Why a Self-Priming Pump Can’t Lift 8 m (and What to Use)

Not long ago we received an enquiry from Australia for a self-priming sewage pump to deliver 50 m³/h with the pump starting at a 6.5 m suction lift and stopping at 8 m. No vacuum system was allowed. We had to decline, and the reason applies to every self-priming pump, not just ours.

The pump does not really suck

A pump on the surface cannot pull water up. It lowers the pressure at its inlet, and the atmosphere pushes the water up the suction pipe. At sea level, atmospheric pressure can hold up a column of water about 10.3 m high, and that is the absolute ceiling with a perfect vacuum.

In a real installation, several things eat into those 10.3 m:

• Friction in the suction pipe and fittings.
• The vapour pressure of the liquid. Warm water boils at a lower pressure, so the margin shrinks as the temperature rises.
• The pressure the pump itself needs at its inlet to avoid cavitation, known as NPSH required. It grows with the flow.
• Altitude. At 1,000 m above sea level there is roughly 1.2 m less to work with.

Add these up and a practical self-priming sewage pump runs out of margin long before 8 m. Our self-priming pumps are rated for a maximum suction lift of 5 m, which leaves enough margin for reliable priming and cavitation-free running. Reaching 8 m would need an external vacuum unit to help the pump prime and hold its prime.

What happens if you try anyway

The pump may prime slowly or not at all, the flow drops, and the pump cavitates. You hear it as a gravelly noise, and over time it eats into the impeller.

Better options for deep pits

• A submersible sewage pump sits in the liquid, so there is no suction lift at all.
• A vertical submerged pump keeps the motor dry above the pit while the pump end stays in the liquid.
• If the pump must stay on the surface, lower its mounting level so the suction lift stays within 5 m.

You can check this in our pump selector: choose a dry installation, enter your suction lift, and it will tell you when a self-priming pump is out of range.