VFD Operated Lift Station Wet Well Size Question 1

[512Engineer]

I’m designing a wastewater lift station with VFD’s. I have not found any printed guidelines, etc. about sizing the minimum working volume for the wet well when VFD’s are used. We are flow matching to an extent, however, it would seem as though I would need a design basis for a minimum volume.

The state requires me to have a 15 minute cycle time for my pumps. The pumps can handle 4 minute cycle times. My idea was to get a variance from the state since I’m using VFD’s and then use a factor of safety of 2 and design around an 8 minute cycle time. This will reduce my wet well volume and still protect the pumps. Does that sound reasonable? The other question I have is what Q, should I design around? For constant speed pumps, we design around the total Q, however, this seems really conservative for stations with VFD’s.

Do you have any guidance or lessons learned that you can share?

Many Thanks.
Charles

 

[bimr]

Variable speed pumps do not require appreciable storage volumes in a wetwell. Use of variable frequency drives for pump speed control allows wet-well detention time reduction to 5 to 15 minutes.

In general, the single pump peak flow approach is most suitable for stations that have relatively rapid flow increase or high headlosses. For such stations, parallel pumping is not as effective, because two pumps operating together yield only slightly higher flows than one pump. If the peak flow is to be achieved with multiple pumps in parallel, the pump station will need to be equipped with at least three pumps: two duty pumps that together provide peak flow and one standby pump for emergency backup. Parallel peak pumping is typically used for large pump stations with relatively flat system head curves. Such operation allow multiple pumps to deliver substantially more flow than a single pump. In addition, use of multiple pumps in parallel provides more flexibility.

You might want to review the Garr Jones book, Pumping Station Design.

http://nepis.epa.gov/Exe/ZyNET.exe/...ge&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL

 

[512Engineer]

Thank you for the response. We will have 5 pumps in parallel (4 firm + 1 standby), about 22,000 gpm firm capacity. It's a rehab, so I'm trying to work with an existing wet well volume to see how much we can increase the pump capacity with our existing storage. I have the Garr Jones text and it discusses wet well volume reduction with VFD's, however, it doesn't go into any details. I've discussed this with other engineers and they typically design the station wet well without consideration of VFD's even though VFD's are being used. That is the way I have always done it as well, but for new construction when you have the ability to build the wet well a little larger. I did consider sizing the minimum storage available around 15 minute cycle times (150 HP pumps), and based on the flow with 1-pump on as a minimum. Cycle time isn't really an issue with VFD's and the pump manufacturer allows a max. of 15 starts per hour. I guess conservatively, I could put a factor of safety and size my volume around 8 starts per hour or something reduced. Any follow up thoughts?
Thanks.
Charles

 

[bimr]

In a larger pump station such as you have, the detention time may be reduced to 5 minutes or so. Other considerations that have to be evaluated will be the standby power and control system to ensure that the pumps are activated in a timely manner.