Automatic Alternation of Pumps

justin lutz

Member
Join Date
Aug 2008
Location
Columbus, Ohio
Posts
93
Greetings all...

So, as I've seen across the forum, this one has been beaten to death. Therefore, I am not going to ask how to program a lead/lag or flip-flop sequence, but rather, I would appreciate some feedback on what I have put together.

What I'm trying to accomplish (using RSLogix500):

1) On first power up, I would like to automatically select pump #1 to begin running. So, let's say the process does its thing for a while and is eventually powered down, or a loss of power occurs. On next power up, I would like to automatically select pump #2 to begin running. The point being - not default to running the same pump at start-up. I don't know if this is really even necessary, as it may not have a big impact if the process is rarely shut down. I do, however, need some way on first start-up to automatically designate a duty pump.

2) Say pump #1 has been pumping (is lead pump)and reaches the end of its cycle. I want to use the "end of cycle" status to alternate from pump #1 being lead to pump #2 being lead pump. In addition to the "end of cycle" status, other triggers will include a pump malfunction, pump failure to run, etc.

I have attached my .RSS (Program File 10) and .PDF files and would greatly appreciate comments, criticisms, advice, etc. Again, sorry to drag out an otherwise tired topic.


Thank you,
Justin
 
looks good thus far I cannot see any problems with what you have the rest looks to be setting your switch conditions for failures etc. One thing you may want to add is to add the real time clock features and record your running hours per pump however thats just a suggestion
 
I’m not very well versed in AB, but I think the First Pass bit is only on during the first scan after power up, so I don’t think you need to create your Pulse On For One Scan bit since it’s doing the exact same thing as the First Pass bit, seems redundant to me.

Second it appears to me that every time the processor powers up it’s always going to enable the Use Pump #2 On Start-Up.

If it were me I would just use a Counter to alternate the pumps, then I would use the First Pass bit to increment the Counter as well as my standard logic.
 
Thank you for the replies, thus far.


One thing you may want to add is to add the real time clock features and record your running hours per pump
I had considered this as well.


I don’t think you need to create your Pulse On For One Scan bit since it’s doing the exact same thing as the First Pass bit, seems redundant to me.
I think I would have to agree with that.


Second it appears to me that every time the processor powers up it’s always going to enable the Use Pump #2 On Start-Up.
This is one of the main reasons for my asking about the flip-flops. I've manipulated it several different ways and I just don't see how it provides any "alternation."

I originally started out with a counter and may go back to it, however it's eating away at me that I can't figure out the flip-flop concept.
 
It’s not providing any alternation because the value of Pulse On For One Scan and Alternator Bit is always the same on start-up. You would need to retain the value of Alternator Bit in order to get your flip-flop to work. Easier to use a counter.
 
Another advantage to recording the run time of each pump is that you could write your program to always start the pump with the lowest hours, power-up or not, as long as the status of that pump is "ready". The main purpose of alternating pumps is to equalize their usage. Always starting the pump with the least hours will automatically achieve that goal. This philosophy is a simple one that will work no matter how many pumps are in a system. Over time the elapsed time on your pumps will remain relatively even.

Just another .02

Stationmaster
 
Stationmaster said:
Another advantage to recording the run time of each pump is that you could write your program to always start the pump with the lowest hours, power-up or not, as long as the status of that pump is "ready". The main purpose of alternating pumps is to equalize their usage. Always starting the pump with the least hours will automatically achieve that goal. This philosophy is a simple one that will work no matter how many pumps are in a system. Over time the elapsed time on your pumps will remain relatively even.

Just another .02

Stationmaster

Yes, this insures that all the pumps fail at the same time.
I never understood why the maintenance group wants this, but I give it to them anyway.
 
Just $0.02 of my own, but if we try to equalize (as best we can) the runtime of the two pumps, are we not increasing the likelihood that the pumps will fail at about the same time? Granted, the chances are not great, but it's certainly possible for one unit to fail as another is being replaced.
 
Why balance runtimes?

If you start out with offsets in the run hours on each pump, then that same offset will be maintained, which can be advantageous to preventive maintenance.

Also, some pump systems need to be run every so often to prevent build-up, prevent seal leakage, or other reasons.

On a brand new system with all new pumps, I can see why you'd want to avoid keeping the usage equalized, but in the real world, those pumps aren't all going to survive until the same day and all quit at once. Some other factors will likely come into play to keep that from happening.

just my 2 cents
Paul
 
I recognize the controversy regarding "wearing all the pumps out at the same time". I personally don't like to keep the run-times equal for that very reason. I only brought it up because the OP was asking about "alternating", and alternating is generally done for that reason. And as mentioned, to make sure each pump gets "exercised" once in a while. I was just sharing another way to do that. Right or wrong, it is a widely used practice in the irrigation industry.

The REAL advocates of equalizing run-times are the big pre-packaged pumping system manufacturers. They sell you a pump station with a steel skid that, without proper attention, will rust out in about 7 to 8 years. They time-share one VFD among all the pumps so that it accumulates runtime whenever any pump is running. They equalize the pump run times so that by the time the skid is rusting out, all the pumps are equally wobbly and tired, the VFD has been run long and hard, and the "fiscally responsible" customer will opt for a NEW pump station, thus producing a new sale. Planned obsolescence.

They don't make any money if the customer sends his pumps and motors to the repair shop and has them rebuilt over the years.

2 more cents....

Stationmaster
 
justin lutz said:
Just $0.02 of my own, but if we try to equalize (as best we can) the runtime of the two pumps, are we not increasing the likelihood that the pumps will fail at about the same time? Granted, the chances are not great, but it's certainly possible for one unit to fail as another is being replaced.

The problem with this type of set up is not that the pumps will fail at the same time (though they might) but that the pump performance will degrade equally. The result is that when one pump does fail or degrades to the extent it needs servicing the remaining pump will perform just as badly. The alternative is to always have one "good as new" pump in reserve to take over when the duty pump needs servicing. Whether this is an issue depends on the application but for systems susceptible to heavy wear such as waste water pumps I would always avoid any form of runtime balancing and implement an effective condition monitoring programme.
Andybr
 
One of the problems I've run into with having redundant systems that are on hold till a failure in the primary system is that several times we've had the primary system break down tried using the second system to find out that it isn't working. We tried to correct this by scheduling a couple hours per week to run the second system but due to manpower problems with operators constantly changing this proved unworkable. However the argument is equally valid both ways. maybe the solution ios to software schedule the second pump to run only a couple of hours per week to loosen everything up but unless you have faith that the operators will reliably run the second pump once in a while then I would definetely consider having some software redundancy
 
Mordred.
Exercise runs are definitely a good idea. They can also be used to perform a useful purpose such as "scouring" pipework or preventing the build up of solids in a tank if it is acceptable to run both pumps in parallel for a short time.
Andybr
 
I'm currently doing a project with a similiar pump control as you described. In my case process going to be down quite often, but I hope PLC going to stay ON. So I did not consider memorizing last pump running.
Rest of the control is similiar to what you described -
Pumps are altered every start, but failure of a lead pump will start lag pump automatically. Additionally I have little setting feature which allow to select mode of operation. Here it is.


Screen_03.jpg
 

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