BEYONDVALVES
BY GEORGE P. DAVET
Belleville Springs in Valves
Belleville springs are used within or adjacent to valves for a variety of
purposes—the most common being to
maintain load on a seal, gasket or packing. They often are used as machine elements in valve actuators or as a component of the valve mechanism and are
designed or arranged to provide useful
mechanical properties within small
spaces. This means they can be retrofit-ted into existing valve designs with minimal revisions. Clearly, these mechanisms serve a vital purpose. However,
many important factors should be considered in selecting a spring.
But even before exploring the design
and selection considerations, it is
important to understand the Belleville
spring itself, which is also known as a
Belleville washer, conical washer, disc
spring, coned-disc spring and other
terms. It is a washer with a conical
shape that produces spring characteristics when loaded axially. The term
Belleville is derived from its inventor,
Julian Belleville. The original application was for a valve.
TODAY’S APPLICATIONS
Among the uses for this mechanism
today are:
Live Loading of Packing
One common application for Bellevilles
in valves is live loading of packing. The
term “live loading” means using a
spring to maintain load on a seal. Most
of these valves use studs or bolts to
exert load onto the gland follower,
which loads the packing (Figure 1). As
the valve is operated, some of the seal
material is lost during each operation.
High temperature and thermal cycling
can accelerate this loss. As this occurs,
preload is lost on the seal. Once the preload falls below a certain threshold, a
leak will occur. Since the stretch in the
stud is small, loss of preload can occur
rapidly.
Bellevilles are used to maintain load
on the gland follower. The springs are
typically arranged in a stack on the stud
to increase the elasticity of the system.
Seal materials are lost over time; however, the live-loaded stud will lose less
load because the deflection of the spring
stack is significantly greater than the
stretch of the stud alone. If load is
maintained on the seal, a leak is much
less likely to occur.
Figure 1. Stacking of Belleville springs on
the gland follower for live loading
Figure 2. Live loading
Flange/Bonnet Gasket Live Loading
Flanges and bonnets are often sealed
with a gasket and these springs are also
used for live loading that gasket (Figure
2). The gasket’s ability to seal partly
depends on the stress maintained on its
sealing surface. Flange bolts are tightened to a given preload to generate this
sealing stress. As with valve packing,
this original preload is lost over time
because of causes such as differential
thermal expansion and thermal cycling
(Figure 3). Once a certain amount of
preload is lost, a leak can occur.
Bellevilles are used to increase the elasticity of the fastening system to reduce
this preload loss.
Live Loading Ball Seats
These springs are also used with ball
seats. Ball seats provide a seal between
the ball and the valve’s body. An entire
article could be devoted to seat design
because there are so many materials
and styles to consider. The load on the
