Special materials for flange-type diaphragm seals
For the user, diaphragm seals allow pressure measuring instruments of all types to be used even under the harshest of applications. Examples include corrosive or aggressive media which could damage the pressure measuring element, e.g. the inside of a Bourdon tube, or high process temperatures which could exceed the upper limit for thermal loading of the measuring instrument components. Diaphragm seals are available in many designs, compatible with the majority of pressure measuring instruments and from various highly resilient materials.
Operating principle of a diaphragm seal system
A diaphragm seal system consists of a pressure gauge with a fitted diaphragm seal and a system fill fluid which transmits the process pressure from the diaphragm seal hydraulically to the measuring instrument (see fig. 1). The process pressure acts on a separating diaphragm which transmits the pressure to the measuring element of the pressure measuring instrument, e.g. pressure gauge, transmitter or pressure switch, via a system fill fluid. In many cases, a cooling element or a capillary is connected between the diaphragm seal and the pressure measuring instrument, for example, to eliminate or to minimise temperature effects from the hot medium to the measuring instrument. In this way the pressure can be determined reliably.
Designs: Flange-type diaphragm seals
Basically, a flange-type diaphragm seal consists of a flange whose connecting dimensions are adapted to corresponding standard flanges. The diaphragm of the diaphragm seal is located in the centre of the flange and forms a flush front seal. The flange-type diaphragm seal is mounted for pressure measurement in lieu of a blind flange. Another version is the cell-type diaphragm seal. It consists of a cylindrical plate, whose diameter is matched to the sealing face area of corresponding standard flanges. The flush diaphragm, matched to the nominal width, is in the centre. The cell-type diaphragm seal is mounted to the tapping flange using a blind flange.
A third version is a diaphragm seal with an extended diaphragm. This version is used for thick-walled and/or insulated product pipelines or vessel walls. These diaphragm seals are designed similar to flange- or cell-type diaphragm seals and are also mounted in the same manner. However, the diaphragm of the diaphragm seal is positioned on a tube, the length of which is dimensioned according to the thickness of the jacket, insulation or wall. The diaphragm is thus flush with the inner wall of the vessel.
A number of details are necessary for the specification of a flange-type diaphragm seal to ensure correct connecting specifications. First, the description must include data on the process connection standard. In Europe, EN flanges are largely used, in other regions of the world standards such as ASME or API are used. Secondly, the diameter and the sealing face need to be specified. For example, EN flanges with B1, serrated or B2 smooth surface are standard. However, there are also other sealing faces, e.g. tongue and groove designs. The nominal pressure rating and the material of the wetted parts are also to be specified.
Additional information is necessary for flange-type diaphragm seals with extended diaphragms; these include length and diameter of the tube as well as the jacket material. Fig. 2 shows a diaphragm seal with all wetted parts made of tantalum (diaphragm, jacket and sealing face). If required, a combination of different materials for the individual components is also possible.
The diaphragm and the connecting flange are elements of the system which come into contact with the medium to be measured. Therefore, the material from which they are made must meet the relevant requirements in terms of temperature and corrosion resistance. Stainless steels are mainly used as standard material in chemical process technology. The worldwide commonly used materials 316L or 1.4404/1.4435 are generally used for the diaphragms as well as the upper body of the diaphragm seal.
A comprehensive range of chemically resistant materials is available for especially critical applications with highly aggressive media. In this case, all wetted parts are made of this special material.
Diaphragms made of special materials
Users have more than 20 different special materials that they can choose from. The most common special materials for diaphragm seals in chemical and petrochemical applications are tantalum or nickel alloys, such as, for example, alloy C276, or Monel ® 400.
A brief overview:
Hastelloy C22, C276
The alloys are used primarily in chemical process technology and chemical apparatus construction. They offer good to very good corrosion resistance at the various operating conditions, e.g. against contaminated mineral acids at higher temperatures, solvents, chlorine and media contaminated with chlorine, formic acid and acetic acid, seawater and saline solutions.
The material is characterised by exceptionally high corrosion resistance which is comparable to that of glass or platinum. Tantalum is not attacked by most organic and inorganic materials at room temperature. This high corrosion resistance is due to tantalum forming a firmly bonding covering layer on its surface, largely consisting of tantalum pentoxide.
Monel alloy 400
This material is resistant against organic and inorganic acids, caustic alkaline solutions and salts over a wide range of operating conditions. The material is better suited for reducing than oxidising conditions and is preferred for maritime and petrochemical applications.
At present, WIKA manufactures more than 70,000 flange-type diaphragm seals annually for use in the process industry worldwide. As they may be subject to extremely tough conditions, specially selected materials and designs offer an effective protection for the pressure measuring instruments. Due to different measuring conditions it is necessary to design the diaphragm seal system optimally for each application to increase the service life using a customised measuring assembly. A number of different and complex factors are essential and must be taken into account for dimensioning the „right“ diaphragm seal for each application.