Utilization of sensors in 2oo3 Voting logic 2

[amar122amar]

Three sensors are mounted on a vessel (V) at elevation of 6000 mm (sensor A), 6600 mm (sensor B) and 7200 mm (sensor C). These three level switches generate a trip signal (2oo3 voting logic) to stop flow from upstream system to vessel (v).
Can we use sensor A to control level in vessel (V)? On actuation (healthy condition) of sensor A, valve at the bottom of vessel (v) will open and on reset of sensor A, valve at bottom of vessel (v) will close.

 

[IRstuff]

Why do you think it wouldn't be usable?

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[LittleInch]

No.

Sensor A is then part of your PCS, but your ESD system.

A 2oo3 trip system would need three sensors all set at 7200.

Sensor B is then your LSHH alarm.

What you describe is not, IMHO, acceptable as an ESD trio system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[amar122amar]

Thanks LittleInch,
Can you elaborate, why it is not acceptable as an ESD trio system.
How it will affect effectiveness of 2oo3 logic system.

 

[LittleInch]

The usual practice is to separate Process control from ESD/ trip systems.

You're using the lower level switch as a process control and hence this is not a shutdown or trip level. Also you can't then use it as part of a trip level.

I've also never seen a 2oo3 system set at the different levels.

A 2oo3 system is intended to give you a higher SIL level, but it is set at one set point, be it level, pressure or temperature. Each sensor should have its own connection to the process.

You should also have alarms set at a sufficient distance below the trip level to give your operators a chance to prevent the trip.

I suggest you discuss with your controls engineer.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Shifting sensor A to process controls will mean supervisory access only to this sensor at PCS will have to be disabled in order to allow operators to change control level settings etc. This arrangement will then make your 2oo3 voting trip loop logic degrade to a 1oo2 voting trip logic with only sensors B and C. Some times, this reduced level of reliability with 1oo2 voting logic may be acceptable - check with your process safety engineer who should advise if this downgrading is acceptable.

Why do you have this unusual 2oo3 voting logic for high level trip on this vessel to shut off inflow ?

 

[amar122amar]

Thanks LittleInch and georgeverghese for your replies.

 

[mk3223]

Typically, the voting logic, i.e. 1oo2, 2oo3, etc., is part of the SIS, Safety Instrumented System, to incorporate safety system in the process design. The required redundant instruments, in additional to the process control instrument are defined by ISA 84 to achieve the required safety and reliability in the plant operation.