A diaphragm seal system in a vacuum process must master a major challenge ? it must deliver a reliable measuring result, even when the pressure drops and the temperature rises simultaneously. The exemplory case of a chemical company producing phthalic anhydride (PA) shows that it is in no way self-evident that requirement could be met on a frequent basis.
PA is primarily used as a plasticiser for plastics. However, it is also found in the production of synthetic resins and dyes. The plant operator obtains this compound via the catalytic oxidation of o-xylene in the gas phase, followed by a vacuum distillation process.
Critical phase for pressure measurement
In this technique, the critical phase for pressure measurement occurs in the distillation column. There, the process temperature can rise to a lot more than 230 �C as the pressure drops to 100 mbar abs. With this type of contrary action, the conventional installed measurement technology reaches its performance limit relatively quickly, that your aforementioned manufacturer was forced to learn ? all instrumentation showed a measurement deviation outside the defined tolerance after only week or two.
Diaphragm seal system adapted in all elements to the vacuum process
The diaphragm seal system is tailored exactly to the conditions of the vacuum process and tested under application conditions.
Consequently, the company approached WIKA with the request for a measuring solution that would also fulfil its task with the required accuracy in the long term under the conditions described. WIKA then drew on its in-depth expertise to create a diaphragm seal system for the vacuum process, consisting of a process transmitter (model IPT-20), a flange-type diaphragm seal with flush diaphragm (model 990.27) and a capillary.
Freebie of this measuring system were selected designed for the application. Silicone oil KN32 was therefore selected as the fill fluid for pressure transmission. Hesitant and conditioning of the diaphragm seal, the capillary, the bond pieces plus the automated filling process were also matched to each other exactly. With sensitive limit ranges, even one drop of liquid an excessive amount of or inadequate can destabilise the volume balance of the diaphragm seal system and thus impair its function. Due to this fact, correct measurement would no longer be guaranteed.
Test under application conditions
After cleaning, all elements of the diaphragm seal system were first fully welded into a single unit as usual, followed by the filling and adjustment of the machine. Before delivery, however, there is another crucial step ? in its laboratory, WIKA simulated a vacuum process for the diaphragm seal system relative to the customer?s conditions and subjected it to a thermal ageing process. This test confirmed the procedure suitability of the measuring system.
Note
Further information on the WIKA diaphragm seal systems can be found on the WIKA website. In case you have any questions, your contact will gladly assist you to.
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How do diaphragm seals work?
Watch the following video for more information concerning the functionality, application areas and benefits of diaphragm seals: