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A diaphragm seal system is normally used when processes do not allow adirect contact between pressure measuring instrument and the processmedia. Examples are processes that deal with extremely hot, very corrosive or abrasive materials.
In the process industry, a wide variety of processes can be monitored using pressure gauges and pressure transmitters. However, when the media to be measured is abrasive, the measuring instruments can wear out extremely fast and must be replaced fairly often which means high costs regarding maintenance and replacement parts.
One possibility in protecting the pressure gaugeor pressure transmitter from sand, solid dirt particles and blunt objects is an assembly with a diaphragm seal. If standard pressure gaugeswere used, the media such as concrete would clog their entry bore andharden. However, with a diaphragm seal, a metal diaphragm separates the measuring instrument from the process media. (fig. a)
Operating principle of a diaphragm seal system
The inner cavity betweenthe diaphragm and the pressure measuring instrument is completely filled with a system fill fluid, The process pressure is transmitted by the elastic diaphragm into the transmitting fluid and from there to themeasuring element, i. e. to a pressure gauge or transmitter. In this way, pressure can be reliably measured. But even a stainless steel diaphragm can wear out over time when constantly subjected to the abrasivemedia.
Diaphragm seal with protection disc
For this reasonWIKA developed a diaphragm seal with a protection disc for use undereven most severe conditions. The pressure measuring instrument is insensitive to blows and impacts on the protection disc. This has a sufficient thickness, in order to resist the abrasion from the process media.
The thickness of the protection disc is however also a disadvantage, since pressures within low ranges cannot be measured so easily. Therefore consultation with our technical advisors is necessary in suchcases.
This type of diaphragm seal (fig. b), which can be combinedwith a pressure gauge, an electronic pressure transmitter or pressureswitch, has been proven for use on cast plaster pumps and concrete pumps as well as for mining and waste removal.
Sandy or powderymixtures, which contain solids and rub like sandpaper against the measuring instruments, are found in the construction industry, for examplewith the transportation of liquid concrete or wall plaster. With thehelp of the pumping pressure quantity of the material conveyed to theprocessing location can be determined. These materials consist of water, sand, gravel, cement and mixtures.
Another example of a process application involving slurries would be a muck removal system used in tunnel construction. The sludge is a mix of water and solid particles which can wreak havoc on a standard stainless steel diaphragm. The pressure transmitters are used here to monitor the removal, thus allowing the operator to slow or stop the boring process if the pipes threaten to become clogged.
Polyurethane diaphragm
An alternative to diaphragm seal with protective disc is an in-line diaphragm seal with polyurethane diaphragm (fig. c). This flanged In-line Seal is installed directly between two flanges and becomes an integral part ofthe pipeline. This seal replaces the conventional T-piece in the process piping system. The polyurethane diaphragm provides excellent protection from deposits in the process. The cylindrical diaphragm minimizesprotrusions or interruptions to the process flow. This elastomer diaphragm is bolted to the seal body.
The inline diaphragm seal with polyurethane diaphragm has proved to be successful in the construction business, more specifically in gold mining. For the extraction ofgold, for example in South Africa, water is added to the mined ore. This mixture is then pumped from the mine to the gold mill, where the gold is separated from the ore. In order to monitor the pressure of thefeed pumps, pressure measuring instruments are required which resist the abrasive effect of the mixture of sand, stones and water.
wikaramic coating
As an alternate solution for pressure measurement of abrasive media, WIKA developed a ceramic coating wikaramic for its metal flush or threaded diaphragm seals. The coated flush diaphragm (fig d) provides a high abrasion resistance and thus protects the sensitive diaphragm from scouring and prevents them from being worn down quickly.
Depending on the measuring combination (mechanical pressure gauge or electronic pressure transmitter), a diaphragm seal with wikaramic coating is suitable for lower ranges starting at 400mbar. Furthermore, the process temperature is not limited by the ceramic coating, rather by the system fill fluid. Typical process temperatures can berealized ranging from -90 up to 400°C.
Wikaramic has been proven in the pulp and paper industry, where the defibration of recoveredpaper takes place. A more common example of this application the daily newspaper which has been printed on recycled paper. During the defibration process of recovered paper, pressure measuring instruments comein contact not only with paper particles and printer’s ink but also staples and paper clips. Who hasn’t put paper with staples or paper clips into the recycle bin? But it is these types of materials that can destroy the diaphragms on pressure measuring devices.
Whereas previously a pressure measurement assembly would be replaced after a short interval of 6 months, the wikaramic coating can increase the life span of the pressure measuring device by years. In the meantime, the first pressure measuring instruments with wikaramic have already been inuse for over 3 years.
While abrasive media can take many shapes and forms and cause conditions to be severe, special designs and materials can provide protection for the pressure measuring instruments.The configuration of the combination of pressure measuring instrumentsand diaphragm seals depends, among other things, on the application conditions in which the assembly must work. The selection of suitable diaphragm seals and the best configuration for the specific applicationreduces downtime. and substantial savings regarding maintenance and repair can be realized.
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