BALL VALVES ...VALVES & ACTUATORS
bushing or bearing. The ball still seals against a squeezes the seat against the ball. There are generated by the differential pressure across the
seat on the low-pressure side, but the trunnions several inherent difficulties in this design for ball. In the case of a high- pressure seal, the
bear most of the load and protect the seat. As a alternative fuel applications. First, pressure from O-ring or spring will provide pressure in addition to
result, the trunnion design can perform well in high- the end screw is static; there can be no means of the gas stream’s, ensuring a better seal under
temperature, high-pressure applications. Under increasing the pressure once it is set. these tough conditions. And in the case of a
these same conditions, the free floating ball design The crush-type design depends on the load damaged seat, the dynamic spring or O-ring
could transfer too much load to the seat, causing created by the original set conditions. In alternative provides extra load to create a seat seal, enabling
damage. On the other hand, trunnion designs fall fuel service situations, the crush-type design must the valve to accommodate a few more cycles
short in very cold, low-pressure applications depend on the elasticity of the seat – its capacity before the seat will have to be replaced.
because the design is not free to compensate for to rebound toward its original shape – to make the While it will compensate for moderate changes
contraction and stiffness in the seat. seal, which may not be sufficient for an effective in temperature and pressure, the energised seat
If designed well, both the free floating and seal. Without manual adjustment to the end screw, design does not perform well on the outer limits of
trunnion designs will have low actuation forces. In there is no way to increase sealing force. the temperature and pressure range for natural gas
the trunnion design, bushings fitted to the applications.
trunnions distribute bearing load evenly over a Correcting for damage The live-loaded design is an improved version
large area. If the bushings are made of a hard, A second difficulty is that the crush-type design of the energised design. Working under the same
lubricious (low friction coefficient) material, such will not apply enough pressure to correct for principles, the live-loaded design employs both a
as PEEK, turning the handle will be virtually damage to the seat. Therefore, when the media is spring and an O-ring to create load on the seat. It
effortless, even under high-pressure conditions. In aggressive, as in a CNG application, cycles may be also uses the differential pressure across the seat
the free floating ball design, the seat is both the limited to one or two before the seat will need to to generate more sealing force. Live loading
seal and the bearing. Ease of actuation depends be replaced. In alternative fuel service provides more performance over the energised
largely on the lubricity of the seat material and the applications, seats are exposed to aggressive designs and is highly effective in compensating for
surface area. A large seat area ensures that the media at high velocities, so it is imperative that wide variations in temperature and pressure. On
load will be widely distributed and actuation will valves contain mechanisms for compensating for the outer limits of the temperature and pressure
not be difficult. the inevitable damage. Damage to the seat may range for natural gas vehicles, the live-loaded
For a seat to be effective, the materials of result from: design performs admirably. Below -29°C, the
construction must be consistent with the Debris in the gas stream spring may lose its elasticity but the larger force
application. One very effective seat material is Hydrates, which are crystalline structures in generated by the differential pressure across the
PEEK or filled PEEK. It demonstrates greater the gas stream caused by pressure drop O-ring provides sealing force to the ball or the seal
stability and consistent characteristics over the full Throttling, which occurs when the valve is and enables a load to be transferred to the seat. At
temperature range (-53°C to 232°C ). It is harder neither fully closed nor open, and high temperature and high pressure the O-ring
and tougher than other seat materials, and Cycling the valve under high pressure, which and the spring work together to provide adequate
recovers well from damage that may be caused by may cause an irregularity on the seat. pressure to make this difficult seal.
cycling, throttling, or contamination. In addition, it Pressure is the main means of compensating for Regardless of the design, seats must be
has a low friction coefficient for ease of actuation. damage to the seat. If the seat is made of PEEK, replaced periodically. The cost of seat replacement
Seat designs also differ according to the filled PEEK, or another moldable material, it will should figure into your choice of a ball valve.
method used to apply pressure to the seat. While tend to further deform and reseal under sufficient In reviewing key issues with regard to the
the fluid stream itself provides the primary force to pressure from the ball pushing downstream. design of the stem, ball, and seat, keep in mind
generate the seal, additional force may be needed The energised seat design constitutes a that the primary failure mode is always leakage.
to allow for a wider range of applications. This significant improvement over the crush-type Other matters may come into play, but what
force comes from added components used in the design. It employs either a spring or O-ring – is most important is a leak-tight seal.
seat design. In the crush-type design, loading inserted between the end screw and the seat –
Swagelok
force comes from the end screw. When it is which becomes a ready source of dynamic energy
www.swagelok.com
tightened and set in place, the end screw and supplies an augmenting force that is
Liquids and Gas Handling
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