Lift station pump failures

[FishScreener]

We have a “Human Waste Disposal Site”, which has a “SCAT” machine, (think specialized commercial dish washer, with a 2-hp grinder on the discharge), to receive the wastes generated by rafters, and an RV dump, It has two lift stations, with tandem 5-hp pumps, which grind the waste, and lift it to a pair of lagoons, about a hundred and fifty feet above the dump station. Lower station takes it from the station, halfway up the hill to the upper station, which sends it on up to the lagoons.

The Lower lift station had a huge issue with dip-a-wipes, stretching if the cutters were the least bit dull, and wrapping around the impeller and cutter in enough volume to break the seal on the motor. So, it was going through two or three pump assemblies a year.

The upper station which was only receiving the waste from the lower lift station had no issues ever. Ran for twenty years without ever needing to lift the pumps out. The guide rails and lift chains rusted out, but the pumps ran flawlessly.

Two years ago we replaced all of the pumps and associated motor controls. The newer models pumps, come with improved grinders, which were designed to handle the wipes. Pull the pumps and change out the grinders once every year to keep them sharp, but little maintenance other than that.

So, with the new pumps and controls, the lower lift station functions as promised. Absolutely no issues. The upper lift station with identical components, is exploding start capacitors, and tripping breakers, one motor has burned out. This is the station which ran for twenty years with no issues..

Anybody have any ideas on what might be causing the upper station to fail? We have checked for clean power, and not found any issues. 240-volts, with a tiny drop on starting, which returns to 240, as soon as the pump runs up. I’m at a loss, and want to figure out a solution and get it installed before Memorial Day.

 

[Twowaxhack]

Any possibility it’s getting power surges ?

Lightning strike ? You have any surge suppressors ?

I don’t know how how it’s controlled, I assume there are float switches. Could they possibly be hanging up and the pump running continuously until the over heat protection kicks out ? Then repeating forever until something blows ?

Shorted wires ?

I believe you can install a meter to see how many times the pump has cycled, etc.

 

[FishScreener]

The pump control panel has counters for how many times each pump has started.

I kept mulling it over this morning. After the waste leaves the pumps, it goes through a ball valve, to a 90. Both discharges run into a tee, and then through a check valve.

I’m wondering if the check valve could be jamming intermittently. The holding capacity is the same for both lift stations. And, all the floats are set to the same heights. If the lower station started sending say 750-gallons up the hill, the upper lift station would kick on when the tank was filled by the 750-gallons before the upper pump turned on. If the upper pump comes on, and the check valve isn’t opening, that pump will run, until the lower station kicks in again, and it pumps another 300-gallons up the hill, and the second pump kicks in. Then the increased head form the two pumps might open the check valve, and the two pumps would run until the shut off float drops.

Since the second pump has started, the sequencer would start the next cycle, on the first pump, and it would again run continually until another cycle of the lower pump, when once again the paired pumps would have enough head to overcome the stuck check valve.

And on a busy weekend, the first pump would run continuously and overheat.

 

[Twowaxhack]

Why wouldn’t you just have two independent systems with the lower pump station pumping to the second lift station ?

Why are both pump stations connected to the line going to the lagoons ? Seems like just the last pump up the hill would go to the lagoon.

 

[Twowaxhack]

The duplex lift stations I’ve done only one pump comes on at a time. It’s a 4 float system on, off, reset that alternates the pumps and a high water alarm.

What I can’t figure out is why you think two pumps are firing at once to overcome a stuck check valve.

If it’s over heating you could install a manual reset on the over heat protection circuit. Most are automatic reset when the pump cools down.

 

[FishScreener]

The pump control panel has counters for how many times each pump has started.

I kept mulling it over this morning. After the waste leaves the pumps, it goes through a ball valve, to a 90. Both discharges run into a tee, and then through a check valve.

I’m wondering if the check valve could be jamming intermittently. The holding capacity is the same for both lift stations. And, all the floats are set to the same heights. If the lower station started sending say 750-gallons up the hill, the upper lift station would kick on when the tank was filled by the 750-gallons before the upper pump turned on. If the upper pump comes on, and the check valve isn’t opening, that pump will run, until the lower station kicks in again, and it pumps another 300-gallons up the hill, and the second pump kicks in. Then the increased head form the two pumps might open the check valve, and the two pumps would run until the shut off float drops.

Since the second pump has started, the sequencer would start the next cycle, on the first pump, and it would again run continually until another cycle of the lower pump, when once again the paired pumps would have enough head to overcome the stuck check valve.

And on a busy weekend, the first pump would run continuously and overheat.

The duplex lift stations I’ve done only one pump comes on at a time. It’s a 4 float system on, off, reset that alternates the pumps and a high water alarm.

What I can’t figure out is why you think two pumps are firing at once to overcome a stuck check valve.

If it’s over heating you could install a manual reset on the over heat protection circuit. Most are automatic reset when the pump cools down.

Ours are 4-float systems. Shut-off, initial call, second call, and alarm. The initial call starts the next pump on the sequencer. If the level in the vault continues to rise, until it gets to the second call, the second pump comes on. If the level reaches the alarm float a red strobe light on the controls pedestal comes on, and the interlock shuts off the lower lift station so you don’t keep pumping waste up the hill and cause the upper vault to overflow. Having a waste spill in the river corridor would cause a huge amount of paperwork, and cleanup costs.

The shut-off float lets the controls know that the vault is emptied and to shut-off the pump(s).

We have 2.5-inch discharges on the two pumps in each lift station. They go out of the vault, and through ball valve which allows you to isolate a pump if you need to, then to a 2.5X4-inch Tee. The line then goes through the 4-inch check valve, and gets lifted to either the upper lift station which is half the head height to the lagoon, or to the lagoons, depending on which station your standing at. The capacity of the vaults at both lift stations is 1500-gallons.

And, I’m thinking we might have sticky backflow valve becasue they are 30-years old, and are the only normally moving parts of the system we didn’t replace.

 

[Twowaxhack]

I dont service systems that large but one thing I was instructed never to do is increase the outlet size pipe larger that the pump outlet. This puts a lot of strain on the pump to open the larger check valve with all that back pressure.

But then again on our duplex systems only one pump runs at a time.

The way I read your post is that only one of your duplex pumps runs unless the second pump is called on by the float.

So sometimes only one 2.5” pump is pumping through the 4” pipe.