What are Safety Instrumented and Critical Control Systems?

By Mark Sen Gupta

Industry Trends

In the early days of process automation, mechanical and solid state relays were commonly used for safety interlock functions. With processes becoming more complex, programmable systems are now common for safety and critical control functions.

Typical applications of such systems in process industries include:

  1. Emergency shutdown
  2. Fire and gas monitoring and protection
  3. Critical process control
  4. Burner management and control
  5. Turbine and compressor control

Critical control includes applications that are not mandated to use Safety Instrumented Systems (SIS) for safety, but are so operationally important that they are implemented in an SIS in order to achieve very high reliability. Example applications are critical pressure and level control loops in power generation applications (nuclear and fossil) where multiple sensors are employed.

Safety Integrity Levels (SIL)

There are four SIL levels, as specified by IEC 61508, IEC 61511 and ANSI/ISA 84.01 safety standards. Demand Mode includes actions taken in response to process or other conditions (generally no more than once per year). Continuous Mode includes functions, which implement continuous control to maintain functional safety.

A large majority of the safety systems that are available today are suitable for both SIL 2 and SIL 3 applications. Additionally, in most applications there is a mix of loops that require SIL 2 or SIL 3 level of protection.

Safety Integrity Levels

Safety Instrumented and Critical Control System Key Components

The following paragraphs describe key SIS segments:

· Hardware (including bundled software)

· Software (unbundled)

· Integration services

· Maintenance services

System Hardware

SIS hardware includes all controllers, workstations, and safety I/O. Bundled software includes control and programming software and may also include HMI. Control software is typically embedded in the controllers and performs monitoring, control, and shutdown functions.

HMI software typically resides in SIS workstations, which are mostly PCs. PCs used for system configuration are also included in the segment. This segment does not include field devices, such as transmitters and sensors.


The software sector includes all programs and packages that are quoted and sold separately from the SIS. They may include lifecycle management software and unbundled HMI packages. Any estimation or configuration software priced separately from the hardware is also included in this category. Unbundled software accounts for a small part of the total SIS business.

Integration Services

The capability to provide a wide range of project services, including process hazard analysis, will be a necessary ingredient for survival for any major SIS supplier in the future. The ability of SIS suppliers to provide superior services, and thus total solutions to end user customers will be a major competitive differentiator going forward.

For end users, selecting a SIS supplier with risk and reliability assessment and integration service capabilities is no longer a matter of convenience; it is a matter of necessity. While companies continue to reduce their engineering staff, advancements in technology and manufacturing are increasing the complexity of most automation projects. That requires more intensive engineering support and state-of-the-art expertise for safety sys-tem implementation.

Supplier services may include:

  • Risk and safety analysis, including layer of protection analysis (LOPA) and hazard and operability studies (HAZOP)
  • Project definition
  • Safety system design
  • Safety system implementation
  • Checkout and startup services
  • Project management

Services include only those provided directly by employees of safety sys-tem suppliers and exclude indirect contributions. For example, project services performed by system integrators (SI) for SIS suppliers are excluded from the overall tabulation. Project services incurred by users, consultants,

Maintenance Services

Although a SIS generally requires little hardware maintenance, configuration updates for changed requirements and periodic checks during process shutdowns are becoming increasingly common. The revenues for maintenance services are growing with the increase in the SIS installed base.

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