Increasing the safety and reliability of ageing facilities with single loop logic solvers
The process industries have experienced significant growth in Functional Process Safety applications. An increased awareness of destruction of property, injuries and loss of life associated with tragic events (which are widely publicised by worldwide media) has driven much of this growth. Ultimately, companies have a moral and legal obligation to limit the risk posed by their operations and the costs of litigation, for when these types of events do occur damages can measure in the billions.
Not surprisingly, operations and plant managers have come to recognise the importance of utilising a properly designed process system that optimises reliability and safety. Companies are therefore actively taking steps to comply with various local and worldwide safety standards such as IEC 61508/61511. They look to equipment specifically designed and approved for Safety Instrumented Systems (SIS), which are used to implement one or more Safety Instrumented Functions (SIF) intended to achieve or maintain a safe state for a process with respect to a specific hazardous event. A SIS is comprised of any combination of sensors, logic solvers and final control elements for the purpose of taking a process to a safe state when predetermined conditions are violated.
Many of the large process plants in Australia operating in the oil and gas and chemical industries were built more than 40 years ago. Much of their infrastructure is ageing rapidly, thus increasing the risk of failure. Such facilities need to demonstrate they have systems and processes in place to maintain continuously safe operations and meet safety standards. Routinely, plant managers will engage their engineering and operations teams to conduct a risk assessment for the process hazards that exist onsite. This will include a review of their safety case documents to make sure they accurately reflect the current systems and processes in use at their facility. These types of Hazard & Operability (HAZOP) studies will identify all potential threats and determine the Safety Integrity Level (SIL) for a specific process. This will help ensure there are adequate control measures to reduce the risk to “as low as reasonably practicable” (ALARP).
With the ongoing efforts to extend the viability of many older plants in Australia, the HAZOP studies will often determine that the existing process control infrastructure may not effectively mitigate process safety risks to meet the required SIL. To mitigate hazards affecting personnel, plant and community, engineering and operations teams will therefore introduce new SIFs or enhancements to existing control and safety measures. These will likely include straightforward steps like installing new hardware including transmitters, sensors, valves and logic solvers that keep the plant’s processes within safe operating limits. Often the HAZOP study may find that the primary BPCS or safety system cannot handle additional process points or logic overhead without an expensive overhaul or replacement. The challenge in upgrading existing hardware and software at many of these process facilities is that they rely on expensive and proprietary infrastructure to run their operations, and the expense of doing a complete retrofit can be extremely cost-prohibitive.
To avoid enormous capital expenditures, process facilities look to safety-certified devices that can be integrated into their legacy systems at a fraction of the price. This is where single loop logic solvers have proven to be an effective yet affordable way to meet the demands for safety systems that adequately reduce risks and meet desired SIL requirements. Single loop logic solvers are independent devices that monitor a temperature, pressure, level, flow, position or status variable. If the input exceeds a selected high or low trip point, one or multiple relay outputs warn of unwanted process conditions or provide emergency shutdown or on/off control. An example of this would be to slam shut or open a valve that mitigates a dangerous process condition.
Safety and reliability are all about reducing risk and increasing resilience. Meeting safety requirements can often be achieved with the simple architecture of single loop logic solvers, and where needed 1oo2 or 2oo3 voting for higher SIL requirements or increased availability. This simple voting approach that employs wiring relays from multiple single loop logic solvers in series is well proven, easily understood, compliant with IEC 61511 and readily expandable. These incremental and cost-effective investments into the process and associated safety and control systems will yield more efficient, safe, and compliant operation at process facilities. Plant owners understand that it takes years to build a reputation, yet one serious operational failure can destroy it.
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