Technique for Calculating FLow From an Existing Sanitary Sewer Lift St 2

[ChipperB]

The purpose of this message is to see if there is a more accurate or better way to calculate lift station flows. I work for a medium size coastal city. Our terrain is very flat and as a consequence we have over 100 sanitary sewer lift stations. Frequently, I am called upon to provide consultant engineers with an estimate of flow from a lift station, so they can size temporary pumps used to bypass flow during sanitary sewer construction projects.

The lift station data I have available to me is from the lift station pump hour meters. The hour meters are read and recorded weekly. When I perform my calculations I use the weekly run times recorded over the previous year.

My technique is shown below:

1. Take the weekly run times for the year for each pump and calculate the average run time for each pump in the lift station.

2. Add the pump run times together to get an average weekly run time for the pumps in the lift station. This assumes only one pump is running at any given time even though we estimate that usually 1 pump is running about 80% of the time and maybe both pumps are running simultaneously 20% of the time.

3. Divide the lift station weekly run time by 7 (1week/7 days) and multiply by 60 min/hr to get a lift station daily run time in minutes per day.

4. Take the lift station pump rated capacity in gpm and reduce by 10% for wear. This is what I call the adjusted pump capacity.

5. Multiply the adjusted pump capacity in gpm by the lift station daily run time in minutes per day to get the lift station average daily flow in gallons per day. This can be converted to gpm by dividing by 1440.

For peak flow, I take the highest weekly pump run time and then calculate as shown in 1-5. The peak lift station flow in gpm is used to determine the appropriate sized pump to use for bypassing flow from the lift station.

I would appreciate any suggestions on how to improve on this method.

Thanks, Chipper B.

 

[semo]

Your calcs sound ok; but, short of investing in an ultrasonic meter, I'd do a drawdown test to actually measure what each pump is flowing.

You need to know the diameter(area) of your wetwell so that you can calculate the volume per foot of depth.

Then you measure the depth increase from the influent flow (pumps off) over a period of time or time over a period of depth to get an influent gpm.

Then you do the same with the pump(s) on, measuring the depth decrease. This gives you the effluent minus influent flow. By adding the influent rate, you know your pump rate.

That will make your calcs a little more accurate.

 

[cvg]

I would not ever rely on rated capacity but do what Semo says. Actual capacity should be checked on a periodic basis and this will help you determine how well your pumps are working and should be part of your O&M plan. Another idea would be to check the power consumption and compare that to the run times to see if power consumption is consistent. This can be an indicater of pump wear, plugging, overheating or other factors affecting the pumping rate of your equipment.

 

[bimr]

If you have all of those lift stations, why don't you invest in a clamp-on flow meter? It seems like meter would pay for itself over time in just labor savings.

A clamp-on flowmeter is probably accurate to 10%.

Trying to calculate the lift station flow rate as Semo is suggesting is a reasonable method. However, it is not always easy to do. It is difficult to measure the dimensions of a dark manhole, difficult to determine the depth in a dark manhole, and difficult to determine the inflow as well. The inflow rate can also change over time.

 

[BULLETflight7]

put a pressure gauge in the valvebox. by doing this, you can get static force main pressure, and the pressure on the system when the pump operates. also do a drawdown on the station. get the drawdown info and the pressure then go back to the pump curve and compare.

http://www.jhwright.com

 

[gerhardl]

All above is sound advice.

Please also note that actual total lifting/pumping capacity might be influenced and reduced when two or more pumps are working at the sime time. If they feed into the same pipeline the second or third pump may give a strongly reduced capacity of back-pressure reasons.

Again the advice is to measure flow.

A practical measured flow may in this way also contribute to 'slim down' the daily pump running mode used today, and give a nice contribution to payback time for investment in measurement devices.

 

[brucedaengineer]

All good advice

Another warning is if your well depth is significant compared to static lift component of discharge head then just drawing the well down will reduce the discharge rate - depends on operating range of start/stop in station

And as others advised need to calibrate one pump, then two in parallel etc.

Definitely do the drop test if you can isolate inflows - or fill from hydrant when inflows very low and pump down - always plot graph of depth vs time to see if flow rate changes with depth - not just start and end of drop test - do test over same depth range as pumps operate

Pressure gauge (sewerage - remember diaphragm type!) is good idea then you can calibrate friction factor for pipeline - providing you have a reliable longitudinal section

Final suggestion is can you measure inflow rate where you are pumping to? If its another well then turn off pumps and measure rate of rise if you can isolate other inflows?