What is a Critical Instrument
What is a Critical Instrument
(OP)
I was tasked with identifying and testing all critical instruments within our facilities. The first issue I am having is whether or not there is a definition of what a critical instrument actually is.
Searching online did not result in anything I would call "official". I did find a couple comments that an instrument is critical if:
* - Results from the instrument affect the final conformity of the product
* - Failure of the instrument results in downtime (ask yourself if you are tasked to get the plant up and running again, if it cannot be done without the instrument)
Is that the definition, officially or unofficially? Is there an official definition within PSM or some other OSHA or US government standard?
Thanks in advance for your help!
-matt
Searching online did not result in anything I would call "official". I did find a couple comments that an instrument is critical if:
* - Results from the instrument affect the final conformity of the product
* - Failure of the instrument results in downtime (ask yourself if you are tasked to get the plant up and running again, if it cannot be done without the instrument)
Is that the definition, officially or unofficially? Is there an official definition within PSM or some other OSHA or US government standard?
Thanks in advance for your help!
-matt





RE: What is a Critical Instrument
The most common definition of criticality I have come across, is related to safety critical elements (SCE). Those elements, by the definition, are "any part of the facilities (including software) the failure of which could cause or contribute substantially to a major accident, or the purpose of which is to prevent or limit the effects of a major accident. Safety critical elements typically include installation hardware and equipment related software, which are designed to prevent or control a release of a major accident hazard, or which are intended to mitigate impacts". Definition of a major accidents normally means the highest-consequence type of events in the company risk matrix, including all red-colored risk categories in the risk matrix.
I'd suggest you go back to the source (to the people who assigned you to this task) and make sure the criticality and how it is determined for each instrument is fully defined before you proceed. Without proper methodology, the work effects will be random and definitely not 100% correct.
Dejan IVANOVIC
Process Engineer, MSChE
RE: What is a Critical Instrument
I'm going to combine what I've found, your comments, and anything remotely supporting some of what I'm saying from my LOPA experience and PSM and come up with some sort of definition we can support as a company. Then, set in stone, we get our list.
Anyone else have any thoughts or references?
-Matt
RE: What is a Critical Instrument
Some possible criteria are:
- Is the instrument designed to provide control or shutdown or communication in an emergency?
- Is the instrument designed to protect process equipment in order to avoid catastrophic failure?
- Is the purpose of the instrument to limit the effects, or mitigate consequences of a catastrophic release?
- Will failure of the instrument potentially cause a catastrophic release in upstream or downstream equipment?
- Will failure of the instrument adversely affect escape and evacuation of personnel in case of emergency?
If answer on any of these questions is YES, then you have a critical instrument (example).
Furthermore, if you decide to expand the criteria and include additional instruments that control product quality and therefore prevent from production losses, you can use the criteria such as:
- Is the instrument designed to ensure the required quality of products and intermediaries?
- Will failure of the instrument cause production losses? etc.
Speak to the panelmen and shift supervisors. They usually have some good ideas on what criteria you can use for criticality assessment.
Dejan IVANOVIC
Process Engineer, MSChE
RE: What is a Critical Instrument
Keith Cress
kcress - http://www.flaminsystems.com
RE: What is a Critical Instrument
RE: What is a Critical Instrument
4.1 Critical Equipment Definition
Critical Equipment [there are some exemptions] is defined as any individual item, device, or system that is directly or indirectly responsible for the containment and/or control of a flammable or other highly hazardous material. Typical examples are: pressure vessels, process piping, control instrumentation, valves, gas detectors, ...
Later in the document:
4.3 Critical Equipment Categories
4.3.7 Controls and Instruments
Devices used to regulate, monitor, shutdown, and/or isolate the PSM covered process. This includes all devices from the initial sensing element to the final control device.
So based on this and the fact that I'm dealing with an oil and gas processing facility, every instrument is considered "critical equipment". But the verbiage, "critical equipment" vs. "critical instrument". I need to understand if there is a differentiation between those, based on OSHA 1910 (I assume). The only exceptions for us are those instruments in water service, where there is nothing categorized as flammable or hazardous (to my knowledge).
RE: What is a Critical Instrument
RE: What is a Critical Instrument
The instruments used only for measurement indication (i.e. PI, TI, FI, LI) should not be on the critical instrument list, because their function is limited to process information. If there is any e.g. temperature indicator that is considered critical while there is no dedicated control loop for maintaining the required (set point) value of that temperature, then something is wrong in the design of process controls.
Criticality of a process control loop can be assessed in the same way as the criticality of the corresponding alarm (refer to alarm management study report, if available)- in other words, deviations from normal parameters are more important/critical for some control loops than for others, and they should be addressed first by the operator. Alarm Management is a (special kind of) criticality assessment for process controls.
The whole purpose of criticality assessment is defining the required performance targets (reliability) and from there defining the required maintenance and/or test intervals, usually in plant CMMS. The higher the criticality of an instrument/loop, the more frequent the preventive maintenance intervals will be. For safety instrumented loops, the required reliability and test intervals are defined by the loop SIL, but for process control system and F&G system this needs to be done separately.
Dejan IVANOVIC
Process Engineer, MSChE