This type of accessory is used when it is necessary to measure the pressure of a fluid. Due to its physicochemical characteristics, it should not come into contact with the sensing element of a pressure gauge, pressure switch or pressure transmitter.
Diaphragm seals protect pressure instruments from one or more harmful aspects of the process media.
When to use a seal?
- The fluid to be measured is highly corrosive and the sensing elements of the instrument are not compatible with the fluid causing premature or adverse instrument failure. The seal diaphragm can be made of a variety of materials and coatings that eliminate or drastically reduce problems caused by corrosive media.
- The fluid is highly viscous and is either impossible to enter the sensing element of the gauge, switch or transmitter, or enters slowly resulting in very slow instrument response times.
- The fluid tends to crystallize, especially during process stops. This can block the pressure ports of the measuring instrument and prevent accurate pressure measurement.
- The fluid tends to polymerize. This can cause a clog and does not allow it to flow freely.
- The fluid is a slurry or sludge.
- The fluid is very hot. This can alter or deform the sensing element of the pressure gauge.
- The measuring point is located in a place that is difficult to access or is not visible. In this case a diaphragm seal with capillary line is required for remote measurements.
- Due to hygiene regulations. Most measuring instruments cannot meet 3A sanitary requirements due to inherent cracks in their design. Another issue is the ability of the measuring instrument to withstand the temperature requirements of steam cleaning used in many sanitary applications. In the case of a pressure gauge or switch, these have cavities that are impossible to access for cleaning.
If the system needs overpressure protection, the seal with protection is the way to go.
What is important for a seal to work properly?
The seal is basically a flexible membrane made of stainless steel or special alloys; it separates the fluid to be measured from the chamber where the sensor element of the pressure gauge, switch or transmitter is located. On one side, the fluid sticks to the membrane, on the other side it is in contact with a chamber that must be filled with a well selected liquid that allows the pressure variations to be transmitted to the sensor element of the instrument. In conclusion, the fluid pressure is hydraulically transmitted to the pressure gauge, switch or transmitter and this pressure generates a bending in the diaphragm that causes a change in the volume of the chamber above the diaphragm. The chamber between the diaphragm and the pressure measuring instrument must be filled by experts in the total extraction of the air inside the chamber above and the consequent unobstructed entry of air pressure from the filling fluid into this chamber. This air extraction is performed by applying vacuum to this chamber by means of a suction pump. To achieve this, specialized equipment is required to ensure total vacuum in the chamber to be filled (chamber above).
The filling process can never be carried out by overflow or gravity, these methods are incorrect and generate future failures in the reading because the inside of the filled chamber is left with air bubbles, which prevents the correct transmission of pressure by the filling fluid.
The displacement volume of the chemical seal compared to the volume of the measuring instrument (pressure gauge or transmitter) is affected by other variables in practical applications. Volume changes of the system filling liquid caused by thermal expansion or compressibility must also be taken into account. To ensure full compatibility between the chemical seal, capillary and pressure measuring instrument, the displacement volume of the chemical seal must always be greater than the sum of the maximum control volume for the instrument and the volume change of the system filling fluid.
The liquids used must be at a certain temperature, perfectly agitated. These fluids can be KN2 Silicone, KN92 USB grade mineral oil or KN3.2 high temperature silicone.
Diaphragm seals can be mounted directly to the measuring instrument and connected to the process in threaded or flanged form; it is also very common that they are connected to a capillary (up to 10 meters with correction) and the instrument is far from the pressure tapping point.
The seals can be used with instruments that measure gauge, absolute or differential pressure.
Types of diaphragm separators