STORM WATER PIPE VELOCITIES

[Berko]

Dear All,

Can anyone help me with a design question. We are looking to pump Storm Water which has large quantities of road grit (varies from sand to granite chips upto 15mm diameter).

The pumps involved will be pumping at a flow of 190 L/s through a 25M vertical rising main.

Can anyone advise:

What is a suitable pipe velocity to keep the sand & grit suspended in the vertical rising main.

Can anyone suggest a suitable Non Return Valve for this grit laden storm water application.

Cheers,

Berko

 

[cvg]

you would be better off allowing it to settle out before pumping. It shouldn't be difficult to provide a large enough sump to allow the sand to settle out.

If you insist on pumping it, you will have to keep your velocity above 3-4 feet/second and the grit will be very abrasive on your pipes.

 

[Berko]

Hi cvg,

Thanks for your comments. Unfortunately this large pump station has no facility to de-grit the flow prior to the storm pumps. In fact the problem with this wet well is that due to a very large, unbenched wet well the client is having to dig pumps out of large quantities of sand & grit on an alarmingly regular basis.

The only realistic option we have is to bench the station, to get the grit moving into the area of the pump's suction & pump it away.

3-4 ft/s seems very low for Storm Water with high levels of grit & long vertical risers. I would normally use velocities in the range you mention for sewage to stop biosolids falling out of suspension, I would think grit would drop at 3-4 ft/s ?

Thanks for your input,

Berko

 

[gbam]

Berko --
Look up the fall velocity of the largest diameter of grit that will be in the system and design to exceed that velocity if you want no grit to fall out of suspension. Review a Sediment Transport Text to determine the fall velocity Vs. size. Has anyone performed a seive analysis on the silt in the sump? This would aid you in determining the size of the sediment.

Good Luck & Have Fun!

 

[civilperson]

Not a good solution, some high density large size sediment will exceed the velocity you provide and settle out. Rather provide grit basin with periodic maintenance to remove the accumulated particles. The extended pump life will pay for the grit removal chore.

 

[msquared48]

In Bioswales, the required maximum is 5 fps so the bioswale can work, to include sediment removal. I would go higher than that figure if you want to keep the sediments suspended.

Mike McCann
McCann Engineering

 

[BRIS]

3 to 4 ft/sec would probably move grit downslope but you are moving up slope - in fact you say vertical.

The upward velocity will need to be higher than the settling velocity (Stoke's Law).

Great if you can keep your pump running at the required velocity for 24 hours/day 365 days/year !!

 

[onyeuko]

Your best option is to provide a sedimentation tank or grit accumulator within or outside the wetwell. Trying to pump the grit along with the wastewater will cause you more problems than will be necessay.

 

[dicksewerrat]

How often are you replacing the impellers? Seems that putting ina grit removal chamber will save money in the long run.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com