how does a 2 out of 3 voting logic works in instrument protection system?

Question

fire and gas system or automatic shutdown system

Answer 1

I'm going to assume that you know what you are asking about and not go into trivial detail.

A two out of three (2 oo 3) voting system can be used in any instrumentated system and it achieves 2 things.

1) If there is a problem, it increases you chances of detecting it and taking remedial action

2) It reduces your chances of taking action (shutting down a fired heater for example) when there isn't really a problem.

In terms of unwanted shutdowns, each instrument has a given probability of failure (once every 5 years is a pretty good number as a rule of thumb). If only one has to fail to produce the unwanted shutdown you will have a spurious shutdown every five years or so. If 2 have to fail for a spurious shutdown, this is reduced to once every 25 years.

Failure to detect a dangerous situation is more difficult to represent, but is also significantly reduced. It has to be said that most of the applications I personally know are for increasing plant reliability rather than increasing intrinsic safety. There are other considerations for safety (which include 2 oo 3) available under the SIL / SIS codes.

Typical measurements in combustion might be fuel pressure (high and low trips), pilot gas pressure (low trip) O2 in flue gas (low trip), air flow rate (low trip) or firebox or convection section temperatures (high trip)

Its not perfect, and how well it works depends on how its defined and how its configured and how well it is maintained.

The best system is with three instruments that are continuously measuring the variable, are checked regularly in the field and are set up with deviation alarms.

The second best system is with switches which give digital output at a certain value but nothing if that value is not detected (no continuous measurement, so the deviation alarm is lost) and with routine calibration and maintenance.

A 2 oo 3 system which is not maintained or tested swiftly loses its advantages.

The signals feed into a PLC which as a result of the input takes action by sending a digital signal (to close a valve for example). The PLC which provokes instrumented emergency action has to be an FSC, a fail safe controller. that means that if its not sure what the situation is, it will shut the system down.

Any help?

Answer 2

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The answer is 6. It can't be 0, because 0/0 is undefined mathematically. The (x-3) factor cancels out, leaving 3+3 = 6. (x² - 3²) / (x - 3) (x + 3)(x - 3) / (x - 3) (x + 3) To answer Tom below: Technically you're correct that if you apply it directly to the problem at hand, the answer is actually not 6, because this system is undefined, but using calculus and the principles of limits, the answer to this approaches 6 from both sides. e.g. 2.9^2 - 3^2 / 2.9-3 = 5.9 2.99^2 - 3^2 / 2.99 - 3 = 5.99 2.999^2 - 3^2 / 2.999 - 3 = 5.999 3.001^2 - 3^2 / 3.001 - 3 = 6.001 3.01^2 - 3^2 / 3.01 - 3 = 6.01 3.1^2 - 3^2 / 3.1 - 3 = 6.1 etc. You can use L'Hopital's rule on the system to actually provide the correct answer, which is 6. In that case, you have: d/dx(x² - 9 / x - 3) 2x / 1 So, if x = 3, the answer is again shown to be 6.

Answer 3

in case of your fire system. If you have a space and install 3 detector heads, your 2 out of 3 logic would alarm if 2 heads detect a fire - even if the third doesn't, because it has for example failed or the smoke or the heat has not gotten to it yet...it would alarm if all 3 detect it and not if only one detects.
I hope that is what you wanted to know.

Answer 4

It is a way of ensuring that a single failed sensor or failed actuator does not shut down a critical safety system. The inputs from the sensors can be logically combined by the processors or by relays so that at least two must agree that the system is safe or endangered before any sensor chain is believed to be true.