Integrated DCS and SIS
The right solution improves plant availability, safety
- Advanced digital technology makes it possible to combine process control and safety instrumented functions within a common automation infrastructure while ensuring regulatory compliance.
- The most reliable approach to control and safety system integration maintains principles of segregation, with safety and control strategies developed by different groups using dedicated methods.
- Operational integration based on the separation principle offers better support for plant life-cycle management.
By Johan School and Erik de Groot
Today's industrial organizations face a host of operational challenges. Ensuring the safety of personnel, equipment, and the environment are priorities for every facility. At the same time, plants must find ways to increase process efficiency, availability, and throughput-helping to improve their overall business performance.
Manufacturers employ varied approaches to interfacing their plant's Distributed Control System (DCS), Programmable Logic Controller (PLC), or relay system with the Safety Instrumented System (SIS). The primary function of a DCS or PLC is to hold specific process variables to predetermined levels in a dynamic environment, while a SIS is a static system that takes action when a process is out of control and the control system is unable to operate within safe limits.
Due to the costs associated with maintaining separate engineering, operation, and maintenance infrastructure for control and safety systems, many companies are now considering a more integrated architecture.
In 1998, the International Electrotechnical Commission (IEC) published IEC 61508, "Functional safety of electrical/electronic/programmable electronic safety-related systems." This document set the standards for safety-related system design of hardware and software. IEC 61508 is a generic functional safety standard, providing the framework and core requirements for sector-specific standards. These include IEC 61511: 2003, "Functional safety: Safety instrumented systems for the process industry sector," a three-part series of standards that provides good engineering practices for the application of SIS technology in the process sector.
ANSI/ISA-84.00.01 was issued as the U.S. version of IEC 61511 in September 2004. It primarily mirrors IEC 61511 in content, with the exception of a grandfathering clause:
For existing safety instrumented systems (SIS) designed and constructed in accordance with codes, standards, or practices prior to the issuance of this standard (e.g., ANSI/ISA-84.01-1996), the owner/operator shall determine and document that the equipment is designed, maintained, inspected, tested, and operated in a safe manner.
The European standards body, CENELEC, has adopted the standard as EN 61511. This means in each of the member states of the European Union, the standard is published as a national standard.
More recently, in 2007, a number of major control system end users and manufacturers, along with ISA, formed the ISA Security Compliance Institute (ISCI) to establish standards, tests, and conformance processes supporting the integration of control system products. The organization's work addresses key issues, such as cybersecurity related to process control and safety systems.
Why separate systems?
In an industrial plant, the DCS and SIS are typically interfaced through a gateway, with each system having its own operator interfaces, engineering workstations, configuration tools, data and event historians, asset management, and network communications.
In addition to segregation of control and safety equipment, there is separation of responsibilities between the personnel who manage these assets. The safety engineer is focused on safe operation, whereas the process engineer wants to maximize plant availability and operational profit.
According to IEC 61511, safety systems should be dedicated to safety-critical assets only. Most DCS systems are not sufficiently robust and fail-safe to operate safety-critical instruments at all times. The standard also urges caution if non-safety and safety functions are implemented in the same safety-related system since this may lead to greater complexity and increase the difficulty of carrying out life-cycle activities, such as design, validation, functional safety assessment, and maintenance.
Today, however, advanced digital technology has made it feasible to combine process control and safety instrumented functions within a common automation infrastructure-all while ensuring regulatory compliance. With this approach, plant personnel can view the status of the safety system and its applications, and combine this information with process control functions. The evolution of the Human-Machine Interface (HMI) allows critical information to be shared between the safety system and controllers and between the safety system and third-party subsystems via a digital bus interface.
Achieving operational integration
Different suppliers offer different strategies for operational integration of plant systems. For industrial organizations, the dilemma is how to achieve an integrated control and safety solution with advanced functionality and productivity, without compromising safety and security.
In a typical industrial operation, four levels of integration are essential from a usability point of view:
- First, the operational integration must allow plant personnel to have a seamless, transparent interface to the process under control. Whether the actual strategy is running in the process controller, the safety system, or on a higher level makes no difference. All required information would be available on the operational level.
- Second, peer-to-peer communication between safety controllers and process controllers is the key to integration. Information from one controller needs to be communicated to peers quickly in order to anticipate process startup or abnormal situations in a controlled manner.
- Third, all data available in the lowest level of process and safety I/O can be transferred to the higher level of operations and turned into information that is usable for various higher level applications.
- Finally, configuration tool integration only has added value if the point information is interchangeable. This means the user has a single point of data entry, and all information entered into the database can be replicated to other databases. The information is available for use at all levels of the safety and control topology.
