Lift Station and VFD's

[tmurf]

I have two questions about a sewage lift station. The station has three identical pumps operated by VFD’s. When flow conditions require more than 1 pump to operate is where my first question leads.

Is it better to hold the speed of the lead pump at 100%, and vary the lag pump, or operate both pumps at reduced speed as required by the flow conditions?

The second is more of a comment than a question.
Would it not be better for a control system to use the full capacity of a wet well during higher flows, rather than a control system that used a fixed level as a set point?

 

[rcooper]

tmurf,

In my experience, it is normal to operate both pumps at a reduced speed. If you operate the duty pump at 100%, you will have to make sure the assist pump starts at a higher speed than the duty pump started at, as it will be pumping against a higher head. Additionally, the assist pump will be pumping a long way off its Best Efficience Point (high head, low flow), so will be prone to clogging, reduced bearing and seal life.

I have used a control system in the past which, once all the pumps have started and reached maximum speed, they all pump down to a single stop point. This was to assist cleaning the bottom of the sump and allow a flush of sewage into the sump from the sewer. Care must be taken though, as step changes in flow to a treatment works can be detrimental to the process.

 

[piffer]

tmurf,
ITT Flygt's pump selection program (FLYPS) does nice graphical presentations of multiple VFD operated pumps. The best way depends on your system (system head curve). There are also advanced pump station VFD controllers (duplex, triplex, etc) on the market that will optimize multiple pump operation (check out multitrode's website).