Centrifugal pump priming

Priming fills a pump's casing, and on a suction lift its suction line, with liquid before the pump starts, and vents the air out. A centrifugal pump has to be primed. Started full of air, it churns the air without creating suction, and its suction lift is much poorer than a piston or plunger pump's. This page covers why that is, the three ways to prime, and what to do once the pump is full.

Why a centrifugal pump needs priming

• Large clearances. A centrifugal pump has large running clearances between the impeller and the casing. Positive displacement pumps have minute clearances, so they can draw a considerable vacuum at the suction and are called self-priming. Centrifugal pumps have much poorer suction lift than piston or plunger pumps.
• No liquid, no cooling. Liquid in the pump cools and lubricates its internal surfaces and cuts wear. A pump with a mechanical seal must never run dry, not even for a few minutes, because the sealing faces overheat and end up grooved and scored. A squeal means the seal is dry, but a quiet seal is no proof that the faces are wet.

Three ways to prime a centrifugal pump

1. The pump sits below its supply. Liquid can flow in by gravity.

2. The pump is on a suction lift, primed from an outside supply. The supply is below the pump, so a foot valve holds the water in.

3. The pump is primed from its own discharge line. A bypass from the discharge line carries priming water to the casing.

The foot valve

On a suction lift, the whole priming problem is that the liquid wants to fall back to the source. The foot valve is a flap type check valve at the submerged inlet of the suction line. It lets liquid flow up toward the pump and closes against reverse flow, so it holds the primed column in place.

The same one-way action keeps the prime between runs, so the operator doesn't have to re-prime at every start. That only works while the foot valve seals well. A pump that held its prime yesterday and needs repriming today points to a leaking foot valve.

Don't confuse the foot valve with the suction strainer. The strainer keeps debris out and is often fitted near the foot valve, but it's a separate part. It goes in the suction line as close as possible to the pump suction.

How high can a pump lift?

Atmospheric pressure on the supply surface pushes liquid up the suction line, so it sets the theoretical maximum lift. At sea level, about 101.3 kPa supports a water column 10.34 m high. Atmospheric pressure falls with altitude: at 240 m it supports about 10 m of water, and at 1 100 m about 9.1 m. Friction and leakage keep real pumps below these figures.

The suction side must also supply enough head. The net positive suction head available (NPSHA), worked out from the suction-side conditions, must be greater than the net positive suction head the pump requires (NPSHR), so the pump doesn't cavitate or become vapour-bound.

After priming: starting the pump

• Start against a closed discharge. A centrifugal pump needs little power at no flow. Starting a large one against a closed discharge valve avoids the power surge, and the motor overload trip, that can come with starting it wide open.
• Open the discharge slowly. Once the pump is up to speed, open the discharge valve slowly. Then check the stuffing box leakage and the other running checks.
• Don't leave it deadheaded. With the discharge shut, the impeller churns the trapped liquid. That energy turns into heat, which boils the liquid and leads to cavitation, vapour-binding and pump damage.
• Never throttle the suction. Closing the suction valve starves the impeller, damages the seals or packing, and causes cavitation and vibration. Flow is controlled on the discharge side.

Positive displacement pumps are different

Positive displacement pumps are self-priming, but they're still primed by hand before starting, because running full of liquid cools and lubricates them. Reciprocating pumps have the smallest clearances and the greatest suction lift. Rotary pumps lift less and benefit from priming when the suction lift is large.

Study it in B2 Pumps & Compressors

Priming is in Part B, B2 Pumps & Compressors: CH01 Types of Pumps and CH02 Pump Operation and Maintenance. By Pass 4's estimate, B2 is 10% of the Part B paper. Try a real Part B question on the 4B practice questions page, or see the Part B plans.

Pass 4 is not affiliated with SOPEEC, Technical Safety BC, or any other exam or licensing body.