VWAM nov 2.indd - Solon Manufacturing Co.

Tech Talk
Using Belleville springs on ball valves
By George Davet, BSME, Vice President and Chief Engineer Solon Manufacturing Company
There are many benefits to using Belleville
springs in or adjacent to valves for a variety
of purposes. Particularity in ball valves, when
Belleville springs are added to the gland as
well as the seat spring, they ensure that the
bolt preload is retained to immensely reduce
fugitive emissions. The load around the ball is
also equalized and seal is upheld while withstanding temperature and pressure changes,
as well as general seat wear.
Before examining the benefits of the
Belleville spring on ball valves, it is important
to understand the Belleville spring. Belleville
washer, conical washer, disc spring, coneddisc spring, etc. are other names for the same
product. It is essentially a washer with a conical shape that produces spring characteristics when loaded axially. The term Belleville
is derived from the initial patent holder,
Julian Belleville, who originally designed the
Belleville spring to be used in valves.
dimensions and the material are known, the
load versus deflection may be modeled. As
the spring is loaded axially, often it will deflect linearly until it is extremely close to flat.
The load will then increase exponentially until it reaches flat. Low stress Bellevilles will
exhibit a non-linear load versus deflection
curve. Belleville springs can also be designed
to provide an optimal spring rate for the performance requirements of the application.
Live Loading Ball Seats
Ball seats provide a seal between the ball
and the valve’s body. An entire column can
easily be devoted to a seat design discussion
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
The Anatomy
of a Belleville Spring
Bellevilles are commonly described by their
inside diameter (ID), outside diameter (OD),
thickness (t), and deflection (h). Once these
metal seats). Since the valve components are
generally machined to a set of tolerances, it
is difficult to guarantee the seat is properly
loaded.
Ball Seat Load
vs. Deflection Curve
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
Load
Why Belleville Springs?
w
Lo
Curve
Load
ess
r
t
S
L
ical
Typ
h
ID
urve
Deflection
OD
t
C
oad
H
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. This is a desirable quality for a spring used in this application since a specific load can be maintained
through a wide range of tolerances.
off flow in either forward or reverse direction.
Another benefit of using Belleville springs on
both sides of the seat is to equalize the load,
while also compensating for seat wear, due
to pressure and temperatures changes. There
are two types of ball valves based on the ball
type: floating ball valve and trunnion mounted ball valve.
Belleville Springs
on Floating Ball Valves
Floating ball valves have a floating ball inside
the valve body. The ball is able to move along
the axis, but does not have any freedom to
rotate against the stem. Belleville springs are
utilized in this application around the seat
springs. When open, the ball drifts toward
the downstream side and tightly pushes
against the seat to ensure sealing reliability.
Under the typical low to medium pressure
application, seals are created by the Belleville
spring-loaded seats pushing against the ball.
Belleville Springs
on Trunnion Ball Valves
Unlike the floating ball valve, the ball of
a trunnion ball valve is fixed. Trunnion ball
valves are used in high pressure applications
and in large sized valves. The ball remains
centered on the shaft within the downstream
Placement of Belleville
Springs around the Seat
Many seat designs within a ball valve incorporate two Belleville springs. The first one is
typically installed on the downstream side
of the seat, with the other one behind the
upstream bearing ring. Using two Belleville
Springs on either side of the seat can form a
seal regardless of the flow direction. This allows for a uni-directional shut-off valve, used
in high pressure applications, or a bi-directional shut-off valve, which allows for shut-
Believe in Bellevilles
Solon Belleville Springs ®
Reduce valve and flange leaks to help you meet
your regulatory requirements.
Benefits:
Reduces total cost of ownership
Reduces maintenance and downtime
Extends packing life
Minimizes fugitive emissions
Solon Disc Springs
Solon Flange Washers
Solon Belleville Springs
Various Materials and Sizes In Stock
Temperature Ranges -240ºC to 650ºC
Visit Solon at Valve World Germany 2012 and Americas 2013
®
Belleville Springs
Reducing Fugitive Emissions
Solon Manufacturing Co.
Chardon, Ohio USA
ISO 9001:2008
440-286-7149
Certified
www.solonmfg.com
[email protected]
Belleville Springs
Loaded on Seats
Belleville seat spring
and the upstream
Belleville seat spring.
