Self-priming pump: how it works, which to choose and why it does not suck

A self-priming pump is the one that can pull up water by itself, even when it is mounted higher than the liquid to be sucked and the suction pipe is full of air. On a boat, it is a feature needed almost everywhere: to wash the deck and chain with seawater, transfer fuel or water, empty the bilge, circulate water in a system. Here we see how it works, what types exist, how to choose it, and why sometimes it stops sucking.

How a self-priming pump works

A normal centrifugal pump must be full of liquid to work: if there is air in the pump body and suction pipe, the impeller spins empty and does not suck. For this reason, as also explained in the Wikipedia entry on centrifugal pump, pumps, normal pumps require a foot valve on the suction line to keep it full. A self-priming pump, instead, is built to expel air from the suction pipe and draw in liquid without external help.

On boats, there are mainly three families:

• self-priming centrifugal pumps have a pump body that retains a reserve of liquid: after the first filling, they recreate the vacuum in the suction pipe by themselves, even at every restart;
• flexible rubber impeller pumps also suck starting dry, because the blades deform creating chambers that move air and water; these are used for engine cooling and many wash pumps;
• diaphragm pumps, used in freshwater autoclaves, tolerate dry running well and operate with a pressure switch.

Where it is used on board

The right type depends on the job. For washing the deck and chain, flow rate and good head are needed; for transferring, the ability to suck from a lower tank counts; for circulating a system, a few liters per minute continuously are enough. For small 12 V systems, there is the Osculati 16.530.12 circulation self-priming pump; for washing and transfers, rubber impeller pumps like the Marco UP1 at 12 V, with a declared 37 l/min and priming up to 1.5 m, and the larger UP1-M at 45 l/min.

When higher flow rates and heads are needed, for example for a fire-fighting system, powerful washing, or service on large boats, professional self-priming centrifugal pumps are used. The Gianneschi CP series is declared self-priming up to 8 m vertical and is available in 12 V version, like the CP 20 A1 G, ; in 24 V, like the CP 26 A2, ; and in 230 V, like the CP 30 A1 G.

How to choose the self-priming pump

Before comparing models, it is advisable to write down four data points: what you need to pump, the height of the liquid relative to the pump, where it must reach, and with what power supply. Then the criteria are these:

1. flow rate and head are read together on the pump curve: the CP 30 A1, for example, delivers about 20 l/min at 17 m head and 60 l/min at 6 m, so the useful flow depends on how high and how far you push the water;
2. the declared suction height is a maximum under ideal conditions: it is better to stay well below, because bends, filters, and long pipes reduce it;
3. the material of the pump body and impeller must be suitable for the liquid, with bronze, stainless steel, or technopolymers for seawater and specific versions for diesel;
4. the expected service, continuous or intermittent, and the tolerance to dry running, which for rubber impeller pumps is very limited;
5. the voltage, 12 V, 24 V or 230 V, and the current draw, which determines the cable section and the fuse.

Installation: where priming is lost

A self-priming pump only sucks well if the suction pipe is perfectly sealed. A loose clamp or a threaded fitting without sealant is enough for the pump to suck air instead of water. For a working installation:

• install the pump as close as possible to the liquid level and keep the suction short and with few bends;
• use a reinforced hose that does not collapse under vacuum, with a diameter not smaller than the pump inlet;
• put an inspectable filter on the suction, especially with seawater;
• tighten every hose holder with stainless steel clamps and seal threaded fittings;
• for centrifugal pumps, fill the pump body before the first start-up, as indicated by the manufacturer.

Why the self-priming pump does not suck

When a pump that was working stops sucking, the causes are almost always few and checked in this order:

1. air in the suction line, from clamps, fittings or cracked hoses;
2. filter or sea intake clogged by algae, sand or bags;
3. damaged rubber impeller, often after running dry even for a short time;
4. pump body emptied on a centrifugal pump, which must be filled again;
5. collapsed suction hose, too soft or too long;
6. low voltage on DC models, due to thin cables or oxidized terminals;
7. on three-phase pumps, reversed rotation direction after work on the system.

For a broader diagnosis on onboard pumps, find a dedicated guide to common problems of marine pumps; if instead the problem concerns the pressure system that does not start or stop, the culprit is often the pressure switch, which we discuss in the guide to the pressure switch of the pressure system.

The impeller: the spare part to keep onboard

On flexible impeller pumps, the impeller is the part that wears out. It works wet and is damaged in seconds if it runs dry, loses elasticity over time and hardens if the pump remains stopped for a long time. It is advisable to replace it at the start of the season or after a number of hours indicated by the manufacturer and always have a spare one. Many impellers have cross-references between brands: for example the 40 mm impeller Trem P0340002 corresponds to Jabsco codes 22405-001, Johnson 09-808B-1, and Volvo Penta 875583. During the replacement, also check the wear plate and the cover gasket: a cover that does not close properly lets air in and the pump loses its prime.

How much does a self-priming pump cost

The price range is very wide. Small 12 V circulation pumps cost a few dozen euros, rubber impeller pumps for washing and transfer usually cost under two hundred euros, while professional self-priming centrifugal pumps for intensive use easily exceed six hundred euros and go beyond a thousand for the most powerful versions. The price mainly depends on flow rate, pump body materials, and type of service, so it is best to start from actual needs rather than the most powerful model.

All models, spare parts, and impellers are in the category marine pumps. If you tell us what you need to pump, the suction height, and the onboard voltage, we will help you find the suitable model.

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