Having purchased a petrol powered 3" water pump with the plan to drain some of the water from the pond to enable the remval of the detritis from a recently felled willow tree, here are a few notes on how to prime the pump.
Priming the pump
If you read the instructions and make Internet searches in how to do this it all seems so simple.
Most (all) of the demo videos showing the priming process a 2" pipe pump were featured. Seeing that a 3" pipe can carry a larger volume of water it makes sense that it will be more difficult to prime. Also using a 5m suction pipe requires more effort to ensure that water finds its way to the pump so that the centrifugal action can draw more water into the pump housing to start the pumping process. A 3 inch pipe represents a lot bigger volume than a 2 inch pipe (Pi R Squared and all that) the amount of air that is in the suction pipe is likewise a lot more. Although the pump is capable of "lifting" water to a maximum height of 8m it needs to have about around 1m of air or less in the suction pipe for the pump to start. Air is compressable water is not.
The "trick" to get the pump to move water is to have it as close to the water level that you want to pump and submerge the suction pipe as close to the pump as you can. If you are using a transparent suction pipe you can see when the water reaches the pump as the air is expelled from it. The pump will change its tone as it starts to pump water. If you are using a lay-flat discharge pipe you can see it fill with water. You can also see if there is any air leakage in the connection of the suction hose, this is primarily between the fitting on the pump and the hose if the clamp is not tightened enough. The use of some tape will help here.
Debris in the impeller
As I am pumping a pond that had a willow tree dropping its leafs there is a large chance that the impeller will get clogged.