Packing material selection is primarily based on the corrosion resistance. The factors below are also important. Reference 16 has an excellent detailed discussion.
Metals. Carbon steel packings are usually the first choice for noncorrosive services. Compared to ceramic and plastic, they normally offer higher capacity and efficiency, a wider range of geometries, higher turndown, are unbreakable, and have a high compression resistance. Stainless steel packings cost roughly three to five times more than carbon steel (16,17); alloys are even more expensive. Generally, stainless steel and other metal packings share the Same advantages as carbon steel except for the low cost. They are used in corrosive services.
Ceramics. Ceramic packings have declined in popularity ever since low-cost plastic packings became available. Compared to plastic, ceramic packings are breakable, have lower capacity, and are unavailable in many of the popular geometries. Ceramic packings are currently specified only in applications where their high chemical inertness and resistance to high temperature gives them the edge (e.g., in sulfuric acid absorption). However, despite their reputation of high chemical inertness, some grades of ceramic may chemically degrade. Case histories of ceramic packing degradation under boiling potassium carbonate solution conditions have been reported (18). A very rapid chemical degradation of ceramics takes place in the presence of hydrofluoric acid.
Plastics. Polypropylene is inexpensive and is most popular when temperatures do not exceed 250°F. Caution is required where hot spots are likely, e.g., due to heat of solution. Other plastics may be better for higher temperatures, but these tend to be more expensive. Generally, plastic Pall® rings are stiffer and resist softening more than other shapes because of their internal "arms."
Some grades of polypropylene tend to promote foaming in alkaline services (16,18). The foaming is caused by leached-off additives of the plastics; the nature of these vary with the supplier and even from batch to batch. Plastic packings tend to degrade in oxidizing atmospheres and when solvents are used, and are best avoided in such application. Plastic also degrades in ultraviolet light and becomes brittle at subfreezing temperatures; careful handling, storage, and transportation may be required. Plastic also tends to become brittle upon aging.
A disadvantage of plastic packings is their poor wettability. While the hydrophilic nature of ceramic, and to a lesser degree metal, promotes the formation of thin liquid film on the packing surfaces even at low liquid rates, the hydrophobic effect of plastic makes such a liquid film more difficult to form. Poor wettability can be particularly troublesome in the initial ("aging") period of operation (3); it may take several days to reach normal efficiency. The ease of handling of plastic packings is an advantage during startup and shutdown. Unpacking techniques such as "sucking and blowing" can easily be used at shutdown. Repacking is easier and faster.
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