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Chemical seals is also known as a ‘diaphragm protector’ which uses diaphragm seals. Designed for use where the process fluid being measured would normally clog the pressure system or might freeze due to changes in ambient temperature and to withstand corrosive effects of certain process fluids. The diaphragm seals the pressure system from process fluid. Any movement of the diaphragm will change the process pressure accordingly and indicated by the gauge pointer

The chemical seal is also known as a "diaphragm protector." Its main components (the upper and lower body and the clean-out ring) are shown in Figure. The pressure instrument is screwed into the upper body, which can be made of standard materials because it contacts only the non-corrosive filling fluid, usually a silicone oil. The top section with the filled diaphragm capsule can be removed with the pressure instrument while the operator cleans out the material accumulated in the bottom housing. This lower body is made of "pipe specification" (process compatible) materials and can be continuously or periodically cleaned by purging.
The seal shown in Figure A is an off-line design; an in-line design is shown in Figure B. In-line devices are less likely to plug, but the process has to be shut down if maintenance is required. The ultimate in self-cleaning designs is shown in Figure C, in which all sharp edges and dead-ended cavities (where solids could accumulate) have been eliminated. The flexible cylinder can be made of a variety of plastics, including Teflon(R), and is available in spool and wafer configurations.

As the process pressure changes, the amount of liquid displaced by the sealing diaphragm is small, and is sometimes insufficient to fill and operate bellows-type sensors. In that case, larger displacement "rolling" diaphragms are used. Volumetric seal elements (Figure 3-17) also can eliminate cavities and sharp edges where material might accumulate. They also are well suited for high pressure and high viscosity applications such as extruders.

Adding seals to a press measurement device can cause the following problems:

• Long or large bore capillaries increase the volume of the filling fluid, increasing the temperature error.

• Smaller diameter diaphragms are stiff and increase error, particularly at low temperatures.
  Filling fluid viscosity, acceptable at normal ambient temperatures, may be unacceptably high at low temperatures.

• Long capillary lengths or smaller bores can cause slow response.

• Uneven heating/cooling of seals and capillaries can cause errors.

• Some fill fluids expand excessively with temperature and damage the instrument by overextending the diaphragm.

• High temperature and/or high vacuum may vaporize the fill fluid and damage the instrument.
  Fluid may contract excessively at low temperatures, bottoming the diaphragm and preventing operation.

• Frozen fill fluid also will prevent operation.

• For a successful seal installation, the following must also be considered:
  Process and ambient temperature range.

• Relative elevation of the seals and the instrument and the hydrostatic head of the fill fluid. Instrument should be reserved after installation to correct for elevation.

• Temperature, pressure, and physical damage potentials during cleaning and emptying.
  Possible consequences of diaphragm rupture in terms of hazard and contamination.
  Identical seals and capillary lengths for both sides of a differential pressure device.
  Seal and instrument performance at maximum temperature/minimum pressure and minimum pressure/temperature combinations.

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