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Assessment of Flexible Lines for Flow Induced Vibration

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The purpose of this document is to establish the analytical methods for determining whether a given design of an annular convoluted metal bellows or flexhose is susceptible to flow-induced vibrations. These analytical methods include predicting the excitation flow range, frequency, and corresponding stress resulting from only flow-induced vibration loads. This then leads to prediction of the expected life of the bellows or flexhose, with a final objective of achieving a theoretically infinite life for flow-induced vibrations.

The analytical assessment in this document shall be performed on all flexible lines consisting of formed annular convoluted metal bellows or flexhose, except those contained in paragraph 1.2, regardless of fluid velocity. It does not consider other bellows or flexhose configurations such as welded disc, ring reinforced, toroidal, etc. For those type configurations which do not fit this analysis, some other approved analysis or testing must be done.

The analytical model does not account for changes in the flexible line during thermal transients. Therefore, the assessment shall be performed three times on each flexible line in a application where its length changes as follows: First, for the flexible line in its free length; second, for the flexible line in maximum thermal compression; third, for the flexible line in maximum thermal extension.

The analytical method in this document was developed only for metal bellows and flexhoses manufactured with formed annular convolutes, as shown in Figure 2. These are the most commonly used type in propellant systems. The analytical model was developed in reference 1. The equations in reference 1 were empirically derived from extensive testing and are the basis of this document.

CAUTIONARY NOTE: The analysis in this document was developed for normal flexible line installations. It does not allow for installations where unusual flow disturbances exist (except for elbows located upstream of the flexible line) or for multi-phase flows.

CAUTIONARY NOTE: This document is intended as a tool for analyzing only one portion of the total design of a flexible line. The engineer must consider all possible load sources other than flow-induced vibration when determining the total system life of the flexible line (see paragraph 3.0). The engineer must also consider other requirements (stability, pressure capability, etc.) not covered by this document in the design of a flexible line.
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Philip J.
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Brenda K Bailey
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