Ladder logic fix required

[Tobester]

I'm a chemical engineer trying to be an electrical. I have a problem here with two automated valves feeding two tanks. I have been able to set them to switch when one tank is full and both open when both tanks are full. However I'm having trouble when they both empty at the same time. The logic requires one full and one empty for switching however in the off chance that both get emptied a valve needs to close. Can anyone suggest anything???



An Aussie Cheers

 

[Steve_D]

Bud, just choose one and if you have both tanks empty i.e. M6a and M6b, turn on Y508 (in parralel with its normal condition i.e. M6A). Just a note.. your really making a rod for your own back with Sets and Resets, just use a normal output with a latch, its so much easier to follow.

 

[RheinhardtP]

Stupid question

If the valves are feeding the tanks, why would you close them if the tank is empty?? What type of valve Energize to open?

Can you draw a mini P&ID, with Flow diagram would help......

Regards

PS SteveD is right this is what a self latching rung look like

Open valve condition == I1
Close valve condition == I2
Output to Valve == Q1

......I1.....................I2..................................... Q1
----[ ]--------------[ / ]--------------------------( )---
...............!
----[ ]-----!
.....Q1

Q1 latches the rung when Open valve condition is high.
Circuit will latch till I2 is high and rung will be broken.
Q1 contact at bottem is called self retaining contact.

 

[Tobester]

Thanks for that, gave the simple option a go and it seems to work fine. I'll have a look a revising the overall program and replacing with self latching rungs, when I'm confident enough to understand them.

Cheers'