Sewage lift station problems

[frayedknot]

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!

 

[NGLENGR]

Obviously the debris is a major problem.

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.

 

[Artisi]

Sounds like you need pre-treatment of the inflow via a shredder / gringer prior to the inflow, it is a bit much expecting a 3hp grinder pump to handle diapers etc.

see attached

http://www.jwce.com/

 

[Ziggypump]

Does your lift station have a screen system? The only sewage lift station I've ever seen had a screen system before the pumps, where some mechanical rakes would pull debris up and dump them in a garbage bin. These rakes were installed for the specific reason of removing the debris that people flush.

 

[oldfitter]

First thing would be to educate the home owners as to what can and cant be flushed into the sewer.
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.

 

[BigInch]

VSDs on a 3 HP pump sound pretty strange to me.

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.

 

[frayedknot]

Thanks for the heads up on the VFDs. Rumor is that the area had power issues when the station was put in (2 other stations are nearby). The local water treatment facility owns the 2 other stations and installed VFDs in all 3 (ours by mistake). My background is in manufacturing and nuclear power and in both we have much stricter design control. I am so frustrated at the lack of factual information from the builder and treatment facility. Is this common in residential construction? I didn't expect nuclear level design control but zero control was a shock.

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.

 

[BigInch]

There is no constant sweet spot with a VSD. That's why you would use one. It would allow the pump to track the "sweet spot" if it changed, but that's a matter of how variable your flowrate and head are.

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.

 

[cvg]

do you have combined storm and sanitary sewer? If so, then you could manage some of the debris by cleaning out the catch basins, possibly by providing trash racks on the storm drain inlets and generally keeping trash in the neighborhood under control. Also, public notification to not place any debris in the sewer system. You indicated that the problem gets worse with a rain. If you have a combined system, then stormwater may be overwhelming the pumps. Otherwise if it is not a combined system, than Infiltration and Inflow (I&I) is your problem. However, it seems to me that with just 80 homes and such a small system that I&I should not be that big of a deal. If your sewer system is private, then you need to do periodic inspections of the pipes and manholes. Recommend video inspection and assocatied cleaning or root cutting as necessary. Also look for broken pipes or cracks in manholes or even missing plugs on cleanouts. If you can't reduce the I&I, you may need to consider larger pumps. First step would be to gather more data, especially operating data such as how often the pumps come on and how long do they run both in dry weather and then in rainy weather.

 

[QualityTime]

Consider installing a trash rack in the wet well. It may have to be manually cleaned unless you can go to an automated type screening procedure

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

 

[frayedknot]

More info: supply is single-phase and VFD converts to 3-phase. Underground lines so getting 3-phase supply would be $$$$. The VFDs are set at 3 seconds.

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".

 

[4JawChuck]

Have you considered upgrading to a air lift style station, the catch basin is one way valved and an air compressor is utilized to pressurize the basin when the basin lift floats indicate high water.

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.

 

[rconner]

Not going to profess much expertise in clogged "grindr pumps", but also wonder if more traditional bigger pumps/pipes might have lesser problems with the specific unmentionables you are dealing with?

 

[QualityTime]

Go to this site to learn about the muffin monster

http://www.jwce.com/category/muffin-monster-grinders/

 

[pumpman50]

I would not use a VFD on such a small installation. Desired velocity upstream should be at least 4 m/sec to prevent blockages so the faster the pump runs the better. Control the pit level with either old fashoned electrodes or better still an ultrasonic device made by Milltronics or similar. These are cheap and cheerful devices and easy to set up by a competant person.
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!