As
the
pressure
builds downstream,
a Belleville spring is
used on the downstream seat, creating
a seal. The Belleville
spring on the upstream seat also flexes with the pressure
movement of the
seat and maintains
the upstream seal.
Under
extremely
high temperatures
and pressures, such
as in nuclear applications, the use of
Belleville
springs
compensate for the
differential
thermal expansion of
the internal parts at
elevated temperatures by absorbing
the heat expansion.
They provide the initial sealing force in
applications relating
to gradual building
Valve World Americas - November 2012 22 www.valve-world-americas.net
VWAM nov 2.indd 22
17-12-12 21:55
Tech Talk
Live Loaded Gland
Seat Spring
pressures. Therefore, Belleville springs on
ball valves ensure that the valve will not get
stuck and can open and close with ease under these abrasive conditions.
Live Loading
on the Gland Follower
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
utilize studs or bolts to exert load onto the
gland follower, which loads the packing. As
the valve is operated, some of the seal material is lost during each operation. This material loss is called packing consolidation. High
temperature and thermal cycling can accelerate packing consolidation. 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 quickly.
Belleville Springs are used to maintain load
on the gland follower. They 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.
Best Materials of Belleville
Springs to use on Ball Valves
One would think that the obvious choice
would to match the Belleville material to the
valve body material. However, this rarely is
a good choice. Bellevilles are highly stressed
parts while the stresses in the valve bodies
are much lower. Therefore, it is important
that the spring material has a high yield
strength. This greatly reduces the candidate
materials. In addition, the spring may be exposed to high temperatures and/or corrosive
media. The spring material must be capable
of withstanding the temperatures and environment under stress without failure over
time. Below are some factors to consider
when selecting a material:
1. What are the minimum and maximum
temperatures?
This is not necessarily the process temperature. The springs may be removed from
the process and so the temperature at the
spring may be different from the process.
Once the temperature is known, the list of
candidate materials will be narrowed based
on the effect the temperature will have on
the mechanical properties. Since each spring
is designed based on the material properties,
it may be necessary to change the design to
accommodate the temperature extremes.
2. What is the environment?
Any time spring material is highly stressed,
corrosion assisted failure is a concern. Some
materials are more susceptible to cracking
in certain environments than others. As expected, the best alloys for highly corrosive
environments are often the most costly.
However, in some cases springs may be protected (plating) or the design may be altered
to reduce stresses to an acceptable level.
3. How fast are the parts needed and what
is an acceptable price?
Many designs may not be available as a
standard. This will always increase lead time.
In addition, some raw materials may meet all
of the performance requirements but will be
unacceptable due to cost or lead time. These
are factors that should be considered at the
design stage.
Questions the Belleville
Supplier will Ask
In order to suggest or design a spring for any
given application, the manufacturer of the
spring will ask certain questions about the
application’s requirements:
1. What are the space constraints?
This includes ID, OD, and height allowance.
2. What is the aim preload for the spring
and the aim deflection?
Each spring design will have a certain load at
a given installed height. Each spring design
will have a certain load at a given installed
height. The aim preload will be based on the
force that is required to seal the seat, packing, gasket, etc. Manufacturing and assembly
tolerances will result in a variation in installed
height for the spring. This will change the resultant preload. The spring design should be
evaluated based on whether this variation in
load is acceptable for the application.
3. What is the operating environment for
the spring?
This includes issues temperature and
corrosion.
4. What is the commercial requirement?
The quantity and lead time requirements
should be considered at the design stage
since these factors affect manufacturing processes, material selection, etc.
Summary
Ball valves may leak due to a variety of reasons. One of the most prevalent causes is
due to a loss of bolt preload. When Belleville
springs are added to the gland as well as the
seat spring of a ball valve, they chiefly ensure
that the bolt preload is retained to greatly reduce fugitive emissions. In addition to maintaining the load, the addition to Belleville
springs on a ball valve can extend the life of
packing, improve performance, and reduce
the overall cost of ownership.
About the author
George Davet, BSME is Vice President and
Chief Engineer for Solon Manufacturing
Co., a manufacturer of Belleville Springs
and Pressure Switches in Chardon, Ohio.
Reach him at [email protected] or
800-323-9717.
Valve World Americas - November 2012 23 www.valve-world-americas.net
VWAM nov 2.indd 23
17-12-12 21:55