Why Is Your Submersible Sewage Pump Losing Flow?

In the world of wastewater management, a drop in flow is the first “silent alarm” before a total system failure. If your submersible pump is humming but the discharge has slowed to a trickle, the issue usually hides beneath the surface.

At Tussu Pump, we’ve analyzed thousands of field cases. Here is why your flow is dropping and how to diagnose it like a pro.

1. The “Hidden” Impeller Erosion

It’s not just about a total blockage. In sewage applications, grit and small abrasives act like sandpaper. Over time, they “shave” the edges of your impeller or increase the clearance between the impeller and the wear plate.

• The Symptom: The pump runs smoothly, but the pressure gauge never hits its target.

• The Fix: Pull the pump and check the wear ring clearance. If it’s over the manufacturer’s spec, even a high-RPM motor won’t push the volume you need.

2. Partial Clogging in the Volute (The “Ragging” Effect)

Unlike a hard rock that stops the pump instantly, “ragging” (wet wipes, hair, and fibers) often creates a partial bypass. These materials wrap around the impeller eye, narrowing the passage without tripping the breaker.

• The Symptom: A slow, gradual decrease in flow over several days, often accompanied by a slight increase in motor temperature.

• Pro Tip: If you’re frequently dealing with fibers, it might be time to upgrade to a open impeller or a grinder pump configuration.

3. Air Binding: The Invisible Wall

If your pump pit is turbulent or the “off” level is set too low, air can get trapped in the volute. Because air is compressible and water is not, the impeller just spins in an air pocket, unable to create the centrifugal force required to lift the sewage.

• The Fix: Check your air release valve. If it’s crusty or blocked, the air has nowhere to go. Ensure the “Stop” level keeps the pump casing fully submerged.

4. The Check Valve Struggle

Sometimes the pump is fine, but the system is fighting itself. A partially stuck check valve (non-return valve) or a build-up of fats and oils (FOG) in the discharge pipe increases back-pressure.

• How to tell: If the pump sounds “strained” (a deeper growl than usual), the restriction is likely downstream in the piping, not in the pump itself.

5. Reverse Rotation (The “New Install” Mistake)

It sounds basic, but we see it often after a control panel repair. If the three-phase wiring is swapped, a centrifugal pump will still push water while running backward, but only at about 30–50% of its rated capacity.

• The Fix: Verify rotation direction. A quick “kick-start” check on the surface will tell you if the torque is moving the right way.

Get Your Flow Back to 100%

A drop in flow is an invitation to inspect, not just ignore. Whether it’s time for a high-chrome impeller upgrade or a simple volute cleaning, Tussu Pump provides the heavy-duty parts and engineering support to keep your wet well moving.

Need a technical drawing or a replacement part list? [Contact our Engineering Team:[email protected]] or [Whatsapp:+86-13633310657].