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Control Logic Diagram

Control Logic Diagram

Control Logic Diagram


I am reviewing a drawing named as "Control Logic Diagram" submitted by a design engineer. (Please see attachment). The drawing (along with other similar drawings) is supposed to be used by SCADA system subcontractor to design the control system. Control logic diagrams i am familiar with are constructed with basic logic gates and function blocks and the logic is very clear.

Does this diagram provide sufficient information for the programmer?

RE: Control Logic Diagram

PA28; I am interested in your question but I am definitely NOT interesting in opening some unknown zip file, nor, I doubt is anyone else around here that foolhardy, but you never know.

I suggest you unzip it for us and post the pages. Use the above. You will then get lots of answers.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Control Logic Diagram

My process & instrumentation experience is a little rusty but I would call this a Piping & Instrumentation Diagram (P&ID) or a Process & Instrumentation Diagram. These are used to define process instrumentation piping requirements and instrumentation signal requirements.

My experience is that piping & instrumentation techs use these diagrams to make their piping, instrument, and signal run installations on the overall system construction.

The short answer to your question is "yes." Examination of the symbols show:
  • function of setting the control set point SP (FLOW SET POINT input to the FC flow controllers)
  • instrumentation signals that give the control system the present value PV such as level sensors LS in those two sumps
  • the FC units output a signal to the variable frequency drives VFD which turn the motors driving the INTAKE PUMPS
  • there is also some sort of control connection LINKAGE CONTROL TO PUMPS that actuate what I assume to be some sort of flow throttling valve on the output side of the pumps
There's probably a few more details that I clearly have missed that will be picked up by my more experienced colleagues here on Eng-Tips.

The ISA International Society of Automation isa.org is your friend for educational materials.

Blue Technik LLC
Virtuoso Robotics Engineering

RE: Control Logic Diagram

Howdy PA28,
I think that it depends upon the programmer, as well as the nature of the contract, as to whether the (attached jpeg) P&ID/Logic diagram is sufficient to permit him (or her) to configure the control system.

Whereas a detailed logic diagram for such a process would likely require many sheets to convey the exact configuration, an experienced programmer should understand that the (attached jpeg) diagram illustrates only the '30,000ft view'.

However, if your programmer is hired on price alone (ie fixed price lump sum), you might be in for some pain.
Are there any other programming standards, or Scope of Work document, to accompany the (attached jpeg) diagram?
Has the programmer done work for this Client before? Is the programmer aware of the Clients configuration standards?
Good luck,

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Control Logic Diagram

Thanks for JPEGing the drawing PA28.

I think that's a great drawing. Wish the ones I usually get were that nice!
I would say that is definitely NOT enough. I would expect a text based detailed description of what the controls need to be doing to go along with the drawing. It would contain stuff like:

A brief description of the expected operational procedure
Expected steady-state flow rates
Estimated setpoints
The important operating points
Expected valve/pump interaction expectations
Expected water levels
Startup procedure

And, I'd add that the subcontractor suggest any additional controls, sensors, or effectors they feel would enhance the system. Often someone who's doing this type of work knows tricks-of-their-trade that can make a system more stable, reliable, or repairable just by adding a few additional things. You really don't want them to tell you this after the system proves difficult to control or open to operational guesstimates. With their input you can decide whether to follow their suggestions or to at least install piping ports and wiring in case the additions become urgent requirements.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Control Logic Diagram

Often you will spend as much time developing the HMI and user setpoint handling for a system as you will on the loops themselves. That aspect of the control system needs to be defined clearly, although that diagram is probably not the place.

I agree with Keith that there's not enough there to develop a working scheme, but there might be enough for a budgetary estimate of the time / work involved.

RE: Control Logic Diagram

Thank you all for your valuable responses.

Clearly the drawing is not enough for the programmer, but sufficient for the assessment of the work ahead.

I think the use of logic diagram for the drawing is in correct, it should rather be described as P&ID with control overview or something. Actual logic diagram may go for many more pages and it should generally be for each piece of equipment. As you can see for the INTAKE pump only the speed control path is presented but pump ON/OFF control is also necessary.

However i have to accept this drawing, as there is no one else who can do a better one and there is no possibility of going for another company. Hopefully the control subcontractor with his experience in similar projects will pull it through and save all.

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