An assessment method for unstable vibration in multispan tube bundles

Abstract

The flow external to the tubes in a heat exchanger tube bundle may cause large amplitude tube vibration. The most severe mechanism is a fluidelastic instability which can damage tubes in a matter of weeks or months. This mechanism has been the subject of many laboratory experiments and theoretical research over the past 30 years, and consequently, there is a body of data ready to be passed on to those involved in the design of heat exchangers. However, this data is not in a readily usable form because laboratory conditions are very different to those found in a typical heat exchanger where fluid properties vary considerably with location. In order to deal with these variations, an assessment equation is derived which takes into account the changing flow conditions along the length of a heat exchanger, the geometry of the tube support system and the dissipation of energy by the vibrating tubes. The assessment equation uses fluidelastic force coefficients. Unfortunately, there has only been a very limited attempt to measure these coefficients directly. Therefore, a method is developed which shows how these coefficients can be deduced from available measured data. This requires the existing data correlations for fluidelastic instability to be viewed in a fresh and more general manner. The heart of the assessment method is a modal analysis of the tubes. This modal analysis is undertaken in two stages and covers both the fluid forces that cause interactions between neighbouring tubes and the variations in tube amplitude controlled by the tube supports. The assessment equation makes the energy dissipation, expressed by a damping factor, the subject of the assessment equation. This is useful because damping data has a wide statistical spread, which introduces considerable uncertainty, and it is necessary to introduce a probabilistic approach when managing the technical risks of fluidelastic vibration.