Most industrial processes incorporate Safety Instrumented Systems (SIS) and Basic Process Control Systems (BPCS) in their operations. This article compares the features of each system.
SIS and BPCS Compared
A Safety Instrumented Systems (SIS) function is to monitor and maintain process safety. Since they are not frequently called to operation, SISs are usually passive and dormant.
Because of this, it's important that devices used in SISs have diagnostics to ensure components are functioning properly, which in turn reduces the frequency of manual verification.
Because of the criticality involve, it is important that changes done after installation comply strictly to the site's Management of Change (MOC) process.
A Basic Process Control Systems (BPCS) function is to control process operations. Since they are frequently called into operation, BPCS are usually active and dynamic.
BPCSs are known by their unique response to different types of digital and analog inputs, and outputs logic functions, which thus makes most failures self-revealing.
Changes to BPCSs are very common and are required to maintain accurate process control.
Both SIS and BPCS systems have multiple layers of protection, but since BPCS systems function mainly as the “control system,” many industrial standards recommend that the SIS systems be separate from BPCS systems:
“A device used to perform part of a safety instrumented function shall not be used for basic process control purposes, where a failure of that device results in a failure of the basic process control function which causes a demand on the safety instrumented function, unless an analysis has been carried out to confirm that the overall risk is acceptable.” – Excerpt from ANSI/ISA 84.00.01-2004 11.2.10
It's always a good idea to write protect field device communication settings to avert possible risk to cyber security threats.
But this is especially true with SIS systems as a means to avert changes being made to the system's devices that would fall outside of the specified safety requirements (as provided by ANSI/ISA 84.00.01-2004.)
Importance Of Diagnostics
In BPCS systems, communication protocols like HART and Foundation Fieldbus play an important role, but not so much in an SIS system.
SIS systems are typically more focus on device diagnostics, since those diagnostics provide information on the health and status of the safety devices in use.
That level of diagnostic information is typically not needed in most BPCS control systems.
Common Cause Failures
These can be caused by a power surge, power loss, equipment vibration, radio frequency interference or temperature fluctuation.
Common Cause Failures also include software bugs or undetected device failures, and can be common in SIF system with a high performance level.
As mentioned above, to reduce risks from these common failures it is recommended to completely separate SIS systems from the BPCS by using redundant devices when and where necessary.
These trips occur in SISs when devices fail within a “specified probability” in a way that it results in an alarm or warning signal.
Often when these trips occur the system needs to be manually reset, however in some more advanced system actions can be taken to automatically reset trips that truly fall in the “nuisance category.
This can be implemented with the use of voting logic that compares device diagnostics and multiple data-points to determine if user intervention is required.
Since SISs and BPCSs are both used to automate industrial processes with the aid of an input, output and logic functions, it is important that engineers and technicians ensure recommended standards and guidelines are strictly adhered to while working on these systems to avert dangerous situations in plants.
Written by Emmanuel Okih
Automation and Control Systems Engineer and Freelance Writer
Edited by Shawn Tierney
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