“Valves and actuators make up a small part of a power station – perhaps only two or three percent by capital cost,” according to Power Engineering International.
Despite this, valves are essential in starting, stopping, and regulating the flow of liquid/gas in a plant. Valves provide a method to isolate components of the plant for maintenance. But just like the rest of the plant, valves also need maintenance. Among required valve, maintenance is packing treatments.
Packing is a set of rings, resilient or semi-resilient material, placed around the shaft or stem to create a seal. These rings push on the stem and the body of the valve, assuring no liquid/gas escapes.
Valve packings are present in four basic styles:
Lubricant impregnates most packing materials. This helps reduce friction and packing wear. These lubricants can be mineral oils, waxes, Teflon, and particles of graphite. The graphite (at least 99% pure) is ideal because it is maintenance-free, permanently elastic, non-hardening, non-aging, highly formable, and permanently resilient.
Valve packing often contains graphite because it is maintenance-free, permanently elastic, non-hardening, non-aging, highly formable, and permanently resilient.
Fabric and rubber packings come in different weight cotton duck fabrics, ceramics, and Kevlar with natural, neoprene, or rubber binders. All of the types are available in die-molded rings, spiral, and coil forms.
Just as the Fabric and Rubber, Ceramic and Kevlar come in two types of packing: Laminated Construction and Rolled Construction.
There are three main types of plastic valve packing: a mixture containing soft-bearing metals, Teflon packing, and Teflon-impregnated packing.
The metals in valve packing are either crimped, spiral wound, or braided to form packings that can withstand higher temperatures than fibers and produce less friction against moving surfaces. Metal foil packings are durable, but are not resilient, so the shaft must run fairly true, and lubrication is an important consideration in their application. Brass valve stems or any valve stem where the base metal of the valve stem and packing is of similar material should not contain metallic packings. Additionally, these packings are common and effective for high-temperature and high-pressure applications.
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