While investigating a well pump failure, it was also learned that the water storage tank collapsed. When the assignment was received, it was alleged that lightning had struck the well pump and storage tank, and caused the tank to fail. Upon arriving at the site, it was immediately noted that the storage tank had imploded. The condition of the tank is shown in figure 1.
Figure 1 Implosion of storage tank
When power was applied to the pump motor, the motor started but, the pump produced no flow. It was also clear that either there was a mechanical problem within the pump or the pump had lost "prime". Prime is the condition in which a vertical column of water extends from the well surface to the entrance of the pump. Since the motor operated when power was applied, lightning could not have caused damage to the motor. And, since the tank was not burned, lightning was eliminated as the cause of the tank failure. It should be noted that the implosion of the tank was the result of a difference in pressure between external and internal sides. In order to cause an implosion, the internal pressure has to be less than the external presure which is at atmospheric pressure, approximately 14.7 pounds per square inch. The only way that the internal tank pressure would fall below atmospheric pressure is if the pump lost prime. In doing so, the column of water between the pump and well surface has to collapse back into the well. By doing so, a vacuum is created within the tank. Since the tank is of fiberglass construction and therefore relatively weak in sustaining negative pressures, the tank walls collapse under external atmospheric pressure. It should be further noted that the vacuum was the result of the failure of the foot valve due to the deposition of sediment on the valve stem and seat. The foot valve is located at the entrance to the pipeline leading from the well reservoir to the pump. The purpose of the foot valve is to maintain the water column to the pump, that is, maintain prime. If the foot valve cannot maintain prime, the water column will fall back into the well creating a vacuum within all piping and the storage tank. Most manufacturers of fiberglass tanks recommend installing a vacuum breaker valve in order to avoid this kind of failure.
