Lift Station 101 2

[cr1973]

I've very little experience in lift station design but was recently asked to review a lift station & forcemain submittal.

The estimated inflow to the 8 ft diameter lift station is 400 gpm. The lift station has 2 pumps, each rated at 400 gpm. Both pumps are off at say elevation 100. Pump #1 on at 101.1, pump #2 on at 101.6.

This doesn't look right to me. First, wet well storage is only ~300 gallons (the bottom of the wet well is sloped and some volume is taken up by the pumps).

Second, pump #1 will run constantly since it's output matches the inflow rate (400 gpm). Pump #2 only runs when inflow is greater than #1 can handle.

Third, pump #1 will wear out much faster than #2 - shouldn't they rotate?

In my experience, a station should cycle 4 or 5 times per hour... pumps shouldn't run non stop.

I think the wet well should hold 10-15 minutes worth of inflow (~5000 gallons).... I also think the pumps should pump out that 5000 gallons within 1 or 2 minutes, so a ~3000 gpm would be needed. Finally I think pumps 1 and 2 should take turns pumping to even out run times.

Am I right, or, am I that out of touch in lift station design?

Thanks

 

[cvg]

suggest minimum pumping rate should be twice the inflow and minimum cycle time 10 minutes.

inflow 400 gpm
pump rate 800 gpm
wet well 4000 gallons
one pump cycle time is 10 minutes, with two pumps alternating - 20 minutes

In addition, 8 foot diameter is relatively small for two 800 gpm pumps. need to consider maintenance as well as need for smooth hydraulics conditions to avoid formation of vortices at the pump inlets

 

[jgailla]

The wetwell seems too small for 400 gpm peak flow.
10' wetwell should work better.
We would design each pump to pump the design peak flow at expected head conditions.
Cycle time should ideally be of 12 to 15 minutes.
The pumps should certainly take turns pumping to even out run times.
For 400 gpm peak design sewage flow, I would have 2-400 gpm pumps and set storage from low level to first pump one equal to about 1200 gallons.

 

[bimr]

Maybe a little out of touch.

Wet well volume depends on the method of pump operation. If you use variable speed pumps and the pumps match the flowrate to the inflow, the volume required is very small. This time is normally less than 1 minute retention. That time will allow for the change in capacity with a pump is started or stopped before the next start or stop point is reached.

For constant speed pumps, the time between starts of the motor should not be less than 15 minutes retention.

There are a lot of other factors to consider. How long the force main is, the percentage of capacity that is being used, the minimum flow requirements of the discharge force main are a few of the variables.

Most of the pump stations have controls that will automatically cycle the pumps so that each pump has the same service time.

 

[cr1973]

Bimr,

These are constant speed pumps, not vfds. So 15 minutes retention comes to about 4 starts per hour.

Can you assume then if 2 pumps, rotate usage, 8 pump starts total per hour (4 each)?

Either way with constant speed pumps, that wet well is undersized by about a factor or 10.

 

[CivEnvEng98]

Assuming that we're talking about your garden variety fixed speed submersible lift station, I offer the following:

I usually select pumps where the Qout is 120% of the peak hourly Qin. If you have a single tie-in pressure and you can select a pump that does 480 gpm, then that's a good start.

If you have Qin (400 gpm), Qout (480 gpm) and using a minimum cycle time of 10 minutes, you can back calculate the minimum V. 1.1 feet is too little. I suggest 2.1 feet or more.

For cycle times, there are 2 main criteria, in my humble opinion: #1 keep the sewage from going septic in the wet well. You can't control how old sewage gets within the transmission system, but you can for what goes in the wet well. Keep cycle times for average daily flow conditions less than 30 minutes. #2, keep the pumps from cycling too much. I suggest that for peak hourly flow conditions keep the cycle times above 10 minutes. Also keep 'em under 30 if possible...sometimes they hit the 40-50 minute range...no big deal in my opinion. However, if you're hitting 60 minutes + then 1) look at your pump Qout. Are you 120% or more of your Qin? If so, then your volume is too much (ie., like trying to use 5,000 gallons as you have suggested) and you need to change the float/probe settings.

I would amend the above comments somewhat if you are required to meet varying tie-in conditions, say tie-in pressures for 2007, then 2015 and 2020. That may require selecting multiple impellers and/or pumps or even the use of VFDs (I can usually avoid going that route....too expesive for such a small station).

 

[cr1973]

Thanks for the input fellow engineers. I am going to send this submittal back for redesign under the opinion that the wet well storage is too small, the pump is a bit undersized, and the pump starts per hour should be in the 4-6 range.

 

[bimr]

cr1973

When you said "pump #1 will run constantly since", I made the bad assumption that you were using variable speed pumps.

If you have 2 constant feed pumps and rotate usage, that would still be only 4 pump starts total per hour, not 8 pump starts.

Good luck on your project.

 

[Pump2005]

The size of the wet well comes across as very small to me. It gives not enough spare room if the peak demand is even slightly exceeded from I/I or if one of the pumps go down.

 

[sekolah9]

I have 20 ft deep lift station.
want to find out the pumping rate.
can i use 20' as my satic head?
when I look at the FLygcatalog,
for total head 20, give me 0 GPM
am I loss in here?
I have never done this calculation
please help.

 

[RWF7437]

sekolah9,
Start a new thread. Provide more information.

good luck