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Passive Priming

Whenever possible, priming should be obtained by allowing the fluid to naturally fill the pump head, driven by gravity alone.

For this to happen, the pump must be installed in the correct location and orientation. In particular:

The pump should be placed well below the free surface level of the fluid in the tank
Air traps in the suction line should be avoided
The pump should be installed in such way that the air inside it is free to escape. This means that either the inlet or the outlet should point upwards
The discharge side of the pump should offer as little load as possible, so that the air can be easily flushed away

In conclusion, the ideal installation for optimal priming will look like in Figure 1.

Figure 1: ideal suction line design and pump installation

If the four points in the recommended guidelines are not followed, the user might incur in the situations described in the following sections.

Pump not well Below the Tank

Gravity feed happens if the pump inlet is well below the free surface level of the fluid in the upstream tank. If the pump is placed next to the tank (as in Figure 2), or under a nearly empty tank (as in Figure 3), then the static pressure will not be sufficient to flush away the air in the pump.

Figure 2: A pump placed at the same level of the tank will not prime

Figure 3: If the upstream tank is nearly empty, the static pressure could be too low to flush the air away, and the vortex in the tank might cause some air to enter the pump

Air Traps in the Suction Line

The suction line must be designed so that bubbles don’t accumulate in air traps. Long flat tubes (Figure 4) and bends (Figure 5) are critical elements in these regards. The suction line should be as short and simple as possible.

Figure 4: long flat tubes can accumulate bubbles and reduce priming capabilities

Figure 5: Such a bend in the suction line will prevent the pump from priming.

Pump Head Orientation

The orientation of the pump head also plays a role in the ability to evacuate the air. If the pump outlet points downwards, an air trap will form in the pump head, preventing priming. The pump should be installed in such way so that either the inlet or the outlet point upwards (as show in Figure 6), allowing the air bubbles to escape by floating away.

Figure 6: The orientation of the pump is important to allow the air to flow out

Restricted Discharge Line

Lastly, the air bubbles in the pump must be able to flow out, flushed by the flow rate. To ensure sufficient flow rate, the discharge line must be as “open” as possible, and offer the least possible system load. If the discharge side is too restricted (as in Figure 7), the air might remain in the pump head.

Figure 7: the discharge line should be as unrestricted as possible, to enhance air expulsion from the pump head

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