An almost universal rule of lift stations is that the engineer designing the lift station does his calculations and adds a safety margin. He or she passes it to someone else in the organization who adds a safety margin. Then it goes back to the city for approval and they add a safety margin.

Finally, there is always the chance that when the PO is with the supplier, he or she says “sorry, we’re out of stock of that model, but I have the next size up – tell you what, I’ll sell it to you for the same price as the smaller pump!”

The net result is pumps which are much too large for the application, running for very short times and not at all at their best efficiency point.

There’s a great article about how VFDs can improve the energy efficiency of pump stations by Joe Evans of Pentair writing for Pumps & Systems. The VFD lowers the effective output of the pump and runs it closer to its best efficiency point.

Another way that utilities solve the practical problem of the over-sized pumps is to add a jockey pump later on. This is a more appropriately sized pump for operation most of the time, with no concerns about worst case inflow condition because the larger pumps will take over.

Some stations are designed like this from the outset – especially stations with infrequent high inflow conditions where the engineer has recognized that large pumps are occasionally necessary but running these large pumps for very short periods is not ideal.

What’s the Lift Station Logic?

The lift station or pump station logic we use in MultiSmart is to group pumps.

So with the case of 2 large pumps and 1 jockey pump, Group 1 would have the jockey pump (let’s call it P1) and Group 2 would have both large pumps (let’s call them P2 and P3).

Group 2 would most likely be set to alternate – and it could be set to alternate based on efficiency to save energy automatically. Group 1 doesn’t need an alternation scheme because there is only 1 pump. (If there were 2 jockey pumps they would probably be set to alternate as well).

We configure the setpoints of P2 and P3 above the jockey pump, P1. And we set a parameter called Max Groups Running to 1, and Block Running Pumps to True (which is the default).

Now what happens is the level rises and P1 starts. The well empties and P1 stops. And the cycle continues. But once a time is reached when P1 can’t handle the inflow, the well level will keep rising and the start point for the lead pump in Group 2 is reached. When that happens, Group 1 (which just contains P1) turns off and the lead pump in Group 2 starts.

So long as this large pump empties the well, once the stop point for the lead pump is Group 2 is reached, Group 1 will again take over again from Group 2.

Depending on whether you have Duty/Assist or Duty/Standby you would set Max pumps to run in Group 2 accordingly. If you want both pumps to run together you don’t have to change the default (no limit), if you only want one pump to run, then you set Max pumps to run in Group 2 to 1. It’s an important point because a lot of stations don’t have the pipework to handle both pumps running together and all you do is double the energy consumed for a 10% increase in flow.

The way the logic works if Max pumps to run =1, and the level for the lag pump is reached, is the running pump stops and the other pump takes over.

Technical Note

The way the logic works by default, in high inflow conditions, the jockey pump, P1, will start each cycle, and then the larger pump will take over. If you don’t want P1 to always start, add some custom logic in IsaGRAF, or the logic engine. That logic would be very simple – once Group 2 starts, hold out Pump 1 (or Group 1), and then when the calculated inflow drops below a certain value, remove that hold out fault.

Summary of Configuration

Create a 3-pump station using the setup wizard
In Settings – > Alternation & Grouping:

• Create a new group
• Move P2 and P3 into Group 2
• Check Group 2 is in “Alternate (Std)” – the default
• Check that Group Alternation is “Fixed (Std)” – the default

Configure your setpoints as you want through Settings -> Setpoints -> Level/Control Setpoints
If you do want only one of the large pumps to run at one time, configure Max Pumps to Run in Group 2 – Settings -> Advanced – > Pump Control -> Group -> Group 2 -> Max pump running

admin In the well efficiency, high inflow, lift station