Priming a pump simply means adding water directly to the pump basket to 'wet' the impeller and pump chamber, and give it something to pump initially. If the pump is above the water level it needs to pull enough of a vacuum to suck the water into the pump basket. When the pump is totally dry, the clearance between the impeller and the housing is too great to generate this vacuum. All the thing is doing is kicking air around...even self priming pumps probably won't pull water when totally dry. It all depends on the pump type and how high above water level the pump is located. Another factor controlling how much vacuum it can pull is what is on the pressure or output side of the pump but the way most systems are plumbed this isn't usually a show stopper to starting flow. Alsd note that is may take a few minutes of running to get the water flowing. Procedure is to just fill the pump basket with water as much as you can, close and tighten the lid, and fire the thing up.
Hope this answers your question.
Al
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