Delayed Fracture of Glass Bulbs Used in Fire Sprinklers

Abstract

Very rarely, glass bulb sprinkler heads activate when there is no fire or elevated temperatures. In such cases, the bulb fragments are never recovered. Speculation may arise that there was a defect in the bulb from manufacturing or installation, which led to its eventual failure. We investigated the growth of such defects using the principles of fracture mechanics and the well-known laws of slow crack growth in glass. These laws include the existence of a threshold, below which cracks do not grow at all, and above which cracks grow at a velocity that is greater than a minimum value. As a result, bulbs either never fail, or fail within a relatively short time after the crack or defect is created. Finite element and fracture mechanics analyses for a typical bulb indicates that the maximum delay is of the order of hours or weeks, not years. Experiments confirm this phenomenon. Over 100 bulbs were intentionally damaged, then placed under a constant load. A total of 44 bulbs, for which the combination of damage and load happened to be above the threshold for crack growth, failed within days (up to 36 days), while all other 58 bulbs survived over 2 years without failure. The theoretical analysis and the experiments demonstrate that fracture of sprinkler bulbs occurs relatively quickly after the introduction of a defect: if a bulb fractures in service without apparent cause, there must have been some event to damage the bulb or the sprinkler head in the days or weeks before the activation.