Cyclic Deflection and Cycle Life

Most deflections are repeated a number of times during the life of the piping system, since the deflections usually are produced by changes in temperature which occur each time the system is started and stopped, and from predictable variations in the way, the system is used. Repetitions can also occur as a result of repetitive mechanical movements and from vibrations. Each time a deflection occurs it is a CYCLE. The number of cycles is important to assure the proper design of the pipe expansion joint since each design has a finite, but predictable life.

Vibrations which cause repetitive deflections can cause premature failure of a pipe expansion joint. Even though these deflections may be small in magnitude, they usually accumulate huge numbers of cycles in a short period of time. Since the bellows are metallic structures, they have specific and predictable resonant frequencies, like the pitch of a tuning fork. When driven by outside vibrations of the same frequencies (or harmonics of them), they can magnify the incoming deflections until they exceed the yield strength of the bellows material and induce early fatigue failure. When a piping system is known to have equipment which can produce vibrations, such as pumps, fans, and other motor or turbine driven devices, their rotational speeds or frequencies should be stated so that PT&P’s designers can assure that the proper pipe expansion joint does not have a resonant or harmonic frequency that is close to those.

CYCLE LIFE AND RATED MOVEMENT – SINGLE EXPANSION JOINTS

PT&P’s single pipe expansion joints are designed for 3000 cycles for any one tabulated movement shown. Cycle life may be increased, or movement may be increased (decreasing the cycle life) by utilizing the graph located on this page. The graph may also be used for superimposing more than one movement condition on the bellows at one time. It is important to remember that the movements shown in the bellows data pages do not allow for any installation misalignment. A proper specification for a bellows pipe expansion joint should reflect what the actual system movements will be. To ensure the highest quality bellows for the least cost, all movement conditions must be taken into account.

Examples of movement conditions that typically occur in a piping system are:

Installation Allow one cycle installation misalignment./p>
Start-Up/Shut-Down Movements produced between the minimum ambient temperature and the maximum start-up temperature. This is normally a very low number of cycles.
Operating Movements produced due to temperature fluctuations during system operation. This is typically a very small movement for a very large number of cycles.
Excursion Movement resulting from a worst-case/ upset scenario, occurring during system operation. This is normally a very low number of cycles.

How To Use Graph

Enter the graph horizontally at the number of anticipated cycles or the movement condition being considered.
Move vertically until the graph is intercepted.
>Move horizontally and read the conversion factor.

Example: Enter graph at 1,000 cycles; read C. F. of 1.

Note: Maximum increase in movements for combined movement conditions shall not exceed 110% of catalog rated movements
Note:Please reference the print version of the U.S. Bellows, Inc. Expansion Joint Catalog for reference tables containing technical data on Expansion Joints.

CYCLE LIFE AND RATED MOVEMENT- UNIVERSAL PIPE EXPANSION JOINTS

PT&P’s universal pipe expansion joints are designed for 3000 cycles for any one tabulated movement shown. Cycle life may be increased, or movement may be increased (decreasing the cycle life) by utilizing the graph shown on this page. The graph may also be used for superimposing more than one movement condition on the bellows at one time. It is important to remember that the movements shown in the bellows data pages do not allow for any installation misalignment. A proper specification for a bellows pipe expansion joint should reflect what the actual system movements will be. To ensure the highest quality bellows for the least cost, all movement conditions must be taken into account.

Examples of movement conditions that typically occur in a piping system are:

- INSTALLATION Allow one cycle installation misalignment.
- START-UP/SHUT DOWN Movements produced between the minimum ambient temperature and the maximum start-up temperature. This is normally a very low number of cycles.
- OPERATING Movements produced due to temperature fluctuations during system operation. This is typically a very small movement for a very large number of cycles.

EXCURSION Movement resulting from a worst-case/ upset scenario, occurring during system operation. This is normally a very low number of cycles.

How to Use Graph

Enter the graph horizontally at the number of anticipated cycles or the movement condition being considered.
Move vertically until the graph is intercepted.
Move horizontally and read the conversion factor.

Example: Enter graph at 1,000 cycles; read C. F. of 0.80

Note: Maximum increase in movements for combined movement conditions shall not exceed 110% of catalog rated movement

Note: Please reference the print version of the U.S. Bellows, Inc. Expansion Joint Catalog for reference tables containing technical data on Expansion Joints.

Cycle Life and Rated Movement: Externally Pressurized Pipe Expansion Joint

Typical movement conditions that occur in a piping system are:

Installation: allow one cycle for installation misalignment
Start-up / Shut down: movements produced between the minimum ambient temperature and the maximum start-up temperature. This is normally a very low number of cycles.
Operating: movements produced due to temperature fluctuations during system operation. This is typically a very small movement for a very large number of cycles.
Excursion: movement resulting from a worst-case/upset scenario, occurring during system operation. This is normally a very low number of cycles.

U.S. Bellows externally pressurized pipe expansion joints are designed for 1000 cycles for any one tabulated movement shown. The movement may be increased, decreasing the cycle life, by using the graph below. This graph may also be used for superimposing more than one movement condition on the bellows at one time.

Using the graph: example: 2500 cycles reads C.F. of 1.23

Enter the graph horizontally at the number of anticipated cycles or the movement condition being considered
Move vertically until the graph I intercepted
Move horizontally and read the conversion factor

Note: Maximum increase in movements for combined movement conditions shall not exceed 130% of catalog rated movements.

Note: Please reference the print version of the U.S. Bellows, Inc. Expansion Joint Catalog for reference tables containing technical data on Externally Pressurized Expansion Joints

How To Select and Order
The first step in selecting the proper pipe expansion joint assembly is to collect and organize as much information regarding the design and operating parameters of the system as possible. As a minimum the following information should be provided:
Size of the line Design pressure Design temperature Design movements and required cycle life Overall length requirement, if any Type of end connections	Bellows material Pipe material Flange specification and material Type of expansion joint Accessories

EXAMPLE

1. Assume the following pipe expansion joint design criteria:

a) Line size: 12-inch nominal diameter

b) Design/operating pressure: 125 PSIG

c) Design/operating temperature: 353°F

d) Movements/cycle life

Installation: 0.25-inch compression; 0.25-inch extension

Start-up/shut down: 6.0 inches compression; 250 cycles

Operating: 1.0-inch compression; 2500 cycle

e) End connections: 150 lb ANSI raised face slip-on flanges: ASTM A-105 (carbon steel)

f) Bellows material: B-168/lnconel 600

g) Type: Single

h) Accessories: Drain and purge

2. Turn to the Conversion Factor Graph, and calculate equivalent catalog movements.

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Total = 5.80

3. Refer to page 93, 12-inch nominal diameter single externally pressurized pipe expansion joints, and go to the lowest pressure rating that is equal to or greater than specified design pressure (125 psig) which will be the 150 psig rated pipe expansion joints. Compare the total equivalent catalog movements to those shown in the catalog, keeping in mind that the tabulated movements shown in the catalog are for 1,000 cycles for one movement only.4. Select the appropriate pipe expansion joint that meets the movement criteria. Since the total equivalent axial movement is 5.80-inches, the unit that is rated for eight inches is the correct selection.5. Include the designations for the type (XS for XFlex/single), drain connection (D), and purge connection (P).

6. The resulting PT&P part number would then be as follows:

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