Wet Well Volume
Wet Well Volume
(OP)
Hello Env. Engs,
I'm designing a wet well for a certain city in southern California. Through looking at the Hydraulic Institute manual, I found equations which would help me size the wet well if I had constant pumps. The equations for constant pumps have pump cycle start and stop times. However, I'm using a VFD pump which doesn't have pump cycle start and stop times. Thus far, I haven't found any equations to size a wet well that uses VFD pumps.
I managed to determine that my wet well diameter is 4' and that the max flow coming into my wet well is 1400 GPM. The sewer main that pumps the sewage into the wet well has a 2' diameter. The pump I selected is a submersible one, I will be using two pumps (one is a backup). The only thing I need to figure out is the height of the wet well. Can anyone suggest me a wet well height that sounds reasonable for my situation?
Thanks!
I'm designing a wet well for a certain city in southern California. Through looking at the Hydraulic Institute manual, I found equations which would help me size the wet well if I had constant pumps. The equations for constant pumps have pump cycle start and stop times. However, I'm using a VFD pump which doesn't have pump cycle start and stop times. Thus far, I haven't found any equations to size a wet well that uses VFD pumps.
I managed to determine that my wet well diameter is 4' and that the max flow coming into my wet well is 1400 GPM. The sewer main that pumps the sewage into the wet well has a 2' diameter. The pump I selected is a submersible one, I will be using two pumps (one is a backup). The only thing I need to figure out is the height of the wet well. Can anyone suggest me a wet well height that sounds reasonable for my situation?
Thanks!





RE: Wet Well Volume
1. How much will your pump move during you 'on'cycle?
2. How long is your 'off' cycle?
3. Is this 2 foot diameter pipe affected by wet weather?
4. What size is your discharge pipe?
You should be able to figure it out if you have this info.
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Wet Well Volume
You need to step back and do a little research. Experienced professionals are now installing self cleaning lift stations because of the operational advantages. There are several papers out there that will assist you in sizing and developing the geometry of the lift station.
http://www.pump-zone.com/topics/pumps/pumps/design...
http://www.hatchmott.com/sites/dev.hatchmott.com/f...
RE: Wet Well Volume
RE: Wet Well Volume
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Wet Well Volume
If you are putting 1400 gpm through a 24" pipe, the velocity is less than 1 ft/sec. You need a minimum of 2 ft/sec for continuously pumped systems or 3.5 ft/sec for intermittently pumped systems to avoid solids deposition. I'm surprised they haven't had trouble already. If you can't do anything about the incoming pipe, at least make sure your new effluent pipe is the correct size.
You should have a 12" or possibly a 16" pipe at maximum to maintain the right velocity.
Without other considerations, I would use a small cycle time such as 8 minutes to size the wetwell. If you do this, it leads to a 8 foot storage height for an 8 foot diameter wetwell, which intuitively seems reasonable for an absolute minimum. The incoming pipe invert should be at least 5 feet deep to begin with.
You probably need an eight foot diameter wetwell to hold the pumps, which will be fairly large for a duplex system.
The few systems I have designed for that large of a flow have been triplex pumps, but I haven't used VFDs.
RE: Wet Well Volume
I realize that I need to do a bit more research into things before I go fully into this thing. Thankfully, I haven't showed anyone important my plans yet and I'm just sort of an intern at the moment.
I like jgailla's response and I actually did make the effluent pipe 12" just like he mentioned since I also noticed that the velocity is very small when coming in.
The wet well I'm making now is looking at a diameter of 6' and a height of 20'. I also wasn't aware that VFD pumps have pump cycle times, I thought only constant pumps have those. I'm looking to use a submersible Flygt pump that is 3' tall and it will be submerged 5' under water (which means there will be 2' of water above it). I will also have a backflow sewer pipe near the top of the wet well so that if the wet well ever overflows, the water can go back into that sewer line. The effluent pipe will be 12" in diameter and will be ductile iron with PVC liner for corrosion protection.
