Soft Tag - What is it?

[leomiranda]

I have one P&ID that shows two pumps. I have one PIT installed in the admission and another PIT in the discharge of the both pumps. Each PIT send their signal to one virtual FI (there is one note saying that this FI is one SOFT TAG) and this FI is sum with the other FI of the other pump and the result is used in one FIC to control one FV valve. I’m lost here. What is soft tag? And what kind of control is this? I know that I can read flow using one PDIT. I’m thinking that this guy is using the two PIT to calculate de flow (calculating the differential pressure using the two PIT) in the pump and using this value to control the valve. Is this?

 

[JLSeagull]

Some type flow element is needed. The output from a pressure differential transmitter across an orifice plate corresponds to flow in a square root relationship. Perhaps the pressure differential across a pump could be calculated along a pump curve for an indirect flow measurement but this would not be precise. This would be thinking "outside the box".

Soft tags are those tags within the DCS software that reflect the inputs, status values, calculated values etc.

 

[controlnovice]

Soft tags represent the control module in the DCS or PLC that either displays the input, manipulates the input or value, or outputs a value.

It is the opposite of a "hard" device. Although not usually described as "hard', "hard" means an instrument you can physically touch.

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[leomiranda]

Thank you!!!!

 

[controlnovice]

In addition, Soft Tags are usually depicted with a different symbol....different from 'Hard' devices.

Soft (DCS/PLC) tags are usually an instrument bubble inside a square. This saves one from having to make notes and write "soft tag".

Do you have or can you get a copy of ISA 5.1?

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[leomiranda]

Yes, I have ISA 5.1