INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

How to handle hardware that spans multiple loops

How to handle hardware that spans multiple loops

(OP)
Hi Folks, two questions:
1. We have several cases where a single physical block of valves, with a single common DeviceNet interface, are connected to multiple loops. What is the recommended way to identify (TAG) the stack?
2. Similar to question 1 we have I/O blocks that connect to multiple loops. In some cases the individual modules connect to multiple loops. What is the recommended way to identify those?

Regards,

Greg

RE: How to handle hardware that spans multiple loops

Q1. If you have to tag the physical block, I'd give a block of valves a "UV" tag - "U" means multivariable. Then, each valve within the stack would be tagged according to the loop that it is in. If you don't have to tag the physical block, just tag each valve according to its loop and identify the assembly it is in with a label outside of the bubble.

Q2. Can you give an example of this?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: How to handle hardware that spans multiple loops

(OP)
Hi xnuke,

Tagging each valve is a bit problematic. One stack we are using is http://www.festo.com/cat/en_us/products_VI10_10 CPV10. It has 8 slots that each hold 2 3-way valves for a total of 16 control points. It is possible to trade out the contents of a slot but in practice the stack is treated a single unit.

The system consists of a PLC connected via DeviceNet to distributed blocks of I/O. In one case the first 3 channels of a digital input module are monitoring a 3 position level sensor and the fourth is monitoring a pressure switch for a different loop. In another case channels 1 & 2 monitor spill sensors, 3 the power good output from a power supply, and the fourth is connected to a smoke detector.

I'm new to using P&IDs, Loop Diagrams, etc... Reading ISA 5.1 and Instrumentation And Control Systems Documentation went a long way towards getting me up to speed but it still isn't clear to me how to document multi-channel I/O type stuff and stuff that is shared by multiple loops.

Thanks for the help!

Greg

RE: How to handle hardware that spans multiple loops

You say that "in practice the stack is treated as a single unit," but I'm guessing that's not really true. Each device always gets its own tag because they will each need to have individual representation in the PLC code, and even though they may be in a block, you're sensing or actuating each device separately. It doesn't matter if they are connected in a valve manifold block or I/O connected to the controller via DeviceNet. I still might tag the valve manifold block UV, since it as a unit could fail and I keep a material history for all equipment, but each individual valve withing the block would get its own tag also since each could be replaced.

For your first DI module, you'd have tags of LS for your level switch inputs and a PS for your pressure switch. For the second, I'd probably tag the spill sensors as YS, the power supply ES or JS, and the smoke detector YS. (Y means event, state, or presence.) For the YS instrument bubbles, I'd add an amplifying label of SPILL or SMOKE outside the bubble to provide more information.

What brand of PLC are you using?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: How to handle hardware that spans multiple loops

Showing Bus architecture on a P&ID doesn't add much value. So, on the P&ID, it doesn't matter that 4 devices/instruments and wired together to one distribution point.

The P&ID should show the Instruments and how they relate to the process equipment/piping/etc... and how they relate to each other in a control loop. ISA 5.1 does recommend line difference shown, such as:

Electrical: ---------------
Internal or software link: ---o---o---o---o---o--- (this is generally shown between two controllers e.i. A primary controller cascading to a secondary controller...both of which remain in the PLC/DCS controller...no physical wiring)
Pneumatic: ---//---//---//---
and there are others as you've seem if you have ISA5.1.

In typical systems, the connection between PLC and Instrument is a pair of wires, shown on the P&ID coming from a Shared Control (Box 4 of General Instrument symbols of ISA 5.1) going to an instrument bubble with --------- between them.

Using Profibus, you could use the ----o-----o----o---- line between them instead, to show that it is a 'soft' connection. The instrument bubble will indicate that it is a field instrument, so users should understand that it is a 'hard' device, but connected 'soft'ly...

Loop Diagrams can be a bit more troublesome: Do you put the distribution point/trunk with all its instruments on one loop drawing? Or each instrument on an individual drawing that references the Distribution point/trunk?

My preference is each instrument has its own loop drawing. The Distribution point/trunk would be referenced on each loop that connects to it. One would still show the ---o---o---o--- line between them on the loop drawing.

Hope that helps. If you have specific questions, PM me with drawings, or include a sketch on this site.

______________________________________________________________________________
This is normally the space where people post something insightful.

RE: How to handle hardware that spans multiple loops

(OP)
I'm still not fully connecting the dots so please bear with me. Let's try a different example: A float switch with four floats. The top one is tagged LSHH because it is used as a hardware cutoff should the vessel get too full. The remaining ones are LIH, LIL, and LILL. On the P&ID I understand that it doesn't matter that they are part of the same physical device. It will take some training to avoid folks assuming they are four separate devices. What I don't understand is where is the link made between the four logically separate pieces on the PI&D and the single physical unit that goes on the electrical loop diagram that my tech uses to wire it up?

Thanks for the help!

P.S. I have not been able to figure out how to PM on this site.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close