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play a crucial role in the efficient separation of oil, water, gases, vapours, as well as contaminants. They serve as an important protective, ensuring compliance with environmental regulations and maintaining operational integrity. The contents that enter these separators originate from wellheads and various stages of fluid processing and refining processes. While separators are instrumental in managing these fluids and gasses, ensuring their optimal performance requires constant monitoring, particularly regarding pressure levels. Ensuring the integrity and safety of separators is crucial, and pressure protection plays a critical role in achieving this goal. Inadequate pressure monitoring can result in severe consequences, including environmental contamination, structural damage, and risks to the safety of personnel and nearby equipment. To mitigate these unwanted or dangerous situations, Safety Instrumented Systems (SIS) are routinely applied. This whitepaper covers the significant importance of pressure protection for separators.
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SEPARATOR PRESSURE PROTECTION Pressure protection within separators is critical for maintaining operational efficiency, environmental compliance, and personnel and equipment safety. Neglecting pressure monitoring can lead to severe consequences, especially when pressure levels exceed safe limits. High pressure in separators can result in structural damage, risking the structural integrity of the separator causing leaks or ruptures which could leak flammable vapours and gases causing environmental contamination. A worst- case scenario would include a potential explosion caused by the leaked gases encountering an ignition source such as a flame or spark.
ROLE OF LOGIC SOLVERS IN PRESSURE PROTECTION OF SEPARATORS A Safety Instrumented System consists of one or more Safety Instrumented Functions (SIFs) that work independently or together to take processes to a safe or desirable state. A typical SIF would involve a sensor such as a pressure transmitter, a logic solver that reads the process variable signal from the sensor, and an actuator or final element that is triggered by the logic solver to take the process to a safe state. For upstream oil and gas locations that only have a single separator with one pressure signal to monitor, single-loop logic solvers like alarm trips have been used to provide emergency shutdown in case of excessive pressure readings. Larger logic solvers such as Safety Programmable Logic Controllers (PLCs) are normally used when there are many points or loops involved in a SIS and come at a hefty price tag. Due to the limited number of safety loops at an upstream wellhead
10
il/Water/Gas separator tanks, commonly
referred to as separators, are vital components in many processes, specifically oil & gas processes. Separators
SEPARATOR PRESSURE PROTECTION WITH SIMPLE AND EFFICIENT VOTING LOGIC
location where separators are found, this would be overkill. However, a new type of logic solver, the SLA Multiloop Safety Logic Solver, has been introduced which can handle up to three SIF loops with 17 onboard I/O points. Unlike a simple single-loop logic solver, the SLA has voting and logic capability similar to a traditional safety PLC, but without complex programming. The SLA implements math with Excel-like formulas and voting logic through checkboxes in a license-free FDT/DTM programming environment as opposed to complex safety PLC programming. This offers a more cost-effective and efficient logic solver solution where more than one separator pressure or loop needs to be monitored. In the following application examples, we will highlight the use of a single STA SingleLoop Logic Solver on a single separator SIF and a SLA Multiloop Logic Solver on multiple separator SIFs.
For both application examples, we will assume that all SIFs need to meet SIL 2. Several factors must be assessed when determining if your chosen equipment within your SIF will meet your SIL requirement. As seen in the chart below, to meet SIL 2 your SIF must be available to take your process to a safe state at least 99 per cent of the time, in a low demand rate. Most logic solvers today are Type B devices, considered complex by IEC 61508 since they contain complex components that have failure rates that are difficult to predict and firmware. As such, it is common for these logic solvers to have internal diagnostics that continuously monitor their health and provide a relay contact if any internal failure or anomaly occurs. Without such internal checks or diagnostic coverage, the logic solver may not meet the required or expected equipment Safety Failure Fraction (SFF) for the SIF.
Figure 1. SIL Levels as defined by IEC61508/IEC61511.
SEPARATOR PRESSURE APPLICATION EXAMPLE To demonstrate how the STA and SLA are applied as logic solvers for separator pressure protection, we will use the following application details. There are three separators located at the site that need safety protection applied. Each separator has a pressure transmitter attached to a logic solver, which sends pressure readings via a 4-20mA signal. When pressure in the separator reaches extremely high-pressure levels, this is considered a dangerous condition. To
September 2024 Instrumentation Monthly
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