Metal Bellows Correctedn 7/11/00 2:40 PM Page 6
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HOW A BELLOWS WORKS
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BELLOWS MOVEMENTS
A bellows is a flexible seal. The convoluted portion of an expansion joint is
designed to flex when thermal movements occur in the piping system. The
number of convolutions depends upon the amount of movement the
bellows must accommodate or the force that must be used to accomplish
this deflection.
The convoluted element must be strong enough circumferentially to
AXIAL
withstand the internal pressure of the system, yet responsive enough to flex.
The longitudinal load (pressure thrust) must then be absorbed by some
other type of device. These devices include pipe anchors, tie rods, hinges, or
gimbal structures. Pressure thrust can be calculated by multiplying the
effective area shown in the catalog by the working pressure.
LATERAL
Pressure Thrust
For the purpose of understanding pressure thrust, a single bellows designed for
pure axial motion can be modeled as hydraulic cylinder with a spring inside.
Force on equipment or adjacent piping anchors “F” = (the effective area of the
bellows) x (the working pressure) + (the spring rate of the bellows) x (the
stroke of the bellows).
ANGULAR
The spring represents the axial spring rate of the bellows. The hydraulic piston
represents the effect of the pressure thrust which the expansion joint can exert on the piping anchors or
pressure thrust restraints (hinges, gimbals, tie rods)
which may be part of the expansion joint assembly.
The area of the hydraulic cylinder would be the
effective area of the bellows. For a 20”, 150 psig catalog
standard expansion joint with 20 convolutions, the
spring force for 1” of axial stroke would be (the axial
motion) x (the spring rate of the bellows) or 1 in. x
1521Ibs./in. = 1521 pounds.
Figure 1. Working pressure acting on the effective The pressure thrust force would equal (the working
area of the bellows.
pressure) x (the bellows effective area) or (150 Ibs./in.
2
)
x (359 in.
2
) = 53,850 pounds.
The pressure thrust force is typically much higher than the spring force.
Expansion joints designed for lateral offset or angular motion are more complicated to model accurately.
However, the effect of pressure thrust is the same.
The pages following the “HOW A BELLOWS WORKS” section describe expansion joints which can be applied
to eliminate the effect of pressure thrust on rotating equipment or other stress sensitive devices.
Pressure Stresses
The ability of a bellows to carry pressure is limited by hoop stress or S2 as defined in the standards of the
Expansion Joint Manufacturers Association (EJMA). This is a stress that runs circumferentially around the
bellows due to the pressure difference between the inside and the outside of the bellows.
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