diaphragms is evacuated and the vacuum can be monitored with a measuring instrument. The type of monitoring can be specified individually, depending on the sensitivity of the process. With regular on-site visual inspection, for example, it is sufficient to check a pressure gauge with green-red display, in other cases an optical or acoustic alarm in the control room may be required. When using media with a high risk potential, the operator can use a pressure switch which will immediately halt the process in the event of any damage. Should the wetted diaphragm become damaged, the second diaphragm forms a reliable seal to the process and maintains the pressure monitoring until the damage has been rectified. Since a break within the system is detected and reported immediately, no microbes can get past the diaphragm undetected. Other options are available which can mechanically or electronically identify a failure in the diaphragm seal. If a seal is broken during maintenance or calibration the operator will immediately see this condition and will prevent a defective unit from being

June 2020 Instrumentation Monthly

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