Constant speed pump

[CuriousElectron]

Greetings,

Can somebody confirm that constant speed pumps are used in lift stations because the objective is to pump the water up to a higher elevation and no flow modulation is required. Seems straightforward, I just wanted to get confirmation from people with industrial/waste water experience. That is another reason such a pump(non-clog type) runs at full capacity, because its primary objective is to maintain a pre-set level in the dry pit.

Thanks,
EE

 

[Artisi]

Provided inflow is constant.

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.)

 

[Compositepro]

Constant speed pumps (centrifugal) are the most common pumps used for most applications because they are simple, robust, and relatively inexpensive. Constant speed does not mean non-modulated flow. Valves are used to modulate flow. The "constant speed" has more to do with the induction motor than the pump. The power used by an induction motor has little to do with its speed. The motor runs at a speed determined by the power frequency. As more load is applied to the motor it will slow slightly and provide more power to keep the speed up. A motor that is running at "full speed" is actually using the minimum amount of power.

Throttling a centrifugal pump does dissipate "some" energy but it does far more to reduce the power used by the pump. The power used by a pump is equal to flow rate times differential pressure. The pump curve of most centrifugal pumps is fairly flat over their useful operating flow rates. This means that the pressure it generates changes little as flow rate changes. Thus throttling can cause large changes in flow with little change in pressure, which means that throttling greatly reduces power consumption.

VFD's are very often not a good solution for saving energy with centrifugal pumps. The pumps must often run at a large percentage of full speed just to generate sufficient pressure to create any flow at all. A VFD will lose about 4% of its full power rating just being in the circuit.

 

[LittleInch]

if the vertical lift part of the discharge is a major part of the head required from the pump then yes, as this is a fixed value then there is no benefit to changing speed of the pump as this might result in no flow if insufficient head is achieved.

Fixed speed is also a lot cheaper.

The objective part is not as clear and how a pump is controlled is very variable. you can't make sweeping generalisations

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[fel3]

Constant speed pumps are very common in small sewage and storm drainage lift stations. And, it is not necessary to have constant inflow, so long as a wetwell with sufficient working volume is provided on the suction side of the pumps. The purpose of the wetwell is to control pump starts and stops in response to varying inflows.* The wetwell working volume is a function of the predicted peak inflow, the number of duty pumps, the pump capacity, and the desired frequency of pump starts and stops.

I have designed a couple dozen small lift stations over the years. All but one were duplex stations with constant speed submersible pumps in a wetwell. The exception was a wetwell-drywell design, but the pumps were still constant speed. For me, "small" is pump capacities up to about 1,000 gpm, pump horsepowers up to about 25 hp, and wetwell working volumes up to about 3,000 gallons. For duplex lift stations, individual pumps are designed to handle at least the predicted peak inflow. For triplex and larger lift stations, the predicted peak flow can be divided between more than one pump.

As inflows increase, the wetwell size for constant speed pumping must increase. At some point, the larger and larger wetwells become uneconomical. I don't know where that point is because I have only designed a handful of "large" lift stations and there is a big gap between my largest "small" lift station and my smallest "large" lift station. My largest lift station (storm drainage) pumped up to 53 cfs (25,800 gpm) with two 100 hp (IIRC) variable speed pumps and a tiny wetwell.


* With the pump off, varying inflows fill the wetwell over some period of time. When the pump ON level is reached, the duty pump starts and draws down the wetwell to the pump OFF level. The pump stops and the cycle repeats. Usually, the duty/standby designations for the pumps swap each cycle. Also, if actual inflow exceeds the capacity of the duty pump, the wetwell level continues to rise to a second pump ON level. This starts the standby pump and then both pumps operate together to draw down the wetwell. If actual inflow exceeds what the two pumps can handle, the wetwell level continues to rise to the ALARM level, and then you're in deep ****.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Compositepro]

The OP has not logged in since posting the question.

 

[CuriousElectron]

Friends,

Thanks to all who responded; I read all responses and benefited from each.

Blessings,
EE