Duty/Standby Ladder logic using Logix 500

[jguidotti]

Hello everyone, 1st time user (from Oz). I am currently writing logic to control 3 potable water distribution pumps with 1 pressure transducer(0-1000kPa)in the line. Having the pumps turn on/off at certain pressures is the easy bit. But I am having issues trying to set up Duty/Standby situation, eg if the pump requested to run fails how do I get the next pump to start(as at the moment it wont start until the pressure drops). To try and make this easier below I have listed what the On/Off set points are and what I wont to achieve.

<700 pump 1 latches
>750 pump 1 unlatches
<650 pump 2 latches
>700 pump 2 unlatches
<600 pump 3 latches
>650 pump 3 unlatches

1. Pumps can start in manual if motor starter ok (no problems)
2. 1 pump to run, demand changes, pumps 2 and 3 start and stop to keep line pressure stable - approx 700kPa. (no problems)
3. If 1 pump is running due to demand and fails (or is being asked for a start and fails) the next pump starts and so on.

If anyone has any thoughts or ladder examples that would be much appreciated.

Cheers

Ozzie Lad

 

[Tim Ledwith]

Oz,

Have your Pumps got to be fixed duty, or can we rotate the set points through each Pump ?
I'm thinking we designate the Pumps as Primary, Secondary and Tertiary and rotate the duty every 1000 operations or so (123,231,312)to even up the wear on the pumps ?
Do the on/off points have to stay at the same settings, or is all you are trying to achieve a steady 700kpa pressure in the line.

Thanks, Tim.

 

[jguidotti]

Tim

Thanks for your reply.

My thoughts as well was to be able to change the pump running duty as you mentioned (123,231,312) but I was going to do that via a selection switch????? as I did write some logic to do it via hours ran etc but it became quite messy.

I have altered the start/stop setpoints for the pumps to kick in all at different setpoints but all switch off at the same setpoint.
eg:
1. <700 latches
>750 unlatches
2. <650 latches
>750 unlatches
3. <600 lateches
>750 unlatches

I thought that this would then allow even if a pump fails that when the next one comes it will now stop at a higher setpoint.

But yes all I am basically trying to achieve is a steady 700-750kpa so it doesn't matter how the pumps operate.........

Thanks

James

 

[Tim Ledwith]

James, I've done something similar in a PLC5. I've attached a screenshot of the part of the program that does the duty rotation. Let me know if it is the sort of thing you were thinking about. The Pump "off" bits are set whenever that Pump is switched off, deselected or faulted - this enables the other pumps to 'fill in' as required.

Tim.

 

[Tim Ledwith]

James, Some more printouts for you to have a look at, one shows one way to define the Pump pressure on/off settings, and the other shows how to drive the pressure on/off operation. Obviously these pumps are set for a lot higher pressure than yours (350 bar), but it should be enough to give you some ideas to work from. Let me know how you go on.

Thanks, Tim.

 

[jguidotti]

Tim,

Thank you very much, hopefully i will get an opportunity to sit down today have a good look over what you have sent me and complete my logic.

You have been a great help.

Cheers

James

 

[Tim Ledwith]

James, No problem, let me know if you need any more info. Tim.