Is there a specific way to depict a split signal for a PID control loop?

[plantprowler]

On many tank level control loops there's a simple PID controller loop with a level measurement element LT and then a LIC which transmits a 4 to 20 mA signal to drive a control valve on a line feeding the tank.

Now in this specific case I'm dealing with it so happens that there's two identical lines feeding the tank with two identical characteristic valves fitted on them. So the idea is to just split the 4 to 20 mA control signal and transmit it two both the valves.

What's the standard way to depict this on a loop? Sketch Below. The block in question is the one I have labelled as "Split". Does it have a standard name / notation? (Note this isn't the final detailed loop drawing; only a diagram made by a process engineer to communicate the control philosophy to the Instrumentation Engineer)

 

[xnuke]

You could use LY. The Y means "relay, compute, or convert." In what you've drawn, the bubble is relaying the signal to both valves. I probably wouldn't show a bubble on the P&ID, there would just be a node where the signal splits to the valves. I try to keep the P&IDs uncluttered but understandable, and I show the details about loop controls on loop diagrams. Also, setpoints aren't typically shown on P&IDs.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TD2K]

My experience is that there is no standard way how the instrumentation engineer or client wants these shown. Yes, I know ISA likely defines something but the individuals involved all seem to have their own preferences.

As a process engineer, I would just draw two dashed lines from the LIC bubble to each control valve and let my instrumentation engineer change it as he wants. I wouldn't include the "split" as I think that is self-evident. When this gets put on a P&ID, I assume you would also produce a control philosophy to explain to the Instrumentation engineer how you expect this loop to operate and there you would get into more explanation what you need it to be able to do. I assume one valve opens and then the other but it could be two sources that you want the ability to control the relative amount of each stream you want to use. What you need to be able to do with this loop is going to drive how you show this on a P&ID.

Your depiction is generally the level I would expect to see on a process flow diagram and adequately gets the concept what you intend this loop to do in my opinion.

 

[plantprowler]

Thanks!

Does this look good then?

 

[itsmoked]

Better would probably both originating from the LIC.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[xnuke]

I'd eliminate the small circle. As far as itsmoked's last comment goes, it's a matter of personal preference. I prefer to see one output signal splitting into two.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[plantprowler]

@xnuke

Thanks! I will get rid of that circle-node.

 

[roydm]

Not to be nit picky
I realize your sketch is just to support the question, and your drawing program doesn't support line types etc. A real P&ID should show a lot more information.

What type of transmitter?
What I see is a controller out in the field somewhere, not even on a panel certainly not in a DCS.
The valve should show more detail, you show a normally closed globe valve with no actuator.
Line sizes etc.
The signal lines should be dashed if it's an electrical signal.
Usually the setpoint signal is only shown if it's a remote
I would show an LY if it's a piece of hardware or something special in software for example split range.

Even if the initial information is wrong it will promote discussion.