Valve stem sealing principle
Time:
2023-03-31
Stem seal - fugitive emissions
The valve stem packing commonly used in process pipelines is a flexible graphite. The typical design method for new valve packing is to place several pressed packing rings, place a woven material ring above the packing ring, and place a woven material ring at the bottom of the stacked packing. This woven material "wipe ring" helps prevent the pressed packing ring from being squeezed out of the stuffing box. Compared to previously used asbestos materials, current valve packing technology effectively prevents light hydrocarbon leakage (fugitive emissions). Key factors affecting the performance of flexible graphite packing materials include the percentage content of carbon, packing density, and corrosion/oxidant resistance. In order to improve the performance of the valve stem packing, the flexible graphite packing system for lifting and lowering valve stem valves has been well discussed in MSSSP-120. Fillers from API 622 should be considered.
Although graphite materials are prone to oxidation at temperatures above 343 ℃ (650 ℃), they have been successfully used in limit gates that use temperatures greater than or equal to 538 ℃ (1000E).
The sealing performance of fillers under dynamic loading and overtime conditions may be reduced due to "packing compaction". This reduces the gland load and leads to packing leakage. This "dynamic loading" process may be a reasonable choice, including a packing gland loaded by a disc spring, providing a method of maintaining the packing gland load to help extend the effective use time of the packing.
The stem of some 1/4 turn valves may be sealed with an elastic O-ring. These sealing rings may have a lower temperature range than PT-FE.
Bellows sealed valves have been used in steam and certain hazardous/toxic conditions to reduce potential packing leakage. In API 602, the nominal size of the valve is NPS 1/2-2. API 602 includes design methods and test details for bellows stem seals in addition to traditional packing.
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