Voting Arrangement
Voting Arrangement
What factors would lead us to select a 2oo3 voting arrangement instead of 1oo2 when considering SIS?
Re: Voting Arrangement
Selecting 2oo3 instead of 1oo2 in a Safety Instrumented System (SIS) is primarily a balance between safety integrity, spurious trip reduction, availability, and economics.
The decision should be based on a formal risk assessment and lifecycle requirements (such as those in IEC 61511 and IEC 61508).
Spurious trip reduction is the biggest driver for selecting 2oo3.
Imagine a high-pressure trip on a methanol synthesis reactor.
If one transmitter drifts high:
1oo2 → One transmitter alone causes shutdown.
2oo3 → The other two healthy transmitters outvote the bad one.
Result would be like avoid unnecessary plant trips, prevent production loss, reduce thermal cycling, increase operating availability.
This is especially valuable when shutdown costs are extremely high, restart takes many hours, catalyst life is affected or large compressors must restart.
The decision should be based on a formal risk assessment and lifecycle requirements (such as those in IEC 61511 and IEC 61508).
Spurious trip reduction is the biggest driver for selecting 2oo3.
Imagine a high-pressure trip on a methanol synthesis reactor.
If one transmitter drifts high:
1oo2 → One transmitter alone causes shutdown.
2oo3 → The other two healthy transmitters outvote the bad one.
Result would be like avoid unnecessary plant trips, prevent production loss, reduce thermal cycling, increase operating availability.
This is especially valuable when shutdown costs are extremely high, restart takes many hours, catalyst life is affected or large compressors must restart.
Re: Voting Arrangement
1oo2 is usually preferred when:
The hazardous event develops very quickly.
Missing a real demand is considered less acceptable than an occasional nuisance trip.
The process can be restarted easily.
Capital cost is a significant constraint.
SIL calculations show it adequately meets the required risk reduction.
Examples include some burner management systems, emergency isolation functions, or smaller process units where trip consequences are relatively low.
2oo3 is commonly selected for:
Methanol plants
Ethylene crackers
LNG trains
Ammonia plants
Large gas compressors
Gas turbines
FCC units
Hydrocrackers
because an unnecessary trip can cost hundreds of thousands of dollars per day.
The hazardous event develops very quickly.
Missing a real demand is considered less acceptable than an occasional nuisance trip.
The process can be restarted easily.
Capital cost is a significant constraint.
SIL calculations show it adequately meets the required risk reduction.
Examples include some burner management systems, emergency isolation functions, or smaller process units where trip consequences are relatively low.
2oo3 is commonly selected for:
Methanol plants
Ethylene crackers
LNG trains
Ammonia plants
Large gas compressors
Gas turbines
FCC units
Hydrocrackers
because an unnecessary trip can cost hundreds of thousands of dollars per day.