Views: 0 Author: Site Editor Publish Time: 2023-04-25 Origin: Site
(1) Live Loading of Packing
The term “live loading” means using a spring to maintain load on a seal. Many valves utilize studs or bolts to exert load onto the gland follower, which applies stress to the packing. As the valve is operated, the seal material consolidates 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 and packing stress can occur quickly.
(2) Flange/Bonnet Gasket Live Loading
Flanges and bonnets are often sealed with a gasket. 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. Similar to valve packing, this original preload is lost over time due to effects such as differential thermal expansion and thermal cycling. Once a certain amount of preload is lost, a leak can occur. Therefore, disc springs are used to increase the elasticity of the fastening system to reduce this preload loss.
(3) Live Loading Ball Seats
Ball seats provide a seal between the ball and the valve’s body. An entire article can be devoted to seat design as there are 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, there will be leakage. If the load is too high, excessive wear will occur and/or the valve will not operate. Often, the seat will have little elasticity (especially metal seats). 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 disc spring 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.
(4) Actuators or Mechanisms
Some low stress disc springs can be designed to “snap over.” This is similar to an “oil-can” effect. Some relief valves are designed with snap acting disc springs to open the valve quickly at a desired load.