Ball Valves for Fire Exposure
[author：topsvalve] [date：10-11-24]
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The soft seals for seat and stem commonly used in ball valves will disintegrate if the valve is exposed to fire for a long enough period. If such valves are used for flanunable fluids, they must be designed so that loss of the soft seals due to an extemal fire does not result m gross mtemal and extemal valve leakage. Such designs provide emergency seals for seat and stem that come into operation after the primary seals have failed.
The emergency seat sealmay be provided by a sharp-edged or chamfered secondary metal seat in close proximity to the ball, so that the ball can float against the metal seat after the soft seating rmgs have disintegrated.
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Figure 3-67. Antistatic Deyice for Grounding Stem to Ball and Stem to Body The stuffing box may be fitted with an auxiliary asbestos-based or pure graphite packing, or the packing may be made entirely of an asbestos compound or pure graphite.
Numerous standards have been established that cover the requirements for testing and evaluating the performance of soft-seated ball valves when exposed to fire. The three basic standards are BS 5146, API 607, and API RP 6F.
BS 5146 is a derivative of OCMA FA VI, which itself was taken from a test specification created by Esso Petroleum in the U.K. This test differed from all former ones by requiring the valve to bein the open position during the test and by using a flammable liquidin the valve. The test owes its origin to the recognition by Esso that, in an actual plant fire, a significant number of valves may be in the open position and must be subsequently closed.Moves are under foot by a number of standard organizations to arrive at an international fire-test specification.
Besides paying attention to the fire testing of ball valves in systems handling flammable fluids, similar attention must be paid to the effect of fire on the entire fluid handling system including valves other than ball valves, valve operators, pumps, filters, pressure vessels, and, not least, the pipe flanges, bolting, and gaskets.
Fire-tested ball valves are referred to as fire-safe. However, this term is unacceptable to valve manufacturers from the product liability standpoint.