Pump control from 4-20ma level

Dakotap92

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Join Date
Jun 2015
Location
Princeton wv
Posts
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Ok so I work at a small water treatment plant and have been studying ladder logic for a couple of weeks because we are maybe makin the switch to a plc because the bristol rtu we have is quite old . I cannot for the life of me figure out how to write the logic for a pump controlled by a tank .... Basically tank is 30ft tall with a 4-20ma pressure transducer.. pump needs to operate when at 25ft and shut off at 29ft. This is probably so simple but I'm new and eager to learn. I have download the rslogix micro to play around with but have no hardware as of now.
 
You will need a PLC with an analog input for the pressure transducer. You can use the SCP instruction to scale it. What shape is the tank?
 
I was think about going with a micrologix 1400. We have about 8 4-20 signals to monitor as well but the main thing I was stumped on was this logic . I tried messing with the less than but I suppose there's something more fitted for this function. The tank is just a cylinder.
 
After you have the scaled to engineering units then think compare instruction(s) with a latch (or seal in).
 
The tank shape does not matter because hydrostatic head pressure sees only the height of the liquid column and he knows the elevation control points, 25 ft and 29 ft.

For sensing, what matters is where the 25 and 29 foot points are in relation to where the pressure transmitter is tapped into the tank (the zero level point) and whether the transmitter itself is level with the tap.

But that stuff is likely there already, because they had a Bristol RTU. I think it's the pump logic, turn on at 25, stay on until 29, turn off at 29, stay off until 25 that he's working on.
 
Yep hit the nail on the head danw. My dad actually installed the bristol stuff along with fix 32 to monitor long ago. Just time to upgrade and my turn to Learn some new things
 
messing with the less than but I suppose there's something more fitted for this function
Click to expand...

A couple of less then's with a seal in around the 25 ft should do the trick.

Give it a try and post what you have.
 
Not all micrlogix 1400's have analog inputs, you can see the range here: http://ab.rockwellautomation.com/Programmable-Controllers/MicroLogix-1400#selection

If you want to monitor all 8 analogs, you will need to buy an expansion card as the 1400 has max 4 analog inputs.

You will need to scale your input from raw to engineering units, you can use the SCP instruction for this. The datasheet for the pressure transducer should tell you its range. Every 100mBar of pressure equals 1 metre height of water. From pi*r^2*H you can then work out the volume of water in the tank. Then you could use the LIM instruction so that when the tank is in the range of 25-29ft, the pump will run.
 
Last edited:
Yep that's exactly what I did earlier today . Just didn't know how close I was to what somebody in the buisness would have done ! I put a latch to bypass everything else in the rung for the hand switch is that correct?
 
Dakotap92 said:
could really use a hand off auto in it too .
Click to expand...

Now you are talking about extra hardware. Manual is usually hardwired,or can be an input to the PLC to branch around the auto logic. Auto is an input to the PLC to enable or disable the above PLC logic.
 
Yep that's what I meant auto would be what we are talking about and switching to hand would be another input to branch around the auto functions
 

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