Pumps not alternating

[fgoodsky]

I have 2 pumps not alternating at a lift station. When im at the panel and use the test switches to initiate the pumps,it alternates in the PLC. But after I leave it just keeps running the same pump until I come back and go into test mode again. would something like that be purely mechanical? or could the plc be corrupted also?

 

[Steve Bailey]

If it used to work properly and suddenly stopped working, odds are that something other than the program changed.

Having said that, I can imagine a scenario where the problem is the program. If the design of the alternating strategy is to count starts and turn on one pump for an odd number and the other pump for an even number. The counter could have reached a maximum value (perhaps 32767) and won't increment any higher, leaving you stuck with the pump corresponding to the odd number.

 

[Rounder]

[SIZE=+3]You'll have to tell lots more about your equipment and how you know they're not alternating. If you're just basing this on number of starts and run times, that can be deceptive if there's Lead/Lag capabilities.[/SIZE]

 

[drbitboy]

@Steve Bailey raises a good point. A similar possibility would be if the accumulated run times of the pumps were kept in 32-bit REALs, and incremented by whole seconds: somewhere around a year ([PI * 10**7]s, assuming two pumps), those runtimes would stop incrementing.



Whether a change in hardware behavior or performance could cause this depends on the program. Can you post a PDF of the program, or the program itself, or image(s) of the relevant code? Can you state the PLC make and model?

 

[fgoodsky]

it is a lead/lag set-up, it does switch the lead when ever i toggle the float test switch that is in parallel with the float that turns on the pump.

 

[Steve Bailey]

Has it ever worked properly?

Can you post a pdf of the program?

 

[Saffa]

it is a lead/lag set-up, it does switch the lead when ever i toggle the float test switch that is in parallel with the float that turns on the pump.

Without seeing a wiring diagram, doesn't that imply that you have a faulty float switch? Is the pump being started by the well level hitting the Standby Float switch maybe?

I'd pull the floats out the well, lay them out and test in sequence. Stop float in up position first, then duty start. Once pump started, drop duty start to hanging position... followed by stop float. Pump should stop.

Repeat and see if your pump rotates duty.

 

[drbitboy]

Has it ever worked properly?

Can you post a pdf of the program?

Do you know how to post a file to this thread?


Unless you respond to these queries, we will not be able to help.

 

[fgoodsky]

Without seeing a wiring diagram, doesn't that imply that you have a faulty float switch? Is the pump being started by the well level hitting the Standby Float switch maybe?

I'd pull the floats out the well, lay them out and test in sequence. Stop float in up position first, then duty start. Once pump started, drop duty start to hanging position... followed by stop float. Pump should stop.

Repeat and see if your pump rotates duty.

I was thinking that it could be the float switch. on the diagram it has two float switches,( probably for the lead and the lag). Could the lead float be bad and then the lag float ends up initiating the pumps, thereby not alternating and it just runs the same pump all day.

 

[fgoodsky]

Do you know how to post a file to this thread?


Unless you respond to these queries, we will not be able to help.

Ive never posted a file to the threads. The files are on my work computer at work. I was just informed of this problem at 4pm, so I didnt have alot of time to look into it, but the bad float switch is looking promising. it just makes the most sense now.

 

[I_Automation]

All the air compressor controls I worked on didn't counts starts, they logged the runtime of each and started the one with the least hours first, then second least, etc. I have had controls running 5 compressors, and each had a switch to take it out of the loop for service.

 

[Saffa]

I was thinking that it could be the float switch. on the diagram it has two float switches,( probably for the lead and the lag). Could the lead float be bad and then the lag float ends up initiating the pumps, thereby not alternating and it just runs the same pump all day.

Quite possible, but would need to see a schematic. Is this all relay based control or is there a PLC / RTU or something like that running the pumps?

Generally I'd expect at least 3 float switches (stop, duty start, standby/assist start) but have seen the stop float ommitted and just run on a timer if the station is very high flow and unlikely that one pump is ever off.

Easy to test when you get back out there.

 

[fgoodsky]

It was in fact a faulty float switch, the one that the maintenance man said he checked and verified was working. thanks for the help.
The lag float switch was starting the lag pump and so it didnt switch the leads

 

[drbitboy]

We still don't have the code, but it sounds like

• The lag float switch input coming on without the lead float switch input being on may be worth an alarm.
• It's a Good Thing that the incoming flow rate never exceeded what the one lag pump could do.
• If the alternating logic is triggered by the lead float switch input coming on only, adding an OR to that trigger, of the lag float switch input, may have kept the alternating logic going, but then the loss of the lead float switch may not have been noticed until the one pump could not keep up.

 

[janner_10]

My moneys on the plc being corrupted!