Page 122 - Valve Selection Handbook

Manual Valves 109

can float against the metal seat after the soft seating rings have disinte-
grated. 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,
andAPIRP6F.
BS 5146 is a derivative of OCMA FS VI, which itself was taken from
a test specification created by Esso Petroleum in the U.K. This test dif-
fered from all former ones by requiring the valve to be in the open posi-
tion during the test and by using a flammable liquid in 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 orga-
nizations 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 stand-
point.
Multiport Configuration

Ball valves adapt to multiport configurations in a manner similar to
plug valves, previously discussed on page 98.

Ball Valves for Cryogenic Service

Ball valves are used extensively in cryogenic services, but their design
must be adapted for this duty. A main consideration in the design of these
valves is the coefficient of thermal contraction of the seat ring material,
which is normally higher than that of the stainless steel of the ball and
valve body. The seat rings shrink, therefore, on the ball at low tempera-
tures and cause the operating torque to increase. In severe cases, the seat
ring may be overstressed, causing it to split.
This effect of differential thermal contraction between the seats and
the ball may be combated by reducing the installed prestress between the