Lift Station Troubleshooting

[JC2021]

I have a pre-packaged lift station that I specified a couple years ago that the owner is having problems with. The system serves a day care that has about 100 kids and 15 employees. It pumps into a gravity manhole about 300 feet away. Everything worked fine for the first year and then two months ago the alarm started going off constantly one night. The owner called the installing plumber and it sounds like he didn't actually do any trouble shooting he just reset the alarm and charged her for the service call. His statement was "There's nothing I can do. It's too small and I told the engineer it was too small when we put it in." So she called me.

Now, I don't have the plans handy, because they were done when I was with a previous company and weren't a set that I kept, but I will get them next week. I'm not normally one to dismiss concerns from an installer, but I do remember the plumber calling me to say that it was too small but when I asked for his reasoning as to why it was too small he didn't have anything other than it just seems smaller than the ones I normally do. So I took him through all my calculations and he didn't have anything more concrete to say.

The code in my area requires a basis of 20 gallons per day per person and a peak factor of 4. I based my calculations off that and it comes out to a little over 6 gallons per minute, which is what I designed the station for. I do you remember that it has 2 Zoeller grinder pumps in it and that they were well more than adequate for that flow rate.

My thought is that if it were actually too small it wouldn't have operated just fine for over a year, and it wouldn't have initially failed at night when no one was using the facility. The main things that I can think of going wrong are a stuck or failed high water alarm switch, one of the pumps has failed, or there is a wiring problem and the panel just thinks that one of the pumps has failed. The system has continued to work fine and not back up for the last 2 months even though the alarm has been tripped and continues to trip every time it's reset. My question is, is there a way to test which of these is the problem? I think I could observe the station run through multiple cycles and if both pumps switch on then it points to a problem with the high water alarm. But if only pump kicks on then it points to a problem with the pump. I'm not sure in a duplex system if I will be able to tell which pump is kicking on and is there an easy way to tell if the problem is with the pump or the pump circuitry? I wish I could give more information about the panel itself, but they called me late Thursday afternoon so I haven't had a chance to be on site.

Are there any other likely problems that I have overlooked?

Thanks.

 

[jhnblgr]

Well pump station size doesn’t seem to be likely the issue if the alarms going off at night when there is no flow since the business is closed.

What was the alarm cleared?

Have the owner get a different plumber or you walk away and have a vac truck clean the pump station.

Possibly steps for troubleshooting
1. Try to manually run the pumps to verify they are operating.

2. Guessing it doesn’t have anything special like vfds or soft starts which then hand works but auto doesn’t since typically people put vfds to auto and soft starts to hand.

3. Take a stick and pull up on the floats to see if the work. One time doing a plant evaluation I had to tell the operator the floats shouldn’t be 4 ft below water level.

4. Pumps could be clogged or thermal overload.

My only thought is if you undersized the pump station you have too many starts per hour and that could burn up the pumps.

Are these single phase pumps? They don’t typically last very long and most places go to 3 phase to get better quality pumps.

108 people x 20gpd x4/ (60x24)= 6 gpm
But since it’s open only let’s say 8 hours a day shouldn’t it be around 18 gpm and 600 gallons of pump volume plus minimum submergence?

 

[JC2021]

Jhnblgr, They are single phase. 3 phase is not available at the location. You are correct there is no VFD.

As far as the numbers, the way our state code is set up is they give you a total per day rate regardless of how long the facility is open and then make you use a safety factor of 4 to account for the peak flow. Otherwise it would have to be on an hourly basis since different facilities could be open very different hours. Using your method if a facility was only open 1 hour a day they would have an extraordinarily higher flow rate which just doesn't make sense. Also I can't imagine that they are actually hitting 20 GPD per person. That would equate to every child using the bathroom and washing their hands 10 times a day. Including the infants.

I'm the absence of any code relating to tank size the rule of thumb at my last company was to give enough storage for 5-10 minutes run time to the pumps so that they don't cycle too often, but cycle often enough to keep odors down. So in this case I think the pumps were pumping at about 25 or 30 gallons a minute, so I probably sized it for 150 gallons plus minimum submergence. At peak flow rate that would give a cycle every 25 minutes which means each pump would be kicking on roughly once an hour. And again that is at peak flow rate so the maximum I anticipate is about eight cycles a day per pump (I would guess realistically they are getting 4-5).

I hadn't thought of thermal overload, but that would be more of an issue with where the off float is positioned I would think and less to do with the tank sizing. Unless they are just running so often they didn't have time to cook down. Just thinking out loud here, but also if it was a thermal issue, I would think you might see both pumps fail relatively close to one another. Not one fail after 12 months and the other keep going fine for another 2 months, which would be 4 months worth of cycles.

I'm impatient to get my eyes on it, but the more I think about it just seems like a failed high water switch is the most likely culprit. I just find it hard to believe that wasn't the first thing the plumber checked.

 

[jhnblgr]

Yeah too many unknowns. I doubt there’s anything wrong with the design, I’m just trying to come up with possible issues.

If you oversize a vfd you can convert single phase to 3 phase to get a better pump.

Could be as simple as a bad relay.

 

[JC2021]

Yeah too many unknowns. I doubt there’s anything wrong with the design, I’m just trying to come up with possible issues.

If you oversize a vfd you can convert single phase to 3 phase to get a better pump.

Could be as simple as a bad relay.

I really appreciate the input. My mind has been churning on it for 48 hours now, and just having someone throw things out to think about has been helpful.