Safety Critical 2

[swtmoon]

In many project, I've met the word "Safety-critical equipment".
And experienced that some particular systems such as flare system, fire fighting system, ESD systems etc. have been considered as "safety-critical equipment".
It is generally agreed but hard to define clearly.

Are there any specific definitions on international standards(eg, BS, API, NFPA etc.)? or text or articles?

 

[epoisses]

The general definition is: Safety-critical equipment is the "last line of defense" to prevent an accident. SC equipment should be independent (i.e. a level controller cannot be a SC low-level cutout at the same time) and should be periodically inspected/tested. SC equipment should therefore be clearly identified and listed (who would remember to test it otherwise) and that also means that you shouldn't just call anything SC too easily, because that would incur very high maintenance cost.
Yep, I guess there must be internation standards that describe things in detail, no doubt somebody can tell you exactly which ones.

 

[hacksaw]

There is a lot of work done on safety. The safety categories are typically broken down interms of the consequences to human life and equipment or cost to repair or replace. Often, but not always risk is defined interms of single item failures. There are key differences between industries e.g. process chemical vs aero-industry.

The general definitions are frequently included in handbooks on process safety, but you can even find excerpts from the various standards in vendor literature.

Good luck

 

[IRstuff]

Failure Modes, Effects and Criticality Analysis (FMECA) is often required in military contracts to assess the possible ways that a system might fail and how that failure might be manifested. The various levels of damage include: damage to equipment, injury to personnel or death to personnel.

When coupled with the probability of occurrence, this develops the overall risks for the system that are to be mitigated. In many instances, simple design modifications can prevent certain effects from occurring. In others, auxiliary equipment become critical because you depend on them to prevent certain classes of injury or damage.


TTFN

 

[swtmoon]

Thank you all,

Through the all posted, I can figure out the definition of SC as;
1. Independent from the process.
2. Can be identified during the risk analysis e.g. FMECA, QRA, Consequence Analysis, Fire Protection Analysis..
3. Provided to prevent certain level of equipment damage, personal injury or death.

Firstly, fire fighting, emergency shutdown/depressuring, flare system comes to my mind. is it right? or what else can be safey critical equipments or systems?

 

[IRstuff]

What you've listed seem mostly to be ex post facto.

Monitors, gauges, alarms, fail-safes, built-in-test, etc., are what can potentially prevent or warning of an impending safety problem.


TTFN

 

[swtmoon]

Thank you TTFN,

But I'm still curious.
Monitors, gauges, alarms can prevent safety problem also process upsets.
But can it be the last line of defence?
In my opinion, alarms and indicators can not prevent escalation of crisis when it already happen.
I feel that fail-safes or fire and gas detections rather can be the safety-critical items.


SweetMoon

 

[epoisses]

I tend to agree with your conclusion, SweetMoon. But definitions in aerospace can be different.

 

[IRstuff]

Safety is not about last lines of defense, but layers of defense. It's about preventing, as well as mitigating the aftereffects of, accidents. That's why a design that has a lower surface temperature is preferrable to one with a higher surface temperature, but with a shield.

Your ability to receive an overtemp warning in a reactor is MUCH preferrable to only rely on a containment vessel after the fact. If you study ANSI Z136.1-2000, which is the ANSI laser safety standard, you'll see that much of the safety infrastructure is about procedures, training, warning lights, interlocks, safety glasses, warning labels and shields. It's much preferrable to have prevented your eye from getting zapped than to know that you've got a medicine cabinet handy for healing the burn.

Fire sprinklers are indeed the last line of defense against fires, but if that's the only line of defense, you're going to lose. That's why smoke detectors are so prevalent, because, in many instances, it can warn you about an impending fire that could be easily controlled or extinguished, rather than waiting for the fire to get so large than you have to depend on your sprinkler system.

As an industry expert puts it, "Only you can PREVENT forest fires."



TTFN

 

[pandora]

swtmoon,

Some good comments here, especially from IRstuff. A bit more of a help might be to check out two things. One is IEC61508 (or IEC61511, which applies to the chemical industry). This sets out semi-quantitative methods for assessing "safety critical" systems, using a fault tree type diagram. Others also want to use the LOPA (Layers of Protection Analysis) which means that you have many layers that are "safety critical" because many layers must fail, so 'normal operation' might still have "safety critical" elements integrated into it.

pandora