Sewage lift station problems
Sewage lift station problems
(OP)
Since I have "engineer" after my name my local community HOA has volunteered me to head up the sewer committee to find a solution to our lift station problems. We own the lift station so we get to pay for all our problems - over $15k this year alone in pumps and emergency calls.
1) 80 homes
2) two 3 HP submersible grinder pumps (don't know NPSH or any other design requirements at this time)
3) VFD installed some time in the past to adjust acceleration of the pumps during starting. How do I find info on how to set up the VFD? I'm not sure if the guys that check our pumps know much because they seem to make adjustments every time they come out for a trip...or is this normal?
We have multiple problems with debris clogging the pumps, causing them to trip offline. At least one pump has been replaced due to insulation failure in the windings. The debris has been anything from diapers, rags, underwear, "flushable" cleaning supplies, etc. The problems seem to get worse after a hard rain. Is there anything we can do to mitigate the problems? Obviously we need to educate the community to not use their toilets as their trashcan but what about from a design perspective? If there is inflow during a hard rain how does that challenge the pumps? I understand cloth items being a challenge but more water shouldn't be, right?
Thanks!
1) 80 homes
2) two 3 HP submersible grinder pumps (don't know NPSH or any other design requirements at this time)
3) VFD installed some time in the past to adjust acceleration of the pumps during starting. How do I find info on how to set up the VFD? I'm not sure if the guys that check our pumps know much because they seem to make adjustments every time they come out for a trip...or is this normal?
We have multiple problems with debris clogging the pumps, causing them to trip offline. At least one pump has been replaced due to insulation failure in the windings. The debris has been anything from diapers, rags, underwear, "flushable" cleaning supplies, etc. The problems seem to get worse after a hard rain. Is there anything we can do to mitigate the problems? Obviously we need to educate the community to not use their toilets as their trashcan but what about from a design perspective? If there is inflow during a hard rain how does that challenge the pumps? I understand cloth items being a challenge but more water shouldn't be, right?
Thanks!





RE: Sewage lift station problems
The VFD can cause problems if the motors are not capable of running with one. I'm not a VFD expert at all, but I do know there are plenty of electrical and mechanical problems (line harmonics, torsional excitation, localized heating, etc.) that can occur in poorly applied VFD solutions.
How are the pumps controlled? Perhaps an investment in a decent strainer would help keep the garbage from hanging the instrumentation. If they don't shut down at the right time, it's possible the pumps are running dry.
RE: Sewage lift station problems
see attached http://www.jwce.com/
RE: Sewage lift station problems
RE: Sewage lift station problems
Next fit pumps capable of handling rags, ie, Flygt N type impellers.
Introduce pump down cycles to clear everything from the well once a day.
Alter stop and start levels so the pumps cycle more frequently to prevent excessive settlement in the well.
Ensure VSDs dont slow velocity through the pump which will cause blockages.
Welcome to my world.
RE: Sewage lift station problems
And isn't that a surefire way to clog them up when they run or start at a slow speed? Seems to be asking for trouble.
RE: Sewage lift station problems
The pit is not pre-screened and I doubt there is room to install a screening system without massive construction.
It appears that the technicians that come out to service the pumps set/adjust the VSD trying to find the "sweet spot" whenever there is a problem. There doesn't seem to be any logic behind it.
Thanks for all the help.
RE: Sewage lift station problems
You could certainly use a VSD on a 3HP, but for a pump station that small it just doesn't seem like it will save you much in power verses the maintenance you would need to do from starting up at low torques and getting all that rubbish stuck in its throat instead of smashing it to pieces. If you need any torque at all in this application, its when you start for cuttin power. After reaching speed, just the spinning momentum would probably be enough to slice through the most typical items.
RE: Sewage lift station problems
RE: Sewage lift station problems
Consider installing a muffin monster or something similiar upstream of the pumps to shred the rags. The only problem with the muffin monster is that you may have to replace the blades and rope could get tied around the blades. I had a similiar application but it was for a very large municpal sewage plant and the dopes...ooops engineers...did not have screening and the dry pit pumps were plugging with rags. The hydraulically operated muffin monsters did the trick for them. Make sure you have a bypass in case the muffin monster malfunctions
RE: Sewage lift station problems
When I referred to the "sweet spot" I meant that the techs would adjust the time based on performance, i.e. It clogged at 10s so they reduced it to 5s and continued until "it seemed to perform best".
RE: Sewage lift station problems
Very simple to operate and maintain and only regular screen cleaning and yearly air compressor overhauls are required to keep it working.
A contract with a local company to keep the unit functional and clean may be cheaper than maintaining grinder pumps.
RE: Sewage lift station problems
RE: Sewage lift station problems
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RE: Sewage lift station problems
You also require a screen of some sort at the inflow. There are conveyor belt systems on the market that will automatically launder the trash from the inflow. All you have to do is empty the launder bin weekly or earlier depending on the volume of the trash. Hope this helps. A long arm and a bad sense of smell is also useful!