Table of Contents
2 out of 2 sov is used to reduce the failure rate of the field instrument. When a single instrument is used, its failure rate is greater than two out of two sov.
Reasons for failure of single instrument :–
1. Single DO(Digital Output) assignment.
2. Single cable from control room to field.
3.Single instrument used in the field.
Benifit Of Two out of two SOV :-
1. Two DO Channel assignment to one output in the control system.
2. Two separate cables from the control room to the field.
3. Use of two separate instruments for one output in the field.
2oo2 sov is made using two 3/2 sov’s. While making it, care has to be taken whether to make its common bypass or not.
Configuration:-
To put the two out of two sov in the final control element, two separate digital outputs are made from the control room and taken through different cables to both the SOVs.
Since 2oo2 sov is used for redundency, it is configured in different slots in DCS / PLC.
Working :-
When both DO are activated–
When both the digital outputs are activate then sov1 & sov2 will be energized and the inlet air from port 1 to port 2 will go to the final output.
When DO1 Activated & DO2 Not activated :-
When digital output 1 is activated and a digital output 2 is not activated then sov1 is energized and sov2 is deenergized then the inlet air will go out from port 1 of SOV 1 to port 2 to the final output.
When DO1 Not activated & DO2 Activated :-
When digital output 1 is Not Activated and digital output 2 is activated then sov1 is denergized and sov2 is energized then the inlet air will exit from port 1 of SOV 2 to port 2 to the final output through port 3 of SOV 1 to port 2.
When both DO is Not Activated :-
When digital output 1 and digital output 2 are not activated, sov1 and sov2 are de-energized, the inlet air will be blocked at port 1 of SOV 2 and SOV 1. No air will go to operate the final control element.
Use Of two out of two SOV
The two out of two SOV is used to work out the failure rate of the field instrument. When a single instrument is used, its spread rate is greater than two out of two SOV.
Advantage & Disadvantage of two out of two SOV
Some of the reasons for failure of a single instrument are as follows –
- Single DO assignment.
- Single cable from control room to field.
- Cable is an instrument used in the field.
Due to failure of work using two out of two SOV.
- Two DO Channel assignment to one output in the control system.
- Two separate cables from the control room to the field.
- Use of two instruments for one output in the field.