Moving on to the definitions…
Definition 3.2.3 for basic process control systems was changed slightly. Let’s look at the change…
Original: … but which does not perform any safety instrumented functions with a claimed SIL >= 1.
CDV: … but which does not perform any SIF.
Hmm. The original states that a BPCS does not perform any safety functionality whose effectiveness is claimed by to SIL 1 or greater, whereas the CDV seems to imply that no safety protective functionality is occurring in the BPCS regardless of the credit given to that functionality. If you then move on to the two new notes, the acknowledgement of BPCS can perform some safety functionality, to some degree, returns.
New CDV Notes:
Note 1 to entry: A BPCS includes all of the devices necessary to ensure that the process operates in the desired manner.
Note 2 to entry: A BPCS typically may implement various functions, such as process control functions, monitoring, alarms or other protective functions.
Altogether the modification is a non-event with the exception that the reference to BPCS being able to perform some protective functionality has moved from the normative requirement into a non-normative informational note. Essentially, weakening the statement. Perhaps something to remember, but shouldn’t altogether change anyone’s view of how BPCS can be used for protective functionality.
Before closing out this blog post I will add a new definition in the CDV for bypassing, which is only remarkable in that it was conspicuously absent from the existing standard.
action or facility to prevent all or parts of the SIS functionality from being executed.
Note 1 to entry: Examples of bypassing include:
- the input signal is blocked from the trip logic while still presenting the input parameters and alarm to the operator;
- the output signal from the trip logic to a final element is held in the normal state preventing final element operation;
- a physical bypass line is provided around the final element;
- preselected input state (e.g., on/off input) or set is forced by means of an engineering tool (e.g., in the application program).
- Note 2 to entry: Other terms are also used to refer to bypassing, such as override, defeat, disable, force, or inhibit or muting.