Metal Bellows Correctedn 7/11/00 2:40 PM Page 9
-––
[
HOW TO INTERPRET A
SENIOR FLEXONICS PATHWAY
BELLOWS DESIGN ANALYSIS
SM
]
–-–
All custom bellows designs should be documented to prove that the critical stress values are within the limits
of the selected design code. Documentation should prove that the design is safe and mechanically stable, and
that the cycle life is in accordance with the specification requirements. The Senior Flexonics Pathway bellows
design analysis shows all the critical information in a summary format. The following explanation is offered
to help customers interpret the information that is shown on the Pathway bellows design analysis so the
information is more meaningful.
This is the actual temperature used for the bellows design. For
certain applications such as refractory lined expansion joints,
the bellows is often designed for a lower temperature than the
media.
This is the allowable primary stress for the bellows material at
the design temperature.
This is the modulus of elasticity of the bellows material at the
design temperature. The room temperature modulus of
elasticity is used to calculate the deflection stresses (S5 & S6).
The longitudinal weld joint efficiency varies based on the
method of inspection and the specified code.
The design movements create the deflection stresses that
determine cycle life. One complete cycle is based upon
moving the bellows from the installed position to the
maximum specified movement and then back to the installed
position.
Material thickness is stated as the standard sheet gauge
thickness.
Hoop Stress (S2) is a critical membrane stress that runs in the
circumferential direction. The S2 value must be lower than the
allowable stress for the bellows material multiplied by the
bellows longitudinal weld joint efficiency.
Pressure Bending (S4) is a critical bending stress that is located
in the sidewall of the convolution running in the longitudinal
direction. It is the stress that makes a “U” shaped convolution
balloon out into an omega shape. The value of S3 + S4 must
be limited to 1.5 times the allowable stress for annealed
The proposed design has this bellows and 3 times the allowable stress for bellows in the
calculated cycle life at the specified asformed condition.
The bellows effective area is the area of conditions.
the bellows that creates pressure thrust
when acted upon by the operating
Deflection Bending (S6) is the primary bending stress
pressure. The system anchors and/or the There are two types of squirm or instability
influencing fatigue life. This stress runs in the longitudinal
hardware on the expansion joint must that can occur for internally pressurized
direction and is most severe near the convolution crest or root.
be designed to withstand pressure thrust bellows. One is called column squirm
Since bellows operate in the plastic range, the value of S6 is
at the operating and test conditions. (similar to buckling of a column) and the
generally well above the allowable code stress value. It is a
other is called in-plane squirm (localized
theoretical calculation based on elastic theory, and the value is
plastic deformation). Senior Flexonics
used to calculate cycle life.
Pathway calculates the maximum stability
Torsional spring rate is offered for those
pressure based on the lower of the two
This is the specified cycle life value. If ASME B31.3 or ASME
pipe stress analysts who are inputting
values. The design stability pressure is the
Sect. VIII are specified, this value should realistically represent
bellows characteristics into a pipe stress
predicted squirm pressure divided by a
the actual number of cycles the bellows will experience in
program. Bellows are not generally
safety factor of 2.25.
service. This is typically in the hundreds of cycles, not
designed for torsional movements. But,
thousands.
the torsional stiffness value can affect the
The Senior Flexonics Pathway spring rate
output of a pipe stress analysis that
calculations are based on the initial elastic
includes an expansion joint.
spring rate criteria from EJMA.
9
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93