Help!

I am trying to add 4 valves and a pump to an existing RO ststem using an Allen-Bradley PLC 5. I only need discrete I/O to open/close the valves and turn the pump on and off when an existing pump secures or starts. My questions are:

1. Can I piggy back onto an existion I/O such as the existing pump?

2. Can I use one I/O for 4 valves to open/close at the same time?

3. Will I need to us additional physical relays for the new valves and/or pump?

4. Should I use 24vdc or 120vac valves? 110 or 220vac for the pump?

5. What should the ladder logic look like fi when the existin HP pump secures, I want the valves to open and the pump to start and run for say 30 seconds to fill the RO with water and then reset to normal operation? Also, there will ve a hand-off-auto switch added and if the system needs to start while the valves and pump are running, it needs to secure and start it's normal routine.

Thank you,

Please don't take what this wrong, but I would hate for you to make a serious mistake here.

From the questions you are asking, it sounds like you need to at least get a consultant into your facility to review your needs and make some recommendations. If you attempt to do this project on your own, even a small mistake or oversight could easily cost you a lot more than hiring it out would cost.

Now to your questions:

1) Yes, you COULD piggyback off existing outputs, but as a rule, this is not a good idea unless you isolate the PLC output from the multiple loads by way of a relay or some other isolation technique. By doubling up on an output, you run the risk of overloading it and damaging your hardware. That can be quite expensive in terms of downtime and component costs.

2) Yes, once again, you COULD gang them all together but it is not a good idea unless you isolate.

3) I would think you should plan on additional relays.

4) If you are going to use isolation, the voltages of your valves and pumps is not particularly important so long as they are correct for the overall system. Where possible, for maintenance reasons, I try to stay with voltages common to the existing equipment.

5) Your ladder logic is totally up to you and your system. To try to give you guidance with it without knowing your existing system would be, IMHO, foolish.

As I said before, it sounds like you are really taking a big risk if you attempt to modify this system without outside consultation. One very reasonable approach for you would be to contact the local distributor for your PLC. Typically those guys have in-house people who can help with small projects like this and they already know their equipment's limitations.

Best of all, they might do it really cheaply.

Steve

Ditto

I second the motion, call in a consultant.