Blast Effects from Rupture of Paper Machine Roll
Paper Making Process
The manufacture of paper from pulp involves the use of a device known as a paper machine, where a mixture of wood fibers in water, known as pulp, is formed into a continuous sheet of paper. Various sections of the paper machine are designed to remove the water from pulp, serving to form the paper sheet. Final drying takes place by passing the paper sheet between steam-heated rolls in the drying section of the paper machine. A photograph showing an example of the drying section in a paper machine is shown below.
The paper machine involved in this story is comprised of many rolls over a distance of several hundred feet. The rolls, which are five feet in diameter and 15 feet in length, are constructed of cast iron and are pressurized with steam at 120 pounds per square inch (psi).
EDT was retained to determine the cause of damage to a building containing a paper machine that had experienced a sudden rupture of one of the rolls. The building owner had alleged that the sudden release of pressurized steam into the enclosed space of the building had damaged the building siding and roof. The portion of the building roof above the location of the ruptured roll had been displaced upward, consistent with damage resulting from the incident. However, portions of the building roof and siding located hundreds of feet from the ruptured roll were observed to be displaced, more consistent with the deterioration of the roof and siding.
Analysis of Overpressure Due to the Incident
The rupture of the roll is similar to an explosion, where the rapid release of pressurized steam from the ruptured rolls expands, and accelerates the air surrounding the roll. The rapid-moving air produces an increase in pressure when the air encounters a fixed object, such as a wall. Such increase in pressure, known as an overpressure, is the force responsible for damage to buildings (and people) when an explosion takes place. The overpressure is measured in pounds per square inch (psi). However, the magnitude of the overpressure decreases in a rapid manner with increasing distance from the origin of the explosion.
Tests conducted by the U.S. Army generated data that relates the magnitude of overpressure to pounds of trinitrotoluene (TNT) detonated, relative to the distance from the TNT. The pressurized steam contained in the rolls has energy, the value which can be calculated knowing the pressure, temperature, and mass of the steam inside the roll. Knowing the quantity of energy present in one pound of TNT, the energy in the steam is then related to equivalent pounds of TNT. Published data that relates overpressure to equivalent pounds of TNT then provides the expected overpressure at various distances from the ruptured roll. Data published by the National Fire Protection Association (NFPA 921) relates the magnitude of overpressure to the expected extent of damage.
Results of Overpressure Analysis
The results of the analysis indicated that the overpressures at the building walls and roof, not in the immediate location of the ruptured rolls, was of a magnitude not expected to result in damage. Overpressures calculated at the walls were in the range of 0.10 to 0.40 psi, where overpressures in the range of 1.0 to 2.0 psi are known to result in wall damages. Further evaluation of the walls and roof by an EDT structural engineer confirmed that the alleged damage was due to long term deterioration of the building, that had pre-existed the incident.
Many times science and analytical techniques provide useful insight as to the cause of damages, perceived upon initial observations, to be related to certain incidents.