I have an external pump that is rated at 3800 gph what size pipe should I
use? My pump actually moves around 1800 GPH currently and forms a suction on
the 2" inlet pipe. My pump, has a 4' tall p-trap, that has a 2" intake that
is located 6" below the water line and can not run with the top off the Pump
Trap. The pump will drain the canister dry. I imagine this is causing the
flow to be restricted also. I am going to but a larger hole in the liner and
put a 3" pipe in then reduce to 2" pipe right before the canister and hope
this helps with the flow. Water only fills about a quarter of the pipe
running into the p-trap when it flow naturally". I am thinking of moving the
hole down further in the liner say 12" to help fill the canister up faster.
I think the canister should be filled up so fast I have to keep the cap on
to keep water from the pond from overflowing not to keep the canister from
running dry.

"Mickey" writes:

I have an external pump that is rated at 3800 gph what size pipe should I
use? My pump actually moves around 1800 GPH currently and forms a suction on
the 2" inlet pipe. My pump, has a 4' tall p-trap, that has a 2" intake that
is located 6" below the water line and can not run with the top off the Pump
Trap. The pump will drain the canister dry.

Is it designed to run without the top? If your pump is above the pond water
level I don't think its possible.

I imagine this is causing the
flow to be restricted also. I am going to but a larger hole in the liner and
put a 3" pipe in then reduce to 2" pipe right before the canister and hope
this helps with the flow.

How long is the 2" piece of pipe that is being replaced with 3"? A few feet
won't make much difference.

Water only fills about a quarter of the pipe
running into the p-trap when it flow naturally". I am thinking of moving the
hole down further in the liner say 12" to help fill the canister up faster.

This will not help unless you lower the pump. Once the intake pipe is below
the surface, lowering it won't change the pressure that the pump inlet 'sees'.
Is the inlet below the surface? You say its only filled 1/4 with water, it is hard to
picture how this can happen.

When the pump is off and the top off the canister, where is the water level?

I think the canister should be filled up so fast I have to keep the cap on
to keep water from the pond from overflowing not to keep the canister from
running dry.

Canister below pond water level? Then it should overflow. Above pond water level
it will suck air as you describe.

What is the outlet pipe size and plumbing (fittings and pipe lengths)?
Could this be reducing your flow? I assume you measured the 1800gph at
the pond, try measuring right out of the pump and see if its substantially higher.

You could also try priming the canister (pump off, block the intake at the pond
and pump outlet and fill canister with water then cap it) to see if that helps.

HTH