PT Notes
Part 3 - Examples of Causes of Dependent Failures in Processes
PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.
Dependent failures can result in catastrophic process safety incidents, Unfortunately, they are challenging to identify and manage. Dependent failures occur when multiple equipment failures or process disruptions are caused by a common factor or when the failure of one component influences the failure of another. These failures occur when the malfunctioning of one component or system leads to subsequent failures in others, due to their interconnectedness or dependency.
Some typical causes of dependent failures are:
Design Flaws: Poorly designed systems that do not adequately consider interdependencies among their components can lead to dependent failures. For example, if an electrical circuit powers several devices but is not designed to handle the full load, the circuit will overload at some point and the devices will not operate.
Shared Resources or Utilities: When multiple processes or systems rely on a common resource, such as electric power, cooling, or other utility, a failure in the shared utility can simultaneously affect all dependent components or systems. For example, if a cooling system serves multiple parts of a process, its failure will impact all those parts simultaneously.
Shared Support Systems: Systems such as scaffolding, platforms, and racks that provide support to multiple pieces of equipment, such as piping, can fail, leading to simultaneous failures of the equipment.
Human Error: Mistakes made by personnel can have cascading effects in interconnected systems. For example, a higher than optimal setting of the reboiler temperature in a fractionator made by an operator could cause the control system to attempt to compensate for the high temperature and the ensuing high pressure by adjusting other variables, such as cooling water flow rates or reflux ratios. These actions could overload the control system, leading to erratic operation or failures in other parts of the process.
Miscommunication: Miscommunication can occur between process personnel. For example, during a routine shift change in a refinery, the incoming operators are not told that a fractionation column is operating under abnormal conditions. Eventually, the continued abnormal operation triggers shutdown of the column which causes a ripple effect through the refinery with downstream units having to shut down as well.
Shared Personnel: If the same individual or team operates multiple processes, a mistake or oversight can lead to errors in all the processes, especially if the processes are complex and require close attention.
Delayed Maintenance: Neglecting regular maintenance can lead to the deterioration of a piece of equipment, which can then cause stress or failures in other parts of the process. For instance, a corroded pipe in a processing plant could burst, affecting nearby pieces of equipment.
Vibration‑Induced Failures: Continuous vibration from machinery can lead to fatigue failures in nearby pipelines or equipment, especially if they share common support structures or are in close proximity.
Control System Malfunction: A failure in a centralized control system can lead to incorrect operation across multiple parts of a process.
Instrumentation Failure Due to Shared Sensors: Controllers that rely on a shared set of sensors or transmitters for process measurements, such as flow, pressure, or temperature, could experience simultaneous control issues if these sensors fail or provide inaccurate data.
Shared Venting Systems: If a plant uses a common vent system for multiple vessels, a blockage or failure in the shared system could lead to over‑pressurization in several pieces of equipment simultaneously, posing a significant risk of rupture or explosion in multiple places.
Common Exhaust Systems: Multiple processes sharing a common exhaust or fume handling system could be affected simultaneously if the system becomes blocked or fails. This could lead to hazardous emissions or buildup of toxic gases in the plant.
External Factors and Environmental Conditions: Changes in environmental conditions, such as temperature, humidity, or corrosion due to chemical exposure, can simultaneously affect multiple components or processes, especially if they are designed with specific environmental tolerances.
Supply Chain and Logistic Issues: Dependencies on certain materials or components can lead to process failures if there is a disruption in the supply chain, affecting all processes in a plant that rely on those materials or components.
Contamination in Feedstock: If the raw material or feedstock used in a process is contaminated, it could affect multiple parts of the process at the same time.
Chemical Cross‑Contamination: In plants where multiple products are handled in close proximity, cross‑contamination could affect multiple processes at the same time.
Chemical Contamination in Shared Pipelines: If multiple processes share a network of pipelines and one process contaminates the shared pipeline, it can affect other processes that use the same network, leading to widespread product contamination or process disruptions.
Cooling Water Contamination: If the cooling water used in different systems comes from a common source and becomes contaminated, it can lead to fouling or decreased efficiency in all systems that rely on the cooling water, potentially causing overheating or process inefficiencies across a plant.
Overload and Capacity Issues: Systems or components that are overloaded or operated beyond their capacity can fail, causing a domino effect in dependent systems. For example, in a scenario where an upset condition, such as a fire, causes multiple safety relief valves to open simultaneously, the effluent handling system may become overwhelmed, leading to a loss of containment. For a large‑scale fire, the demand on fire protection systems, such as sprinklers or firewater pumps, might exceed their capacity leading to inadequate fire suppression in multiple areas.
Laboratory Information System Error: Inaccurate or delayed data from a central laboratory can lead to inappropriate adjustments in multiple parts of a process, affecting product quality and process safety.
Chemical Spills: A chemical spill or release in one area of a plant can have cascading impacts on multiple other parts of the plant.
Fire and Explosion: A fire or explosion in one part of a plant can quickly propagate to other areas, especially if flammable materials are present in other areas or if protective barriers are inadequate.
These examples illustrate the importance of understanding, identifying, and addressing dependent failures in risk assessments and safety management practices for process plants to ensure safe and efficient plant operations.
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