Valve Magazine - Summer 2012

service temperature up to 550° F (288° C). 3 Meanwhile, a 15% glass-filled/85% virgin PTFE can handle a service temperature up to 518° F (270° C). 4 This means that if the normal operating temperature of a process is between 518° F (270° C) and 550° F (288° C), a failure can take place in that soft good if it was originally specified as a 25% glass-filled but was replaced by a 15% glass-filled.

BEYOND

SEALANTS

Rubbers are very common and are widely used as sealants. Nitrile rubber is one example. The general term for such a seal is acrylonitrile butadiene copolymer. The percentage of acrylonitrile in nitrile rubber varies between 18% and 50%. This can have a significant influence on the physical properties of the finished nitrile rubber material. 5

The higher the acrylonitrile percent-
age in nitrile rubber, the better the
resistance to oil and fuel. At the same
time, elasticity and resistance to
temperature environments; therefore,
the Belleville material chosen should
not only be high strength, but able to
withstand the environment without sig-
nificant property changes. Also, the
material needs to be commercially
available in a form conducive to manu-
facturing the spring. Finally, potential
materials costs are always an impor-
tant consideration.

Figure 4. Load vs. deflection

compression set is adversely affected by
that percentage. Figure 1 shows the dif-
ferent percentages of acrylonitrile and
the effects on both the resistance to oil
and the elasticity. 6 This means that if a
nitrile rubber with 50% acrylonitrile
was replaced by a nitrile rubber with
20% acrylonitrile, the resistance to oil
and fuel will increase and the prob-
ability of swelling will increase rapidly.

IN CONCLUSION Manufacturers select soft goods in valves and actuators to suit the applications for which they are designed. Replacing those soft goods with products that are locally fabricated can be risky because of the different properties of different filling percentages, a situation made more complicated because those filling percentages are not usually published and provided in the manufacturers’ BOM. VM

GARY BURROWS is senior applications engineer for Emerson Process Management–Valve Automation–Bettis. He has 30 years of experience in actuator and valve design, field service and application engineering. Reach him at gary.burrows@ emerson.com.

AMR ATIAH is an instrumentation engineer at Saudi Aramco–Ras Tanura Refinery. He has been in the instrumentation field 10 years and is currently on an 18-month internship assignment with Emerson Process Management. Reach him at amr.atiah@aramco.com.

REFERENCES

1. www.buckeyerubber.com/ProdDisc. htm#Teflon

2. www.darcoid.com/images/uploads/pdfs/ Fluid%20Power%20Sealing% 20 Materials% 20(PTFE%20-%20Filled% 20&%20Non-Filled).pdf

3. www.k-mac-plastics.com/data%20sheets/ teflon-25%25GF-Data-Sheet.htm

4. www.matweb.com/search/datasheet. aspx?matguid=313108f956a6497d8883d 535c753dac5&ckck= 1

5. www.parker.com/literature/ORD% 205700%20Parker_O-Ring_Handbook. pdf

6. www.123seal.com/seal-gasket/wp-
content/uploads/HYDRAULIC-GROUP_
resize1.jpg
Engineers commonly attempt to select
a material that matches the body or
hardware material used to construct the
valve. However, this is not always a suit-
able spring material. For instance, a
valve that employs 316 stainless steel for
the body material does not mean that
same material would be a good selection
for the Bellevilles because 316 stainless
steel has a low yield strength unless it is
cold worked. This limits the available
material thickness. Also, the material
may be difficult to source in this form,
which can drive up cost and manufactur-
ing lead times. Another issue is that the
material may be more susceptible to
stress corrosion cracking in its strength-
ened condition. In that case, it might be
necessary to employ more exotic alloys in
corrosive applications.

THE GEOMETRY

Bellevilles are commonly described by
their inside diameter (ID), outside diam-
eter (OD), thickness (T) and deflection
(h). Once these dimensions and material
are known, the load versus deflection
may be modeled. As the spring is loaded
axially, it often will deflect linearly until
it is extremely close to flat. The load
will then increase exponentially until it
reaches flat.

CONCLUSION

Belleville springs are used in various valve applications. The most common purpose is to maintain load on packing, gaskets, ball seats or any other seal, a practice referred to as “live loading.” Although it is important to consider the limitations of any Belleville design, live loading itself can be very effective when the right choices are made. VM

GEORGE DAVET, BSME, MBA, is vice president/chief engineer for the Belleville Springs and Pressure Switch Divisions for Solon Manufacturing Company ( www.solonmfg.com), located in Chardon, OH. Reach him at gdavet@solonmfg.com.