Diaphragm seals, also called chemical seals or gage protectors, use a thin flexible diaphragm to isolate the pressure element from the process fluid (see Figure 400-8). The space between the diaphragm and the sensing element is filled with a suitable noncompressible liquid. Diaphragm seals can be an integral part of the pressure instrument or be connected by capillary tubing. Capillary tubes up to 25 feet long are available. In vibrating service, diaphragm seals should be remotely mounted and have armored stainless steel capillary tubing. Diaphragm seals have the following
• Water, if the leads are subject to freezing
• Process streams that are corrosive to the pressure element
• Dirty process streams containing solids that can plug the pressure element
• Viscous process streams that can solidify in the pressure system
• High-temperature process streams that exceed the maximum temperature rating of the instrument
Diaphragm seals can be used at temperatures ranging from -40°F to 1500°F. They can have either a ¾-inch or 1-inch NPT screwed or flanged process connection, depending on the piping classification. The bottom housing and the diaphragm material should be 316SS or better and should be compatible with the process fluid. Filling fluid identification or maximum temperature limit should be stamped on the body or nameplate. The diaphragm should be welded or attached to the body so that fluid will not escape when the diaphragm is disassembled.