It should be noted integration of the DCS and SIS does not imply a single common system. Rather, the two systems are integrated for ease of use and convenience. The configuration software may have different types of logic blocks, with some meant exclusively for use in safety functions and others used in normal control system functions. If the logic solvers/controllers need to communicate with other logic solvers, then it has to be over a communication bus that is robust enough to carry safety-critical data reliably. Thus, the integrated system is not really totally integrated but is much more closely aligned than earlier totally stand-alone systems.
Choosing right approach
There are undoubtedly many good reasons for tight integration of control and safety from an operations and productivity point of view. Solutions for operational integration encompass key areas across the plant, including operator interfaces, data processing, peer control, diagnostics, post-mortem analysis, fire and gas systems, alarm management, power supplies, and simulation/optimization.
However, industrial plants cannot achieve their operational objectives and still minimize safety risks without choosing a solution driven by the separation principle. Some technology providers seek to integrate control and safety by using the same hardware and software platform, network environment, operating system, and engineering tools for both environments. However, this strategy increases the possibility of systematic controller failures, including those resulting from design errors.
Experience has shown the most robust and reliable approach to DCS-SIS integration maintains the well-established principles of system segregation, with safety and control strategies developed by different groups using dedicated methods.
The specific criterion for effective operational integration includes:
- Separate databases: Plants should employ separate databases to store safety and control strategies and also use dedicated control and safety configuration tools. Maintaining separate tools with separate databases prevents unauthorized changes or corruptions, decreases safety risks, and prevents common cause failures.
- Database integrity, security: It is important to protect safety software from viruses and harmful hacking. This includes checking the integrity of the software before installation, after installation, and during run time. The integrity of all data accessed through the safety configuration tool, as well as the integrity of an application loaded into the safety system, must be safeguarded from unwanted changes to protect the entire safety application during the entire life cycle.
- Managed and protected application environment: Protection of the safety system from off- and on-process changes is another important requirement. In particular, users need a login protection mechanism to protect the safety application from accidental or unauthorized changes when it is unmanned over a specified period.
- Dedicated software and hardware: Using dedicated and specifically developed hardware and software, according to the IEC 61508 safety standard, reduces the risk of a common cause failure. Dedicated hardware and software for safety and control also protects the safety system from any defects in control-related operations.
- Secure network environment: A good practice is to protect the safety system from outside threats through the use of an embedded hardware firewall, which isolates the safety application during runtime execution from external devices. With this embedded firewall and the use of a SIL-certified safety protocol, the data integrity between control and safety is protected and guaranteed.
- Safety device certification: Additional cybersecurity assurance can be gained by utilizing safety solutions that have received the ISCI's ISASecure Embedded Device Security Assurance certification. This certification recognizes the integrity of the embedded safety device and its development life cycle and involves rigorous testing of communication robustness, functional security, and software development security.
Benefits to end users
As described in this article, industrial facilities can realize significant advantages from seamless, yet secure, integration of plant control and safety systems. Some of the potential benefits include: accurate time synchronization, elimination of data mapping duplication, common HMI, and reduced operator and maintenance training requirements.
With secure integration at the control data and operator levels, plants can implement a single, common operational interface for control and safety. Operators can view the status of the safety system, as well as monitor alarms and events related to the process. Any alarms or events from the safety system are automatically integrated into the control system HMI. It also makes no difference where an application is running; all required information is available to the operator. This allows for a wide range of applications to be monitored plant-wide from any operator console, from rotating equipment and compressor protective systems through emergency shutdown systems to large plant-wide fire and gas applications. And with integrated simulation tools, plant managers are able to verify and optimize hazard identification, train operators, and verify the responses.
Last, operational integration based on the separation principle offers better support for plant life-cycle management. Users can choose to migrate their control and safety systems to newer releases independent of one another. Migrations can even be performed on-process without effecting normal operations. This contrasts with common platform solutions, which frequently require migration of the control and safety infrastructure as a whole.
Prudent industrial organizations will continue to adhere to the separation principle for control and safety systems while at the same time achieving operational integration, which combines hardware and software diversification with integrated solutions for the operator interface, data processing, data analysis, and alarm management.
ABOUT THE AUTHORS
Johan School is a TÜV certified Functional Safety Engineer and holds a bachelor's degree in electrical engineering. As a member of national and international standards committees, he takes part in developing definitions of international safety standards. He has been with Honeywell for more than 18 years, works for the Honeywell Safety Center of Excellence, and is located in the Netherlands. Erik de Groot is marketing manager, safety management systems, for Honeywell Process Solutions, responsible for the FSC and Safety Manager product lines. He has been active in the process industries in process development and automation for 26 years, including 15 years with Honeywell, where he started as application engineer. Contact both at email@example.com.