Pressure Measurement Process Safety With Dry Measuring in Sanitary Applications
In manufacturing processes for food, pharmaceutical and biotechnology products, safety for the consumer has the highest priority. In these processes, measurement technology delivers valuable information for high-quality and efficient production. For the specific requirements with respect to hygienic design, there are several things to consider with the measuring instruments. This article explains important correlations and presents a reliable pressure measurement solution.
Pressure measuring instruments which are used in sanitary applications must be made from inert materials. For example, only measuring instruments which have wetted parts of high-quality stainless steel are used here. Also, the cases are made from corrosion resistant stainless steel, since the equipment is often cleaned from the outside.
Additional demands are also imposed in terms of the smoothness of the surfaces. In the standards, such as EHEDG Doc. No. 8, “Hygienic equipment design criteria”, a roughness of Ra <0.8 μm is considered as sufficient for cleaning in place (CIP) processes. This applies particularly for the wetted surfaces, but also for the instrument’s case. Certain processes in biotechnology demand an even lower surface roughness of Ra <0.4 µm. This requirement is easy to understand, since within the process, there must not be any point where particles or other constituent elements of the medium can be deposited.
Why Sanitary Pressure Measurement is Important
From such deposits, impurities can quickly cultivate — and these can endanger the entire process. Also, with a change in the medium, such deposits can become a problem: When the plant is cleaned and rinsed, they can only be removed with a very strong and expensive cleaning effort. In the worst case, this could even contaminate the following batch.
Process Connections — Dry Is Safe
Also the junction of the measuring instrument with the process, through the process connection, must not represent any risk from a hygienic point-of-view. Here, dead-space free, flush diaphragm solutions have proven effective. Through the relevant standards, such as DIN 11864, process connections which are particularly suited for sanitary applications have been defined.
Diaphragm seals assembled to the gauge, with the appropriate process connections, represent a further solution. Diaphragm seals separate the pressure measuring instrument, i.e. the pressure transmitter or pressure gauge, from the measuring medium. The isolation is achieved by means of a flexible metal diaphragm. The internal space between the diaphragm and the pressure measuring instrument is completely filled with a system fill fluid, which transmits the pressure to the measuring element of the measuring instrument. But not without risk: A rupture of the diaphragm could result in problems, since in such an event, the system fill fluid needed in a diaphragm seal could find its way into the manufacturing process.