Constant Speed Pump Control

The bands

The off level sits above the pump minimum submergence; the lead-on level sits above it by a volume that gives an acceptable cycle at the inflow that produces the most starts, which is an inflow of about half the pump capacity; the lag-on level sits higher by enough volume to let the lead pump prove itself before the second is called; the high alarm sits above that with time to respond; and the overflow is above everything with as much margin as the well provides. The bands are entered on the screen with limits, shown on the level display, and written in the control narrative.

Starting

A soft stop matters more than a soft start on many force mains: a full-speed pump stopped by opening its contactor lets the column reverse and slam the check valve, and the pressure wave travels the main. Soft starters with a ramp-down, slow-closing check valves, and surge relief where the main is long address it. The generator sees the inrush too, and a staggered start with a soft starter is what lets a modest generator carry two pumps.

Alternation and staging

The lead alternates each cycle so that both pumps run daily and wear evenly; the lag starts when the level reaches its band, which means inflow exceeds one pump or the lead is not delivering; and a pump that fails is skipped with the other promoted and an alarm raised. A triplex station adds a standby stage. All of this lives in a small controller with a relay backup on floats, or entirely in relays with an alternator on a simple station, and the logic is tested by faulting each pump.

The concerns

Septicity

Wastewater that sits in a wet well for hours at low inflow goes septic and produces sulfide, which corrodes concrete and smells. A minimum cycle frequency, achieved by a lower off level at night or a timed pump-down, keeps the detention within limits; the trade is starts.

Grease and solids

Fats and rags float and collect at the off level and on the walls. A periodic pump-down below the normal off level, to the minimum submergence, skims the accumulation into the force main; the sequence runs it on a schedule and never with the level control blind.

Surge

Full-speed stops on a long main; addressed by soft stops and valves as above.

Starts

A band that has drifted or a float that sticks raises the starts per hour past the motor rating and the motor fails hot; the starts count per pump is trended and alarmed.

Ragging

A pump that runs long with rising current is wrapped in rags; some controllers run a reverse or a repeated start sequence to clear it, and the current trend is the detection.

When variable speed earns its cost

• Flow must be limited to what a downstream plant or main can take.
• The force main or the electrical system cannot tolerate full-speed starts and stops even with soft starters.
• The station has enough friction head that the energy saving is real, which is rarer than the sales literature suggests.
• Two stations in series need to be matched so that the upper one does not flood the lower one.
• Even then, a periodic full-speed run to scour the main is part of the sequence.