INGENIEUR
INGENIEUR
Figure 1 : Anatomy of an Accident : Example of fire / explosion due to pipe rupture
normal operations , process hazards are contained and controlled but they are still present . An initiating cause or event such as a process upset ( e . g . pipe rupture ) will result in a deviation ( e . g . abrupt pressure change due to an imbalance in pressure with the atmosphere that will result in a process hazard ( e . g . a flammable gas in storage ) becoming a loss event ( e . g . a leak of a flammable vapour that ignites and explodes ) with subsequent impacts ( such as injuries and structure damage ).
Good layers of protection design prevents or mitigates all forms of loss of containment by controlling the process so that it remains within its normal operating conditions and contains or controls hazards . Figure 2 shows the basic concept of layers of protections .
Once an initiating cause occurs , normal operation cannot continue without a process or operational response . If an initiating cause develops , then the Basic Process Control System ( BPCS ) is the first layer of defence to help bring the process back into control . The control basically provides the early process or operational response . The BPCS addresses process deviations as they occur and has either built-in control systems or requires operator actions to bring the process back into control . Many protective features are built into basic process control systems which depend on automation to maintain process variables at set points , which are fixed at specific values to ensure safe and normal operating conditions . If operator actions are required , the alarm system is installed as the second layer of protection . The alarm brings the unusual situation to the attention of relevant persons at the plant who will analyse the situation and decide on appropriate actions to regain the defined safe condition .
The direct result of an initiating cause is a deviation - an unusual situation , outside design set-point or operational parameters . If a deviation continues uncorrected ( i . e . the BPCS failed to regain control ), it will cause a loss event - irreversible releases of process material / energy . If this situation occurs , the third layer of protection plays an important role . Since the normal control could not be regained , the appropriate next course of action is to obtain a safe process condition . This usually involves shutting down the process in order to bring it into a safe working condition . Normally , Safety Instrumented Systems ( SIS ) are used to shut the process down . The SIS aims to avoid any loss of containment .
If the process cannot be shut down in time , then a relief system may be called into action . The relief system tries to divert the material in containment to achieve a reasonable pressure and hence protect the containment . This mitigation / protection layer may allow some impact event to occur i . e . partial release of material / energy , but they reduce the severity of the event . Pressure relief systems for example , prevent catastrophic rupture of the vessel , but they may release the
72 VOL 68 OCTOBER - DECEMBER 2016