Ingenieur Vol 68 Oct-Dec 2016 | Page 75

Figure 2: Line of Defence / Layers of Protection in Process Safety vessel contents to the environment. Although this results in a “loss of containment,” the fluid is discharged at an appropriate/minimum quantity and to a specific safe location. The aim of relieving the system is to avoid the catastrophic rupture of the system by depressurising it. As depicted via the example of a relief system, the layers of protection basically employ actions to minimise the impact of a loss of containment event. Such layers of protections limit the extent of the loss event. There are common cases in which process plants provide secondary containment or diking for flammable tank areas to ensure only small areas are affected due to any discharge of hazardous and environmentally unfriendly material. The mitigation action also includes installation of fire water systems which consist of deluge systems or water curtains and long-range water cannons. Another layer of protection is provided when operations are fully prepared for an emergency. There must be an effective emergency response plan which requires reliable communication from the detection of loss of containment to the discharge of the released gas to warn surrounding areas of potential problems and to trigger-start early and proper responses to minimise the impact of the loss event. As highlighted earlier, process safety is to prevent and mitigate accidental or unintentional loss of containment of hazardous materials. The implementation of layers of protections provide barriers to prevent undesired impact events from reaching people, the environment, or equipment thereby accomplishing process safety objectives. Why Accidents still happen Some recent process safety accidents include the Texas City (2005) and Buncefield (2005) explosions. Even though the layers of protections system may have been implemented for many decades and as a result most hazardous, out of control conditions are known and protected, serious accidents still happen today. And one can therefore ask oneself, why do these accidents still happen? The occurrence of these accidents was not due to unknown physical or chemical process hazards. The Buncefield explosion, for example, was due to the ignition of a vapour cloud emanating from spilled gasoline due to overfilling a storage tank [6]. And it has long been recognised that fires or vapour cloud explosions can occur as a result of spillage of flammable materials from aboveground storage tanks in tank farms. Common causes that could unleash spills include overfilling, leaking from worn-out and corroded containment, and loss of containment due to pipeline ruptures. In the Buncefield incident, investigators said faulty gauges and safety devices led to overfilling a fuel 73