3 replies to this topic

[Mayank Joshi]

Hi,

   I want to know how does having separate transmitters(Flow, Pressure) for PLC increase reliablity ?

 

Egs if their is a trip of Low Flow then in some plants there is a flow transmitter for DCS and 3 separate transmitters for PLC (2 out of 3 voting) but in some plants there are only 3 transmitters which are common for both DCS and PLC.

 

Also do we feed the Raw value coming from field directly to the PLC(without compensation) , how does this help.

 

Is there and OISD norm which requires separate system(PLC) for S/D and DCS for control??  

[AlertO]

hi Mayank

 

Share or common element or transmitter depends on the SIL level. For SIL level 1 and 2, we can use them commonly under voting  algorithm; otherwise, we have to separate them. 

[Mayank Joshi]

Can u please elaborate

[AlertO]

OK. let me explain more.  

 

normally as your understanding is correct we should separate SIS and BPCS to eliminate the common failure risk. as per the statement in paragraph 11.2.10 of  IEC 61511.

 

“A device used to perform part of a safety instrumented function shall not be used for basic process control purposes, where a
failure of that device results in a failure of the basic process control function which causes a demand on the safety
instrumented function, unless an analysis has been carried out to confirm that the overall risk is acceptable.”

 

It emphasizes about the analysis, right?

The analysis shall show if the sensor diagnostics can reduce the dangerous failure rate sufficiently
and the SIS is capable of placing the process in a safe state within the required time.

 

Anyway in Shell DEP 32.80.10.10-Gen. Section 6.7.3, it summarizes this exception as:

 

"Sharing of sensor elements (common transmitters) shall only be permitted when all 3 of the
following conditions exist:
•  SIL 1 or 2 (not SIL 3)
•  Safe and dangerous fault tolerant sensors are applied (e.g. 2oo3)
•  Sensors used for control via a middle of three voting algorithm"  

 

However, to permit this exception shall be under the consultant of experience safety people.

 

Hope this may help you