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Wet Well Volume

Wet Well Volume

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?


RE: Wet Well Volume

A couple things you need to know.
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.

RE: Wet Well Volume

Where did you come up with the 4-foot diameter? That diameter is too small as you will not be able to fit your pumps inside of it. And VFD's are considered to be cconstant pumping systems.

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.

RE: Wet Well Volume

I don't know much about force mains, but a 24" sewer dropping 1400 gpm of flow sounds like an awful lot for a 4' diameter wet well.

RE: Wet Well Volume

I would check ITT-Flygt re sewage pumping systems, look at their website as they might have a programme there to help you.

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

Four foot diameter is too small and will probably not hold pumps capable of pumping that flow.
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

Thanks for the feedback everyone,

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

An 8 minute cycle time is much too large for a V/S pumping station. The 24-Inch incoming gravity sewer is probably sizes correctly if it is installed at minimum pipe slope.

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

You are correct, you do not have to consider the cycle time when you are using VFD pumps. VFD pumps should be sized to operate at the instantaneous system flow.

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

Yup, VFD lift stations are indeed suppose to be able to handle exactly how much influent flow is coming in. But if both pumps (including the backup one) fails then the wet well would overflow, but if I had an exiting trunk line near the top of the wet well, then the water could leave from there if the wet well water level gets that high.

RE: Wet Well Volume

Allowing the lift station to backup into the sewer would seem to be a risky proposition. If the lift station backs up, it may cause sewer backups in every tributary to the lift station. The sewer may in turn back up into residences. The lift station owner would probably be liable for the cleanup costs of the sewer backups.

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

The backed up water won't be going back into the same trunk that it came out from though, it's going into another sewer trunk that leads to possibly some other water treatment plant in the district.

RE: Wet Well Volume

I was referring to upstream of your planned lift station, not the downstream side.

If you allow the lift station water level to rise, you will surcharge all of the incoming sewers.

RE: Wet Well Volume

"....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)...."

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

Regarding "You are correct with respect that there are no restrictions in the number of times you can start and stop a VFD pump."

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

odors and corrosion can be a big issue with oversized wet wells. More float and a bigger wet well is not always a good idea.
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

In all due respects bimr I think if you check with the motor manufacturers VFD driven motors do NOT have restrictions on the number of starts and stop. I had this confirmed by US Motors a number of years ago and by major pump manufacturers. I think it is because the inrush on startup is slowly reamped up. Therefore the heat generation on the winding insulation is not as severe

RE: Wet Well Volume

"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."

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

Qualitytime, you are correct regarding starting motors with VFD's.

"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

bimr..thank you!

RE: Wet Well Volume

The following was explained to me a long time ago about the difference between a constant speed motor and a VFD motor:

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

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