Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

What is a Critical Instrument

What is a Critical Instrument

What is a Critical Instrument

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!


RE: What is a Critical Instrument

There are many definitions of criticality of a device, instrument, equipment, or a system. Usually it means 'safety critical' but it can refer to something else as you noted.

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.

Process Engineer, MSChE

RE: What is a Critical Instrument

Thanks Dejan, that's EXACTLY my feeling (your last paragraph), however, I think they're depending on me to be the "decider", if you will.

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?


RE: What is a Critical Instrument

Below is an example of SCE classification/selection - see if you can use similar approach:

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.

Process Engineer, MSChE

RE: What is a Critical Instrument

If this is meant for process safety critical instruments, the first thing to do would be run a formal SIL classification exercise, if there isnt one already. When the dust has settled on this exercise, the SIL loop classification for each loop should then be marked on the plant shutdown matrix and / or the plant cause and effect matrix, and preferably described as such in the plant process safeguarding memorandum. One or more of these documents should then be the primer for the information you need.

RE: What is a Critical Instrument

So from our PSM guideline (this is a PSM facility, just found this document), here's a breakdown of what is written:

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

Strange, I've not come across devices functioning purely in monitoring or regulation service classified as process safety critical. Suggest checking with the custodian / author of this PSM guideline. These are devices acting as operational barriers against process parameter deviation, but not safety critical.

RE: What is a Critical Instrument

The definition "Devices used to regulate, monitor, shutdown, and/or isolate the PSM covered process" seems pretty vague, in the sense that - if no specific criteria are applied - literally every single instrument will be accounted for as a "critical" instrument, purely on the basis that the facility in question is an Oil and Gas facility. This definition/approach defeats the purpose of classification and criticality assessment. This is also an example where the PSM system fails to provide clear definition and scope, because it tries to be universal instead of specific.

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.

Process Engineer, MSChE

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


eBook - Functional Prototyping Using Metal 3D Printing
Functional prototypes are a key step in product development – they give engineers a chance to test new ideas and designs while also revealing how the product will stand up to real-world use. And when it comes to functional prototypes, 3D printing is rewriting the rules of what’s possible. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close