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Single transmitter for two ESD outputs
Single transmitter for ESD on HH and LL

Apologies up front, I'm a process engineer. Our client wants to use a single transmitter for a HH and LL output to ESD for compressor suction pressure.

Is this possible, permissible or should there be two separate transmitters, one for HH and the other for LL?

Do you know why your client prefers a single transmitter and what precisely is the device.

One way round the subject is HAZOP meeting where the fail safe / fail to danger and the implications can be discussed at length.

If the process impacts, the safety impacts, equipment damage and the production loss impacts from a false trip or a failure to trip are low enough then it's OK.

If impacts are high as determined by a HAZOP then using a 2oo3 transmitter arrangement is recommended.

3 transmitters with different 2oo3 logic for each high or low trip.

good luck

By W.L. Mostia on 22 June, 2018 - 3:23 pm

It is not an uncommon practice to use one transmitters for both HH & LL trips. The typical justification is that the HH and LL trips are mutually exclusive scenarios (e.g. cannot occur at the same time). In considering the risk of a single LOPA/SIF scenario (either HH or LL), the risk reduction (RRF) considers the transmitter for the HH or LL without concern for the risk reduction provided for opposite scenario, e.g. single cause and effect LOPA scenarios. However, when looking at the overall risk in the plant, the failure of the transmitter defeats two protection scenarios, which increases the overall plant risk and thus is not a good idea. Thus I would recommend two transmitters. Note that two transmitters also can provide 1oo2D voting and a transmitter deviation alarm, which can detect transmitter problems as well as redundant H or L alarms.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Eng. (TUV Rheinland)
WLM Engineering Co.

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

Thanks for your clarification. You have mentioned that it is not an uncommon practice. In which scenario it could be technically justified? If the transmitter PFD & HFT satisfies the SIL requirements for both the loops, is it allowed in compliance to IEC 61511?

By W.L. Mostia on 6 August, 2018 - 1:46 pm

In my opinion since SIF's are based on single scenario risk, a single transmitter can be used both for high and low pressure trips and meet IEC 61511. You might note that the trip valves are commonly shared, but they are also commonly redundant. I believe that in designing a system, you need to look at cumulative risk. If this question is driven from a cost perspective, e.g. someone wants to save the cost of a transmitter on a project, you have to ask if that person has safety first in mind?

William (Bill) L. Mostia, Jr. PE
ISA Fellow
WLM Engineering Co.

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson
Any information is provided on a Caveat Emptor basis.