Flow-Induced Vibration in Evaporations

The concern for flow-induced tube vibration has become a serious consideration in the design of shell-and-tube equipment. These problems can lead to tubes and tubejoints that leak, increased shellside pressure drop, and intolerably loud noises. The result is that equipment must be removed from service for repair and modification.

Flow-induced vibrations can damage tubes in evaporators. All tubes vibrate under all flow conditions! However, we are concerned with vibrations which cause significant tube damage. As larger evaporators, greater flowrates and higher shellside velocities become more prevalent, damaging tube vibrations are more likely to occur. No evaporator design is complete without considering the possibility of damage as a result of flow-induced vibrations.

Damage is more likely to occur with gases or vapors on the shellside than with liquids. Flow-induced vibrations also occur with liquids on the shellside, but the damage is often limited to localized areas of relatively high velocity. In severe cases, tubes can leak within a few days or even in a few hours after the equipment has been placed in service. More often, damage will appear a year or so after startup. Additional tube damage will develop after the initial damage has been repaired, but the number and frequency of further damages will decrease with time.

In a number of cases, heat exchanger tube failures attributed to flowinduced vibration have resulted in consequential damage to other equipment within a plant. Failures of this nature have proven to be the most destructive, most costly, and have required the longest plant shutdowns for rectification.

Currently available methods for predicting flow-induced vibration damage are inadequate for predicting failures. At best, they identify the equipment that are susceptible to damage. The primary reason for this lack of precision is that flow-induced vibrations are extremely complicated. Much has been learned, but the probability of its occurrence is still not known. However, the cost penalty for equipment designed to completely avoid damaging vibration is modest and is almost always easily justified.

Some of the problem areas concerned with prediction of vibration include:

1. the complex pattern of flow through a tube bundle
2. the complicated fluid mechanics of a bank of vibrating tubes
3. the role of damping
4. the rates of wear and fatigue.

Nevertheless, it is possible to develop design criteria, especially when tempered with experience, to ensure that equipment will be safe from vibration damage.

Flow-induced vibrations problems in tubular equipment are commonly thought of as consisting entirely of mechanical failure of the tubes. However, the vibration can increase the shellside pressure drop, sometimes as much as double. Further, an acoustically vibrating unit can produce an intolerably high noise level. With an increasing emphasis on noise control, acoustic vibration must be an important consideration in design of tubular equipment.