This article is about Process Control System Segregation in Terms of Risk Areas. Topic is based on international codes, standards and SAES-Z-001 Aramco Document. Very useful for Electrical, Instrumentation, Process and Control Engineers.
Process Control Systems are responsible for controlling and monitoring the production of industrial processes. To ensure optimal process availability, risk areas should be segregated into different levels for increased system and process availability. This article will discuss the three levels of risk area segregation and the requirements for each level.
Level 1 Risk Area Segregation
Level 1 (L1) segregation provides the greatest degree of segregation and is used to segregate plant operations based on a 50% production loss rule. Equipment located in separate L1 Risk Areas require separate UPS Power Circuits, Power Supplies, Power Distribution circuits or panels, Operator workstations and operator alarm panels / systems, Process controllers (DCS), Safety Instrumented Systems (ESD), Compressor or Turbine Control Systems and associated I/O subsystems for each, Process Control Network equipment and cabling, System and marshalling cabinets, and Auxiliary systems interfaces.
Level 1 segregation does not apply to software applications which require a ‘global’ system database. The exceptions to Level 1 segregation are DCS Configuration Database, System Diagnostics applications, Analyzer Management Systems, Instrument Asset Management System (IAMS), Alarm Management system, MVC software, Data Acquisition and Handling Systems (DAHS), Power Monitoring Systems and Condition Monitoring Systems since failure in any of these does not adversely affect the ability to operate the plant. Furthermore, a double failure of any redundant component in one L1 Risk Area shall not affect the operations of equipment in any other L1 Risk Area.
Where a single operator console is used to monitor two or more L1 risk areas, each risk area must have a dedicated Operator Workstation. Requirements for Operator Workstation redundancy (ie the backup operator workstation) can be met using a workstation dedicated to another risk area as long as that workstation has full monitoring and control capabilities of both risk areas.
In addition, where a single operator console is used to monitor two or more L1 risk areas, control network communications equipment and cables should be segregated between risk areas. Communications cables can terminate on a common network switch associated with the console provided the switch is supplied in redundant configuration and both are dedicated for the operator console.
Level 2 Risk Area Segregation
Level 2 segregation is used to segregate parallel processing units within a Level 1 risk area and to segregate major process equipment installed within Utilities plant areas. Equipment located in separate Level 2 (L2) Risk Areas must have separate Process Controllers and associated I/O modules, I/O communications equipment and communications cabling, and Marshalling Cabinets.
Where two or more operator consoles are used to control equipment within a single L1 risk area, equipment operated by each console should be segregated into separate L2 risk areas. In addition, parallel processing trains within an L1 risk area which have been segregated into separate Level 2 risk areas require separate Emergency Shutdown Systems for each L2 risk area.
An exception is allowed for segregation of I/O communications cables when I/O modules are located remote from the controllers and fiber optic cables are used for communications. In this case, controllers in two separate L2 risk areas may share the same fiber optic cable provided that dedicated fiber strands are used for each controller, no Fiber optic converters are shared between controllers, and communications between the controller and I/O is redundant and the redundant cables are installed in separate routes.
Level 3 Risk Area Segregation
Level 3 segregation is used to segregate parallel process equipment or equipment installed in redundant configuration in order to increase process availability. Level 3 segregation requires segregation of equipment at the I/O card level. Any equipment which serves the same purpose but is provided in redundant configuration to increase a process system’s availability should be segregated into separate L3 risk areas.
Redundant or parallel processing equipment are equipment such as Booster pumps, Shipper Pumps, Sales Gas Compressors, Feed Gas Compressors, LP, HP and Pipeline compressors in a GOSP, column bottoms pumps, reboilers, filters/separators, condensate pumps, etc., which are installed in redundant configuration.
Equipment located in separate L3 risk areas should not share the same I/O card. Field cables for equipment located in separate L3 risk areas may be terminated in a common marshalling cabinet and use a common (redundant) external field power supply if required.
Final Thoughts
Process Control Systems should be segregated into risk areas to increase system and process availability. Three levels of risk area segregation should be applied for optimal process availability. Level 1 segregation provides the greatest degree of segregation and is used to segregate plant operations based on a 50% production loss rule.
Level 2 segregation is used to segregate parallel processing units within a Level 1 risk area and to segregate major process equipment installed within Utilities plant areas. Level 3 segregation is used to segregate parallel process equipment or equipment installed in redundant configuration in order to increase process availability.
- International Codes and Standards Used in Process Control System.
- Spare and Expansion Capabilities of Process Control System.
- Process Control and Equipment Protection.
- Control Console Technical Specification for Industrial Control Projects.
- Operator Graphical Displays for Process Control System.
- Guidelines for Process Alarm Systems: Alarm System Management.
- Distributed Control System (DCS) Historization and Trending.
- Process Control System Access and Security.
- Process Control System Integration and Interface with other Disciplines.
- Technical Requirements for System, Network and Server Cabinets – PCS.
- Electrical Wiring and Power Distribution for Distributed Control Systems.
- Process Control Network Cabling Requirements | PDFBAG