Flow meter input to PLC

Thread Starter

Jonplynch

Joined Jan 28, 2012
93
Hi guys

I am having a bit of trouble with a flow meter that is metering in a fluid and giving an input to a PLC which in turn is giving an output display in litres on a HMI screen. When i start the process the pump begins to run and the meter starts to count but cuts out after about 5 seconds. On watching the PLC the designated input is pulsing and the motor is running (very slow i might add). The switch on the flow meter is only new as the one before it had failed. The flowmeter itself is only a small simple meter which is in good condition and i recently changed the seals and diaphragm inside. I was having the same issues before i changed the seals and switch. The motor is a fixed speed motor and is running at the same speed it always has been and i have no way to increase speed to dictate flow and see if this would help. I don't have access to the PLC in any way or have drawings. But everything appears to be working as it should bar the cut off after a few seconds. All lines have been cleaned and if i keep resetting it i do eventually get a flow into the tank so there are no blockages. Is it possible the pump isn't fast enough to satisfy the flow meter and therefore the PLC input?
 

strantor

Joined Oct 3, 2010
5,695
when you say "cut off after a few seconds" do you mean the process actually stops (motor turns off) or do you mean it simply stops counting while the motor continues to run and pump fluid?
 

strantor

Joined Oct 3, 2010
5,695
My SWAG is that the PLC isn't seeing enough pulses from the flow meter (PLC programmer's assumption is that the pump lost prime or a blockage has occurred) and is shutting down the pump to prevent it burning up. I would investigate the flow meter again. I know you just rebuilt it, but maybe you made a booboo; maybe the moving parts have too much resistance from over-tightening or something. I would also investigate the motor; why is it turning so slowly? is it overheating? Drawing excess amps? is the pump worn out so that it moves much less fluid than it used to?
 

Thread Starter

Jonplynch

Joined Jan 28, 2012
93
Yeah I am going to take the meter out and bench test it later tonight but as I said earlier the same problems were occurring before I took the meter apart and before the switch failed. I changed the insides of the meter just to be sure and before I closed it back up everything was moving nicely. The motor was always slow well since I looked at it and it's not overheating or drawing excessive heat or current but a part of me believes it is just too slow to keep the flow meter happy but I could be wrong
 

gerty

Joined Aug 30, 2007
1,302
Or.. Is the plc programmed to shut down after a preset number of pulses (indicating tank full). If so make sure the number is still correct.
 

Thread Starter

Jonplynch

Joined Jan 28, 2012
93
Yeah I haven't got a spare motor or pump to test probably be another few days before I can get a motor at least

The pump
itself is in good condition as stated it runs very slow so has never worked really hard and it only pumps the same fluid all the time which is nim abrasive. And gerty the process stops when the set lwvel is reached through the interchangeable levels on the HMI
 
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