Lift Station Pump Cycling Time

[JustinGonzales01]

I am currently designing a lift station for a small subdivision. I have calculated the flow into the wet well to be 36.4 gpm (it is a very small system). The diameter of the force main leaving the wet well is 3 inches. I was able to calculate the total dynamic head which then allowed me to choose my pump from the Flygtt pump selector. My question now is how do I go about sizing the wet well or calculating the cycle time if I have two of the same pumps which will be ran in a lead/lag system?

 

[ashtree]

Is your 36.4 gpm the peak flow or the average flow taken over 24 hours?
You will need to establish a peak flow rate, probably something like 3-5 times the average flow rate.
The depth of the incoming sewer needs to be determined?
What is the maximum number of starts per hour you will accept for your pumps? Probably 5 - 8 starts an hour would be a good start.
The pump performance needs to be established with one pump and two pump operation.
When you say lead/ lag do you mean duty/ standby or both pumps on duty with second pump coming on after a time or upon a certain level being achieved?

The other issue will be is whether or not you are going to use a package or precast station, or will you custom build to suit. If you are using packaged or precast then some of your dimensions will be established for you and you will need to work within those constraints.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[JustinGonzales01]

36.4 gpm is the peak hourly flow that is being designed for, the system is very small. We are not interested in sewage because this lift station is being used for residential waste water so the only water reaching the wet well is from the gravity lines. I agree with the 5-8 pump starts. Ideally I would like to keep it under 10. In this lead lag system I am implying that the first pump will be triggered by a float switch then when the water reaches a higher depth in the wet well the second pump will be triggered by another float switch. I have all the information regarding the pumps used just needing help understanding how I can calculate the cycle time.

 

[JustinGonzales01]

This will be custom built.

Thanks for your time,
Justin Gonzales

 

[ashtree]

You need to compare how much is coming in during the peak to how much is being pumped out. But you also need to consider how much you can store in your given pump station above the low level cut off and your intended start level.

If your pump station could store 100 gallons between the low level and the pump start level and flow in was 10gpm it would take ten minutes at peak flow to get the level to the point where the lead pump would start. If the pump had a capacity of 20gpm it would take 10 minutes to pump out because with the 10gpm flow in this would give it an effective flow rate of 10 gpm. Therefore in this scenario the pump would pump for 10 minutes and be off for 10 minutes which would equate to three starts per hour. Total cycle time 20 minutes

If the pump had a capacity of 100 gpm the number of starts per hour increases. It would take 10 minutes for the pump station to fill but at a 90gpm effective out flow rate the pump would only pump for a little over a minute(66 secs). Total cycle time 11.1 minutes which is 5.4 starts per hour.





Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MartinLe]

Last year I had a similar (but simpler, 1 pump) question ... I fiddled around in excel for half an hour to simulate the water level over time, gave up and graphed the whole thing on paper (water level vs time, different (pos. and neg.) slopes for different net flows).
Then I could easily find the LS+ (pump on) point for a given allowed starts per hour.
The math is of course exaclty as in ashtrees post above, I just couldnt do it in my head.