Sizing Sanitary Force Mains

[GoldDredger]

I am doing some preliminary cost estimating for a lift station and force main. I've sized the lift station by estimating daily demand and peaking factors (4) to get a GPD for the lift station.

In sizing the force main I've been looking at velocity based on converting the GPD into Cu.Ft/second, then dividing by area of force main to come up with a ft/s.

My question is, since I've already considered peaking factors in estimating the GPD for the lift station, is is reasonable then to simply convert GPD into cu-ft/s for a velocity calculation, or should I be factoring in another peaking factor.

Also, what force main velocities do you consider prudent? I was thinking under 12 ft/s, but perhaps even that is too high for the water hammer affects of pump shutting on/off.

 

[bimr]

The "estimating daily demand" is an average flow over 24 hours or average daily flow.

The peaking factor is driven by the population served and begins at approximately 2.5 times for large populations. You have to decide if the peaking factor of 4 is adequate based on the population served.

Multiplying the average daily flow by the peaking factor gives you the peak hourly flow.

If the sewers are old and you have excessive I/I, then it may be necessary to add an additional safety factor for I/I.

If you are pumping any distance at all, you would want the velocity to be 4-5 ft/sec for economical pumping. A velocity of 12 ft/sec is way too high and will generate excessive headloss and require uneconomical pumping HP.

 

[77JQX]

Use the inflow to size the wet well and pumps based on allowable starts per hour. Then size the force main based on the pump flow and bimr's velocity recommendations. The gpm driven through the force main is determined by the pump, not the inflow to the wet well.

 

[jgailla]

You want to shoot for 3.5 ft/sec minimum unless your pumping will be fairly continuous. With a peaking factor of 4, it sounds like you have a pretty small population you're collecting from, so you will have intermittent pumping. 3.5 ft/sec is enough, according to the codes, to resuspend solids. For continuously pumping stations, 2 ft/sec is sufficient as solids will stay suspended at this velocity.
You always want to use the lowest velocity practicable because the friction head with higher velocities will make your pump larger and therefore more costly up front and especially in operating costs over time. Just pick the largest pipe size that will still have the minimum velocity at the design flow.
Try reading the Ten States standard for wastewater. This document is incorporated into the state codes in many states.
As 77JQX says, your pump will determine the flow in the force main. For costing purposes, you'll need a horsepower for the pump, so you might as well pick a pump and fit the pump curve to the system curve to determine the operating point. There are so many different pump and impeller choices for sewage pumps that you should be easily able to get a pump that delivers pretty close to your design inflow.