Sharing of Transmitters for Different Control Loops

[Sawsan311]

Dear All,

I am seeking your views in the acceptance of good engineering practice to have a common transmitter used to send signal to two different control loops whih perform different functions. For example, a flow meter sending signal to minimum flow recycle control loop, same transmitter is used to send the flow feedback to a flow control loop regulating the same pump speed to achieve a specific set point. Do you agree that transmitter burn out would cause losing both loops and hence as a good engineering practice, it is not recommended to have such sharing of sensor among wo BPCS.

Thanks

Regards,

 

[TiCl4]

I don't see why you would need independent flow sensors here - a loss of flow indication will likely already shut down the pump, meaning flow control and minimum flow recycle control will not be needed.

 

[thebard3]

I cannot understand what you are attempting to do. It seems that you want a common sensor, the signal from which is affecting 2 different control elements and either will affect the same process value? I expect each would fight the other.

Brad Waybright

The more you know, the more you know you don't know.

 

[georgeverghese]

No harm done with the use of transmitters for more than one control loop, as long as the transmitter is not in any process safety function. Saves on CAPEX and OPEX maintenance.

 

[Sawsan311]

Dear All,

thanks for your replies, considering the failure of the common shared transmitter can be latent (unrevealed) and if the transmitter is not provided with its own diagnostic coverage of the dangerous faults, then surely we would agree that sharing among two BPCS is not recommended.

Answering your Mr.Bard3 question, the same transmitter is used for sending a signal to minimum flow recycle valve and at the same time to pump's VFD. We agree that in case of low flow, the VFD controller would still dictate ramp up the speed while the controller towards minimum flow would dictate recycle operation. This ends up in having the two controller conflicting each other and mandating some kind of loop decoupling to override one another to accommodate the low flow scenario.

 

[Compositepro]

You seem to be seeking confirmation of the answer that you have already decided is correct. There is no inherent reason to not share the transmitter. Whether you should or not depends on many economic and safety factors, which we have no knowledge of.

 

[TiCl4]

Sawsan311,

We surely do NOT agree that sharing a transmitter reading among two different BPCS is inherently wrong or unsafe. The application needs to be considered based on the individual case. Certain requirements, such as adhering to a SIL-level or coupled-failure scenarios, may dictate more than one flow device and transmitter.

However, in your particular case, where you have a flow reading that governs both the pump speed and minimum recycle flow - potential control/interlock SIL rating issues aside - there is NOTHING inherently wrong with a shared transmitter.

 

[georgeverghese]

If the calibrated transmitter range required to operate two control loops is wide and results in reduced sensitivity in process parameter readout for one or more of the control loops, then it may be better to use to 2 separate transmitters. This is the only reason I can see for dedicated transmitters.