Control diagram symbols

Hello guys,

I need your help about understanding symbols that are used in control diagrams. It seems that these symbols are somewhat standard.
For example, look at the example in attached picture. LT and FT are symbols for level and flow transmitter. SP is setpoint. Feedwater valve is an actuator. There's summing block, square root (for flow) and gain. Now the part I don't understand is the one which contains letters: ATA (three roms). It seems that setpoint is generated from that block. What these three letters represents?
I saw similar schemes before, but never really thought about exact meaning.

Is this some kind of standard blocks for control (something like UML diagrams for software engineering)?

Thanks

http://www.plctalk.net/qanda/uploads/control_diagram.jpg

I'm not very familiar with that type of engineering diagram, but my guess, would be that the ATA block is a pneumatic positioner for the control valve....

-MUR

That is the old Scientific Apparatus Manufacturers Association (SAMA) diagramming system. According to my reference (Control of Boilers, Dukelow 1986), the diamond indicates "Manual Signal Processing". The letter A indicates "variable signal generator" and the letter T indicates "transfer". The reference cited above has the exact same figure that you have posted, but does not go into specific detail about interpretation. My own naive reading is that the first <A> indicates manual entry of the set point, the <T> indicates transfer from control signal to physical valve action, and the second <A> may indicate manual PID tuning or bias.

Here's a diagram labeled with those function Matt described. (original has poor resolution of text characters)

Thank you guys,
I have found same info in one Siemens file on some web page.
Here's a complete explanation:
"The three diamonds ganged together represent the adjustable setpoint (left A), the adjustable manual output (right A), and the auto/manual transfer switch (T)."

P.S. Matthew, you have that Dukelow's book. Are you working in some kind of power plant by any chance?

Pandiani said:

P.S. Matthew, you have that Dukelow's book. Are you working in some kind of power plant by any chance?

Click to expand...

Nope, just happened to be interested in the topic and found a good deal on a used copy.