The ability of a pump to self-prime can be affected by several factors.

1. AIR LEAK IN SUCTION LINE
As fluid recirculates in the pump and forces air out of the discharge chamber, it’s trying to create an area of low pressure. However, if With all types of pump, the suction line must be air-tight.  If air continues to be drawn into the pump, the pressure is never reduced and fluid is not drawn up the suction line.

2. DEBRIS IN THE IMPELLER

If the eye of then impeller is plugged with debris, it removes the hydraulic capability of the impeller to create an area of low pressure.

3. PUMP IS AIR BOUND
A pump can become air bound if it experiences any of these conditions:

3.1 NO AIR RELEASE LINE
If there is no air release line, air cannot be vented to the atmosphere, and instead collects on the discharge side.

3.2 DISCHARGE LINE IS PRESSURIZED
Valve on the air release line is closed, and the valve on the discharge line is closed, again, giving no place for the air to go and get out of the pump.

3.3 XCESSIVE IMPELLER CLEARANCE
If there is excessive clearance between the impeller and the wear plate, the pump has a difficult time creating a low pressure area. This is typically caused by wear, but could also be due to improper reassembly.

4. PLUGGED RECIRCULATION PORT
During the priming process, as discussed above, fluid is recirculated through the volute casing. If the recirculation port becomes plugged, the eye of the impeller is unable to create an area of low pressure in which to pull liquid up the suction line.

5. LIFT TOO HIGH FOR PUMP SPEED OR IMPELLER DIAMETER
If you’ve undersized the pump for the suction line, it will not be able to create the low pressure area it needs to prime. It’s important to understand the suction lift requirements before selecting a pump for the application.
The ability for self-priming pumps to prime hinges on all the right conditions. The pump must be able to evacuate air from inside the pump, create a low pressure area at the eye of the impeller, and also be properly sized for the right NPSH conditions.

When pumping water in cold environments, it’s important, if the temperature may fall below freezing, to drain a pump or to provide some form of heating.  Damage may occur if water freezes in the pump or pipework.

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