Valve Magazine - Spring 2012

DESIGN CONSIDERATIONS Some of the factors to consider when selecting Belleville springs are:

Figure 3. Ball seat load

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.

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.

Actuators or Mechanisms

Some low-stress Bellevilles can be designed to “snap over.” This is similar to an oil can effect. Relief valves can be designed with snap-acting Bellevilles to open the valve quickly at a desired load.

The ball seat can be live loaded with
a Belleville to ensure that the load is
within the proper range. Since these
Stacked Bellevilles also may be used
anywhere a spring rate is required
because they can produce a variety of
load characteristics. This is especially
true in cases where the maximum load
for a coil compression spring is too low.

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Loading/Stacking

Bellevilles can be stacked in series or stacked parallel to alter the loading characteristics. Two springs stacked in parallel doubles the load required to flatten the springs with no deflection increase. Two Bellevilles stacked in series will produce twice the deflection for the same load. The parallel/series arrangements also may be combined, but the most efficient use of material is to employ series stacks, which reduces any sliding friction between springs (Figure 4).

Materials

Bellevilles are often highly stressed; thus, the material selection is an important factor. Valves are often subjected to corrosive and/or extreme

MSS/ANSI Co-branded Standards

MSS has other standards currently undergoing the ANSI canvas process and awaiting
approval as co-branded
MSS/ANSI standards.

For further information on MSS membership or MSS standards, contact Bob O’Neill, MSS executive director at 703/281-6613, or on the web at www.mss-hq.com. MSS is an ANSI accredited standards developer.

www.mss-hq.org.

MSS SP- 55 Quality Standard for Steel Castings for Valves,

Flanges, Fittings, and Other Piping Components,Visual Method for Evaluation of Castings. This highly regarded standard practice provides reference photographs of surface irregularities commonly found in steel castings.

MSS SP- 58 Pipe Hangers and Supports- Materials, Design, Manufacture, Selection, Application, and Installation. This standard practice provides guidelines for all phases of pipe hanger design and usage.

MSS SP-114 Corrosion Resistant Pipe Fittings, Threaded and Socket Welding Class 150 and 1000. This standard practice establishes requirements for the design of class 150 and 1000 corrosion resistant fittings including materials, pressure-temperature ratings, dimensions, and testing.