- Published on Monday, 25 June 2012 08:49
- Written by George P. Davet
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 retrofitted 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.
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.
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 seat must be within a certain range for the seat to be effective. If the load is too low, leakage is possible. If the load is too high, excessive wear will occur or the valve will not operate. Often, the seat will have little elasticity (especially when made of metal). Since the valve components are generally machined to a set of tolerances, it is difficult to guarantee the seat is properly loaded.
The ball seat can be live loaded with a Belleville to ensure that the load is within the proper range. Since these loads are usually much lower than those used to seal packing stems or gaskets, the spring stresses can be lower. Some Bellevilles with very low stresses can produce a load curve with a rate of nearly zero. This means that as the spring deflects, there is little change in load, a desirable quality for a spring used in this application since a specific load can be maintained through a wide range of tolerances.