No. Many pumps are not self-priming, and assuming that they are can result in a pump running without liquid, failing to produce flow or suffering damage.
A standard centrifugal pump is generally not self-priming. If the pump sits above the liquid level and the suction pipe contains air, it normally cannot remove that air and draw liquid up into the pump.
Self-priming centrifugal pumps are designed specifically to deal with this situation. Some positive displacement pumps can also self-prime, although their dry suction capability varies considerably between pump types and designs.
So if a pump is described as self-priming, don’t stop there. You still need to know how much suction lift it can handle and whether it will work with the actual liquid and installation.
What does self-priming actually mean?
Priming means removing air or gas from the pump and suction line so liquid can enter the pump.
For a centrifugal pump to generate its normal hydraulic performance, the impeller needs to be surrounded by liquid. An impeller spinning in air cannot generate the same pressure difference that it produces when operating in liquid.
This becomes important when the pump is installed above the liquid source.
Imagine a pump sitting two metres above an open tank. Before startup, the suction pipe may contain mostly air. The pump has to remove this air before atmospheric pressure can push liquid up the suction pipe towards the pump.
Most standard centrifugal pumps cannot do this by themselves. The Hydraulic Institute notes that most centrifugal pumps are not self-priming or submersible and may require manual filling and venting after losing prime.
If the pump is installed below the liquid level, the situation is different. Gravity can keep the suction pipe and pump casing full of liquid. This is normally called a flooded suction arrangement.
In that case, self-priming capability may not be required at all.
Why standard centrifugal pumps generally cannot self-prime
A conventional centrifugal pump transfers energy to liquid using a rotating impeller.
When the casing contains liquid, the impeller accelerates that liquid and develops pressure and flow.
Air behaves very differently.
Its density is much lower than that of water or most process liquids. A standard centrifugal impeller spinning in an air-filled casing therefore produces very little suction pressure difference.
The pump may run, but the liquid does not rise far enough through the suction line to reach it.
This is why a conventional centrifugal pump installed above its liquid source normally needs to be primed before startup.
Priming methods can include:
- manually filling the pump and suction line
- using a foot valve to help retain liquid in the suction line
- using an external vacuum priming system
- changing the installation to provide flooded suction
- using a pump specifically designed for self-priming duty
Air entering the suction line can also cause a centrifugal pump that was previously operating correctly to lose its prime.
How does a self-priming centrifugal pump work?
A self-priming centrifugal pump uses a casing and internal recirculation arrangement designed to separate air from liquid during startup.
The casing contains a quantity of liquid.
When the pump starts, that liquid recirculates through the pump while mixing with air being pulled from the suction pipe. The air separates from the liquid and is discharged while the liquid continues circulating.
As more air is removed, pressure in the suction line falls.
Atmospheric pressure acting on the liquid surface then pushes the liquid up the suction pipe towards the pump.
Once the suction line fills and liquid reaches the pump, it begins operating more like a conventional centrifugal pump.
The U.S. Department of Energy definition of a self-priming pump describes a pump designed to lift liquid originating below the pump inlet. It also notes that these pumps generally require initial manual filling of the casing but can subsequently re-prime without an external vacuum source or further manual filling.
This distinction matters.
Self-priming does not necessarily mean the pump can start completely dry.
The casing may still need to contain priming liquid before startup.
Can positive displacement pumps self-prime?
Many positive displacement pumps have better self-priming characteristics than centrifugal pumps.
Positive displacement pumps move a defined volume by trapping and displacing fluid mechanically. Depending on the pump design, this action can also move air and reduce the pressure in the suction line.
Examples of pump technologies that may have self-priming capability include:
- diaphragm pumps
- peristaltic pumps
- gear pumps
- screw pumps
- progressing cavity pumps
- rotary lobe pumps
However, they should not all be treated the same.
Internal clearances, valve design, pump speed, sealing, whether the pump is wet or dry, liquid viscosity and pump condition can all affect suction performance.
The Hydraulic Institute includes pump priming, dry running and air or vapour handling as separate operating considerations when assessing positive displacement pumps.
A pump being classed as positive displacement therefore does not automatically tell you how many metres it can lift liquid during startup.
Check the manufacturer’s rated dry suction lift or wet suction lift for the actual pump.
Self-priming does not mean unlimited suction lift
This is where the term “self-priming” can become misleading.
A self-priming pump cannot pull liquid upward indefinitely.
The pump creates a lower pressure in the suction line. Atmospheric pressure acting on the liquid surface provides the force that pushes the liquid towards the pump.
There is therefore a physical limit to suction lift.
For water at sea level, the theoretical limit created by atmospheric pressure is approximately 10.3 metres. A real pump cannot achieve this full theoretical value because the system also has friction losses, vapour pressure requirements and other losses.
The available suction conditions are commonly assessed using Net Positive Suction Head Available (NPSHA).
NPSHA describes the amount of pressure available at the pump suction above the liquid’s vapour pressure.
The Hydraulic Institute’s NPSHA calculation includes atmospheric pressure, suction pressure or elevation, velocity and the liquid’s vapour pressure.
Increase the suction lift and the pressure available at the pump inlet falls.
Increase the liquid temperature and its vapour pressure generally rises, reducing the margin further.
Add a long suction pipe, undersized pipe, valves, strainers or fittings and friction losses increase.
All of these factors can reduce the practical suction lift.
The liquid matters too
A suction lift quoted for cold water should not automatically be applied to every fluid.
Consider:
- liquid temperature
- vapour pressure
- viscosity
- specific gravity
- dissolved or entrained gas
- suction pipe diameter
- suction pipe length
- fittings and valves
- pump speed
- elevation above sea level
A pump that primes successfully on cool water may behave very differently when handling a hot liquid close to its boiling point.
Likewise, a pump tested with a short suction pipe may take much longer to prime when connected to a long suction line containing a large volume of air.
This is why suction lift should be treated as an operating condition, not only a pump specification.
Check the actual suction conditions
If a pump is listed as self-priming, check the suction lift it is rated for.
Then compare that with the actual installation.
For example:
- Pump is 3 m above minimum liquid level.
- Liquid temperature is 40 °C.
- Suction pipe is 6 m long.
- Suction pipe contains two bends and a strainer.
- Pump may start with an empty suction pipe.
- Liquid viscosity is 50 cP.
All of these can affect whether the pump will prime reliably.
A pump being called self-priming is only part of the answer.
Key takeaways
- Most standard centrifugal pumps are not self-priming.
- A conventional centrifugal pump normally needs liquid in the casing before it can operate correctly.
- Self-priming centrifugal pumps use retained liquid to remove air from the suction line during startup.
- Many positive displacement pumps can move air and self-prime, but suction capability varies by pump design.
- Self-priming does not mean unlimited suction lift.
- Check suction elevation, NPSHA, liquid temperature, vapour pressure, suction losses and the pump’s rated priming performance before selection.
