(91c) Shrapnel:  Accounting for Blast Fragments in Facility Siting Studies

When a pressurized vessel ruptures, the resulting stored energy is released. This energy has the potential to cause a shockwave and accelerate vessel fragments. The need to account for the damage resulting from the shockwave from a physical explosion has long been recognized and methods have been developed and published. There is a general awareness that projectiles are also created during a rupture event, but these blast fragments are typically only addressed anecdotally, in describing the damage done by explosion after the event has occurred, not as part of anticipating the damage done by an event in the course of conducting a facility siting study.

Shrapnel damage by blast fragments is only of concern to the extent that it goes beyond the blast zone caused by the shockwave from an overpressure event. The horizontal range of the fragment field depends on the energy released during the overpressure event. However, the number of fragments that can do damage is finite and the probability that a fragment will do damage is considerably less than 100%. Understanding how to estimate the horizontal range of a fragment field, to estimate the number of fragments with the potential to do damage, and to estimate the probability that a fragment will cause damage are all essential to accounting for blast fragments in facility siting studies.

This paper develops a simplified method for predicting the extent of shrapnel damage by blast fragments resulting from a pressure vessel rupture. There are six steps to the method, and it can be used without depending on complex or expensive modeling software.

1. Determine the maximum horizontal range of blast fragments.

2. Calculate the fractional distance to a potential target, based on the maximum horizontal range of blast fragments. When the fractional distance to a potential target is greater than 1, the shrapnel case may be ignored.

3. When the fractional distance to a potential target is less than 1, use look-up tables to determine the probability of fragments going at least as far as the potential target. There are separate look-up tables for horizontal vessels and for vertical and spherical vessels.

4. Determine the probability that a blast fragment sufficiently large to do damage would travel in the direction of a potential target. The probability of a blast fragment going in the direction of a potential target depends on the number of blast fragments and the area of the target presented to the blast and the distance of the target from the blast.

5. Calculate the target density of the process. Not all space presented to a fragmenting pressure vessel would result in a catastrophic release if entered by a fragment.

6. Calculate the probability of damage by blast fragments as the product of target density, probability of blast fragments going far enough, and probability of blast fragment going in the right direction.

When the probability of damage is sufficiently low, the shrapnel case may be ignored.