900 Section V The Impact of Chemical Engineering Design on Society causing catastrophic failure of the vessel. Such a failure can cause direct injuries, release toxic material, cause a fire, or lead to a BLEVE and/or a VCE. To reduce the chance of a runaway condition during process upsets, the temperature difference between the reacting mixture and the cooling medium should be kept small. This may seem counterintuitive. However, consider a case where the temperature driving force is 1°C. If the temperature of the reacting mixture increases by 1°C during a process upset, the driving force for cooling has doubled! If the heat-exchange system had been de- signed for a 10°C driving force, that same upset would result in only a 10% increase in cooling. In systems with a chance of runaway, increased heat transfer area is the cost of an inherently safer system. A common scenario for an accident involving an exothermic reaction is the loss of coolant accident (LOCA). Unless the cooling system is backed up to the extent that it is essentially 100% reliable, one must consider this scenario in designing the vessels and the pressure-relief systems. 26.3.2 Pressure-Relief Systems During a severe process upset, the pressure and/or temperature limits of integrity for ves- sels can be approached. To avoid an uncontrolled, catastrophic release of the contents or the destruction of the vessel, pressure-relief systems are installed. Usually, these are relief valves on vessels or process lines that open automatically at a certain pressure. Down- stream, they are connected to flares (for flammable or toxic materials), scrubbers (for toxic materials), or a stack directed away from workers (for materials such as steam that present physical hazards). The design of the pressure-relief system is especially important, because the worst-case scenario must be considered, which is sometimes the simultaneous failure of multiple relief systems, as was the case for the Bhopal tragedy in 1984. The design of such systems is complicated by several factors. The devices are de- signed to operate under unsteady conditions. Therefore, a dynamic simulation is required. Also, the flow through the relief system may be single-phase or two-phase flow. For two- phase flow, not only are the calculations more difficult, but also more factors affect the pres- sure drop, such as whether the line is horizontal or vertical. In addition to the relief valves (which are called safety valves, relief valves, pressure-relief valves, or pop valves depending on service), rupture disks are used to open the process to the discharge system. Rupture disks are specially manufactured disks that are installed in a line, similar to the metal blanks used between flanges to close a line permanently. However, the disks are designed to fail rapidly at a set pressure. Ide- ally, the rupture disk allows no flow when the pressure is less than the set pressure, and it ruptures immediately, offering no resistance to flow, when the pressure hits the set point. 26.4 PROCESS HAZARD ANALYSIS Under the "Process Hazard Analysis" requirement of the Process Safety Management of Highly Hazardous Chemicals regulation (29 CFR 1910.119), employers must complete such an analysis of all covered processes using one or more of the following techniques: · What-If · Checklist · What-If/checklist