Two out of three logic a popular control scheme used in critical facilities .They are voting schemes which ensure a process condition status .These control scheme is mainly used in shutdown systems and high risk process facilities.This is for avoiding unwanted shutdown and at the same time a confirmation of a trip condition is also established .
Process condition are signalled to control system through three identical devices .The logic initiates an output by 2oo3 voting schemes .Means it needs two values to be matched to a setpoint out of three for the output initiation.
For example a high pressure alarm should cause a shutdown initiation .And we are using 2oo3 logic . Three identical pressure sensors used . And if two of them goes high ,then shutdown will be initiated .If only one sensor goes high nothing will happen . Because chance of a sensor to get faulty. And there is a chance to make false alarm .Also there is a chance not to detect a alarm .By employing Two out of three logic ,we can avoid the effects caused due to the sensor faults .2 o o 3 logic is for safety purposes It also it helps to avoid unwanted trip or shutdown or output initiation ..
Ladder logic- 2 o o 3
Let us design a ladder program which represents 2 o o 3 logic .
Problem
.A pipeline carries a compressed gas . when pressure reaches above some setpoint ,then it should cause a shut off valve to close .
Three identical pressure transmitters are used .
Valve .-Normally close .SOV operated .The valve will close on de energizing the SOV.
When pressure is normal SOV must be energized .Means valve open.
When any two sensors gone high out of three sensors then valve should close by de energizing the SOV.
The below diagram shows the required ladder logic .PSH1 PSH2 PSH3 represents the discrete bits of pressure transmitters high alarm status . They are the discrete status derived from assigning a setpoint on the analogue value of the transmitter .The states of these bits are as follows
1 – when pressure is normal
0 – when pressure above the set point .
Let us examine some of the process conditions and analyze how the 2 o o 3 logic works.
When all the transmitters senses pressure below setpoint .
This means the condition are normal .
SOV energized
Valve open.
See below figure.
When any one of the transmitters senses pressure above setpoint .
This also means conditions are still normal because only one sensor reads high .And it is not sufficient to initiate a shutdown .
SOV is still energized
Valve still open.
See below figure. PSH 1 is high .But the valve doesn’t closes .
When any two of the transmitters senses pressure above setpoint .
This means conditions are not normal .Because out of three sensors two are reading high value .This is regarded as a confirmed case .And action is initiated .
SOV de energizes.
Valve closes
See below figures
. In figure 1 PSH 2 and PSH 3 goes to high .
PSH1 and PSH 2 reads high value see below figure.
Two out of three logic can be implemented through ladder logic through different ways .We have to consider the input status and output status at normal and alarm conditions .Which means whether input is energized or de energizes in normal or alarm conditions. Or whether energized output or de energized output is needed in alarm or normal conditions etc.