Feedback is appreciated on this new updated design.
RE: Wet Well Volume
For a medium size pump station such as your are describing, you should have at least 2 pumps plus a standby. A full understanding of the base and peak flows are also necessary to properly size the pumps and wetwell.
For V/S pumping, the wet well need only be a sump for pumping at the instantaneous system flow rate. The only storage required is enough to allow pumps to be shut down or started up without an excessive change in water level, and the water surface area needed for that purpose is quite small. Furthermore, the change of water surface elevation can be reduced to the amount needed for regulating the speed of the pumps.
With V/S pumping, the storage time will be based on providing sufficient time for a change in capacity when a pump is started or stopped. When a pump is started, the motor must be ramped to the desired speed, and the pump currently online must be reduced in speed. The time required for this is usually 1-2 minutes.
Fo C/S pumps, the pump station should be optimized so that the number of permissable motor starts per hour is not exceeded. C/S wet wells are usually significantly larger than V/S wet wells. The cost of V/S pumping equipment is usually much less expensive than the cost of the C/S concrete wet well.
RE: Wet Well Volume
Pump cycle times should be evaluated when installing C/S pumps. Operating pumps with excessive pump starts will shorten the service lifetime of the pumps.
Not familiar with PVC lining of ductile iron piping except for relining projects. Where did you get this PVC lining information?
Also don't understand the overflow concept. Lift stations are typically designed to handle the influent flow.
RE: Wet Well Volume
RE: Wet Well Volume
You would have to know what elevations will cause sewer backups and overflows before you can consider this overflow scenario.
For a lift station of this size, you should have at least 2 pumps plus a backup. The lift station should also be equipped with an emergency generator.
RE: Wet Well Volume
RE: Wet Well Volume
If you allow the lift station water level to rise, you will surcharge all of the incoming sewers.
RE: Wet Well Volume
You are correct with respect that there are no restrictions in the number of times you can start and stop a VFD pump. But having said that, to me, I would be very conservative in sizing a wetwell to give you plenty of float. It would be better to have more volume than not enough volume just in case something unexpectantly happended or the inflow characteristics are not accurate. Nobody is going to thank you for trying to save them money. when the sewer system backs up in someones house, by cutting down on the wetwell size to save costs. All of the logic that you used with that approach won't make sense when this happens. Save yourself some second guessing and some sleep and size the wetwell conservatively.
RE: Wet Well Volume
A VFD pump has the same limitations on starting and stopping as any other pump motor. VFD Pump Stations are usually designed to operate continuously so you generally do not have to be concerned with start/stops.
Too frequent start/stops will burn up a VFD pump motor in the same manner as a C/S motor.
A sewage lift station has less than 30 minutes of volume in any case. Being conservative in sizing the wet well is not going to prevent a sewage spill. Proper design of the lift station will do that.
RE: Wet Well Volume
you may need a second pump (vfd or not) which starts when flows exceed the capacity of the first pump. Make sure it does not start and stop frequently.
RE: Wet Well Volume
RE: Wet Well Volume
From a practical operations point of view, sometimes equipment such as VFD's, MCC's, motorized valve operators, PLC's, SCADA, level measuring instruments or human operators act up. Or calculations are incorrect. All I was trying to say was reducing the size of the wetwell to barebones minimum because one assumes everything will work 100% 27/7/365 does not give you much play leftover to react when something goes wrong.
RE: Wet Well Volume
"When a VFD is applied to a motor, surge current is almost eliminated depending on the inertia of the load and the time required for the motor to reach full or required speed. The result is that there is virtually no limit to number of motor starts per hour, regardless of size."
RE: Wet Well Volume
RE: Wet Well Volume
VFD motors, as I understand it, are just "regular" motors with larger external casings. The larger external casings allow the heat to be dissappated faster. So for a 20 hp VFD motor you would probably be getting a 25 hp casing